Thorn Cycles Forum

Technical => Wheels, Tyres and Brakes => Topic started by: Andre Jute on October 30, 2011, 07:31:54 AM

Title: In praise of riding low pressure tyres fast
Post by: Andre Jute on October 30, 2011, 07:31:54 AM
Warnings from Andy Blance about the dangers overinflation pose to your rims may give a few of you the impetus to lower pressures, fit fatter tyres, or even graduate all the way up to balloons. See for instance p8 of the Mercury brochure: http://www.sjscycles.com/thornpdf/ThornMercuryHiRes.pdf

In return for years of good information and a great deal of entertainment on this board, mainly on the Rohloff forum, I thought I'd put something back by sharing some observations on riding fat low pressure tyres, which I've done for years. My everyday bike, a Utopia Kranich, is in fact designed from the ground up to have 60x622 Schwalbe Big Apples as its only suspension. I live among the hills of West Cork, and ride on very poorly surfaced lanes, either badly potholed or, if newly fixed, rough with big tar-set chips. This is an edited version of my reply on rec.bicycles.tech to a question by Pete Cresswell about whether the weight of fat tyres would slow him down:

***

PeteCresswell wrote:
> I'm tempted to put out the big bucks for a set of Big Apples just
> to see how good/bad they are compared to more normal tires. From
> what I've read, speed for a given effort isn't too bad, but
> acceleration/liveliness suffers noticeably.  "You pays your money
> and you takes your choice...."

There are explanations on the net by Jobst Brandt and Sheldon Brown about why slick or nearly-slick tyres of any width offer less rolling resistance than patterned tyres. And wider slicks have less rolling resistance than narrow slicks. Deeper technical explanations in my Designing and Building Special Cars, if you can find a copy.

Admittedly, Big Apples are *heavy*. There is a certain amount of effort
required to get them moving. Still, once they're moving, the same
effect works in reverse, and they just keep on rolling; the Big Apples
contribute much to making your bike and you feel like a powerful
rolling force, just about unstoppable. I was used to paying for
Marathon Plus, so the Big Apples didn't strike me as expensive. I will
say though that, if you're worried about the weight, you should spend
the few bucks more and get the Big Apple Liteskins which are pounds
lighter per pair, and the superlight racing tube as well for another
substantial weight saving. My tubes are three years and 5000km of
potholed roads old and I've never had a flat. My Liteskins show no
great signs of wear either, and they haven't been mollycoddled.

As for the handling, I am very impressed with the Big Apples. I can't
make a direct comparison, because the two shorter-wheelbase, naturally
more nippy bikes that I have don't offer wide enough forks for Big
Apples, whereas the bike I have the Big Apples on has a very long
wheelbase and a very laid-back geometry, good for stable fast touring
on sweeping bends rather than darting in and out of traffic.
(Actually, since I fitted a electric motor, I've surprised myself by
doing a bit of nipping in and out of traffic, so the tyres are more
capable than the extent to which I've been using them, limited more by
my accelerative legpower from low speed than by their size and
construction.)

On tarmac, which is all I know, the roadholding and handling of the
Big Apples are beyond anything on a bicycle you're likely to know. On
fast sweeping downhills, and on tight corners too, on bad road nobody can stay with me. Everyone who tries to keep up arrives at the bottom of the
big hills around here white and stressed, and with their coccyx
hurting. I've never, including some scary moments with a tractor in
the dusk of a winter's eve, even approached the real limits of the Big
Apple roadholding. And, I must tell you, I keep mine pretty soft
(usually under 2 bar for an all-up weight in the order of 130kg), so I
sacrifice no comfort for speed. Those big round tyres appear to cling
like shit to a baby's blanket with any part of them that you care to
roll the bike over onto. I've never been down on the Big Apples. (The
one time I should have gone down, after a spectacular downhill slide of sixty
metres on icy slurry, I crashed into the wheel of a tractor parked
across the road, and landed on my feet, with my bike upright too.)

The speed, the security, the recoveries possible from situations that
would put you down on another tyre (riding off a broken verge of road
with the front wheel), any one of these capabilities would make the
Big Apple worth the price. But we haven't even talked of the main
reason to fit it.

> The two chief offenders that I see around here are tree roots
> under blacktop - pushing it up into ridges; and concrete slabs
> with many cracks that mis-align over time.

My bike is designed from the ground up to take the biggest Big Apples,
60x622mm. They are its main suspension. My Brooks saddle is the three
helical spring model B73 but the seatpost is solid. There isn't even any gel
in my handblebar grips: they're Brooks' solid, edge-on leather rings,
hard as rock but surprisingly comfortable in combination with the Big Apples. I cycle with unpadded thin leather dress gloves.  The frame itself is of lightweight steel and crossframe design, triangulated in three dimensions and capable of resisting more than 5000lbs per inch of twist (same as a big Rolls-Royce car), in short ultra-stiff. The entire suspension is thus in the Big Apples.

I am ultra-sensitive to vibrations in my hands. The roads here are
pretty rough. Even when they're newly made, they're uneven and the top
surface hardly ever smooth; the lanes  and minor roads I ride would,
in the States, get the official in charge of them summarily dismissed
five days a week. But in the nearly three years I've had the Utopia
Kranich with the Big Apple Liteskins, this is the first time I've
given a thought to residual stress injury in my wrists from vibrations
on my bike, and then only to say I gave it no thought for 34 months.

Microvibrations are an important problem on a bicycle, and it is one the Big
Apple designer understands or has lucked into an answer to. You
can't storm a Big Apple equipped bike across the sharper speed bumps
without feeling the effect, but I do ride mine faster across the
speedbumps at the supermarket than any other cyclist in town. But
that's the sort of bump you are aware of, and take measures to
ameliorate. What you can't see, and what mechanical or hydraulic
suspension on bikes is too stiff and slow to handle well, is
microvibrations from the road. This the Big Apples handles
brilliantly: one day you just remember that your hands and wrists
haven't hurt for quite a while. It's the least visible of all the Big
Apple advantages but to my way of thinking the most dramatic.

Out on the open road, I ride the Big Apples straight through potholes
that would stop a Marathon Plus equipped bike by throwing off the
rider, and that my roadie friends have to slow to ride around. I feel
no pain. The ability of the Big Apples to take anything in their
stride without disturbance to the rider or his line is a big part of
their ability to set impressive point to point times, a consistent and
significant fraction better than the widely respected and very capable
Marathon Plus which was my previous favourite.

As you can see, I'm very impressed. I've already bought replacements
for my Big Apple Liteskins and the Superleicht racing tubes (type 19A)
-- same again. They come in boxes the size of shoeboxes for
substantial pairs of boots. They sit reassuringly on top of my biggest
bookcase.

I know, everyone fits 29er tyres to rims for skinny tyres, and ERTRO made a special exception to the rules about matching tyre width to rim width for it. I don't care what ERTRO does for commercial reasons. For engineering reasons, more than adequately explained in Andy Blance's note reproduced above, it is not clever. A rim narrower than 40% of the tyre width defeats the purpose of the Big Apple -- which we have seen is a cushy, fast, secure ride -- by forcing you to inflate the tyre to a much higher pressure, which not only cuts severely into your comfort, but into your speed as well, because the jarring from the hard tyres throws you off line.

Big Apples cry out for rims at least 25mm wide, preferably wider. For the record, my 60mm/2.35in Big Apples ride on Exal XL rims, 25mm wide on the inside, and I'm just about to build an electric front wheel on a Rigida Big Bull rim, also 25mm wide on the inside, because I can't source an Exal XL in 622 size. On wider rims you can reduce the pressure still more. Well made very wide rims are available: try the unicycle suppliers.

Andre Jute
There are photographs of my roads on my personal netsite:
 http://coolmainpress.com/BICYCLING.html
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on October 30, 2011, 10:44:12 AM
I am not technically minded and respect the opinions of people more knowledgable in this field than I.

However, I find Andy Blance's comment  "... perhaps the more uncomfortable the ride, the faster they think they are going." to be more than slightly condescending.

I had Panaracer Paselas on my RST which I inflated to roughly 55 psi front and 60 psi rear: with this setup I felt the bike was slower than I hoped it would be - certainly much slower than my audax bike running 700x25 tyres at 100 psi.  I found that when riding with friends some on 700 x 32 touring tyres I had to pedal vigorously to keep up while they freewheeld on long descents - I suspect there may be other factors in there too e.g. seals/bearings in the Rohloff, drag from Schmidt Son dyno-hub.  However, since I switched to 1.6 Supremes inflated to 80/85 I found the bike rolls better, and I feel I get more response when out of the saddle climbing; but the proof of the pudding for me is that my average speeds increased with increased tyre pressue.

In summary, I am not qualified to question the wisdom offered above, but according to the clock it doesn't work for me.

I am therefore more than a little perplexed, maybe the last straw has descended on the RST's back.


P.S.  I have now sent a query to Schwalbe about this, will update if/when I get a response. 
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on October 30, 2011, 06:36:22 PM
I'm so glad you started this as a separate thread, Andre, and as usual, you've expressed your point skillfully and enjoyably.  Your experience has been a more extreme example of what I've found (I have yet to try a tire as wide as the Big Apple), yet I have also experienced baffling disappointment at times, much as Relayer has expressed.  Here are extracts from some off-list correspondence I generated on the subject --
-------------------------------------
...The second revelation that changed my cycling life in a positive way was learning about the importance of tire pressure.  In 1989, an American magazine, _Bicycling_, published an article on the topic by their contributing technical editor, Frank Berto, who for many years worked as a quality-control and measurement engineer for the petroleum industry in California.  He consulted experts at Michelin (French) and National (Japanese) tire companies and summarized their recommendations in an article and accompanying chart.  These are widely available on the 'Net in either their original form or adapted and commented on by others, the most well-known being Seattelite Jan Heine, who has pushed the concept forward through replication across a variety of other tires, mostly of 650B designation.

After extensive testing, the engineers concluded tire pressure should be adjusted so the rim "drops" about 15% under the total intended load including bike, rider, and luggage.  They concluded a tire drop of 15% makes for the best combination of performance characteristics including low rolling resistance, comfort, and cornering traction.  As you might imagine, the amount of air needed in a given size tire will vary depending on the weight of the bicycle and rider and any load it carries.  In other words, a lightweight rider alone will use less pressure than a rider who is carrying a full load of touring gear, food, and water.  Of course, air pressure will also vary according to tire width and section profile (or tire height).

Of course, all this must be taken ceteris paribus, as there are many contributing variables, including rim width and depth between the sidewall bead-retaining "hooks", the tube used, overall mass, profile shape and section, and perhaps most telling -- tire construction.  As an aside, I once tried some 700x35C Michelin City Pilot tires, and felt as if I were riding through molasses.  Or across a mattress.  No, it was more akin to riding across a mattress on which an entire jar of molasses had been spilt in cold weather, an altogether horrid experience.  I actually had to pedal to coast, which is to say I could not coast, only decelerate rapidly absent intervention.  In contrast, Schwalbe Marathon Plus tires in the exact-same size were a relative pleasure to ride, and the key difference between them was construction in general and the composition of the flat-resisting layer in particular.  As an additional data point, I found the 700x35C Marathon Plus tires slower than my 26x2.0 Duremes, but this is on different bikes with a host of other variables in play.  The difference between these tires was not due simply to feel or vibrational frequency leading to differences in the relative perception of speed.  No, it was something quantifiable with instrumentation over an identical closed course as well as a negative difference in qualitative feel.  There was minimal testing bias as well, since I had just purchased the City Pilots and did not relish laying out additional coin for the Marathon Plusses.  The City Pilots simply proved unlivable and irredeemable in their sluggishness for my intended use.  They were, simply, Evil.

It took me awhile to understand the entire concept, as it was opposite to what I had always supposed and thought and opposite conventional cycling dogma in the Modern Age.  It is opposite to what most people think, and it does not seem logical or sensible at the apogee of an era where tires grew ever more skinny and pressures bloomed, all in an effort to banish the last iota of rolling resistance.  Eventually, I began to grasp the logic behind it and decided to give it a try when I replaced my 700x25mm tires with 28mm, and then 32mm, and adjusted the tire pressure in each.

Suddenly, riding was so much more comfortable.  My hands didn't get as sore because there was less vibration through the handlebars.  My bottom was more comfortable because there was less road shock coming up through the frame, seatpost, and saddle.  I could corner faster and with more confidence because small bumps and pebbles didn't deflect the bicycle sideways.  Best of all, my components began to last longer because they were also spared the extremes of road shock that came with higher, inappropriate pressures.  Even the tires themselves lasted longer, and I developed fewer flat tires because the tire better could conform to sharp objects instead of being punctured like a balloon stretched taut.  When operated at the appropriate pressure, the tire functions as a true suspension with minimal increase in rolling resistance.  Above or below that optimal point (again, all other things being equal), the bicycle will dip into the outlying extremes of the performance-rolling resistance curve and the outcome will be less than stellar.

The basic concept also dictates that narrow tires must be run at higher pressures to prevent pinch flats.  Fatter, wider tires can be run at lower pressures because they have greater air volume.  The label on a tire may only indicate a single recommended pressure, and it might not be appropriate for the load being carried.  As Berto referenced in his original article, the maximum pressure indicate may more accurately reflect only a safety margin above which the tire may be reliably expected to blow off the rim and therefore may be only a hedge against possible torte claims.  Schwalbe is very good about printing a range of pressures in their tire sidewalls, which reflects their understanding that one single pressure is not appropriate for every situation.
-------------------------------------
The thing that astounded me was the idea that a wider tire with lower pressure could actually be faster than a narrow tire run at high pressure.  So much depends upon tire construction and design and on the pressure used and, of course, those many other variables mentioned above.  Get it right, and things are golden; the ride of your life.  Get it wrong, and you'll be visiting that molasses-strewn mattress.  In general, though, the idea of wider tires run at appropriate pressures has been one of the Great Positive Revelations in my cycling life.

As a side note, my 1989 Miyata came equipped with tires of a radial construction, rather than the usual bias ply design.  While they were remarkably "fast" rolling, I quickly replaced them as the radial construction made for a squirmy and insecure ride, especially when banking into turns.  As with a radial car tire, the rim could move laterally atop the planted tread and felt dreadful in a single-track vehicle like my bicycle. I recall motorcyclists expressing similar dislikes and reservations over radial-ply tires when they were first introduced for that market.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on October 30, 2011, 07:30:43 PM
Relayer,

Your concerns and observations are certainly valid _for you_ and should not be discounted.  I too have had similar baffling experiences at times with certain combinations of tires, tubes, rims, section width/height/profile, etc. and I look forward eagerly to any insights Schwalbe can provide.  Please keep us apprised of any developments.

One observation I can make is that when one changes from one tire width to another, handling also changes in ways that are not always obvious, and this can affect the speed-comfort equation as well (in ways other than rolling resistance, of course).

One of these is the idea of pneumatic trail.  As a hobbyist framebuilder, I quickly learned the importance of intended tire size and profile when drawing-up my frame designs, as tires affect many parameters including outright clearances as well as handling in the finished frame.  Since tire height (profile) is closely correlated to section width (in many cases, at a near 1:1 ratio), wider tires are also taller, and the inverse is true for narrower ones.  Changes in tire width/height affect bottom bracket height (or "drop" when measured in isolation as a frame-design parameter), front-center clearances (with concerns about toe-tire overlap at the font wheel) and effective measurements of trail as a function of head angle extended to a ground plane intersecting with vertical measurement at an offset dropout as a function of fork rake.  Fitting a larger tire is akin to fitting a bigger wheel, and this has a manifold effect on handling due to pneumatic changes in trail.

Take to extremes (and assuming one had clearances to allow for it), changing from, say, a 25mm tire to one of 50mm will radically alter the effective geometry of an already-built frame, a parameter often overlooked in the tire width debate.  I have seen poor-handling bikes salvaged and good ones wrecked through inappropriate tire choice.  Shimmy with a handlebar bag, for example, can be induced or cured with a sizable change in tire section width due to a concomitant change in height/profile.  Mixing tire widths F/R alters handling in surprising ways that do not occur with mixed tire pressures, since those are load-proportionate and tend to equal out.

Apart from the science, bicycles are magical things.  Unable to stand alone (absent a kickstand), they are dynamically stable with a rider.  If you saw one ridden for the first time in a circus, you couldn't help but applaud.

Best,

Dan.

Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on October 30, 2011, 07:53:35 PM
This has become a well-discussed topic in a number of other online forums and sites, and those observations are helpful.  By request and for the convenience of those following the thread here, I've reproduced a number of the more pertinent links below:

http://www.google.com/url?sa=t&rct=j&q=heine%20the%20performance%20of%20tires&source=web&cd=2&sqi=2&ved=0CGAQFjAB&url=http%3A%2F%2Fwww.bikequarterly.com%2Fimages%2FBQ64TireTest.pdf&ei=ALOrTu_bGarTiAKf_LmACw&usg=AFQjCNF-3CY6_XEBdQ4Y7Uuzw_AspdvJrg

The above link will take you to an interesting PDF article on tire width and pressures written by Jan Heine. Although it does not include 26" tires (only 700C and 650B tires are included), the ideas still apply to 26" touring tires.

Jan Heine's testing and research also led to the surprising conclusion that ultralight (latex) tubes may actually _increase_ rolling resistance instead of reducing it (see link to PDF of original article above). Jan Heine's ideas are controversial, and need to be repeated a number of times using scientific rigor and method, but they do reflect a growing body of user experience and perception. It is all very interesting to me, and it flies in the face of conventional bicycle orthodoxy. Absent more formal testing, his work is about the best we have at the moment.

Thorn designer Andy Blance tends to agree that wider 26" tires may be more appropriate and faster (or as fast) as narrower 700C wheels in certain circumstances, and details his thoughts in an article presented on the Thorn website here: http://www.thorncycles.co.uk/why26inchwheels.html

This link...

http://www.google.com/url?sa=t&rct=j&q=heine%20the%20performance%20of%20tires&source=web&cd=3&sqi=2&ved=0CGcQFjAC&url=http%3A%2F%2Fwww.adventurecycling.org%2Fresources%2F200903_PSIRX_Heine.pdf&ei=ALOrTu_bGarTiAKf_LmACw&usg=AFQjCNG-WnUZeDbpJhiu5EJYvbuiVPSPSA&cad=rja

...will take you to another presentation of Heine's article adapted from Berto's data.

Jan Heine greatly expands his ideas on optimum tire pressures in an article from his blog and includes additional links on the topic, here:

http://janheine.wordpress.com/2010/10/18/science-and-bicycles-1-tires-and-pressure/

http://janheine.wordpress.com/2011/02/01/a-journey-of-discovery-part-3-wide-650b-tires/

Additional, interesting discussions of tire pressure, size/width, rolling resistance, and comfort can be found here:

An update of Berto's original article with wider, 26" tires included:
http://www.google.com/url?sa=t&rct=j&q=heine%20the%20performance%20of%20tires&source=web&cd=15&ved=0CH4QFjAEOAo&url=http%3A%2F%2Fwww.bccclub.org%2Fdocuments%2FTireinflation.pdf&ei=sLurTrXgFeemiQLYrL32Cg&usg=AFQjCNG0brdux1kpIIRk4JMOvrS1cmY86w&cad=rja

http://www.bikeforums.net/showthread.php/592097-Informal-comparison-23mm-v.-28mm-tires.

http://www.bikeforums.net/showthread.php/537756-Tandem-Tire-Size-Pressure

http://www.williambenedict.com/bikes/tires.html

http://forums.roadbikereview.com/general-cycling-discussion/reduced-air-pressure-reduced-rolling-resistance-229343.html

http://janheine.wordpress.com/2011/01/20/tire-sizes/

http://www.livestrong.com/article/350585-bike-tires-that-support-a-lot-of-weight/

A particularly good discussion of pressures and rolling resistance appears here: http://www.pickledpedallers.co.nz/General03.html

http://community.terrybicycles.com/wordpress/?p=114

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on October 30, 2011, 08:24:48 PM
Another reason why it is desirable to have lots of air volume in a touring tire...

I have attached a photo I took during my June 2010 bicycle tour of Oregon, California, and Nevada's Great Basin Desert.

It shows damage caused to the belt of my 700x32C tire.  Road construction caused me to ride in the gravel at the side of the pavement, and to avoid a car, I ran over the leg of a portable traffic sign advising motorists to use caution.  I was riding slowly, but the sign support was steel, and of course my bicycle was heavily loaded.  Although the tire was properly inflated, there was not much air volume in that size, and the impact caused the stiff Kevlar belt to split and the tube herniated through the hole.  I knew it would probably fail very soon, but I wasn't in a good place to fix it, so I rode on another 12km till it blew.  Of course, that was when I discovered all my spare tubes were defective -- the valve stems had not been properly vulcanized so they pulled out even before I could inflate them ( http://www.thorncycles.co.uk/forums/index.php?topic=3752.0 for a pic).

