Author Topic: In praise of riding low pressure tyres fast  (Read 32365 times)

Andre Jute

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In praise of riding low pressure tyres fast
« 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
« Last Edit: November 28, 2011, 02:36:03 PM by Hobbes »

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #1 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. 
« Last Edit: October 30, 2011, 01:41:15 PM by Relayer »

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #2 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #3 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.


Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #4 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #5 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)

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Re: In praise of riding low pressure tyres fast
« Reply #6 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

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #7 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:



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.
 

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #8 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.
 

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #9 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

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #10 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!!     ::)

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #11 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.
« Last Edit: January 28, 2012, 06:11:54 AM by Danneaux »

Eric

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Re: In praise of riding low pressure tyres fast
« Reply #12 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.
 

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Re: In praise of riding low pressure tyres fast
« Reply #13 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #14 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.