Author Topic: In praise of riding low pressure tyres fast  (Read 44493 times)

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #15 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.

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #16 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

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!

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #17 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #18 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #19 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.

julk

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Re: In praise of riding low pressure tyres fast
« Reply #20 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.

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #21 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

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #22 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.
« Last Edit: November 28, 2011, 04:28:44 AM by Danneaux »

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #23 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.

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #24 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.

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #25 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #26 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.

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #27 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.

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #28 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.
« Last Edit: November 29, 2011, 05:03:36 PM by Relayer »

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #29 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?!?!?