The damage to the pictured tire was caused by impact, rather than puncture.  The tube puncture happened after the belt parted.

This is one reason why I chose the 2.0 Duremes for the Sherpa.  They might not have fared any better, but the extra air volume and lower pressure would have increased my chances of avoiding damage that caused this flat tire.  I believe the wider Duremes will help when carrying a heavy touring load, and they seem to roll very nicely when I am riding the Sherpa unloaded.

In my experience, it is also possible to damage a Kevlar belt in a similar way by over-inflating a tire.  The Kevlar belt is very stiff and the rubber is very elastic, and excess pressure can overstress the belt and cause it to split or cause the overlying tread to separate, ruining the tire.

Best,

Dan.  (eager to get away from a string trimmer and yardwork on a very rainy Fall day)
Title: Re: In praise of riding low pressure tyres fast
Post by: slim on October 31, 2011, 12:02:40 AM
Relayer,

An link on the Schwalbe site with some excellent information:

http://www.schwalbetires.com/tech_info/tire_dimensions
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on October 31, 2011, 12:40:09 AM
I see two factors that would lead to fat tires stressing narrow rims. Certainly the angle with which the tire casing leaves the rim can cause the tire pressure to pull the rim apart. But a fat tire also has more tension in the casing than a narrow tire, for the same pressure. Here is my argument:

(http://i140.photobucket.com/albums/r6/kukulaj/tirepressure.jpg)

Imagine the circumference C getting a little bigger. The tension of the casing has to stop this from happening. A bigger circumference gives a bigger diameter - the whole tire is yielding a bit to the pressure. This change in energy is force times distance - everything here is per unit length for some very long cylinder. The same force times distance formula can then take that energy back to the tension in the casing. For a fixed pressure, the tension in the casing is proportional to the radius (or width) of the tire.
 
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on October 31, 2011, 12:54:11 AM

An link on the Schwalbe site with some excellent information:


It looks like the Rigida Andra 30 rims have an inner well width of 19 mm, so the range of widths in the ERTO standards manual would be 28 to 50, or in inches 1.1 to 2.0. A wider width then brings in the lower  pressure limits, according to Schwalbe.
 
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on October 31, 2011, 01:02:51 AM
@Relayer: Don't forget to share when you get a reply from Schwalbe. I would say, in general, that the answer lies, first, in sidewall construction, and secondly in the technical details of the anti-puncture band. I'd have no difficulty in believing that someone at Schwalbe knows a thing or two about tyre compounds... I've had very good luck with their tyres. Also, the comfort band of inflation, while wide enough as a percentage, falls off fast on either side; for my circumstances, with the bike and gear and rider probably weighing 125-130kg in everyday trim, i.e. very heavy indeed, the comfort band is an amazingly wide 1.6 bar to 2.1 bar, below this getting a bit close to snakebites caused by crashing through potholes at speed, above noticeably less comfortable. I think the Schwalbe's recommendation is pretty conservative, and a Dutch cyclist I know who doesn't mollycoddle his mountain bike with the same 29er Big Apples also inflates in the same band. I normally inflate once a month to a fraction over 2 bar and at the end of the month the tyres are down to around 1.7 bar but you can't tell from the ride or roadholding until the pressure falls to 1.5 bar, which (on no evidence but simply because one must make a choice) I consider the lower limit for my weight; I carry a BBB electronic manometer in the on-bike toolkit for a check, but in fact the gauge on my SKS Rennkompressor standpump is good enough and close enough. For the speeds I get up to, on most hills in the 45-70kph bracket, I haven't found it necessary to have different pressures front and rear. I could probably tune out most of my bike's understeer with differential front and rear pressures but I suspect I would lose part of the predictability and security, and at those speeds they're pretty valuable.

@Danneaux: I enjoyed your posta, Dan; thanks for the references, a couple of which I don't remember seeing before. I didn't arrive at balloons with a bang, but over a period of years and via several bikes with ever wider tyres. I was fortunate that I arrived in cycling 20 years ago from motor racing, where my specialty was suspensions, so that at least I didn't suffer the tyre prejudices, especially with regard to slicks on tarmac, that so bedevil discussions between cyclists. I still remember Jobst Brandt, the ex-Porsche engineer who was behind the slicks that Avocet was (I think) the first to introduce to American cyclists, having tremendous difficulties explaining to engineers on rec.bicycles.tech why slick tyres work better... As for explaining why bikes cannot aquaplane, life isn't long enough; I'll leave that to someone who is paid to do it, like Andy Blance. (Now there's gratitude for all the years I've been copying his excellent-value, longlasting component choices, and learning from his articles how to fit a bike!)

@JimK: Long before you get to failures, I reckon fitting fat tyres on narrow rims is a waste of money because inevitably you must inflate to a higher pressure to keep the tyre on the narrow rim, which loses the comfort advantage of the wider tyre. My rule of thumb is that the rim's minimum width should be 40% (across the beads, not the outside measurement) of the tyre's ERTRO width. Chalo Colina, a noted American bicycle mechanic who himself weighs 350 pounds plus, and has extensive experience of Big Apples, likes wider rims still, obtained from the unicycle crowd (38mm inside width on a Kris Holm rim!), which in turn lets him come down to 2 bar, an amazing thing with his weight.

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on October 31, 2011, 04:11:55 PM
Today I reduced the Supremes from 80/85 to 55/60 psi to be within Andy Blance's limit for the protection of my Rigida Grizzlys. 

First of all I was astonished at how firm they felt having just been reduced a whopping 25 psi!  I then went for a run on one of my shorter usual routes and the road noise from the tyres was much reduced (I don't recall this happening when I tried it with the Paselas) and there was no apparent increase in rolling resistance.  The tread on the front tyre didn't indicate any wider area of rubber in contact with the road.  The comfort aspect was improved, especially for my hands.  The average speed was on a par with recent spins on the Supremes at highest pressure, and it was quite windy out today.  Overall this ride was a definite improvement on past experiences.

I will therefore not be going above 55/60 and will be happy in the knowledge that my rims are safe - thank you Andy Blance.  I am also very happy with my ride today and am a convert to the tyre drop or low pressure theory, thanks to all the input on this thread and previous input from Vik which unfortunately I didn't retry when I changed tyres.

Next steps will be to try to gauge the lower pressure limit that will suit me, and maybe an optimum level/range, I think it might be fun doing this.  Only thing is, I am beginning to wonder if the Supremes will be too slick/slippery for winter!!     ::)
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on October 31, 2011, 04:30:01 PM
Relayer,

Thanks for the latest report; I'm really pleased for you!

I, too, have found different tires/tire construction can produce remarkably different results that can transform a bike's ride and handling one way or another, and different tires can respond differently to changes in air pressure, depending on construction, materials, and design.

Here's a little observation, born of boredom on a long day ride:

It seems when I have optimally matched tire to rim and have the ideal pressure, the worn/clean portion of the tire (the part that wets first or dries first when meeting a puddle or having ridden through dirt) is the same width as the rim, regardless of tire size.  I've often pondered this as I rode along and looked down at the spinning front tire.  It would be fun to take this observation further and see if there is a correlation between road surface contact width, optimal pressure, and rim width.  Anyone else noticed this?

One last suggestion, Relayer -- Duremes are essentially the same tires as Supremes, but with a relatively light tread, which may provide a bit better mechanical connection on loose surfaces such as dirt.  They will not, however, provide more traction on wet pavement.  Debate is pretty much settled on that matter, concluding that slick bicycle tires do not hydroplane. [EDIT:  Slicks surely do lose traction on wet grass, mud, and dirt, even on a tandem.  It is a bit disconcerting to find the tandem's torque on startup was enough to spin the rear wheel in place without going anywhere.  Marvelous for the ego until I realized it was gear-boosted torque and not raw muscle power. Even the light tread on the Dureme would have helped the mechanical link between tire and surface].

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Eric on November 01, 2011, 08:44:08 AM
After experimenting - I've settled on 45front 55 rear on my new RST with 1.6 Supremes......fast, comfortable and no pinch punctures (tempting fate :-\?). 90 kg man who travels with handlebar bag and two panniers; mainly on good roads with the odd track.
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 02, 2011, 11:10:21 AM
I have not had a response from Schwalbe yet, however slim posted a link to their tech site above from which I found an interesting page about rolling resistance  http://www.schwalbetires.com/tech_info/rolling_resistance#why

While it is brief it gave me a couple of gems of information:-

1)  it confirms that wider tyres roll better than skinny ones, but the important qualification to this which is often missed by authors or not picked up by me is this applies at the same tyre pressure - clearly given the limitations on tyre pressures with 26" rims we are talking apples and pears between 26" and 700c wheels.  By looking at the wider [tyre] picture my belief that speed and comfort are inversely proportional has been restored.   ;D

2)the resistance chart shows rolling resistance is very low compared to air resistance which rises steeply with force/speed. Tyre width is mentioned in relation to air resistance - this doesn't worry me but sometimes I do wonder about air resistance with wide mudguards.

3) there is a nice paragraph about tyre deflection with a chart.  (Dan: maybe the answer to width of tyre contact area for the same tyre at different pressures could lie in this - does it flatten solely lengthwise?)

I am happy that my RST sits comfortably between my fast tourer/audax bike (700 x 25) and my sadly underused MTB - the RST gets by far the most use of the three.   ;)

Dan, I have ordered myself a pair of Duremes and platform pedals for the MTB to try out over the winter, already put a B17 on it.  Medium term this could be a first step towards it transferring to a fat tyred load carrying tourer frame; longer term to becoming car-free?  Who knows.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 02, 2011, 07:35:06 PM
Hi Relayer,

That Schwalbe chart you found is a real gem.  Here are some anecdotal, random thoughts and observations in response...

1) The whole matter of rolling resistance illustrates the importance of careful research design and stating conclusions "cetaris paribus" ("with other things the same") to minimize unaccounted variables as contributors.  Trouble is, there are so many variables, and so much tire research falls into the category of "casual", lacking the rigor of scientific method and good, replicable research design methodologies.  You're absolutely correct, Schwalbe present their findings holding constant for tire pressure.  In the last chart on the page you linked, Schwalbe state, "The following gives a rough overview of tires and their relative rolling resistance. A direct comparison is impossible though, as the tires have different widths and some are used with very different inflation pressures.".  It is devilishly hard to *not* mix apples and oranges in some way, and Schwalbe have used great care to make cleart a number of variables can affect the outcomes.  I think their results are among the more reliable I've seen, and square with my perceptions while riding their tires.  It would be nice to see their results independently replicated, but I can understand why they might not wish to participate, as their products are the result of proprietary research and trade secrets.

2) Schwalbe indicate a major factor in rolling resistance is not the size of the contact patch, but the shape, which affects distortion.  Put another way, they indicate that for an identical contact patch _area_, rolling resistance varies as a result of shape.  This correlates well with the research Avocet did when introducing slick bicycle tires to the public, and goes directly to their argument (since widely replicated and proven) that bicycle tires a) do not hydroplane as car tires do as a consequence of contact patch shape and loading, and b) tread does not affect hydroplaning in this application.  Schwalbe indicate contact patch shape (and therefore distortion and rolling resistance) can be influenced by tire load, section width, pressure, and the width of the rim the tire is mounted on.

As an aside, the _cornering_ dynamics of single-track vehicles like bicycles and motorcycles are different than for cars, thanks to camber, or leaning into a turn.  General Motors developed a concept car with a leaning body and Mercedes-Benz' 2002 F400 "Carving" concept car deliberately exploited the higher cornering forces made possible by active camber control.  Downhill skiiers do much the same to initiate a turn.  In effect, a bicycle's contact patch changes shape radically as a result of introducing tilt into the equation and this effect is seen only to a much lesser degree in multi-track vehicles like ordinary cars and trucks, whose treads are planted near-flat, varying from vertical by only a few degrees during dynamic suspension loads placed by cornering forces.  I have often pondered this when watching Wheelchair Olympics, where most of the chairs have wheels with a good degree of negative camber, which should increase straight-line rolling resistance.  I'm guessing they allow better approach and access by the rider's arms, and this more ergonomic design overcomes any increase in straightline rolling resistance, especially as the motive force is direct and not multiplied by gearing.  Wheelchairs are also multitrack vehicles, and I am guessing the paired, negatively-cambered rear wheels might also provide better straightline capability, coupled with whatever trail is afforded by the single front wheel of a racing chair.

3) It seems the radial deformation at the contact area of a narrower, higher-pressure tire could be greatly reduced if the tire were so hard as to mimic a solid hard-rubber design (you'd not wish to ride it!).  Yes, the contact patch would still be longitudinal  compared to a softer, wider tire, but would be minimized due to a complete lack of conformation and sidewall deformation, the footprint and contact-area shape being governed largely by elastic hystersis of the rubber rather than varying and conforming as a result of reasonable air pressure.  With essentially infinite inflation pressure, rolling resistance should drop. 

4) Rim width is important in _shaping_ the contact patch of a tire.  Though pressure forces will act equally on the tire casing, the actual _shape_ of the casing (and thus the tread-contact patch)  can be altered as a consequence of bead width (and construction).  In the past, I went berzerkum on the subject (a kinder term might be <ahem> "intellectually curious") and inked the treads of various tires with water-soluble paint and then rolled the weighted tires across butcher paper.  The actual tread-contact are did vary in width for the same tire at the same pressure and the same load as a function of rim width; contact patches became wider with wider rims, and narrower (more radial or lengthwise) with narrow rims.  At the time I did the tests (which were essentially an exercise in play, as I did not employ any rigorous scientific method), Panaracer once offered a tire for triathletes with an oval cross section that was much taller than it was wide.  I tried inking one of these and the footprint was consistent with actual section width, but there was not the usual near 1:1 correlation with profile we are used to seeing in conventional tires.  I suspect the radial tires that came with my 1989 Miyata 1000LT would have produced a footprint shaped wider than an identical tire of bias-ply construction, as is the case with radial vs. bias ply automobile tires.  As noted in my earlier post, these tires felt horrid because the rim would move laterally over the casing while the tread stayed firmly planted.  It constantly felt as if the tire was slipping sideways beneath me, only to be caught at the last moment.  They were very comfortable otherwise, and *did* stay planted; just felt weird.  I once experienced much the same effect when driving a truck with radial tires in heavy crosswinds when pulling a trailer with bias-plies.  It not make for a happy mix.

5) As a fearless undergraduate, I once engaged in an intellectual argument with my physics professor, who insisted that regardless of weight distribution, all four tires on a car carry the same weight *and* exert the same pressure on the ground.  I worked as an independent car mechanic to earn my tuition, and held firm to the idea that at the same pressure, bulging sidewalls at the heavier end of the car were a clear indication of greater load being placed on those tires.  We parted the argument as gentlemen, each impressed by the conviction and complete idiocy of the other.  Ah youth, impetuous youth!  I did manage to duplicate the old argument that pressure exerted was directly proportional to surface area, and cited an advertisement by a trucking association debunking the claim that heavy trucks break up the roadways and "proved" their assertion by showing a woman in high heels exerted greater pressure on a surface than a loaded semi-tractor equipped with large-section tires.  Wikipedia grounds this argument in bicycle terms, saying, (wrt) "Air pressure in a bicycle tire relative to atmosphere (gauge pressure). A bicycle may actually have higher pressure (psi) than a car tire. Even though the bike carries less weight, the tires are so much thinner that the force is concentrated in a smaller area, and more air pressure is required for support. ( http://en.wikipedia.org/wiki/Orders_of_magnitude_%28pressure%29 ).  I think the Wikipedia statement should replace "thinner" with "narrower in cross section and profile and therefore volume". 

6) Back in my uni days, a friend was pursuing his doctorate in kinesiology and recruited me as a test subject.  We had many good-natured discussions about this very subject, and he felt that in terms of human energy use and therefore fatigue, one should not consider rolling resistance apart from the effects it places on the human body.  The thrust of his argument at the time was that a tire run at higher pressure might well have less rolling resistance, but would use more of the rider's energy due to the fatiguing effects of greater vibration and the need to fight outright wheel deflectiona nd a bucking saddle and handlebars on rough roads.  He said that higher pressure had the effect of beating up the rider, and the price paid might well be measurable in overall energy used to transit a given distance, cetaris paribus, of course.

7) I have often pondered the effects of fender width on wind resistance, and how can have a much greater effect on forward progress, speed, and rider energy use than rolling resistance.  At one point, I substituted a rear fender for a front one, completely covering the leading edge of the tire, and found it did make a measurable difference, though not nearly as much as when the sides of the "wheel pant" were faired in as well.  Tire width and tread design are also contributors to wind resistance, particularly at the top-leading edge of the tire, where contra-rotational speeds are greatest relative to airflow in forward motion of the bicycle.  This is why you will sometimes see little leading "spats" that look like tiny mudflaps ahead of car tires and below the fenders on cars designed for high fuel economy and low drag. All bets are off for an upright, loaded touring bike, which has a Cd approximating that of a barn door, square-on to the wind.  For those interested in some reading on high-speed aerodynamics (hand raised), the research on low-drag bulled designs and rocket nosecones is fascinating stuff. 

8) Besides, width, rim depth at the tire well has an effect on a tire's overall volume and cannot be fully discounted, as the rim provides the "fourth side" to the tire.  In the past, tourists tended to favor channel-section rims (with a drop center) for comfort over the later, stronger box-section designs.  I wonder if the greater overall air volume afforded by the greater interior rim volume might have given some credence and validity to this notion?

9) Most of the arguments regarding rolling resistance hold true for smoothly surfaced pavement, and become far less important on rough roads covered with rocks, dirt, gravel, or sand.  I know my road slicks felt like mush when I rode the Dutch sand roads of Zeeland, the Veluwe and the Green Heart fully-loaded.  It was a battle just to make continuous forward progress. The tires would often spin while digging holes for themselves while I churned forward in as low a gear as I had, trying to balance torque against RPMs.  Dry sand roads were nearly impassable while damp ones were akin to solid, cured concrete in making forward progress.

10) So far (subject to an expeditionary trial, scheduled for late next Spring), my Sherpa's 26x2.0 Duremes seem to be the solution for me.  I am currently running the rigid model with wire beads, and will use them till they are worn out.  I purchased two folding models at a very good price to carry on tour and will use them as replacements when needed.  The Duremes appear identical to the Supremes in basic construction and tread compound, differing only in tread design and thickness.  They seem to be an ideal compromise between the road slicks I have always used in all conditions and a dedicated expedition/rough road tire like the XR.  Early trials both unloaded and fully loaded over pavement, grass, dirt, heavy and light gravel and sand are promising and indicate the tire may be a "Jack or all trades, master of none" which is pretty much what I was looking for to cope with these varied conditions while on-tour.

Fun topic!

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 02, 2011, 09:54:59 PM
For those who have written to ask the source of the chart at the beginning of this thread...

I found Andy Blance's warning regarding tire pressures for *700C tires* is listed on page 8 of the new Autumn 2011 Mercury brochure PDF:
http://www.sjscycles.com/thornpdf/ThornMercuryHiRes.pdf

Andy's warning regarding tire pressures for *26-inch tires* is listed on page 6 of the new Autumn 2011 Nomad brochure PDF:
http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf

I presume he will include the warnings in all the brochures as they are revised for Autumn 2011 release.  At present, I find the warning only in the two brochures listed above.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 02, 2011, 11:56:00 PM

6) Back in my uni days, a friend was pursuing his doctorate in kinesiology and recruited me as a test subject.  We had many good-natured discussions about this very subject, and he felt that in terms of human energy use and therefore fatigue, one should not consider rolling resistance apart from the effects it places on the human body.  The thrust of his argument at the time was that a tire run at higher pressure might well have less rolling resistance, but would use more of the rider's energy due to the fatiguing effects of greater vibration and the need to fight outright wheel deflectiona nd a bucking saddle and handlebars on rough roads.  He said that higher pressure had the effect of beating up the rider, and the price paid might well be measurable in overall energy used to transit a given distance, cetaris paribus, of course.

I recently read the 3rd edition of Wilson's book Bicycling Science:
http://www.librarything.com/work/75715 (http://www.librarything.com/work/75715)

The energy loss from bouncing around is not just due to added rider fatigue. The body is acting like a shock absorber, soaking up energy. Similarly, one's panniers will soak up energy from bouncing around.

Wilson's book has some amazing mathematics on bicycle tire contact patches, etc. I confess I just skimmed all those formulas!
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 03, 2011, 12:05:33 AM
An excellently argued report, Dan, and good to read too. I had the privilege of being on the same newsgroup with the brains behind the best products of Avocet's best years, Jobst Brand, and hearing at first hand the arguments for slicks, against aquaplaning, and so on.


