I'm not sure if I'm understanding the question...but, if you mean do energy losses beyond the breakpoint depend on the particular tire? Only so much as to where/when the breakpoint pressure is gone beyond. After that, those losses far overwhelm the inherent flexing losses.


That makes sense. In a resonance condition, the tire is being flexed more than in the non-resonance case. Also, any damping in the structure as it is might also be a factor.

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Although if we're trying to generalize, this probably gets less true as you look toward beefier tires. The more hysteresis you have, the more hysteresis you save by increasing pressure. And, if a stiff tire with lots of damping performs worse as suspension on pavement to begin with, then the penalty for exceeding a breakpoint might be lower as well.

Not a lot of breakpoint testing gets done on slow road tires, though. Racers might not find answers to questions like "does a Schwalbe HS 404 even have a breakpoint pressure on asphalt" very valuable.
Jan Heine had a chart where a Rubino was staying within a 5W range for all pressures from 80PSI to 130PSI, while an open CX was - in addition to being faster in general - about 10W faster at the sweet spot than at the slowest higher-than-breakpoint pressure.

Yeah...from what I know about the methodology used there, I'm not sure how comfortable I'd be in drawing conclusions from it...

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It's imprecise and doesn't allow for effective isolation of performance factors, but if done carefully it's not obvious that it would be horribly inaccurate for comparing significance of differences in breakpoint effects, if they do exist on that magnitude. At any rate, I'm not saying that it definitively proves anything.

I am just thinking about it.

On one side on bad roads with too high tire pressure one gets high crr values. I think this is what you call breakpoint? Is it the same as what is called impedance losses by others? But the impedance losses shall occur beside other places in the body of cyclist. Other cyclist other impedance losses with the same tire? Then one would need something like a cbr (c-constant body impedance resistance).

On the other side, is it a breakpoint? Ok, when you test a tire with a certain pressure at your test speed you get high crr, i.e. the breakpoint. If you increase the speed, is the high crr still there or can it decrease again?

Was this where you saw that chart? https://www.renehersecycles.com/...-pressure-is-faster/

That is the chart I was thinking of, yes. I think it was first published in one of the Bicycle Quarterlys.

That data is pretty coarse and noisy, but it could be very interesting to see more precise information plotted in a similar style of chart across a wide variety of tire styles.

Nope. We went over this in a thread not too long ago.

On smaller irregularities and bumps, the spring rate is dominated by pressure... this is from Josh Poertner's tests. It makes essentially no difference whether it's a 23 or 28mm tire; it's the pressure that matters. A 28mm tire at 80psi will have more comfort, lower Crr, and better pinch flat resistance than a 23mm tire at 90psi.

It isn't an earth shattering difference though, and I never run anything bigger than a 25mm.

Yep... it's almost entirely pressure dependent for small irregularities.

hi Josh, can you use the pressure calculator to extrapolate from 29 to 26 for other non-standard tire sizes like 24 and 20"?

thanks,
Eric

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We just don't have enough data to really draw the curves of understanding between diameters at this point.. the data driving the algorithm for the calculator is over 90% 700c/29" and 650b and even then, the actual tire diameter range is quite small as the 700/29er data is heavily weighted toward 32mm and smaller road tires and the 650b stuff is pretty much entirely 42-55mm off-road tires.

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That was going to be my question.

Based on the Michelin charts
https://i.stack.imgur.com/O9WIL.jpg


it suggests 115psi which is what I've been running at forever.


That chart suggests
Rear Tire Pressure (PSI): 108.5
Front Tire Pressure (PSI): 106
and given the fact I'm in Chicagoland and roads are crappy I would think that 100psi might even be more appropriate.

Am I reading this wrong?

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My concern with the RR tests being done in Finland and the Netherlands is the use of the Diamond Plate drum. Diamond Plate typically has approximately 4mm bumps. This is much larger than even the worst roads I ride on here in the US. Go ahead - take a ruler outside to the road and look for yourself.

While the so-called hysteresis losses should extrapolate to smoother surfaces, does anyone know this for sure? Reading this thread you can see that the whole wheel/bike/rider combination is very complex. IMHO, a wheel held rigidly (no fork), with the tire riding over 4mm bumps, doesn't simulate what most of us are doing.

Has anyone done similar tests with a smooth(er) drum, and also at higher speeds (more in line with what the riders on this site ride on and travel at)??

The fact that most of the common tires tested by myself and BRR typically differ from each other by the same percentages, points to the fact that when tested at pressures below typical the breakpoint pressures experienced outside, both the smooth rollers and diamond plate covered drums give a valid estimate of the "outside" Crr values, with the only difference being a multiplication factor to go from the test values to the expected outside values. This makes sense if you realize that all the diamond plate is doing is causing "extra" flexing of the casing, as compared to the smooth roller, with the resultant addition material hysteresis losses.