6) Back in my uni days, a friend was pursuing his doctorate in kinesiology and recruited me as a test subject.  We had many good-natured discussions about this very subject, and he felt that in terms of human energy use and therefore fatigue, one should not consider rolling resistance apart from the effects it places on the human body.  The thrust of his argument at the time was that a tire run at higher pressure might well have less rolling resistance, but would use more of the rider's energy due to the fatiguing effects of greater vibration and the need to fight outright wheel deflectiona nd a bucking saddle and handlebars on rough roads.  He said that higher pressure had the effect of beating up the rider, and the price paid might well be measurable in overall energy used to transit a given distance, cetaris paribus, of course.

No doubt in my mind, from experience with bicycles on ever fatter tyres, and also professionally as a psychologist with an interest in ergonomics, that your friend is right about the cost of a harsh, disturbed ride.

Stands to reason, dinnit, and if I had a few hundred grand for extended research in some agreeable warm place with good cycling, I'd prove it!

I've also started wondering, from the discussion here reinforcing discussions elsewhere over recent years, whether  the parameter set in which high pressure reduces rolling resistance isn't much more limited than we've come to presume.

BTW, it isn't a new idea that a soft, long-travel suspension handles the road better than a hard, short-travel suspension, which is the subtext of my original article in this thread. Colin Chapman's Lotus cars depended on it, and the Chapman Strut (like a McPherson strut but applied to the rear suspension) uses the soft, long travel approach to save both money and weight. There is even a later bicycle equivalent, in that the first offroad downhill racers, on the first mountainbikes, took their bikes from the beach cruisers which already rode on balloon tyres as a long-travel suspension.

Andre Jute

PS Thanks for answering the source question.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 03, 2011, 03:03:17 AM
Jim, your calculations look spot-on to me, and I would only add that rim design (i.e. a channel section rim vs. a box-section design) can effectively change the air volume independent of outside rim diameter or tire profile and width, but I certainly agree with your postulation that for a fixed pressure, casing tension is proportional to tire section (where section is a function of area as determined by both width and profile).

Jim, a great observation as always, wrt the rider's body, the bike, and the luggage acting as shock absorbers.  Anecdotally, I know I arrive at the bottom of a rocky, uneven downhill faster and less fatigued when I post instead of remaining seated.  It is just so much easier than being rattled to death, and removing my weight from the saddle and pivoting through the pedals, cranks, and BB make life much easier for the bike as well.  I have a couple earlier editions of _Bicycling Science_ and return to them again and again to marvel over the authors' approach.  There is so much to ponder in the book that it is hard to absorb it in one go and so I'll read, then ponder, and then re-read some time later with a new perspective.  I love books and treatises that make me think like that.

Tying this further to the thread at hand...

Also much-beloved is my 1974 copy of Fred De Long's _Guide to Bicycles and Bicycling_.  He presented some remarkable engineering work on the science of bicycling, and approached it with an engineering background (in the 1930s, he was a manufacturing engineer for SKS bearings).  He was best known as an avid club cyclist who worked to better rationalize and apply CPSC (Consumer Product Safety Commission) regulations in the States and was a tireless advocate for cycling.  In my early years of riding, I learned to really think about bicycling in scientific terms from his publications.  His section on the "Human Engine" was a nice collection of contemporary thinking on cyclists' kinesiology, circa 1974, and even included data that would later be known as VO2MAX uptake.   In his chapter on rims and tires, he devoted considerable space to what he called "Serviceability and Rolling Resistance".  Among his more interesting observations is this one:

"The 26x1-3/8-inch blackwall tire can be expected to have a rolling resistance 40% greater than that of a good 27x1-1/4-inch tire.  Eight percent of this is due to size difference, but the balance is due to the heavier walls and treads used to give more service [on the 26" tire].  A balloon tire with a lower inflation pressure may require that the cyclist expend from 2-1/2 to 3 times more energy. ...the tire design varies with the intended service." [pg. 167]

I believe this -- and the era in which he was writing -- contributed to the prevailing philosophy that tires with large section width, run at lower pressures are inefficient compared to more sporting, narrower tires of larger diameter.  I believe the real truth was obscured by the relative differences in construction materials.  Large-section balloon tires of the era were simply horrid from an enthusiast's perspective.  They were made of poor-quality materials, were extraordinarily heavy, and the sidewalls were so stiff that I recall myself being surprised when a tire that appeared full when parked simply collapsed under my weight, flat of all air; the stiff sidewalls alone maintained the shape of the carcass until overloaded when I mounted the bike.  By contrast, even the poorest quality 27" tire of the day was a lightweight marvel in comparison and employed much higher quality materials in its construction.  Tubulars -- especially Mrs. Pye's handmade silk sewups from New Jersey used on board tracks by Six-Day riders back in the '30s and '40s and the much later cotton training sewups and Clement Criterium Setas and heavier Roubaix road tires pretty much blew them away, but were the dedicated province of the hardcore club rider or racer here in the States at that time, not the general recreational rider or rare tourist.  For some years -- too many in my opinion -- bicycles in the States were considered sidewalk toys, intended largely for children in the eyes of the general public.  This has improved greatly, but still holds to an extent in some areas, where they are instead viewed as expensive recreational toys, much like skis, and similarly intended for use in relatively restricted venues such as parks and bike paths (Why cartop a bike?  Why not ride from your front door?).  I ran into this on my 2010 Great Basin tour, where I was told by elderly ranchers that bicycles belonged on sidewalks and not amidst desert sagebrush.  They simply did not think it possible, and rated as nil my chances for a successful transit.

Sorry for the digression, but it does put into perspective some of Fred's views at the time he wrote his _Guide_ and places them in the larger context of American cycling at the time, some 37 years ago.  Among his further observations, he notes, "...The actual [recommended] pressure [in a tire] will depend on load.  Note that rolling friction decreases as pressure is raised, but the shock and vibration also increase."

In more contemporary treatises, Tony Hadland has spent considerable space on the subject, starting about a dozen years ago.  I had the privilege of corresponding with him briefly some years ago regarding the origins of a Folder of unknown provenance in his possession.  His root page is here:
http://www.hadland.me.uk/
Relevant portions of his website are here:
http://www.hadland.me.uk/lafford.htm , where he presents his observations on tires most suitable for recumbents and small-wheelers, and here:
http://www.hadland.me.uk/page15.html , where he talks about small wheels for adult bicycles.

Though Tony's work is geared toward smaller wheels than we might consider and use here on the Thorn Forum, his general observations and recommendations are still valid, and especially interesting (to me, anyway) is his recounting of the theories of Frenchman Paul de Vivie ("Velocio", see http://en.wikipedia.org/wiki/Paul_de_Vivie  for background) from the 1920s, in which he advocated balloon tyres (sic) of up to 2.25" (57mm) cross-section on small rims.  Hadland editorializes, "The idea of a reduction of tyre diameter being matched by a corresponding increase in cross-sectional area certainly has merit. The volume of air and pressure remains the same as in the conventional tyre, while the wider cross-section compensates for (and can even improve on) the otherwise harsher ride of the small wheel."   Most relevant to our discussions in the earlier posts on this thread, Hadland goes on to observe...

"...As for rolling resistance, a reasonable prima facie indicator is the length of the tyre print (under a known weight) divided by the inflated tyre radius.10 For a given tyre pressure and load, the contact patch area is approximately constant, regardless of tyre diameter. (For example, a tyre inflated to 50 psi and carrying a load of 100 lbs has a contact patch with an area of approximately 100/50 square inches, ie. 2 square inches, whatever format the tyre may be.11) However, with the Vélocio approach to small wheels the patch is wider but shorter. Thus compensation is obtained for the otherwise higher rolling resistance.

However, this compensation depends on superior lightweight tyre carcass construction. This is difficult to achieve because, the larger the cross-section, the stronger the carcass must be to hold a given pressure. For economy of manufacture, the strength of wide section tyres often comes from thicker, heavier and less flexible materials, and results in a higher rolling resistance," (in this latter section, Hadland references the work of Rob Van der Plas, 'Rolling Resistance', _Bicycle_, UK, February 1984).

And there we have it, in my opinion.  As evident in Schwalbe's tests and charts, primary factors in rolling resistance (at whatever level) are not the area of the contact patch, but the shape and how it influences deformation of the carcass, coupled with the quality of materials used in constructing a tire.  From this we can infer that poorly designed tires using inferior materials will be slower regardless of the pressure used, and the inverse -- carefully designed tires made with high-quality materials will not only roll more easily and comfortably from the get-go, they will allow for greater exploitation of those characteristics through proper inflation -- *and* will tolerate lower pressures and increase handling and ride comfort *without* incurring the same penalties in rolling resistance suffered by their poorly-constructed kin.  Fatter-section tires run at lower pressures (provided they are of quality materials and construction) will have a wider contact patch that causes less deformation and therefore lower rolling resistance than a narrower tire, which is characterized by a more longitudinal contact patch more subject to deformation and, therefore, relatively higher rolling resistance, other factors being held equal.

The fact that not all of us have had equal success in experiencing low pressure, wide tires that ride fast may be due to the idea that not all tires are created equal or have design parameters and materials optimized for the purpose.  Certainly, I've ridden some tires that were bog slow *regardless* of pressure (see my earlier post in this thread referencing the 700x35C Michelin City Pilots) and they were irredeemable regardless of my adjustments and the tubes and rims selected. When it all goes right -- and that seems to depend on a number of variables and in some cases, purpose-matched components like rims and friendly frame clearances -- it is a postulate that holds true in spades and is a genuine joy to experience. 

Andre, I took the opportunity to peruse your off-list site, quickly became lost in it, and thoroughly enjoyed your writings and photography.  Your incomparable Utopia Kranich is about as close as I can imagine to a true systems approach to maximize the superb qualities of large-section tires operated at low pressures.  It has to be _the_ optimum design for this philosophy and is an incredible machine.  I've enjoyed your observations and insights to this thread, and especially the reference to Lotus Engineering and Colin Chapman's designs ("Simplicate, then add lightness" was one of his engineering battle cries).  Carrying the comparison a bit further, I was struck by how much like a bicycle my French friend's Citroen 2CV appeared when we examined it in his barn.  Basic, simple, lightweight, and with incredible versatility and economy -- all in a vehicle originally intended to help farmers get their pigs and produce to market in the most economical, reliable manner possible.

Lets all work on getting that extended research grant on the general topic; the reference to an "..agreeable warm place with good cycling" sold me on the idea!

And...I'm delighted with my Duremes run at appropriate pressures and can't wait to try them "in anger" so to speak, under load and in a proper expedition setting.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 03, 2011, 06:54:43 AM
For those of us not yet tired of the subject (pun intended), a nice parallel summary of our collective discussion appears in the form of an entry on Jan Heine's Wordpress blog, "Off the Beaten Path" published October 18, 2010:

http://janheine.wordpress.com/2010/10/18/science-and-bicycles-1-tires-and-pressure/

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: julk on November 03, 2011, 11:07:03 AM
Dear all,
I have been fascinated reading all the posts on this subject and am a convert to lower pressures for at least the last 20 years of my cycling.

For me, as I have aged, the most important feature of tyre volume/pressure has become comfort, I can forgo the speed aspect which was important to me in the flush of youth!

I suspect this change of mind also relates to the gradual deterioration in tarmac road surfaces over the last decade or three. I don't seem to find the smooth tarmac roads of my early riding memories.
Julian.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 03, 2011, 11:52:40 PM
Andre, I took the opportunity to peruse your off-list site, quickly became lost in it, and thoroughly enjoyed your writings and photography.

Thanks Dan. Just so nobody thinks I'm denying them the same riches (heh-heh!), my personal netsite is at
http://coolmainpress.com/andrejute.html
and within this there's a bicycle page at
http://coolmainpress.com/BICYCLING.html

Your incomparable Utopia Kranich is about as close as I can imagine to a true systems approach to maximize the superb qualities of large-section tires operated at low pressures.  It has to be _the_ optimum design for this philosophy and is an incredible machine.

The Kranich was my second choice. I had a design for a fully triangulated 12mm tube stainless steel bike, something like a Pedersen, with the tubes externally lathed to make butts, but absolutely nobody wanted to build it for me, especially after they heard that I trusted the FEA about as far as I could throw it, and intended determining the final weight of the tubes by trial and error. The Kranich was the nearest I could get. The only other bike on my shortlist was the unisex model of the Thorn (I wanted a low stepover), and I'm an artist with an engineering sideline, not the other way round, so I just purely hate visible welding. I know, I know, lugs and brazing cost so much more, and are no more efficient, so that welding makes commercial sense. Every year about this time I think about buying another bike, but for the last several years now only the Utopia Kranich and Phoenix (a semi-recliner), the Thorn Raven, and the Pedersen have appeared on my shortlist. Maybe I'm lacking in imagination or daring, but my list is limited by the non-negotiable specification that the bike must take balloons together with mudguards.

I've enjoyed your observations and insights to this thread, and especially the reference to Lotus Engineering and Colin Chapman's designs ("Simplicate, then add lightness" was one of his engineering battle cries).  Carrying the comparison a bit further, I was struck by how much like a bicycle my French friend's Citroen 2CV appeared when we examined it in his barn.  Basic, simple, lightweight, and with incredible versatility and economy -- all in a vehicle originally intended to help farmers get their pigs and produce to market in the most economical, reliable manner possible.

I never had the patience for Lotus cars; I like my engineering either solid or clever; even Chapman admitted that light and cheap was a certain recipe for disaster. Clever accounts for keeping several Citroen over the years, including a GS, a DS (out in Australia where back then it would cause at least half the populace to regard you as "probably queer"), and several SM when I lived in France and the UK. It was reflections on cars that by cosseting the driver permit him to set times apparently beyond their power that first set me on the path that ended with a bike designed from the ground up for the biggest balloon tyres available.

Ride tall.

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 28, 2011, 04:21:22 AM
Hi All,

I see Schwalbe's website has recently added an entire section to this very topic, available here:  

http://www.balloonbikes.com/en/

This six-part section cites many of the advantages already indicated by our esteemed Mr. Jute, and the FAQ section ( http://www.balloonbikes.com/en/faqen.html ) makes a number of additional points and clarifications, including the following:
- - - - - - - - - -
Do wide tires have only advantages?

Of course not. Racing bikes use quite narrow tires, because they are lighter and accelerate faster. But at normal speeds of up to 20 kph wide tires roll easier and are above all more comfortable.

What is the difference between Balloonbikes with 50 and 60 mm wide tires?

60 mm wide tires provide maximum Balloonbike comfort. But also the lighter weight 50 mm wide tires had double the air volume of a standard 37 mm tire. The 50 mm wide tyre is a good choice for anyone who wants a sporty yet comfortable ride. Between these two sizes there is also a popular 55 mm width.
- - - - - - - - - -
This last statement on air volume for 50mm wide tires made me sit up a little straighter.  "Sporty yet comfortable" sounds eerily like my 26x2.0 Duremes!  My coast-down tests indicate they roll every bit as well as my 700x32C Bontrager Select K4 tires on another bike, yet the Duremes have a far more comfortable ride and much less vibration through the handlebars and my hands don't hurt as much after a long ride.  I rode mine at 3.4bar today, but may well drop that to 3.1bar for even greater comfort.  Rolling resistance has so far seemed unaffected by running at less than the tire's maximum rating of 5bar, which I am reluctant to use for risk of eventual rim failure.

Of further relevance to the foregoing conversations, Schwalbe indicate "...Wide tires perform best on wide rims, and this also prevents problems. However, in principle, it is possible to fit wide tires onto the commonly used 19C rims".  Mindful of Andy Blance's cautions in the recently revised Thorn brochures, Schwalbe also recommend only 2.0-2.5 bar inflation as the most comfortable range at normal loading, minimizing outward stress on even (relatively) narrow(er) rims, thereby reducing the risk of rim fracture.  Wide mountain bike tires are usable on relatively narrow rims because they are run at low pressures.  The real problem comes when running narrow rims with wider tires at high pressure.  The outward leverage caused by higher pressures can readily fracture a rim at the sidewalls or in the center well, especially on a bicycle carrying a heavy touring load on rough roads.  A good photo of this sort of fracture is here:  http://www.flickr.com/photos/tylerkellen/4265397463/

Schwalbe's new section makes clear true balloon tires require a systems approach to use effectively -- they offer a number of problems to be resolved, including frame and fender clearance and standover, since they are not only wider but taller.  Lacking a purpose-built frame, it seems equally clear that large-section tires offer advantages for cyclists who do not race and accelerate frequently, but  place a high value on comfort and low rolling resistance.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 28, 2011, 05:27:54 AM
Schwalbe have also revised their tire pressure inflation FAQ, and made their recommendations more clear at:

http://www.schwalbetires.com/tech_info/inflation_pressure

Schwalbe are careful to note their recommendations comprise only a general guide.  Nevertheless, their recommendations are for an “average rider” weighing about 75kg/165 lb and are listed in a chart.  Schwalbe add:
Quote
If the rider is heavier or carries luggage, a higher inflation pressure should be used. For each additional kilogram that the tire must carry (bike, rider, luggage), the inflation pressure should be increased by approx. 1%. It is recommended that higher inflation pressures are used on very small diameter tires such as recumbants and folding bikes.

They further indicate
Quote
This weight is mainly influenced by the weight of the rider and any luggage. Contrary to a car, the vehicle weight is only a minor part of the total weight.

I'm guessing Schwalbe figured inflation pressures sans bike weight, but obviously bikes vary; my Sherpa weighs in at 18.1kg/40lbs dry and bare of luggage.  That's pretty substantial and alone would call for another 18.1% increase in tire pressure.  I wish Schwalbe had also given a base bicycle weight for their calculations so I would know how much to add to their figures to get to a recommended pressure.  Ah, me.  I will write them to see what they say.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 28, 2011, 02:09:28 PM
Thanks for posting these Schwalbe links. I think the 1% increase is for each 1 kg per tire. I.e. a total of 18 kg weight would be 9 kg per tire or 9% increase in pressure. Of course that would need a bit of fine tuning to deal with front/rear weight distribution.

Putting the pieces together, it would seem that narrower rims imply a lower weight carrying capacity, because they require lower pressure. A 50 mm width tire on a 19 mm wide rim has a limit of 4 bar. 3 bar is recommended for nominal load. So one can increase the pressure about 33% from nominal to the limit. That would carry 33 kg per tire, or 66 kg total, or about 145 lb. I should deduct about 30 lb for the base load of a heavy rider on a heavy bike. OK, 115 lb is still more than I would be likely to carry, at least over any real distance.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 28, 2011, 02:11:48 PM
I don't have time to look it up now, Dan, (installing a new Mac and must pay attention if I don't want to strip it out again...), but you might get a further hint by checking the max load of the Schwalbe tyres.
Some of those numbers are amazing.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 28, 2011, 06:44:08 PM
Thanks, Jim...on reflection (and with your insight) it seems reasonable Schwalbe's recommendations are *per tire* rather than *per pair*.  Otherwise, the combined weight of myself, the bike, and full touring load put me off the charts, and way beyond the recommended pressure printed on the tire sidewall...and that cannot be right.  After all, that same load with "only" 50psi/3.4bar did not result in even a 15% drop in rim height from the floor, insofar as I could determine (devilishly hard as I sat atop the bike, a friend on all fours with a caliper, trying to measure rim height as I held a level across the handlebars, bifocals adding to the adventure for all.  Tire Follies or Science.  I prefer ehm, "Science".  Yes, that'll do nicely.  In warmer months, these Inquiries take place outside.  The neighbors are used to it by now, and their curtains hardly wiggle anymore.  Most of them will return a wave, so there has been progress).

Off to check those load figures, Andre... good idea.

Thanks, guys!

More in a bit as insight strikes...

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 28, 2011, 07:54:09 PM
Okay, let's use me as an example (I'm pretty average in my dimensions and my Sherpa is pretty typical of a heavy-touring setup and the results will benefit me directly, so why not?  ;) ).  The kg/lb conversions will be off a bit due to rounding on my part.

If we figure the Schwalbe recommendations are *per tire*, lets take 50% of the weights below for convenience sake.  They could always be adjusted proportionally to account for extra weight on the rear tire.

I weigh 78kg/172 lbs in typical riding gear; compared to Schwalbe's average rider of 75kg/165, I am 3kg/7lbs over.  Half that difference is 1.5kg/3.5lbs.

My Sherpa weighs 18kg/40lbs dry (empty bottles) and unloaded except for my underseat bag with spare tube, multi-tool, patch kits and manometer.  Divided by half, we get 9kg/20lbs.

Schwalbe say the pressure must consider the load on the (one) tire of (half the) bike, rider, and luggage.  In this case, that is my own 1.5kg/3.5lbs half-overage plus 9kg/20lbs for the half-bike, totaling 10.5kg/23.5lbs.  According to Schwalbe we increase pressure by 1% per kg beyond the chart listings, so that would be a 10.5% increase.  The chart indicates my 26x2.0 tire (mounted on Rigida Andra rims, so we'll go by the actual section width of 47mm) would have a recommended  base pressure of 3.5bar/50psi.  A 10.5% increase to that would result in a 50/50 F/R per-tire pressure of 3.87bar/55.25psi for me on the unladen bike with no luggage and empty bottles. 

By interpolation (he doesn't list a 47mm section-width tire), this would put me past Andy Blance's *maximum* recommended inflation figures (see the Nomad brochure at http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf page 6).  And this is just me, on an unladen Sherpa.  Imagine if the bike were loaded.  Yikes!

Plainly, there is some discrepancy between Schwalbe's hedged recommendations and Andy's reality-based hard-limit maximum warnings.  I wonder what pressure would result in a true 15% drop, the amount recommended by Berto's original article, which tire engineers thought resulted in the optimum balance of comfort and low rolling resistance.  It is also worthwhile to go by the actual caliper-measured section width of the tire _casings_ (sans tread) as mounted on your bike, as that often varies from the published size.  Tires also grow in width from new; mine measured an actual inflated 1.75" (45mm) at first.  By the end of the week when I measured again, they had grown to their present 47mm and seem to have stabilized there.

Dazzled,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 29, 2011, 05:01:39 PM
Dan

Schwalbe:
I suspect the guidance on the Schwalbe site is slightly misleading i.e. I would have thought that surely where they are quoting a recommended tyre pressure for a rider of a given average weight they must have factored in the weight of an average bicycle the rider would have to be riding??  Hope you are successful in getting clarification on this.

Rim Width:
I am also unsure why Andy Blance's safety limit should be universal, e.g. the Rigida Andra is advertised as a heavy duty rim and is 25mm wide, the Grizzly is more of a sporty rim and is 22.7mm wide - if you add 10% extra width to the Grizzly's width you must be adding to the strength and/or pressure tolerances of the rim?  After all, that is why we have wider rims to take wider tyres isn't it?

DT Swiss Rims Data
Andy Blance states a maximum tyre pressure of 32/37 psi for a 2.35" tyre, yet in the DT Swiss chart there are many rims which take up to 2.35" tyre with a rider of 90kgs up to 4 Bar (58 psi so far as I can convert).  There are also rims for 2.5" and 3.0" tyres with riders of 110/130kgs up to 3 Bar/43.5psi.  That is a significantly different limitation.

http://www.dtswiss.com/getdoc/1613ff79-dfd7-480c-9f43-f44f19ebaf1d/TechnicalDatasheet.aspx/  

It is interesting to note that most of the 26"/4 Bar rims here are also narrower than the Andra (sporty rather than touring?).

Berto:
I would also hazard a guess that Schwalbe design their tyres to have significantly less "drop" than the 15% espoused by Mr Berto (similar advances in technology as affects Fred de Long's Guide as you mentioned earlier).

Plenty of food for thought in this thread yet methinks.
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 29, 2011, 05:25:48 PM
By interpolation (he doesn't list a 47mm section-width tire), this would put me past Andy Blance's *maximum* recommended inflation figures (see the Nomad brochure at http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf page 6).  And this is just me, on an unladen Sherpa.  Imagine if the bike were loaded.  Yikes!
Dan.

Dan
As an afterthought, imagine these calculations if you had a 26" wheeled touring tandem?!?!?
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 29, 2011, 05:51:42 PM
The Rigida Andra 30 has an "inner well width" of 19mm:

http://www.sjscycles.co.uk/rigida-andra-30-26-(559)-mtb-css-rim-black-32-hole-rohloff-drilling-prod13269/

It sure seems like the rim strength ought to enter the equation too!

The 1% per kg formula also gets me thinking. The tire carries a load equal to the contact patch area times the pressure. So, to keep the contact patch area constant, the pressure should increase proportionally with the load. If the starting load is 50 kg (per wheel),... that would call for a 2% increase for each kg increase in load? Where is my mistake?

Keeping the contact patch area constant is one goal in determining tire pressure. Another is keep the rim elevated, to avoid pinch flats etc. Seems like when one hits a bump, the contact patch area increases which lets the pressure push the bike upwards. So an interesting number is the ratio between the cruising contact patch area and the maximum contact patch area (just before the rim hits ground).

Or much of the time when riding over bumps, the bike doesn't really get pushed up to clear the obstacle, but the tire deformation just takes up the variation in the road surface. This is clearly one major advantage of fat tires. Especially with a heavy load, the bike is just not going to move up and down to get out of the way of rocks or whatever. The extra height of a fat tire can completely absorb small obstacles, and gives the bike more time to move to avoid larger ones.

Yeah, lots to chew on!


Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 29, 2011, 07:12:50 PM
still chewing...

The contact patch of a fat tire is both longer and wider than that of a narrow tire. The tire can start pushing the bike up and over an obstacle as soon as the contact patch reaches the obstacle. Thus the longer in space the contact patch, the longer in time the bike has to move upward to avoid the obstacle.

Seems like two sorts of obstacles need to be looked at: sharp obstacles that are smaller than the contact patch, e.g. an sharp step up, and long obstacles, like a sharp step down.

There's a nice problem. Just drop the loaded wheel onto a smooth surface. Will the rim hit the ground?

Let x be the tire width, i.e. the distance from rim to the ground when the wheel is completely unloaded.

Model the tire as a simple spring. The tire pressure becomes the spring's stiffness, k. We'll inflate the tire to achieve some fixed fractional deformation of the tire, i.e. k varies as the inverse of x.

The energy of the dropping bicycle, proportional to v**2, must be taken up by the spring, whose energy is k*(x**2). But since k is inversely proportional to x, the maximum energy the spring can absorb is proportional to x. I.e. the maximum velocity, with which we can allow the bike to drop, is proportional to the square root of the tire width. That velocity is proportional to the height from which the bike is dropped.

Looking at that Schwalbe inflation chart, they do have pressure roughly inversely proportional to tire width. The pressure seems to go up a bit faster as width goes down.... lots of other factors at play here!
 
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 29, 2011, 07:26:25 PM
Quote
As an afterthought, imagine these calculations if you had a 26" wheeled touring tandem?!?!?

Oh, but I do!  I bought it used from a family where each member weighed well in excess of 375lbs/170kg, so a pair was a serious load.  And they had really ridden it, including at least one Seattle-to-Portland ride of about 325km/202miles.  I repainted it, converted it to a road orientation, and relaced the wheels, truing and tensioning them with the original spokes (they were originally built with uh, "inconsistent" lacing).  I've used it a lot, with heavy stokers and on really rough roads.  The bike alone weighs 20.9kg/46lbs set-up for touring with Arai drum brake, racks, bottle and cages, pump, fenders, etc. as pictured.  My Sherpa is perilously close to that at 18kg/40lbs in a similarly ready state.

When my Dutch pal visited from The Netherlands in 2007, we decided on a couple night's tour up into Oregon's Cascade mountains on a mix of paved and very rough gravel roads.  My tandem runs CR18 Sun Metal rims (25mm outside width) with 36 spokes and Matrix (Trek) 26x1.5 Road Warrior slicks that measure a true 38mm/1.5" in section width, and I ran them at the listed sidewall-listed maximum of 5.86 bar/85psi front and rear.  We never had a problem, and this thing was the very definition of Heavy Metal.

It is pretty fair to say we took everything but the kitchen sink with us, including his heavy Dutch Army surplus tank-driver's boots, since that's all he had for hiking at our destination. Towing my homemade trailer loaded with 56kg/125lbs alone, our total weight was in the neighborhood of 272kg/600lbs.

I'm not proud of the weight, but it was a wonderfully fun trip, and a nice introduction to tandem touring and camping for my friend.  We ate like gluttons and lounged in a camp equipped for kings (we even took two tents, albeit small 1-person models).  We had neither tire nor rim problems, though the distance was short.  Maybe I've been extraordinarily lucky all these years, but the fact also remains that despite the weight, these were fairly narrow tires on fairly wide rims, so that may well be a factor.

For my part, I am just tickled by this topic, and look forward to hearing more ideas on the subject.  I'll dig further, too.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 29, 2011, 08:13:49 PM
When hitting a sharp step up at high speed, the bike cannot respond quickly, and the height of the step that can be managed is simply proportional to the tire width.

Another interesting case is an abrupt change in direction, e.g. the start or end of an inclined plane, or where the angle of inclination suddenly changes. This seems more like a step-down in that the spring of the tire will have time to push the bike in the new direction. The limit must be something like the sine of the angle of changed direction being proportional to the square root of the tire width.

A pot hole is a nice case, i.e. a step down followed by a step up. A very long pot hole, where the bike gets to riding along the bottom before hitting the far edge, is just the simple combination of the two steps. A shorter pot hole will let the bike hit bottom but still be bouncing when the step-up comes - clearly the worst case is when the step-up arrives just as the bike is at the bottom of the first descending compression.

Then another case is when the bike never hits the bottom, it just flies off the step-down and hits the step-up. Since the bike has started to fall, this is like a step-up onto an inclined plane. The step-up eats some of the tire width, and then the rest had better be enough to push the bike in the right direction. If I get insanely desperate I will come up with a formula but maybe I should go for a ride instead!
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 30, 2011, 12:13:56 AM
Jim,

I've enjoyed spending time thinking about your posts on this topic; they're well-reasoned and information-dense, and give a lot to think about!  You surely have a good mind.

Your thoughts and calculations appear reasonable to me, but the larger problem we all face is we are not tire engineers at the company in question, so we don't have access to their test data and must rely on our own speculation.  For example, we know the shape of the tire patch has something to do with rolling resistance, but we don't know for sure what size that patch should be to optimize a given parameter.  I wish we did, 'cos I could get a lot deeper into all this; its a great topic!  Like you, I keep turning from the computer to my copies of _Bicycling Science_ and other sources in search of answers.

You wrote a few things that really got my gears turning.  Apologies for taking your quotes out of context...
Quote
The contact patch of a fat tire is both longer and wider than that of a narrow tire.
Yes, but as I recall reading, it is the shorter patch length that reduces rolling resistance of a fat tire versus a narrow one for a given inflation pressure.  I'll check my notes, but I think this point emerged from Jan Heine's tests mentioned earlier in this topic thread.  Hmm.
Quote
This seems more like a step-down in that the spring of the tire will have time to push the bike in the new direction. The limit must be something like the sine of the angle of changed direction being proportional to the square root of the tire width.
Yes, but remember there is some hysteresis in the equation as well.  Yes, there are some momentary localized pressure spikes, but the tire and tube and rim together comprise an endless closed column that is itself flexible to a degree.  Wouldn't the overall pressure remain fairly constant for a given static pressure regardless of momentary dynamic loading? In other words, if I put a small amount of air in a tire and attached a manometer, I don't think I would see a pressure rise when poking the tire with my thumb (just did it; no change in instrumented pressure).  While I would be compressing the casing/tube locally, if the overall air column is not constricted except by its own flexible casing, it should bulge out an equal amount, distributed about the remainder of the tire/tube circumference.  Moving on from this a bit, I'm not sure any impact-related bulge remains at "ground zero" long enough to alter the contact patch too much apart from any gap that is bridged by the tire. I once saw some fascinating high-speed camera footage of a motorcycle tire as it passed through a pothole, and the resulting casing deformation appeared to take place on the tire *after* the wheel passed the pothole; in other words, the tire had rotated, and the bulge resonated through the tire after impact.
Quote
There's a nice problem. Just drop the loaded wheel onto a smooth surface. Will the rim hit the ground?
;D Well, not unless something goes terribly wrong!  All kidding aside, this is a fascinating problem.  I once cut an old rim in two and did the same for a tire and tube, then carefully mounted one inside the other and then cyclically loaded the assembly so I could better visualize how pinch flats ("snakebite punctures") were caused.  This was a crude little experiment, but it appeared pretty clear that with insufficient inflation, the tire beads would bow inward under vertical load, causing them to be unseated from the rim's bead-retaining hooks.  Even on my little model, the tube was quick to try and fill the gap and became caught between the tire bead and seat when the momentary vertical load was eased.  Because the tire was loaded vertically and equally, both beads bowed inward, and trapped the bulging tube equally, which is why those sorts of punctures are nearly always paired.  I realize you were postulating hypothetically, but in reality, I do think the rim could  hit the ground if the impact was sufficient to cause a pinch flat (or if the tube was underinflated).  By the way, a sew-up tubular tire never suffers from pinch flats 'cos there are no beads to trap the tube.
Quote
Model the tire as a simple spring. The tire pressure becomes the spring's stiffness
I agree the inflated tire will act as a spring, but I am not at all sure that spring action is linear.  That could be devilishly hard to model, given the dynamic operation of the spring-as-tire coupled with variations in carcass construction, tread, and overall design.  It would be interesting to build an instrumented test rig for this, to augment the calculations.  I know intuitively that rough roads have the effect of increasing rolling resistance, but the obstacles encountered will surely have a greater effect than dynamic changes in contact patch size, shape, and orientation, though each of those are affected by impact.  A large-diameter rotating drum or billiard-table smooth surface would make empirical calculations of pressure and rolling resistance much easier.  In the past, I had fun inking various tire treads then loading them vertically through the axle on a dummy fork to see how load and pressure affected contact patch shape and size.  I wish I had also tried doing this with the same tire mounted on rims of varying width.
Quote
It sure seems like the rim strength ought to enter the equation too!
It surely does!  We already know rim width makes a difference, and it is worth noting it forms the rest of the cylinder containing air and therefore plays a part in determining overall air volume.  Rim width also plays a role (though less important) in determining casing shape and therefore determining contact patch shape, size, and orientation as well.  The same nominally-sized tired will plant itself differently depending on whether it is mounted on a narrow, medium, or wide rim.  handling and rolling resistance are affected as well.

I registered for an account at Schwalbe, their email contact link is broken (could they have seen me coming?), so I guess I will need to phone them to inquire further.  That will be tomorrow or the next day, 'cos in between working and perusing this Forum, I've been out riding.  ;)  Some really cold weather is coming, so I've been trying to cram in as many shakedown rides as I can to get the Sherpa sorted for my needs (and to play with tire pressure, of course!  Got the motion-detecting alarm sorted as well; it is fun and will be useful at night when wild-camping solo).  

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 01:44:35 AM
Here is a simple analysis of the shape of the contact patch:
(http://i140.photobucket.com/albums/r6/kukulaj/contactpatch.jpg)

r is the radius of the tire, and R is the radius of the wheel. Again, I assume we'll inflate to the point where the load compresses the thickness of the tire by 15%.

The contact patch grows in both directions as the width of the tire increases. But the width of the contact patch increases more quickly: linearly with the tire width. The length of the contact patch only increases as the square root of the tire width.

Unfortunately my copy of Bicycling Science is stuffed in my storage unit ten miles away, probably buried in a heap of boxes. Not an ideal situation! But it is fun to try to think through this stuff!

Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 01:57:08 AM
I'm not sure any impact-related bulge remains at "ground zero" long enough to alter the contact patch too much

I agree that the tire pressure really isn't changing dynamically in any significant way. Sure, sound waves must be bouncing around inside the tire, but I don't think there is any significant effect. The total change in volume due to loading and compressing the tire is not very significant either.

But the bike is getting accelerated up and down on our bumpy road. Gravity is constant, so the force of the road, transmitted through the tire, has to be changing. That force is just pressure times contact area. So the contact area must be fluctuating just in parallel with the variations in vertical acceleration.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 02:06:47 AM
I know intuitively that rough roads have the effect of increasing rolling resistance, but the obstacles encountered will surely have a greater effect than dynamic changes in contact patch size, shape, and orientation, though each of those are affected by impact.

I am not addressing rolling resistance at all. I remember there was a lot of analysis in Bicycling Science of rolling resistance... and I remember getting rather lost in those formulas! It is surely an interesting question, how does rolling resistance change with load and with tire width.

It is also true that a rough road is tough slogging, not just because of rolling resistance, but also because the load is not rigid but soaks up energy from vibrations. The same sort of thing could happen on a perfectly smooth road, if one were pushing hard on the pedals and then letting up - the back and forth accelerations could get cargo sloshing around and absorbing energy.

But I am not looking at efficiency here. I am just thinking that a key criteria for a tire to carry a load is that you really don't want the rim to hit the ground. That can bend the rim, or give one a pinch flat, and sure it rattles the rider too! So I am just trying to see how a wider tire is more capable of keeping the rim from hitting.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 30, 2011, 02:44:25 AM
A quick look at www.schwalbe.co.uk shows that site has all their technical information bundled into a single tidy PDF file, and it is more complete than the technical information listed on Schwalbe's US website.  

The UK PDF makes for interesting reading relevant to our present topic and is here:
http://www.schwalbe.co.uk/_webedit/uploaded-files/All%20Files/Technical%20Info.pdf

Page 10 indicates, "A dense carcass is important for low rolling resistance".

Pages 16-17 are devoted to a discussion of rolling resistance and contact patch shape and size, as well as energy requirements for two tires at various pressures, with the following quote:  "At 2 bar a 60mm wide tire rolls as well as a 37 mm tire at 4 bar".

Schwalbe's tire life estimates are intriguing as well, and they indicate a tire can be ridden so long as there is rubber present on the tread.

Of particular interest to me was the news that a Schwalbe tire can be stored for up to five years with no problem, and perhaps longer if stored properly.  I stored some spare tires for the tandem some 18 years ago, and came across them last Spring.  They mounted with no problem and aired up just fine.  No cracks or issues of any kind, but I had stored them in air-evacuated plastic bags in a cool, dry place in the garage.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 03:42:30 AM
That PDF is very nice - thanks for posting that link!

Fascinating, the table of rim vs. tire width is different in that pdf, compared to http://www.schwalbetires.com/tech_info/tire_dimensions - the crucial difference I see is that the pdf says a 19mm rim can handle up to a 60 mm tire, while the usa website says the 19mm rim is limited to a 50 mm tire width. The Rigida Andra 30 rim has a 19mm interior width, so that is a significant difference!

The discussion of contact patch shape in the pdf is quite interesting. Note that when they say that a narrow tire has a longer contact patch, that's assuming that the pressure is the same in both tires. If, instead, the pressure is kept inversely proportional to tire width, then the wider tire will have a larger contact patch...

ah, it seems my contact patch shape calculation, in a previous post, must be wrong. The shape must be roughly elliptical and the area must be proportional to the product of the length and the width. But I have the patch width increasing proportional to the tire width and the patch length proportional to the square root of the tire width. This would have the area increasing with the 3/2 power of the tire width. But if the pressure is inversely proportional, the area must be proportional linearly. Rats, back to the drawing board!
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 30, 2011, 03:55:39 AM
Schwalbe's 2012 UK _product_catalog_ also has some information relevant to this discussion.  It is available here:
http://www.schwalbe.co.uk/_webedit/uploaded-files/All%20Files/Bike2012-English.pdf

There is a nice photographic comparison of the entire line, starting on page 41 for the "Marathon Concept".  The Evolution series is presented as their "Best" and includes Racer, Supreme, Dureme, and Mondial (XR replacement).  The next tier down is the Plus series, which includes the Plus and Plus Tour, the latter with a more aggressive tread. The Original series includes only the Marathon with Green Guard puncture protection.  The Evolution series has the least rolling resistance and lightest weight, while the Plus has the greatest flat protection and durability.  Their chart shows the original Marathon is like the Evo series, but heavier and less easy-rolling.

Page 44 indicates...
Quote
The Big Apple started off the Balloonbike trend ten years ago: Comfortable cycling without using complicated technology! Air cushion tires are used as natural suspension. Inflated to around 2 Bar a Balloonbike rolls really easily and with a full suspension effect. A normal tire with a width of 37 mm must be inflated to a rock-hard 4 bar in order to roll similarly well.

This means at the same pressure, the narrower tire will be at a disadvantage, so Schwalbe is using the same pressure for their rolling-resistance comparison rather than adjusting it proportionally.

The Energizer series (page 42) is optimized for low rolling resistance and high grip.  I presume this comes at some expense of overall longevity and flat resistance.  If one needed really grippy, fast-rolling tires, these might fill the bill for, say, commuting.  Probably not so good near the curb as in the traffic lane.  

Yeah, Jim, there's quite a few differences in Schwalbe's UK info vs the US.  I'm still pondering it all, and plan to check out their sites in other countries as well, though my limited foreign-language skills will make it a tough slog (Google Translate doesn't work on PDFs).  In your last post, you observed...
Quote
Note that when they say that a narrow tire has a longer contact patch, that's assuming that the pressure is the same in both tires. If, instead, the pressure is kept inversely proportional to tire width, then the wider tire will have a larger contact patch...
<nods>  Yes, I think that is the basis for their argument that fat tires can roll more easily than skinny ones; they're using the same pressure in their comparison (see above).  When I read that, I had one of those "head-slap" moments; well, duh!

Still wrapping my own mind around all this,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 04:20:21 AM
Aha... if you use my contact patch formulas...

(http://i140.photobucket.com/albums/r6/kukulaj/moretirepressure.jpg)

L is the constant load, P is the pressure. The pressure should be proportional to the -3/2 power of tire width. It's a nice fit to the Schwalbe recommended pressure:

(http://i140.photobucket.com/albums/r6/kukulaj/Book1_24639_image001.gif)
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 30, 2011, 10:19:43 PM
While I am thinking about it, I wanted to clean up my math a bit:

(http://i140.photobucket.com/albums/r6/kukulaj/pressuremath.jpg)

I decreased my tire pressure a bit before my ride today - to 50 psi in the rear and 45 in the front. Felt just fine, though that was only about 5 psi less than what I had - I couldn't really swear that I noticed any difference!
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 02, 2011, 09:37:47 AM
Jim,

You may well enjoy seeing this:  
http://books.google.com/books?id=uDQEAAAAYAAJ&pg=PA1007&lpg=PA1007&dq=How+a+pneumatic+tire+supports+its+load&source=bl&ots=N9LCRVitB6&sig=fljjpXQy-7IfG20L3zwo5f5qsfo&hl=en&ei=UzR-TOCgLMKqlAeh6a3tCw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CB0Q6AEwAjgK#v=onepage&q&f=false

It is a Google Books excerpt from vol. 26 of the 11th Edition (1910-1911) _Encyclopedia Britannica_, page 1007.  The article envisions the tire as a spring and shows their calculations for determining the ellipse of the contact patch, among other things.  It really is a nifty little article on tire design, construction, and science for anyone interested in bicycle tires.  It also addresses our discussion of rolling resistance through a thumbnail presentation of the work of a Professor Osborne Reynolds.  Reynolds was a pretty interesting fellow himself -- an Irish mathematician who contributed greatly to the field of fluid dynamics.  See: http://en.wikipedia.org/wiki/Osborne_Reynolds  His research helped me a great deal some years ago when I examined boat propeller design and the effects of cavitation on propulsion efficiency.  I remembered his work on fluid dynamics and managed to track down this reference pertinent to our discussion of rolling resistance and tire design.

Reynolds inverted the problem we're dealing with and made his initial calculations upon an iron roller resting upon a sheet of India rubber.  He postulated rolling resistance is the result of slippage and distortion at the contact interface, and will vary due to hysteresis.  The upshot of it is his steel roller had the least rolling resistance due to the least hysteresis (deformation) caused by vertical compression (as a solid, the steel was uncompressible within any measurable limits for the test), followed by a pneumatic tire pumped hard and then an underinflated pneumatic tire.  

What really made me raise my brows was his statement saying a solid-rubber tire had the greatest rolling resistance of all!  Professor Reynolds says...
Quote
...If the vertical compression cd of the tire be denoted by y, the energy lost may be said to be proportional to Hy. Comparing three tires of steel, solid rubber and air respectively rolling on a smooth, hard surface, H (hysteresis) is probably smallest for steel and largest for rubber, y is least for steel, greater for a pneumatic tire pumped hard, greater still for solid rubber and for a pneumatic tire insufficiently inflated.  The rolling resistance of the steel tire will therefore be least; next in order come the pneumatic tire inflated hard, and the pneumatic tire inflated soft, while the solid rubber tire has the greatest resistance.

Whoa!  

Intuitively, the solid rubber tire "should" behave like a pneumatic tire inflated rock-hard so it has essentially no vertical compliance beyond the natural hysteresis provided by compression of the rubber tread, since any suspension effect of the casing would be essentially "locked out".  However, according to Osborne's thesis a solid-rubber tire is instead more akin to an underinflated pneumatic tire in terms of rolling resistance because of higher energy loss due to greater hysteresis for the same vertical compression (which I take to be deformation as a function of vertical load).  I need to ponder this a bit more.  

Jim, with regard to your concerns about the rim bottoming out (I'm thinking aloud here), the air inside a tire provides the opposing tension that supports the load.  The air in a tire serves mostly to keep the casing intact on the rim everywhere and/but actually supports the load directly only at the contact patch, where deformation of the casing transfers the load upward from the contact patch and on to the tire sidewalls and then on to the rim and centrally-supported axle and thence to the frame or fork.  If the air pushes equally outward all around the tire casing, then the only place where it can actually support the load _is_ at (or through) the contact patch...the overall pressure in our tire remains the same, despite the local deformation in shape.  The outward force of the air at the contact patch must equal the weight load in order to support that load to some degree.  Since pressure in the tire remains constant, the sidewalls must necessarily bulge outward when the center of the tread is deformed flat under load.  That is what transfers the load...the sidewalls bulge as the tire deforms under load and the vertical air pressure support (or tension) of the casing is reduced at the bulge; it pushes out instead (same overall pressure).  This is not so different than the way tensioned bicycle spokes support an axle load through a wheel, only in this case the air is providing the tension on the tire casing.  The tire tread at the contact patch isn't compressing...it just has less vertical tension that it would with no load.  Another way of putting it is the load _hangs_ rather than _stands_.  It just hangs less at the contact patch.  ;)  The _area_ of the tire contact patch is a function and result of both pressure and load.  The _shape_ of the contact patch is a product of tire design (i.e. conventional bias ply versus the radial-ply tires that came OEM on my '89 Miyata 1000LT), width, diameter, and pressure.  It is important to remember the inflated pneumatic tire cannot support a load separate from the wheel it is mounted on.  In the case of a bicycle, we're talking about two tensioned structures supporting the central axle load.

Whew!  My head hurts.  I'm putting this one to bed tonight, and then myself.

Best,

Dan,
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 02, 2011, 11:11:05 AM
For those still following the discussion, there is more to chew on here:

http://www.sheldonbrown.com/tires.html

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on December 02, 2011, 11:44:07 PM
I don't mean to belabor this endlessly, but I feel like I am getting close to understanding the relationship between pressure and tire width... yet there is still something off, and I am hoping that by showing my thinking explicitly, maybe somebody will find my error.

Seems to me that the contact patch area can be estimated quite closely. If we apply enough load to compress the tire by x, then the contact patch area should be 2*pi*x*sqrt(r*R),
where r is the radius of the tire and R is the radius of the wheel:

(http://i140.photobucket.com/albums/r6/kukulaj/contactpatch2c.jpg)

Suppose we have a wheel of radius 13 inches and a tire of radius 1 inch. Suppose we load the wheel so the tire compresses by 15% of the 2 inch tire width, i.e. 0.3 inches. Then the contact patch area should be  2*pi*0.3*sqrt(1*13) = 6.8 square inches.

Whew, that can't be right! That would carry about 100 pounds at 15 psi.



Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on December 03, 2011, 12:19:15 AM
FWIW, if you look at the Encyclopedia Britannica article that Dan so kindly posted, the contact patch area formula that I posted is exactly what appears there. Even more strangely, at the end of the article there is a table of tire sizes, pressures, and loads. Looking at the first column, a 28 inch wheel with a 2.5 inch tire and .19 inches of deflection, I compute a 5 square inch contact area. The table shows 70 psi carrying 360 pounds, so that is very much in line with 5 square inches.

That size is not far off a bicycle tire. To carry a 100 pound load, that would put the pressure down to 20 psi. But that is only .19 inches of deflection... the usual 15% Berto rule would call for .38 inches..  or 10 psi.

Maybe my problem is just the 15% rule. If that's really 15% of the tire radius, rather than of the diameter, that would pretty much put things in order. It seems I am off by a bit more that 2x, but still, that gets me in the ballpark.

Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 03, 2011, 12:46:45 AM
Jim,

If I understand the 15% rule of thumb correctly, then it is a 15% drop in the rim height measured at the bottom, under load (compared to unloaded, however that is defined).  That would indicate radius rather than diameter.  Though it a terribly difficult to measure empirically.

The idea in measuring the rim at the bottom (or the axle centerline) is it provides a true measurement of casing tension reduction under load.

As for the actual amount of pressure in a tire...did you see those photos on the Schwalbe websites?  Their photo of underinflation looked noticeably flat to me!  The photos showing a properly inflated tire could encompass a pretty broad range of pressures: http://www.schwalbetires.com/tech_info/inflation_pressure

It is also interesting to read this...

http://performancesimulations.com/fact-or-fiction-tires-1.htm

...a third-party presentation and discussion of  Avon tires' research showing contact patch pressure not only does not remain close to constant under increased load, it never even approaches that of the air pressure in the tire.  Relevant to the foregoing, the author states,
Quote
Contact patch pressure is constant and equal to air pressure?  No, not even close.

Wide tires have a greater contact area?  From this data it appears very likely.  Which would mean the "wide tires are softer and therefore give more grip" argument is bunk.  The contact patch is bigger, and the contact patch pressure is lower.  Avon has several sets of tire data available.  Feel free to do your own analysis on any of the other tires.  It appears likely from looking at this data is that if lateral sidewall deflection were included, the contact patch size might change by a rather small amount when the load is doubled in a typical operating range.  The contact patch certainly does NOT get twice as long and the contact patch size certainly doesn't double.  The above tables show the absolute limit to how much the length and areas could increase.  In reality the changes must be even less.

Wow.  Again, he's talking contact *pressure* and not *area*.  This data and observations seem to apply most directly to Formula 3000 car racing tires, which are of vastly different construction than bicycle tires, though the basic ideas still have relevance.

I fear we may be the only two nutters still in this game, Jim.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on December 03, 2011, 01:39:37 AM
I'm reading this with fascination but not contributing anything because I must still set up networks, and then catch up on a couple of lost weeks... Thanks for the break, fellows.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on December 03, 2011, 01:57:29 AM
Our teenager lifts weights in the basement, so it was easy for me to acquire 100 pounds of iron. I parked my Nomad next to a bookcase and marked the position of the rack top against the bookcase as I loaded the rack with the 100 pounds. The whole thing was a bit sloppy, but my best measurement is that the rack moved down about a quarter inch. This is with about 50 psi in the tire. Grrr. There is a factor of 3 loose here someplace! The quarter inch is very much in line with the 15% rule and Schwalbe's recommended pressure. It's just way out of line with my theory.... and with the Encyclopedia Brittanica theory, for that matter.

Humbling!
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 03, 2011, 02:08:25 AM
Thanks, Andre!  With three of us in the game, we've got the critical mass to be scientists, not nutters.   :D   Good luck on the network setup; looking forward to hearing from you when things ease up a bit.

Good thinking about loading the rack, Jim.  It is near-enough directly above the tire to get a pretty clear and accurate measurement.  'Sure beats my gymnastics.

Just for fun...if you have a stamp pad or water-soluble paint, it would sure be interesting to ink the tire tread and get some imprints on graph paper. 

Best,

Dan (who usually solves problems by calculation, formulae and inference, but *loves* rigging apparatus for empirical testing).
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on December 10, 2011, 04:09:27 AM
Still chewing! The section "How A Tire Supports Its Load" in

http://www.sheldonbrown.com/tires.html (http://www.sheldonbrown.com/tires.html)

really knocked me for a loop. Sure, the tire has to exert a force against the ground to support the load; that is just the contact patch, area times pressure. But the tire must also push up on the rim with that same force!

Obviously my first model is just way too simple. Let me try again.
(http://i140.photobucket.com/albums/r6/kukulaj/newtirecalculation.jpg)

On top there is a rim which contacts the tire at point B. On the bottom is a contact patch. The tire leaves the road at point A. I just show half the tire since the whole affair is symmetric.

Where the tire is not touching the ground, it will have a circular cross section. This circle will be tangent to the gound at point A.

What I calculate here is the cross-sectional length of the tire, which has two parts - along the contact patch, and then the circular part. The height of the rim off the ground and the contact patch width must cooperate to keep this cross-sectional length constant. I would like to solve this equation, expressing the contact patch width in terms of the rim height. That sure doesn't look feasible, though!

So I did the next best thing, which was the numerical approach. I just used dumb trial and error to find some patch widths and rim heights that would give the same tire length. For easy comparison, I present the drop in height, or compression, rather than the height. I plotted the contact patch width versus this compression, and also plotted my old formula. You can see that the new relationship gives much smaller patch widths. This is for a rim (half)width of 9.5 and a rim height at zero patch width of 50 - roughly my Marathon Supreme 2 inch tires on the Rigida Andra 30 rims.

(http://i140.photobucket.com/albums/r6/kukulaj/image001-2.gif)

This makes the contact patch areas about a third of my old calculation - which brings the pressures right in line with reality.

But it sure seems like this problem is overconstrained - I still haven't factored in the requirement that the tire support the rim with the required force! The more I dig into this, the more confused I get!

Truly humbling!


Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 10, 2011, 05:40:13 AM
Jim,

You (like myself and others interested in the science of tires) may wish to take a look at a publication by the US' National Highway and Traffic Safety Administration and the Department of Transportation. ( http://www.google.com/url?sa=t&rct=j&q=pneumatic%20tire%20science&source=web&cd=3&ved=0CCsQFjAC&url=http%3A%2F%2Fwww.nhtsa.gov%2Fstaticfiles%2Fsafercar%2Fpdf%2FPneumaticTire_HS-810-561.pdf&ei=L9XiTrr_IIWNigL9_ODBBg&usg=AFQjCNFP7z68TbA8W2Df1q8J0hT1SREPjA&cad=rja ). It is an update to the previous reference standard in the field for automotive tire design, S.K. Clark's, _Mechanics of Pneumatic Tires_.  I especially enjoyed the sections on waves in rotating tires (and how their propagation is damped by hysteresis) and rolling resistance.  Though the application is specific to automotive tires, the maths for determining and analyzing the characteristics of casing and carcass design are largely applicable to bicycle tires.

Quote
The more I dig into this, the more confused I get!  Truly humbling!

Jim, you're in good company; the modeling of tire behavior and characteristics -- even those as relatively straightforward as rolling resistance and tire patch shape, area, and loading are not simple.  The authors of the above publication indicate (pg. 410)...
Quote
A tire is remarkably complex in its structure and geometry. A model embracing too many of the features becomes over-complicated and the basic understanding is lost.

For a real appreciation of the difficulties and complexities in modeling tire forces, see F. Koutny's treatise, _Geometry and Mechanics of Pneumatic Tires_, a Czech engineer from Zlin, original home to Tomas Bata, founder of a shoe-building firm that later produced the the Bata Biker cycling shoes popular in the late 1970s; one of the first truly mass-produced, walkable cycle-touring shoes with...rubber soles.  For fun, see the references... http://en.wikipedia.org/wiki/Zl%C3%ADn ...and... http://en.wikipedia.org/wiki/Bata_Shoes

Respect! for tire engineers, and Schwalbe have some good ones.  This stuff goes all the way back at least to 10 December 1845, when Scot Robert William Thompson ( http://en.wikipedia.org/wiki/Robert_William_Thomson ) was awarded a British patent for the first pneumatic tire (an air-filled tire for horse-drawn carriages).  This, some 40 or so years before John Boyd Dunlop's application for bicycles (the passage of time was responsible for Dunlop receiving a patent; the rediscovery of Thompson's previous patent was responsible for the revocation of Dunlop's).  This stuff is fascinating to me, and so are the people involved -- true founders and giants in their burgeoning fields.    

<whoo!> A deep breath and small diversion in the discussion of bicycle tire rolling resistance and tire science.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: stormdog on December 12, 2011, 08:31:09 PM
Hi

I have been following this subject, and was wondering if any conclusions have been reached on the pressures we should inflate our tyres to. I realize it will depend to a degree of weight carried.

I have a Thorn Raven Tour with Rigida Andra 30 Rims and Schwalbe Hurricane 26x2.00 tyres, I was wondering what pressures other people use depending whether commuting or touring, I do quiet a lot of camping touring so am sometimes heavily ladened but carry very little on my daily commute over the Welsh hills.

Thanks for your views

John
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 12, 2011, 09:20:48 PM
John!

A worthy call for practicality in application...

I'm running Andra 30 rims and Schwalbe Dureme 26 x 2.0 rigid tires that measure a true 47mm in section width.  I weigh 172lbs/78kg in riding kit and my Sherpa weighs 40lbs/18kg with a pump, fenders, lights, and empty bottles.  I have been really happy recently running pressures of 45psi/3.1bar in each tire, front and rear.  With the bike loaded to the hilt (109-154lbs/49kg-70kg with full water tanks and a week's food), I seem to be doing alright with pressures pushing toward 55psi/3.8bar or so F/R.

I'm still fine-tuning and fiddling myself, but that's what's working nicely at present for a comfortable, smooth-rolling ride at each end of the spectrum.  I have found pressure variations make little difference in the outward appearance of fatter tires, unlike narrow, higher-pressure road-bike tires.  Looking at the Schwalbe guidelines and photos, I think at my weight unloaded, I could easily and safely do day rides on the Duremes at the minimum 30psi/2.0bar without damaging the sidewalls.

Hope this helps,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on December 12, 2011, 09:40:07 PM
All my analysis is fun and I hope leads to some increased understanding, but I would still advise sticking rather closely to the Schwalbe recommendations:

http://www.schwalbetires.com/tech_info/inflation_pressure

I think you want to increase those numbers with load. The base figure seems to be about 90 kg total on both tires combined. If you are a heavier fellow or are carrying a big load, you'll want to notch the pressure up a bit. Schwalbe recommends 3 bar for a 90 kg load. If you have say 30 extra kg aboard, crank that up to 4 bar. The limit for a 19 mm rim like the Andra Rigida 30 is apparently 5 bar, so you won't want to carry more than 60 extra kg.

Of course all of this is rather more precise than is really meaningful! But it's a starting point and then you can experiment to see what works for you and the conditions you ride in.
Title: Re: In praise of riding low pressure tyres fast
Post by: stormdog on December 12, 2011, 10:12:16 PM
Thanks Dan and Jim,

I will now do some weighing to check my tyre loading, and start doing some testing

Will let you know my results.

May be a few days as the forecast is for very stormy weather this week, and my route to work and back is over some high exposed roads, so may be a little bit of a problem trying to compare like for like.

thanks for your help

John

Title: Re: In praise of riding low pressure tyres fast
Post by: revelo on December 22, 2011, 09:38:41 PM
Interesting discussion. I recently completed a 1000 mile tour of the Mojave desert area of the Southwest United States, about 70% dirt road, 30% paved roads. Initially, I ran Schwalbe Marathon Extreme 2.25 (57-559) at about 42 PSI (about 2.8 bar) for the rear and 40 PSI (about 2.7 bar) for the front. When I had problems with sand, I deflated to 30 PSI (about 2.0 bar) for both tires, which helped a lot. Later, when I returned to the paved road, I reinflated back to the original pressures, then deflated again when I hit sand, etc. This got old fast, so I finally decided to deflate to just 36 PSI (about 2.5 bar) and see how that worked on sand. And it seemed to work just as well as 30 PSI. Then I left the tires at 36 PSI when I returned to pavement and the tires appeared to run as fast as at higher pressure. So that is some field experience in favor of lower pressures.

This tour was on a MTB. I just ordered a Nomad MK2 and will have it fitted with Schwalbe Mondial 2.15 (55-559). I will probably run these at the same pressure as the Extremes, or maybe slightly higher, say 38 PSI (about 2.6 bar) for the rear and slightly lower for the front, say 34 PSI (about 2.4 bar). My weight is about 85 kg dressed and a typical load would include 6kg of camping gear, 4 kg of food and 12 or so liters of water, so the total load would be 107 kg above the bike weight (about 18kg I imagine). 125 kg - Schwalbe's 75kg = 50kg. Divide this load in half gives 25kg additional per tire above Schwalbe's base loading. 35 PSI recommend pressure for 55-559 * 1.25 = 44 PSI. This sounds right for someone traveling exclusively on paved roads. But for someone like me, who is mostly on dirt, with much of that dirt being sand, 35 PSI or so is probably better.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on December 22, 2011, 10:52:24 PM
Hi Frank,

I've enjoyed looking at your blog and could see the Nomad coming; glad it will soon arrive.  Will you be using the same luggage setup on the Nomad?  Looks as if the MSR Dromedary would fit the frame triangle the same way as on the Novara Portal.  Same for the hiking staff.  The Thorn rear rack will transfer right over.

Also, on your new Nomad...have you decided to try front panniers and rack, or will you stay with your existing handlebar-bag setup?  With the water mid-frame, you're probably pretty well balanced F/R on weight, so you should be set with pretty equal tire pressures. <nods>  Yes, the Mondial 2.15s should give you a nice wide footprint on the soft stuff, especially at 30psi.  Please keep us updated as to how the tires do with this pressure under load.  I am especially interested in how the sidewalls fare, not only with this pressure, but with the Great Basin's prevalent goat-head puncturevine.  I intend to carry some Mr. Tuffy liners for use when I get to where the goatheads are worst, but will leave them out as I transit to and from Oregon and Claifornia through the Cascades.
 
'Shame we can't find dehydrated water in the outdoors stores.

Looking forward to pics when you get it all set up.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: revelo on January 01, 2012, 12:10:28 AM
I've enjoyed looking at your blog and could see the Nomad coming; glad it will soon arrive.  Will you be using the same luggage setup on the Nomad?  Looks as if the MSR Dromedary would fit the frame triangle the same way as on the Novara Portal.  Same for the hiking staff.  The Thorn rear rack will transfer right over.

Also, on your new Nomad...have you decided to try front panniers and rack, or will you stay with your existing handlebar-bag setup?  With the water mid-frame, you're probably pretty well balanced F/R on weight, so you should be set with pretty equal tire pressures. <nods>  Yes, the Mondial 2.15s should give you a nice wide footprint on the soft stuff, especially at 30psi.  Please keep us updated as to how the tires do with this pressure under load.  I am especially interested in how the sidewalls fare, not only with this pressure, but with the Great Basin's prevalent goat-head puncturevine.  I intend to carry some Mr. Tuffy liners for use when I get to where the goatheads are worst, but will leave them out as I transit to and from Oregon and Claifornia through the Cascades.
 
'Shame we can't find dehydrated water in the outdoors stores.

Looking forward to pics when you get it all set up.


I don't mean to hijack this thread, but I will address the above questions:

I'm not sure what version of my website (http://www.frankrevelo.com/hiking) you saw. I've been updating it and there is a now a trip report available (http://frankrevelo.com/hiking/trips_2011_11.htm). The hiking staff idea was an experiment and has been rejected. I encountered very few stray dogs, which was the main reason I wanted the staff in a quick-draw position for use as a weapon.

I do not want front panniers. My maximum water carrying capacity is 19 liters, but I never actually carried more than 16L, and about 6L of that I kept in the front triangle. That is sufficient weight to keep the front of the bike from rearing up too much, since my rear rack load is not that heavy. What I discovered is that a little rearing up is actually beneficial when traveling on sand. If the front is heavily loaded, then when you hit a patch of soft sand, the bike stops dead in its tracks. The only time I've had problems with the front rearing up was while testing my bike, unloaded, on steep and rugged uphills roads in the mountains around where I live (Reno, NV). But I can't pedal up these kinds of roads with the bike loaded, so that is a moot point. When I encounter this sort of road with a loaded bike, I just get off and push.

I am not a believer in tire liners, especially since Schwalbe tires already have a good protection belt. I think stans sealant in inner tubes is a much better idea for dealing with goathead. Read my report on flat prevention in goathead territory at http://frankrevelo.com/hiking/biking_flatprevention.htm

Now back to tire pressure. I'll definitely report my experiences with the Schwalbe Mondial 2.15. As I wrote before, I'll probably start with 36 PSI (2.5 bar) or maybe a little lower on the front and higher on the back, and then lower the pressure until I get something that works reasonably well on dirt roads. What I won't be doing is adjusting the pressure back to something higher when I go from dirt to pavement, since that is too much trouble. Also, lower pressure is safer if I need to jump off the pavement onto a soft road shoulder due to traffic coming up from behind. The more I think back on my recent trip, the less important going fast on pavement is, even assuming (and this is what this discussion is all about) that higher inflation makes for faster travel. What really matters to me is not bogging down in sand.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on January 01, 2012, 10:16:39 AM
Hello, Frank, I'm the OP. I wouldn't worry too much about hijacking the thread; I'm anyway constitutionally averse to staying on some narrow subject because you always miss out on what the other fellow knows but is reluctant to say for fear that he'll be accused of being OT.

The thread is about everything, positive and negative, to do with wide tyres and low inflation. It's just fortuitous that I have some fast downhills on rough roads and have found the Big Apples to be faster over them than one would expect from the common leaning of roadies towards narrow high-pressure tyres.

A prosperous new year to all, and may you cycle safely.

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: E-wan on April 01, 2013, 03:30:40 PM
if you're worried about the weight, you should spend
the few bucks more and get the Big Apple Liteskins which are pounds
lighter per pair, and the superlight racing tube as well for another
substantial weight saving.

Thinking of getting a pair of 26” x2.15 Big Apple Liteskins for my nomad for the summer (assuming the snow eventually goes)

Are there lighter presta tubes that you can use with them that will still retain their pressure as well as normal 54-559 Schwalbe tubes?

When I have tried light weight tubes in the past I have given up as they have needed toped up with air every few days.

Thanks

Ewan
Title: Re: In praise of riding low pressure tyres fast
Post by: julk on April 01, 2013, 05:11:28 PM
Ewan,
I use some of these
http://www.sjscycles.co.uk/schwalbe-sv14a-xxlight-presta-tube-26-tyres-40-559-to-54-559-prod14897/ (http://www.sjscycles.co.uk/schwalbe-sv14a-xxlight-presta-tube-26-tyres-40-559-to-54-559-prod14897/)
and they are half the weight of a 'standard' tube.
They keep the air in for ages.
Julian
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on April 01, 2013, 05:52:38 PM
Thinking of getting a pair of 26” x2.15 Big Apple Liteskins for my nomad for the summer (assuming the snow eventually goes)

Are there lighter presta tubes that you can use with them that will still retain their pressure as well as normal 54-559 Schwalbe tubes?

When I have tried light weight tubes in the past I have given up as they have needed toped up with air every few days.

You're thinking of "racing" tyres.

These Schwalbe "ultraleich" that I recommend lose air very slowly. I have a proper SKS Rennkompressor (and a nice digital gauge as well) but I use it about every month or two months and I've never applied the minipump carried on the bike to these tubes except when I tried it to see if it actually works. They're now 3 years old and have done 6300km, and they hold air as well as when new. In any event, these fat tyres are very forgiving, and the Schwalbe minimum recommendations are just that, recommendations; you can ignore them with impunity. I weigh 14 stone and have ridden my 60mm Liteskins down to 1.5bar on occasion, and for months on end at 1.6bar, and the fellow who put me onto the Big Apples weighs 350 pounds and rides his at 2bar; more normally I start mine at 2.03bar, which is where I find I have the best compromise between comfort and secure handling on potholed but fast downhill lanes, and a month or two later find they still have 1.8bar in them. Tyres that fat just shrug off rough roads and abuse. It may well be that even lighter tubes, if properly made, would also be satisfactory; I don't know because I tend to stick with Schwalbe and they don't make anything lighter than ultraleich in my size, though they do in yours (see the XX Type 14A).

The ones I use are Schwalbe 62-622 Type SV19A i.e. Presta, 140g each v 220g for the standard T19 tube. They make a definite difference.

American Schwalbe site http://www.schwalbetires.com/bike_tires/tubes

German Schwalbe site http://www.schwalbe.com/gbl/en/produkte/schlaeuche/index.php5?flash=1&ID_Land=38&ID_Sprache=2&ID_Seite=148&tn_mainPoint=Produkte&tn_subPoint=Schlaeuche

It looks like the standard Presta balloon tube for 559mm rims is Type SV13 at 190g, with Type SV14 as the Extralight (bad translation for "ultraleich") tube for 559mm rims, at 130g each. Type 14A appears to be the XX "racing" tube, 95gr. So the tube you want (if the idea is to use the ones I have good experience with) is Schwalbe Type SV14 54-559. It comes wit 40mm or 60mm Presta valves; that's what the SV in the codes stand for, Schlaverand, or French, or Presta valves, which are all the same thing.

If you decide to try the 95gr tubes, don't forget to report here on your experience.

Andre Jute

PS Have you checked the clearances for 2.15 tires?
Title: Re: In praise of riding low pressure tyres fast
Post by: E-wan on April 01, 2013, 06:45:02 PM
If you decide to try the 95gr tubes, don't forget to report here on your experience.

Andre Jute

PS Have you checked the clearances for 2.15 tires?

Thanks for the advice both of you

Decided to give the Schwalbe SV14A XXLight Presta Tubes a shot, as you say Schwalbe tend to make them fairly well and I've never had trouble with their other tubes loosing pressure


I hope 2.15 bigg apples will fit,  currently running 2.1 ice spikers and they fit ok and only clog the mudgards in soggy snow or lots of mud.

Ewan
Title: Re: In praise of riding low pressure tyres fast
Post by: E-wan on April 05, 2013, 03:23:23 PM
Thought I had it sorted but it looks like they don't make folding big apple tyres any more!

I can find the light skin folding version of them still available in 2.0 that's a little narrower than I was hoping for.

Anyone seen any of these still available in 26×2.15 folding?

Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on April 05, 2013, 04:11:50 PM
Thought I had it sorted but it looks like they don't make folding big apple tyres any more!

I can find the light skin folding version of them still available in 2.0 that's a little narrower than I was hoping for.

Anyone seen any of these still available in 26×2.15 folding?



I don't think you'll notice the difference unless you previously had the 60mm Big Apples fitted. I don't have direct experience but it seems logical. It is commonly said by experienced cyclists who have tried both that in normal use the 50mm Big Apple offers 95% of the value of the 60mm. You have to be really heavily loaded (and one of the guys saying it has weighed twice as much as I do, and I'm no featherweight) or ride through obstacles like a maniac, really to get the value out of the 60mm width (and all the associated costs of building a wider bike), or so they say. In any event, though I reckon you should design for the 60mm if you buy a custom bike, on the general principle that the best will soon prove to be barely good enough, there's no disguising the fact that most people consider the 50mm plenty good enough.

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 05, 2013, 05:16:04 PM
Hi Ewan!

To echo Andre's comments (and he has the most experience of any of us wrt balloonbike tires), Schwalbe say the following...
Quote
What is the difference between Balloonbikes with 50 and 60 mm wide tires?
60 mm wide tires provide maximum Balloonbike comfort. But also the lighter weight 50 mm wide tires had double the air volume of a standard 37 mm tire. The 50 mm wide tyre is a good choice for anyone who wants a sporty yet comfortable ride. Between these two sizes there is also a popular 55 mm width.
This comes from Schwalbe's Balloonbikes™ FAQ: http://www.balloonbikes.com/en/faqen.html

They have an entire (and newly updated) site on the topic here: http://www.balloonbikes.com/en/

It is a bit hard to dig out, but they consider their 50mm/2.0 Dureme to also be a balloonbike tire, albeit on the "sporty" side. I'm having success running mine between 2.0 and 2.5 bar (~29-36psi) and seem to have settled on F/R pressures of 29/32psi recently when riding my Nomad with a lightish day load. For reference, I weigh 172lb/78kg. The unladen bike weighs 45lb/20kg. With water, a HB bag, and the usual amount of "stuff" along -- and water -- the bike weighs about 65lb/29kg. Add these figures together to get the total bike weight, then halve it for average static load per tire (weight distribution aside). Of course, with a heavier touring load, I'd run the tires at higher pressure.

Running the tires -- particularly the front one -- at these pressures has the effect of a rising-rate spring under compression. In other words, the tires absorb niggling, low-amplitude/high-frequency vibrations wonderfully. I don't even feel expansion joints between concrete slabs, and chip-seal (the bane of American cyclists) feels as smooth as the best-finished asphalt. However, over large bumps, the tires compress initially, then very rapidly firm-up while still protecting the rim adequately. You do still feel the bigger bumps, but the effect is blunted, for lack of a better term. I'm learning all sorts of things from my experiments. For example, on rough logging roads, I sometimes feel the effects in my neck. I had supposed a front pressure adjustment would best address this, but in fact, a (relative) reduction in rear tire pressure made the biggest difference in correcting what amounted to a pitch-moment.

Amazingly, the bike rolls well with the 2.0 Duremes tires at 2.0-2.5bar. Drop below 2.0, however, and it feels like I'm pushing ("wheelbarrow effect") even though sidewall distortion isn't noticeably increased or even visible.

I'm going to buy a pair of mechanical/analog bathroom scales today so I can see what my actual front/rear weight distribution is at various loads. Digital strain-gauge scales don't work well for this, as they "hunt" for awhile before settling down, and if the weight shifts during the calibration process, the result can be wildly "off". I need something a bit more dynamic so I can see what's happening with weight shifts and load distribution.

I still largely go by Frank Berto's 15% drop method as a guide in determining maximum tire pressures; I use his figures as a starting point. Frank updated his tire pressure article from the original that appeared in American publication Bicycling magazine many years ago. It is a bit dated (2006) by current standards, but now includes minimum pressures for 26in/MTB tires in addition to optimum pressures for narrower tires. see: http://www.bccclub.org/documents/Tireinflation.pdf The graphed output is linear, so the trendlines can be exended for higher loads.

A bit more about Frank Berto here: http://www.thedancingchain.com/About%20Me.htm

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on April 05, 2013, 07:22:26 PM
In other words, the tires absorb niggling, low-amplitude/high-frequency vibrations wonderfully.

This modest-sounding advantage of balloons has far-reaching effects. I'm a writer. A writer is in fact a sort of manual laborer. He operates a keyboard while looking at a blank wall or out of the window. My hands are extremely sensitive to tiny vibrations. Until I went over to balloon tyres, bicycling used to wreck my hands and wrists. Schwalbe's Marathon Plus, which have their advantages too, are especially bad in this regard. An hour on the bike with Marathon Plus fitted was about as much as I could take before the tingling in my wrists became unbearable. We too have the cheap chip seal road surfaces...

But it isn't just me and my professional sensitivity in my hands and wrists. If you're not irritated by these micro vibrations, your whole ride becomes more agreeable, you ride better and further, and you ride safer too because you're not irritable and doing stupid things like cutting off large SUVs. I was saying to my wife yesterday that I haven't given the physio, who once ruled my life, a thought since I bought a Herman Miller Mirra chair and went over to a bike without suspension struts but with balloon tyres.

There are many other benefits to balloon tyres, most of them well canvassed in this thread, but the microvibration damping is so important, it bears stressing -- because it's absence doesn't stress the cyclist!

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: E-wan on April 05, 2013, 09:22:04 PM
Thanks for the replies and your thoughts and observations.

I had initially been thinking of 2.0 Duremes but thought of opting for Big Apples instead when I noticed they were available in 2.15. My rational at the time was that over the winter I have been running 2.1 ice spikers and have on a few occasions when going over a kerb or root etc felt the rear tyre make contact with the rim. This was with a modest lode in 2 small rear panniers. Admittedly I have been riding the ice spikers at around 2 bar (30 psi) which Is lower than I would intend to run Summer tyres at.

However I just went and measured the ice spikers and found that they are 46mm in diamater across the carcass on my regida andra rims (measures with a pair of callipers pinching the sidewalls). If I measure the distance across the nobbley spiky bits on the edge of the tread this is 54mm. This might suggest that a 2.0 dureme or big apple might actually have more air in it than a 2.1 ice spiker. Therefore seems to make sense to try a 2.0 tyre rather than hint around for one in 2.15 which would be heavier anyway.

Any idea what the difference is between 26x2.0 Duremes folding and 26x 2.0 big apple liteskins? It looks like the duremes are lighter at 630G where the big apples are 675g at 2.0. Also the tread on the duremes looks better suited to a little off road work than the big apples but it’s hard to tell without experience.

Thanks

Ewan
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 05, 2013, 10:08:52 PM
Ewan,

I just calipered my 26x2.0 Duremes mounted on Rigida Andra rims measure 47.73mm at 32.1psi/2.21bar. At their full 70psi/5.0bar rating, they measure 49.97mm, so that's when they are 50mm/2.0 inches on Andras.

Mounted and inflated tires often vary from their stated widths, usually in the narrower direction if less than full rated pressure. From the sound of things, your 2.1in Ice Spiker with similar pressure has nearly identical air volume to my 2.0 Duremes.

I've found the tread on the Duremes provides a better mechanical lock with dirt than a road slick or nearly slick tire like the Supreme, yet has tread blocks arranged to create a center ridge while riding. A photo of mine spinning is here: http://www.thorncycles.co.uk/forums/index.php?topic=3896.msg18965#msg18965
...photo linked directly here: http://www.thorncycles.co.uk/forums/index.php?action=dlattach;topic=3896.0;attach=1318 Not much tread contacts the ground in road use. On soft soil, the entire tread width comes into play, providing more traction.

It is not a tire or tread specifically aimed at off-road or on-road use, but a nice compromise between the two, making a good all-'rounder that rolls silently without any buzzing noise or feel. My guess as to the weight difference between the two? Given equal bead materials, I think the more open tread of the Dureme may contain less rubber overall than the Big Apple's larger, incised tread blocks.

Have you looked at the Big Ben? Similar to the Big Apple, but with a different, taller tread design for increased traction. Unfortunately, it seems to be available only in a wire bead listed at a more portly 720g. Schwalbe's North American site lists the 26x2.0 Dureme folding at 590g: http://www.schwalbetires.com/bike_tires/road_tires/marathon_dureme If only all 6 of mine (4 wire, 2 folding) did not wobble...visually distracting but doesn't seem to matter in use.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: julk on April 05, 2013, 10:28:31 PM
Ewan,
I have recently moved to 26x2 folding Duremes and have been very impressed with them.

I was riding 26x2 folding Hurricanes before - the Duremes feel faster, more supple, easier to steer and just as comfy at the low pressures talked about in this thread. I guess tyre technology has moved on again.
Julian.
Title: Re: In praise of riding low pressure tyres fast
Post by: E-wan on April 05, 2013, 10:52:01 PM
Think I will Go with the 26x2.0 Duremes then & try these with extralight tubes.

Dan is that an clinometer on your bars? where did you get it from And how do you keep it level & Stop it from twisting through road vibrations.

I did consider the Big Ben but it seems to only be available in wire rather than folding at the moment.

Thanks

Ewan
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 05, 2013, 11:03:43 PM
Quote
Dan is that an clinometer on your bars? where did you get it from And how do you keep it level & Stop it from twisting through road vibrations.
It is! A SkyMounti from Austria. See: http://www.thorncycles.co.uk/forums/index.php?topic=3896.msg20102#msg20102 All info at the link above. They are available in two handlebar diameters and black or red cases. All the viewing windows are clear atop neon yellow-green for high contrast with the black printed numbers and indicators.

It reads directly in slope percentage from the forward and rearward edge of the bubble, which floats in a thick mineral oil and is very stable. The unit itself mounts on a rubber strip and is secured -- securely! -- with two screws that need only slight tightening (overtightening will crack the mount).

I love my SkyMounti inclinometer and have one on each of my bikes. I has added greatly to my cycling enjoyment, and weighs very little.

All the best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 06, 2013, 02:37:51 AM
Hi All!

I now have a pair of identically-reading analog scales and am having fun looking at my weight distribution on the Nomad. What I wanted to look at was the static dry weight of the bike alone and how it was proportioned Front/Rear and the same with me aboard. The point of the exercise was to see the proportion of weight carried by the respective tires so I can adjust tire pressure accordingly to minimize rolling resistance while maximizing comfort. For fun, I did the same measurements with my favorite derailleur rando bike for comparison.

It is early days (I just got the scales), but here's what I found this afternoon...

With only myself (heavier in street clothes and with stuff in my pockets and on my belt) and a dry day-ride load aboard (underseat tool bag, but no other bags and with empty bottles; those are dynamic loads and I wanted to establish a static baseline), F/R weight distribution when riding on the Nomad's brake hoods is 39%/61%.

The front tire carries a lighter load and so can do with less pressure to aid comfort, while the more heavily-laden rear requires more. Further, the proportion is roughly 40/60 -- just as conventional wisdom has long held.

Out of curiosity, I measured the F/R weight bias of the Nomad alone. As above with a dry weight and no bags, the Nomad weighs in at 46lbs total, proportioned at F20lbs/R26lbs, or 43%/57%.

In contrast, the favorite of my 700x32C derailleur rando bikes measured similarly weighs 31lb total, proportioned 14lb/17lb, or 45%/55% alone. The bikes are not comparable in any way, so I can't really say how much the heavier Rohloff rear hub contributes to the 2% greater rearward weight bias (the Nomad has a steel vs. alu rear rack and longer rear stays as well as a ring-lock aft of the bottom bracket)

With me on it, the weight distribution on the rando bike is F81lb/R124lb, or 40%/60% (39.5%/60.4%).

Though we're comparing apples to oranges here and aren't holding constant for anything -- frame size, wheelbase, chainstay length or frame materials --the Nomad with me on it carries only 1% more of its weight on the rear wheel, despite all the weight in the Rohloff hub. In other words, the Rohloff hub along with everything else that makes the Nomad a Nomad, only results in a 1% difference in F/R weight bias when I'm on the bike.

Now, you'd think a 39%/61% weight bias would mean the same for tire pressures, but it doesn't work that way. Running the 26x2.0 front tire at, say, 2.0 bar/29psi would require the rear tire have ~73psi/5bar. This would be unnecessarily harsh and non-compliant at the rear. The front tire carries only 83lbs and the rear only 131lbs -- neither carries the entire load of 214lbs. Instead, if one wishes to adjust tire pressures proportionately, you take the difference between the front and rear: 61%-39%=22% so you take 22% from the front and add to the rear. Going with Schwalbe's 29/36psi/2.0/2.5bar is pretty close for F/R distribution.

Fun stuff!

Best,

Dan. (...who never tires of tire-talk)
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on April 06, 2013, 02:55:23 AM

Now, you'd think a 39%/61% weight bias would mean the same for tire pressures, but it doesn't work that way. Running the 26x2.0 front tire at, say, 2.0 bar/29psi would require the rear tire have ~73psi/5bar.

I don't get your math here, Dan. 60% is 1.5 times 40%. So I would guess that pressures should be proportional, e.g. 2 bar and 3 bar. I run my tires at much closer pressures, but maybe I need to try some more radical experiments like you are doing.

Anyway, where did your 5 bar pressure come from?
 
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 06, 2013, 03:39:23 AM
Hi Jim!

A good question, Jim! There are some additional factors to consider in setting tire pressures proportional to weight distribution with wider tires. My thoughts typed faster than my fingers, so what I should have written with greater explanation was this:

Many people make the mistake of pumping the rear tire to maximum pressure (~73psi/5bar in the case of a 26x2.0 Dureme), then work back from that to set the front pressure in the same proportion as the bike's load distribution (39% of 73psi is ~29psi). It doesn't work that way, though I heard it repeated once again when I went to pick up my new tandem tires.

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Going with Schwalbe's 29/36psi/2.0/2.5bar is pretty close for achieving ideal F/R distribution according as determined by 15% rim drop on my bike for my weight, considering my weight distribution when on the bike.

Now, does Berto's 15% drop result in an ideal tire pressure (one that optimizes comfort and minimizing rolling resistance) on wider "road" tires like the 2.0 Dureme? I he PDF linked above, Berto says no: "I don't think that the 15% drop criteria is valid for mountain bike tires because most mountain bike rims are narrower, relative to tire width, than road bike rims". The rim well contributes to a mounted tire's overall air volume, though it is devilishly hard to model what effect rim-well reservoir has on tire behavior, actual drop, and things like comfort and rolling resistance. Among other things, the rim sidewalls that contain this reservoir (which is not partitioned) are rigid, whereas the tire casing is differentially flexible -- the tire sidewalls behave differently than a belt-reinforced tread-center, though pressures act equally on all parts.  Unfortunately, this and some other factors in MTB-width tires at the time of publication (2006) resulted in Berto coming up with a minimum pressure graph instead of an optimum pressure graph as he did for road tires. Road tires just were not commonly available in such wide widths at the the time of his study and didn't make his test sampling.

I do think the relatively greater size of tire-to-rim may account for why I achieved 15% drop at less pressure than one would have expected by tires shown in Berto's road-tire graph of optimum pressures. In his treatise, Frank admits, "The numbers aren't precise". He cites 15% as ideal for the road-bike tires in his test, but --depending on the valence placed in rolling resistance or comfort -- cites a range of 13%-17%. From what I have read of Schwalbe's published tire research, it appears wider tires do behave differently at similarly proportional tire pressures thanks to their wider-but-narrower tire contact patch.

Fascinating topic, Jim. Sorry I wasn't more clear in my initial explanation.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on April 06, 2013, 04:03:45 AM
Many people make the mistake of pumping the rear tire to maximum pressure (~73psi/5bar in the case of a 26x2.0 Dureme), then work back from that to set the front pressure in the same proportion as the bike's load distribution (39% of 73psi is ~29psi). It doesn't work that way, though I heard it repeated once again when I went to pick up my new tandem tires.

Wow, that is some odd thinking! Pumping up the rear to the max, OK, that is a little silly but it does reduce the rolling resistance. But then to reduce the front to its fraction of the *total* load, rather than to its ratio with the *rear* load, that is perverse! What a world!

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Fascinating, really! The drop is such a small number and a bit awkward to measure - I'm glad you've figured out a system for measurement with some precision!

I am still perplexed by how the tire holds the rim up. But in any case, it sure seems to me that the spring constant for the tire ought to be proportional to the pressure, i.e. changing the load and the pressure in the same ratio should leave the drop constant. But that's not what you are observing! Reality can be quite surprising!

Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 06, 2013, 04:36:14 AM
Quote
to reduce the front to its fraction of the *total* load, rather than to its ratio with the *rear* load, that is perverse! What a world!
Agreed, but it sure seems to be common. Similarly, most people hammer in the maximum pressure printed on the sidewall -- if they remember to check their tires at all. A bike-shop friend actually recommends this to customers, since it at least means better rim protection and fewer "snakebite" flats from under-inflation. He says it beats the alternative, seeing customers' tires with as little as 15-20psi, cracked sidewalls, and complaints "something must be dragging". He has a point.
Quote
The drop is such a small number and a bit awkward to measure - I'm glad you've figured out a system for measurement with some precision!
The only way I could figure to do it was the similar to Berto's method:

1) Get two bathroom scales that read equally under the same weight (they're not as precise as I'd like, but at workably close).
2) Mark the center of each scale with an "x" across the top so the center of the axle/tire contact patch is on the center of the scale. Neglect this, and repeatability goes out the window. Block-up the bike's brakes so the wheel won't turn.
3) Mount the bike while on the scales and hang onto a wall. Get someone to note the scale readings while viewing vertically to discount parallax with the indicator needle/dial.

Once you know how much weight is going on each tire, remove the front wheel and mount it on the test rig (I used a spare fork with a modified stem clamped to the end, set on the ground so it supported the wheel vertically with the tire centered on one of the scales.

I then rigged one of my digital micrometers on the center of the rim top at the bottom (noting the reading in relation to the scale top), and pressed the forks till the scale readings for load at the axle matched what they were when the wheel/tire was mounted on the bike. At that point, I pushed the micrometer down on the rim and noted the reading. I then released the pressure on the tire and confirmed the "return" height equaled the initial height and subtracted the loaded height. Presto: tire drop at a given load, and repeatable for accuracy.

Fortunately, I use Rigida Andra rims and the same tires front and rear, so I was able to do my tests with just the front wheel/tire, changing pressure as needed to simulate F/R position on the bike.

Berto's method was a little different. He used a hydraulic jack and a dial caliper, but the basic methodology was similar. I'm not sure he allowed for the preload of the bare bike on the tires, however. I considered both, and finally went for a 15% drop overall with the bare wheel/tire loaded with the total weight relative to its position to see if I could duplicate drop using the weight registered on the scale with me atop the bike. I'm still pondering if this skews the figures compared to preloading the tire with the bike's weight, but figured this method was consistent with Berto's that measured overall load on the tire with bike *and* rider combined.

Careful as I was, there are still many potential sources of error in my measurements. For example, the center-top of the scales may be deforming under pressure, or the steerer of my test-rig fork may be bearing some small part of the vertical load, skewing the results a bit, though I took care to load the fork at the dropouts so as to load the axle in turn.

And, of course, all my carefully-gathered figures are for naught once I load the bike with bags and water!

I was curious as to where my body fit in relation to the Nomad's axles, so I grid-lined a profile photo and was surprised to find myself well within the wheelbase and pretty equally so (attached pic), though the bulk of my mass lies rearward of the center point between the lines.

Whew! Tire pressures are complexicated!

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: macspud on April 06, 2013, 05:13:44 AM

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Going with Schwalbe's 29/36psi/2.0/2.5bar is pretty close for achieving ideal F/R distribution according as determined by 15% rim drop on my bike for my weight, considering my weight distribution when on the bike.


Strange, I would have thought that if 29psi holds up 83lbs then that would mean that the tyre is sagging until 2.86 square inches of tyre surface is in contact with the road and therefore to get the rear tyre with 131lbs to also sag until 2.86 square inches tyre surface is also in contact with the road would be the way to calculate it so for 2.86 square inches to hold up 131lbs would require 46psi.
So to get both tyres to sag equally until the footprint is 2.86 square inches you'd have F/R 29psi/46psi.
Obviously from what you've said this is not the case so I'm missing something somewhere. Is there a difference in the way that the tyre behaves at different pressures i.e. the sidewall deflection?
I'm confused  ???
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on April 06, 2013, 06:38:40 AM
Quote
Is there a difference in the way that the tyre behaves at different pressures i.e. the sidewall deflection? I'm confused
Me too, Mac!

Discounting any of the myriad possible problems in my test rig outlined briefly above (which could well have skewed my results), I wonder if the relative (in)elasticity of the belt might have something to do with how much and where the tire deforms at a given pressure and load. The tire carcass is not homogenous in its construction -- there's a stiffer belt and tread in contact with the ground, and the casing is domed in shape. I wonder if the different materials and their elasticity and placement might constrain the tire's vertical movement at some pressures and loads.

What I'd like to do (if I can figure a way to hold everything together while I do it) is to caliper the carcass width under load at various pressures and see how it compares to rim drop.

Best,

Dan. (...who is looking forward to playing more)
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on April 06, 2013, 01:26:14 PM
how much and where the tire deforms at a given pressure and load.

Yeah, I think you're on to something here. Recently you observed that the width of a Dureme changes a bit as the pressure changes. Once the shape starts changing like that, it's a different tire!

I am sure that Mac's pressure and area theory remains correct. That is the ground-tire interface, where the load must be supported. A wider tire will increase area more quickly as the drop increases, so there is geometry in play.

But there is also the tire-rim interface, which also must support the load. That is much more mysterious for me! Anyway, if the pressure changes the geometry, then that is going to shift the relationship between force and pressure... things go non-linear! Wild!
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on June 05, 2013, 12:32:07 PM
Somehow I got myself going again on this topic! I fit my general formula to the Berto chart at

http://www.vintagebicyclepress.com/images/TireDrop.pdf

and figured out how to compare the results visually:

(http://i140.photobucket.com/albums/r6/kukulaj/Nomad/oldvsnewberto2cropped_zpsf45f2604.jpg)

My formula tweaked to fit Berto is:

P = 110 * L * W^(-1.5)

P is tire pressure, in psi
L is single wheel load, in pounds
W is tire width, in mm

* is multiplication
^ is exponentiation

Another nice data point - for the 102 mm Pugsley tire, my formula give about 10 or 11 psi, which is on target. So the formula should work across a very wide range of widths and loads.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on June 05, 2013, 11:51:48 PM
I put out a table of values for this formula for a variety of loads and tire widths

http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html (http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html)
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on June 05, 2013, 11:52:57 PM
This is a tremendous effort, Jim -- My!

Still reading and digesting...

All the best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Peddrov on June 06, 2013, 10:12:07 AM
Thanks Jim, this is some good data right there.

What's your opinion on conflicting manufacturer recommendations. On my 2" Schwalbe Marathon Supremes, it says 2.0 - 5.0 Bar or 35-70 PSI. Funnily enough this in itself is a contradiction, because 2.0 Bar are 29 PSI, not 35. That aside, I would probably be ok with 29 PSI on my front wheel according to your table. But what if someone doesn't weigh much? For example, my girlfriend + bike without any panniers would tip the scale at about 145lbs. So, for the front wheel, that would be about 20PSI, which is way below what Schwalbe says is ok.

Back in the day, when I didn't know much about such things, I toured on Schwalbe Marathon Plus tires with (presumably) very low pressure. I didn't have any gauge etc. so I don't know exactly how badly I abused them, but they did develop cracks after a few months and I eventually threw them away.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on June 06, 2013, 12:25:58 PM
Thanks Jim, this is some good data right there.

What's your opinion on conflicting manufacturer recommendations. On my 2" Schwalbe Marathon Supremes, it says 2.0 - 5.0 Bar or 35-70 PSI. Funnily enough this in itself is a contradiction, because 2.0 Bar are 29 PSI, not 35. That aside, I would probably be ok with 29 PSI on my front wheel according to your table. But what if someone doesn't weigh much? For example, my girlfriend + bike without any panniers would tip the scale at about 145lbs. So, for the front wheel, that would be about 20PSI, which is way below what Schwalbe says is ok.

Back in the day, when I didn't know much about such things, I toured on Schwalbe Marathon Plus tires with (presumably) very low pressure. I didn't have any gauge etc. so I don't know exactly how badly I abused them, but they did develop cracks after a few months and I eventually threw them away.

The Marathon Plus is a high pressure tyre, very stiff. I'm not surprised to hear that, under-inflated, they cracked. I found that under about 55psi they just stopped working.

I routinely run my 60mm/2.35in Big Apples down to 1.6 bar, sometimes 1.5, and they're never inflated to more than 2.1 bar. My bike and I together pull the scales north of one-eighth of a ton, and the small roads and lanes on which I ride are riddled with large potholes through which I often ride at speed, but in 6400km I've seen no signs of the tyres feeling the strain. My tires look like they'll make 10K. There's a family resemblance in the carcass of all those fat Schwalbe tyres so I expect that you would do no harm through experimenting by lowering the pressure in small steps. Certainly, Berto's 15% rule would result in a lower pressure than Schwalbe recommends. Schwalbe, a German manufacturer, is almost congenitally bound to practice a large measure of covering its corporate ass with big safety margins in its operating suggestions.

In general I would say that, unless you load your bike to the margin of the tyres' capability, you can forget Berto and Schwalbe both, and reduce tyre pressure on tyres 2in and wider to where you ride comfortably, which will likely be well above snakebite territory. I've found little advantage in wear, longevity, punctureproofing, rolling resistance, power requirement, roadholding and handling (roadholding is what the bike does right, handling is whether and how fast you can make it react when you exceed the limit of roadholding), in inflating the exemplary Big Apple Liteskins to even the lowest pressure Schwalbe suggests.

On a side note: I'm not at all certain that even with a heavy bike/gear/rider I have yet undercut the Schwalbe suggestions far enough to reach Berto's 15% drop. Those balloon sidewalls are quite a bit stiffer than I expected to find before I fitted them.

In summary: blow off some air and see.

Andre Jute
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on June 06, 2013, 06:48:53 PM
Yeah I think the ranges given by the tire manufacturer are pretty loose. Actually the whole subject of tire pressure is pretty loose, like nobody really knows. Schwalbe's chart of recommended pressures matches my table pretty well except about 30% higher pressure. The whole subject of rim width has to be brought in too. All you can really do is take in information to get you started and then do your own experimenting. I guess if you are pushing limits very far in any direction then just don't put yourself in risky situations, e.g. high speed down a curvy road or way out in the wilderness a hundred miles from a telephone. Once you have a lot of experience to know a configuration is working then you can start to rely on it more.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 08, 2013, 09:52:03 PM
Just saw this... lots of measurements and graphs etc.

http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf (http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf)
Title: Re: In praise of riding low pressure tyres fast
Post by: il padrone on November 08, 2013, 11:17:48 PM
I guess if you are pushing limits very far in any direction then just don't put yourself in risky situations, e.g. high speed down a curvy road or way out in the wilderness a hundred miles from a telephone. Once you have a lot of experience to know a configuration is working then you can start to rely on it more.

Hundreds of miles away in a wilderness of desert I was running my Marathon Mondial 2.15 tyres at sub-20psi while carrying a load of 50-60kgs of gear plus me. They ran well, but the footprint was still not big enough for the sand

(http://i1327.photobucket.com/albums/u666/petesig26/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020145_zps23dce545.jpg) (http://s1327.photobucket.com/user/petesig26/media/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020145_zps23dce545.jpg.html)
(http://i1327.photobucket.com/albums/u666/petesig26/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020160_zpsce9c8cbb.jpg) (http://s1327.photobucket.com/user/petesig26/media/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020160_zpsce9c8cbb.jpg.html)
(http://i1327.photobucket.com/albums/u666/petesig26/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020208_zps23df62c6.jpg) (http://s1327.photobucket.com/user/petesig26/media/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020208_zps23df62c6.jpg.html)
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 08, 2013, 11:57:36 PM
Just saw this... lots of measurements and graphs etc.

http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf (http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf)

What I've been saying for years...

Thanks for that, Jim. As always, science comes like a breath of fresh air into the musty corners of street corner gossip.
Title: Re: In praise of riding low pressure tyres fast
Post by: il padrone on November 09, 2013, 12:47:37 AM
Interesting however - the benefits of lower pressures and wider tyres are valid for gravle roads and especially off-road tracks. But on road the higher pressures produce less rolling resistance.

Different set-up required for different environments. Who woulda thought it??
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 09, 2013, 01:35:50 AM
One of my common mental orbits is around the topic of riding efficiency, like joules per kilometer or whatever. Of course one can put a power meter on one's bike along with a GPS and just measure the efficiency quite directly. But the causes and contributions aren't exposed.

One of my fantasy bike gadgets is a wind speed/direction meter. It's one thing to ride 20 miles, but then to ride 20 miles with 4,000 feet of climbing, that's saying a lot more, and then to have a 15 mph headwind most of the way! I wouldn't mind being able to document my bragging rights!

A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide. My more common riding experience is on hard surfaced roads that are not at all smooth. Some kind of vertical accelerometer should be almost trivial in this time of gadgetry beyond belief! More bragging rights! But also another type of surface where wide, low pressure tires work well.


Title: Re: In praise of riding low pressure tyres fast
Post by: il padrone on November 09, 2013, 02:04:56 AM
A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide.

At times, but not always.

(http://i1327.photobucket.com/albums/u666/petesig26/Otways%20and%20%20Shipwreck%20Coast/P1010225.jpg) (http://s1327.photobucket.com/user/petesig26/media/Otways%20and%20%20Shipwreck%20Coast/P1010225.jpg.html)

(http://app.getsmileapp.com/images/58dcf05b22d61e454481feee2007d97d5e168ffa233f3e49c167d20289f55cdf/jpg/2048x1536)
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 09, 2013, 03:04:15 AM
That Australian terrain is just awesome! Thanks for the photos! Fat tire country, for sure!
Title: Re: In praise of riding low pressure tyres fast
Post by: il padrone on November 09, 2013, 03:47:41 AM
Yep, used the fat tyres - 2.15"

Red dust country

(http://i1327.photobucket.com/albums/u666/petesig26/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020153_zps39c7537c.jpg) (http://s1327.photobucket.com/user/petesig26/media/Red%20Centre%20Way%20and%20the%20road%20to%20Old%20Andado/P1020153_zps39c7537c.jpg.html)
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 09, 2013, 09:38:29 AM
One of my common mental orbits is around the topic of riding efficiency, like joules per kilometer or whatever. Of course one can put a power meter on one's bike along with a GPS and just measure the efficiency quite directly. But the causes and contributions aren't exposed.

One of my fantasy bike gadgets is a wind speed/direction meter. It's one thing to ride 20 miles, but then to ride 20 miles with 4,000 feet of climbing, that's saying a lot more, and then to have a 15 mph headwind most of the way! I wouldn't mind being able to document my bragging rights!

Haha I think you're going in the opposite direction to me Jim!

I'm trying to stop myself getting hung up on average speed readings (since it seems to be getting less as the years go by) and therefore a power meter would probably be a bit depressing.  Similarly a wind speed meter, when I'm battling into a headwind I can happily curse the weather people for getting it all wrong without a meter to prove they were right and I am a wimp.  ;)


A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide. My more common riding experience is on hard surfaced roads that are not at all smooth. Some kind of vertical accelerometer should be almost trivial in this time of gadgetry beyond belief! More bragging rights! But also another type of surface where wide, low pressure tires work well.

While I love 700c 25mm tyres for smooth roads, I am also becoming interested in fat tyres and offroad cycling. Offroad exploration and the possibilities for that around my neck of the woods could happily occupy me for a long time to come!

But I look forward to more of your science Jim, and bragging rights too!

Jim

P.S.  Great photos as usual Il Padrone   ;D
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 09, 2013, 09:56:20 AM
Quote
...a power meter would probably be a bit depressing.  Similarly a wind speed meter...
Oh, I want.need one of those windspeed meters, Jim (Relayer): http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Delectronics&field-keywords=wind%20speed%20meter&sprefix=wind+speed+me%2Celectronics&rh=i%3Aelectronics%2Ck%3Awind%20speed%20meter
Quote
when I'm battling into a headwind I can happily curse the weather people for getting it all wrong without a meter to prove they were right and I am a wimp.
Don't forget the effect of riding into the wind is additive, Jim. The formula is: Forward speed + headwind ? [not equal to] wimp.

So, how do I measure the wind with what turns out to be remarkable accuracy? I turn and ride with the wind, sticking out my wet tongue. As soon as I feel a breeze on my tongue :P I know I've just started to exceed the (now) tailwind and check my speed. If I time it just right, the reading is accurate to within 1-2mph/1.6-3.2kph or so. How do I know? I call occasionally phone a friend or family member and have them check either the online airport readings or online realtime readings at Weather Underground home reporting stations in my immediate area. If the readings match, I know I've got it pretty close.

And people wonder how I entertain myself when touring solo.  :D

All the best,

Dan. (...who now wonders if drivers think I'm sticking my tongue out at them)
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 09, 2013, 03:03:30 PM
So, how do I measure the wind with what turns out to be remarkable accuracy? I turn and ride with the wind, sticking out my wet tongue.

Your dedication to precision measurements in bicycling science is an inspiration to us all, Dan. I've written to the I. G. Noble Prize Committee about you.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 09, 2013, 03:57:09 PM
Having given my enthusiastic general support to this report, it is time to mention a rather serious quibble.

The reporter starts off by saying that roadies have always thought that the harder the tyre is inflated, the easier it is to propel, and that mountain bikers thoughtlessly transferred this preference to offroading. So far, so good. (1)

After the tests are in, he then concludes that offroad tyres should be wide and only modestly inflated. That too is right.

Then, for on-road tyres, he makes the same mistake he rightly accuses the reactionary roadies of.

Apparently his "authority" for this error is the graph on p7 (2), labelled "The influence of tyre pressure". It shows a dramatic fall in rolling resistance as the inflation pressure falls on gravel, a lesser slope but in the same direction on grass, and a still lesser but opposite slope on the road.

Actually, his conclusion in relation to on-road pressure is already a mistake from this evidence alone. Eyeballing the graph, the difference between 2bar -- which the maximum to which I and many others inflate 2.35in/60mm Big Apples to carry heavy bikes and heavier riders with even heavier loads, and 4 bar -- is about 5% cost in rolling resistance power.

Another graph shows us that on the road rolling resistance represents only about 12% of power consumed. So what we're talking about is a power cost of about 0.6% extra. That's a trivial cost for all the other benefits in comfort, roadholding and handling from using the lower inflation.

It's already a ludicrous conclusion from a very small margin. If I didn't like the rest of this fellow's article so much, I'd condemn him for not putting his brain in gear. Unfortunately there's more, that any experienced cyclist will immediately see:

But that's on a test track's ultra-smooth road surface. On any rougher road, such as is found in real life, the rolling resistance disadvantage will almost instantly tilt the other way, towards the off road condition, towards an advantage. So now, for real roads, one chooses the low-pressure balloon in its own right, and again comfort, roadholding and handling factors tend the same way.

Still not the whole story. The tests were taken at 5km to eliminate wind resistance considerations. (Unnecessarily so in my opinion, but never mind.) The point is that the higher the speed -- and road speed is certainly higher on average than either 5kph or 5mph -- the greater the rolling resistance advantage of all low-pressure tyres, and the correspondingly greater the comfort, roadholding and handling advantage of balloons.

The reasons to fit balloons on a bike for use on the road, and to keep the inflation low, are overwhelming when you put your mind in gear, including on this evidence which at first sight can be (and by this fellow is) read the opposite way.

QED.

FOOTNOTES
1. It isn't quite that simple, but never mind. The real story has to do with beach cruisers, which already had low-pressure slick balloons, giving birth to mountain bikes, and mountain bikes being perverted by ignorant marketing fashions over the decades, but, as I say, never mind the truth, we'll go into it some other day if anyone cares.

2. For some reason my Mac won't copy or save this PDF from Safari, nor mail it, nor anything, so my page numbers refer to what Safari shows.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 09, 2013, 05:37:18 PM
Hi Andre and All!

Further research in the area of bicycle tire rolling resistance is always welcome, but I also noted some apparent inconsistencies in this summary report, especially given some of Schwalbe's own previously reported findings. My concerns are similar to yours -- again, based on Schwalbe's summary report, findings that seem to have been repeated widely without much actual review or analysis.

I have methodological concerns regarding the small sample size of the tires and some confusing representations of rolling resistance shown in some of the charts. For example, if the meadow results in the greatest rolling resistance (in terms of watts used, according to the article text), why does a gravel road show greater rolling resistance? (see attached chart, reproduced from the Schwalbe document).

The lower rolling resistance of the wider tire cannot be examined apart from its larger diameter (wider also means taller), and so one can't conclude width alone is responsible for reduced RR 'cos the wider tire's larger diameter also reduces RR on rough surfaces by bridging gaps. One possible way to address this would be to vary wheel size to allow for variations in width while holding constant for outside diameter. For example, my 26x2.0 Schwalbe Duremes are within about 1.0mm of the same OD as a 38mm Grand Bois Lierre tire mounted on a 650B rim. Of course, in that comparison, the tires are of different construction, tread pattern, and brand.

If air pressure varies, then effective tire diameter also varies, but I don't see this addressed in the summary report.

Not to quibble unduly, but we don't know if the meadow was wet (with rain or dew) or dry or something in-between or if it was the same for all tests. Real-world testing is less consistent than roller-based lab testing and can lead to greater variation in the variables.

These examples illustrates the difficult in obtaining good data when threats to internal validity are high. The study seems to be based on a pretty small sample size and relatively few miles as well.

I dearly wish we had access to Peter Nilges' original work so we could review it in context; it is not really possible to critique the study fairly based on third-party summary reports. I'm doing a search now...

Best,

Dan. (...who really appreciates JimK finding this report, finds it fascinating, and wants to know More)
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 09, 2013, 06:47:11 PM
Huh.

Apparently, this study goes back further than I initially thought, perhaps to 2009. It was the subject of a protracted discussion on the MTBR Forum in 2011: http://forums.mtbr.com/wheels-tires/schwalbe-tire-pressure-study-question-689628.html

One poster there also spotted the anomalous chart showing gravel had more resistance than the grassy meadow, mailed Schwalbe for clarification, and received a reply saying the chart was labeled in error.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 09, 2013, 06:59:48 PM
For example, if the meadow results in the greatest rolling resistance (in terms of watts used, according to the article text), why does a gravel road show greater rolling resistance? (see attached chart, reproduced from the Schwalbe document).

Dan

I would be so bold as to say the keys on that chart are wrong.  Under the chart the narrative suggests that 18 Watts of power can be achieved by reducing tyre pressure on the meadow ... the difference in the Meadow line on the chart is less than 10, whereas the Gravel line is the only one with that kind of variance.  Methinks switch Gravel and Meadow and it makes sense.

Doh!  You have just posted that Schwalbe said this.

Jim
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 09, 2013, 07:03:15 PM
Quote
I would be so bold as to say the keys on that chart are wrong.
Hi Jim (Relayer)!

<nods> Yes, there is a lot of confusing information in that report, and the charts and text don't always agree.

Still trying to locate an online copy of the original thesis or dissertation.

All the best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 09, 2013, 07:11:36 PM
I also wonder why compensation was made for heavier tyres, and to what degree?  .... Further down it gives an example of a modest amount (?) of extra power required to get a tyre 500g heavier up to speed, does this mean that the power factor for heavier tyres has been reduced?

Also, it is stated that rolling resistance has a continuous effect and lighter weight is only a factor in acceleration ... but to my mind the weight of the tyre will have an effect on momentum long after the acceleration phase is over?

I have no idea how significant the effect of momentum would have compared to resistance, but if weight is a factor them momentum must also figure?.

Jim
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on November 09, 2013, 07:20:38 PM
Quote
I have no idea how significant the effect of momentum would have compared to resistance, but if weight is a factor them momentum must also figure?.
I'd think it would affect the results to a degree, though how much on rough ground, I'm not sure. There's inertia of the mass as a whole (i.e. forward movement of bike and rider combined) then there's rotational inertia, the "flywheel effect" from very heavy wheels.

My guess is the very (!) low speeds used in the study would minimize the effects of each.

Heavy flywheels (and things that act like them) do store energy, but the downside is they have a dampening effect on acceleration when they need to be spun up again.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 09, 2013, 10:47:52 PM
I operate hefty 25mm wide Exal rims, heavy Sapim spokes, and very heavy 60mm Big Apple tyres on substantial hubs (Rohloff rear, electric front). My experience is that after the initial breakaway from a standing start, which these days I achieve in top with just a touch of the motor, the bike picks up speed deceptively fast on the pedals in 11th gear without the cyclist breaking into a sweat. The spacing of the Rohloff gears and the ease of changing gears are factors, of course. Once going the thing has a momentum of its own that can catch out the unwary cyclist who begs a ride on my bike in tight corners where such a long wheelbase bike must be turned in early, and the deceptive speed, and reluctance to shed it merely for stopping pedaling, aggravates the effect. (It's like putting a guy used to a small hatchback into six litres of Mercedes.) Roadies especially, who never fail to comment on how heavy my bike is, are caught out time and again by this momentum in a bike they expect to be slower than their carbon confection. And it's all in the wheels, because the frame itself is built of specially developed very thin wall Columbus steel that is, surprisingly, lighter than my two aluminium similar purpose (Dutch stadssportief) bikes' frames. So, I conclude that weight low down in the wheels has a positive effect.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 10, 2013, 03:45:27 AM
Hey, check it out, an Andra 40, 25 mm width and the spoke drilling for Rohloff!

http://www.ryde.nl/en/products?product=47 (http://www.ryde.nl/en/products?product=47)
Title: Re: In praise of riding low pressure tyres fast
Post by: Relayer on November 10, 2013, 10:06:18 AM
I've just found an interesting piece from an old 'Rivendell Reader' via another forum about heavy wheels/tyres and their momentum uphill on a Pugsley.

http://fcdn.mtbr.com/attachments/surly/239838d1172936789-rivendell-reader-pugsley-pug2.jpg

OK, it's not scientific, but Grant Petersen's experience surely makes his findings credible.

Jim
Title: Re: In praise of riding low pressure tyres fast
Post by: NZPeterG on November 10, 2013, 10:55:34 AM
Hi all,
90% of riders run too higher pressure in there tyres.
Lower pressures is faster and better....
Just lower your tyres and ride!



Pete....;-)....:-)....).:...



.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 10, 2013, 12:00:23 PM
Hey, check it out, an Andra 40, 25 mm width and the spoke drilling for Rohloff!

http://www.ryde.nl/en/products?product=47 (http://www.ryde.nl/en/products?product=47)

That looks like the business. And in a proven design too. I hope the rim will be generally available. It is hell getting Exal rims unless you're a manufacturer.


I've just found an interesting piece from an old 'Rivendell Reader' via another forum about heavy wheels/tyres and their momentum uphill on a Pugsley.

http://fcdn.mtbr.com/attachments/surly/239838d1172936789-rivendell-reader-pugsley-pug2.jpg

OK, it's not scientific, but Grant Petersen's experience surely makes his findings credible.

Jim

His findings don't surprise me at all, nor that he should tell the truth about them, and admit he doesn't know the reason, and not offer all kinds of fraudulent rationalizations. I'm a longtime admirer of Mr Petersen.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on November 10, 2013, 03:15:24 PM
That Andra 40 is not available in the CSS coating. Someplace I read that it comes with a power coat all over the sides so you can't use rim brakes at all but only disk or drum brakes. Rats! --- but that might just have been the way that one shop got them. Unclear.
Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on November 10, 2013, 04:32:30 PM
Thanks for the warning, Jim.
Title: Re: In praise of riding low pressure tyres fast
Post by: Joe B on February 21, 2014, 10:15:47 AM
Although I’ve not been able to read through all eight pages yet I’m finding this discussion most illuminating.
My two solo bikes run on 700x28 Vittoria Rubino’s and 26x1.5 Pasela’s
While the tandem uses 26x1.75 Pasela’s and the wife’s and daughters bikes have 1.75 Marathons.
I also have a set of Marathon XR’s for use when touring.
With all of these tyres I have always tended to inflate to the maximum as indicated on the sidewall however having read the superb articles posted by Andre, Dan and others perhaps this is not the best approach.


Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on February 21, 2014, 02:01:50 PM
With all of these tyres I have always tended to inflate to the maximum as indicated on the sidewall however having read the superb articles posted by Andre, Dan and others perhaps this is not the best approach.

Happy to be of service, Joe. Unless you're really very heavy or carry lots of gear on the bike, with tyres in the 1.75in region you can start by inflating to the minimum on the sidewall and from there see how you go. It costs nothing to experiment. Share your results in this thread so we can build up the community store of knowledge.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on March 15, 2014, 04:37:01 AM
Hi All!

Earlier in this thread, I mentioned the tire pressure article by Frank Berto, which I found a great aid to my riding. Our own JimK has also developed a tool for determining optimal pressure -- that ideal point where comfort is maximized *and* rolling resistance is minimized.

I recently found an Android phone/tablet app that does the same thing, based on a 15% sidewall drop under load, like Berto's. It is called Bicycle Tire Pressure Calculator (Berto Tire Pressure). It costs USD$2.50 for the full version that handles unlimited number of bicycles, but if you only need pressures for two bikes, there's an otherwise identical free/demo version:
Paid: https://play.google.com/store/apps/details?id=com.edisongauss.bertotirepressure
Free: https://play.google.com/store/apps/details?id=com.edisongauss.bertotirepressure.demo

This app takes into account general bicycle type, bike weight, fore/aft cargo weight and rider weight. You can choose wheel diameter and specify actual tire width in mm. Data can be entered in lb. or kg and pressures in psi or bar. choosing bike type gives a proxy for riding position. I chose "French Randonneur" as the closest match to my Nomad's setup. Andre's would probably be closer to "Dutch City". Remember to tap the bike graphic to update recommended pressures after making changes.

For my Nomad, I found the app recommendation agreed perfectly with the pressures I'm already using -- 29/34psi -- when I entered the Schwalbe Dureme's published width at 50mm. At the actual measured 47mm, it comes out at 30/36. On my other bikes, it was a bit under what I'm using and makes me eager to give it a try to see the results for myself. It may not be exact, but should give a good starting point for experimentation and provides loads of play value for techie types like me.

Best,

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on March 15, 2014, 05:03:26 AM
Cool! Here's some more info:

http://www.edisongauss.com/berto-tire-pressure-app/ (http://www.edisongauss.com/berto-tire-pressure-app/)

I will be studying this!
Title: Re: In praise of riding low pressure tyres fast
Post by: JimK on March 15, 2014, 05:12:03 AM
They're guessing that a 26" tire should get inflated 10% less than a 700C tire. My estimate is that it should be more like 5% less.

Looks like their crazy formula is not much different than mine. I tried to derive one from physics & just fit to Berto using a single free parameter. I would guess they're using 3 free parameters and less physics. The difference probably emerges just at the edges. Nowadays with the fat tire bikes, what was once an edge... it'd be interesting what that app recommends for pugsleys and such like!
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on March 15, 2014, 05:17:11 AM
I agree,  Jim. I love this stuff,  and playing with tire pressures has transformed my cycling and made it more pleasurable and efficient over the years.

All the best, 

Dan.
Title: Re: In praise of riding low pressure tyres fast
Post by: mickeg on March 15, 2014, 01:24:51 PM
Thanks for listing that Android app.  Now if only I knew what type of bike my touring bikes and my foldup bike are so I knew which geometry to select.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on March 15, 2014, 07:04:08 PM
Quote
...if only I knew what type of bike my touring bikes and my foldup bike are so I knew which geometry to select.
Hi Mickeg!

That is a dilemma with this app. The frame geometry profiles do affect the app's pressure recommendations. For example, if I substitute "Dutch City" for "French Randonneur", the pressures change to F/R 24/40 to reflect my change in weight distribution. In my case, I already knew through testing what my tire pressure "should" be, and my positioning on the Nomad with its drop handlebars is very close to that of a traditional French Randonneur, so that may be why my current pressures were essentially confirmed by the app.

The "Obsessive/Custom" option throws up a graphic of a long-wheebase Surly Big Dummylike cargo bike with F/R recommendations of only 9psi, way off what I would have expected unless the bike was running Large Marge tires. Hmm.

Best,

Dan.

Title: Re: In praise of riding low pressure tyres fast
Post by: Andre Jute on March 15, 2014, 08:20:37 PM
That app sounds like a bit of a suck and it and see gimmick for innocents. No innocents around here, though some have guileless smiles. (You can trust them. They ride a Thorn.)

In theory, I like the 15% rim drop prescription. In practice, it's purely for obsessives with time on their hands. I'd rather spend my time cycling. So what if my tyres give 1% less milage than someone who spent hours, perhaps days, measuring a 15% rim drop for all possible loading conditions? A pair of Schwalbe Liteskins 60x622 with a matching pair of Ultraleicht tubes T19A cost Euro 66 landed in rural Ireland and those tyres are still marching on strongly at 7169km, so the saving over around 10,000km is 66 cents.

Instead I propose that you test by riding, which is fun rather than work. Schwalbe, the makers of most of our tyres, give good information. I presume Pasela's makers do too, and if they don't, Dan published a reference to an Andy Blance article on tyre inflation not too long ago in another thread. So look up the makers recommendation for your tyres and and inflate to that, front and rear. Take a ride, take several rides, ride for a week or a month to see what happens when the tubes let out a little air by osmosis. If by that time you forget to follow the rest of my prescription, you've arrived, that's the right pressure for you. If you're inclined to experiment, first increase the pressure 10% and take a short ride. The purpose is merely to prove that an increase is required so infrequently as to be an ignorable condition.

Now reduce the pressure to 90% of the maker's recommendation. Why such a large initial reduction? Because those guys cover their backsides. Their marketing department doesn't want their tyres to get a reputation for giving tubes snakebites, so they bump up whatever the guys in the lab tell them is a safe pressure. Right. Now, at 90% of the recommendation, you're at the native, natural inflation of the tyres on your bike for your weight and circumstance.

Now go down another 10% of the maker's recommendation, 20% altogether. If you love this level, you can very likely safely ride here. The downside might be slightly faster tyre wear, and some tyre sidewalls may be damaged by under inflation, though generally they are damaged only by gross under inflation, not 11% real under-inflation. (This is 100 recommended minus 10% CYA margin = 90% of what the guys in white coats thought right. 10/90 is 11%.) It is unlikely that you will cause more drag by 11% under inflation, and very unlikely that you will reduce drag by overinflation; overinflation will cause faster tyre wear than under-inflation, just like on a car.

In summary: 90% of manufacturer's recommendation is very likely to give you the correct 15% drop at the rim, and to be the native, natural, necessary inflation of your tyres. A reduction to 80% of manufacturer's recommendation will give you more comfort with no or very small penalties in efficiency or longevity of tyres.
Title: Re: In praise of riding low pressure tyres fast
Post by: mickeg on March 16, 2014, 02:15:58 PM
Thanks Dan, I will just use the French Randonneur geometry for my touring bikes.  I have drop bars and use the drops about a third of the time, so that makes sense.  Since the only difference between geometries would be a slight shift of pressure to front from back or to back from front, it is very possible that the difference in pressures from one geometry to the next is within the tolerance of error in my reading the pressure gauge on the pump.  So, not a big deal if I pick the wrong geometry.

My newer Road Morph G and Lezyne Micro Floor Drive have better gauges than the one in the photo, but the one in the photo is on the pump I use 90 percent of the time.

I know that you have calibrated gauges, but I am not going to worry about minor gauge error.
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on March 16, 2014, 07:16:31 PM
Sounds a sensible approach to me, mickeg.

For what it is worth, among my collection of non-certified gauges, there's enough variation to make a substantial difference -- as much as 4psi. Still, if one consistently uses a single gauge, the pressures within a range are pretty consistent even if they vary across the dial or in absolute terms. Any gauge will give one a yardstick for consistency and will allow one to  index and measure cause-effect wrt pressure adjustments.

I was delighted to find the Android app simply for the fun if gives and was encouraged to see others are examining the question of pressure on comfort and efficiency also. I may have simply gotten lucky to find the pressures/configuration matched my Nomad, but it has made me wonder about diddling with the pressures on my other bikes as well to see the effect of their recommendations. I think the tool may be most valuable as a means to get one close. Fine-tuning for one's own needs from there may give the best result. For example, my rando/touring bike runs 32mm road slicks on 20mm box-section rims. The tires add air volume, but the rims take it away compared to a wider, channel-section design. I may have less margin to play with in that case.

The Vittoria tire company also has a free Android app for adjusting tire pressures: https://play.google.com/store/apps/details?id=com.vittoria.itirepressure I have not yet tried it, but it appears to neglect some variables. I suspect it may be geared more toward users of Vittoria tires. Curiosity will soon get the better of me and I'll download it to try.

Schwalbe have an app for Android and iPhones (displaying in German) that seems to show local sales outlets. One screen shows tubes and another a tire pump with the admonition to check pressure every 30 days: https://play.google.com/store/apps/details?id=de.diemedialen.schwalbe.app

Best,

Dan. (...who is far from being "air" to the throne in the tire pressure kingdom)
Title: Re: In praise of riding low pressure tyres fast
Post by: John Saxby on March 16, 2014, 07:42:08 PM
Quote
"air" to the throne in the tire pressure kingdom
  No worries, Dan -- to air is human after all, and a throne would surely be out of place in the Republic of Oregon, nicht wahr?  (Just a wee end-of-sentence tag there, to help you on your Euro-trek.)
Title: Re: In praise of riding low pressure tyres fast
Post by: Danneaux on March 16, 2014, 08:05:10 PM
Brilliant, John!  ;D

All the best,

Dan. (...who dearly loves puns)