My own testing, and some testing done by Andy Coggan a long time ago suggests that the "smooth roller to flat outside" data correction is going to be in the range of ~1.3 to 1.5X. The correction for factor for a diamond plate roller is going to be less.

The above is part of the reason I don't see a need for adding "texture" to the roller testing. The smooth rollers tell you all you need to know about the tire material properties when used reasonably (i.e. not over-inflated). But...people look at the smooth rollers and think "that doesn't reflect reality" and try to add in "roughness", even though it's not telling one anything additional in the end.

Now then, if one is interested in finding the "breakpoint pressure" of a particular tire/loading/speed setup, that's going to require a test fixture that incorporates some SIGNIFICANT damping between the wheel and the load mass (or forcing element, such as an air cylinder, like is used at Wheel Energy, from what I understand), so that it can mimic the damping losses in a human body. I have yet to see a test setup that does so...besides, field testing for Crr using Virtual Elevation methods outside do a pretty good job of that evaluation, so why not just do that instead?

Roller testing for tire ranking (to find the good ones), then VE testing for pressure evaluation is my recommendation...or, just use the Silca pressure calculator for the latter to save even more time ;-)

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In addition, I think there is an assumption that this all applies equally to the rear tire, when in fact that test ideally would have the tire driving the drum, and not vice versa, since most of us spend more time pedaling than coasting - again pointing to how complex this all is.

I do not want to diminish this testing, but point out issues that might explain why my legs conflict with some of the numbers I've read, especially as to "Brand X vs. Brand Y" tires.

Thank you!!

On my test rig with a smooth wooden drum normal force is applied through weights which can freely move vertically on linear translators. It is not funny when 50 kg become bouncing up and down in resonance if there is some kind of excentricity (for example when the tire is not perfectly round, but crr gets high in such a case). I can adjust a damper like it is used in motor cycles to avoid bouncing of the steering rod.

Actually...my testing is done on a bike with small rollers (i.e. the wheel IS driving), and again, the fact that many tires have similar percentage differences when compared between my results and BRR's tests implies that driven vs. driving isn't a large factor, if at all.



I would speculate that your observation is the result of either a.) human perception difficulties (i.e. what's "feels fast" isn't necessarily so), or b.) tire construction differences between what was tested on the rollers and what you had on hand. I've personally experienced "unannounced" tire construction changes that can easily affect the Crr.

No problem :-)

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Tom A. wrote:

Actually...my testing is done on a bike with small rollers (i.e. the wheel IS driving), and again, the fact that many tires have similar percentage differences when compared between my results and BRR's tests implies that driven vs. driving isn't a large factor, if at all.

I did not know that - I'm going to have to take a serious look at your blog. Also, for anyone still following this, I had contacted Jarno Bierman at BRR, and he says the raised areas of the diamond plate on his drum are only 1mm high, not 4mm.

Hanginon, thanks for the update on Jarno's drum!

To reiterate what Tom has been saying, I think that it's easy for us to get caught up in thinking about this in absolute terms rather than relative ones.. I see this in wind tunnel debates all the time as well.

Tom and I have shared/compared data for more than a dozen years, plus we have the work of Wheel Energy, Jarno, and others.. plus SILCA has more than 4000 data points from on-road testing of Crr using virtual elevation techniques with pro road teams, triathletes, olympians.. etc

What I can tell you from all this is that while VE/Chung testing is the best way to really get an absolute answer, the relative differences between tires really doesn't change (as long as we are below the break-point pressure) depending on the measurement style.. so a tire that tests 3% faster on Tom's smooth roller with wheel driving, will similarly test ~3% faster on Jarno's bumpy machine with machine driving and similarly on Petri's machine at Wheel Energy, and similarly ~3% faster in VE testing on road that we might do with a team.

The one caveat to all this is that the more bumpy the surface, the better the good tires test.. so 3% faster on smooth/mildly rough roller testing might be 4-5% better on Wheel Energy's 'Roubaix' surface, or even more in actual testing in the Arenburg. These very fast tires also tend to have flatter impedance curves once the break point has been passed.. so they tend also to penalize you less for pressures that are too high.

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Your welcome. While he refers to it as "Diamond Plate", now that I've had a closer look at the picture of it, the pattern is not the same as the stuff typically found in the US - and 1mm bumps are certainly far more like what I ride on. Obviously I should have waited for his response before typing. Sorry about that. None the less, this subject is very interesting, and I've learned a lot!

To add to the confusion, Aerocoach just posted a result saying that Corsa Speed Tubulars recorded the same resistance as CS clinchers with latex tubes or tubeless.

More and more happy with my H3 / Pro Disc for £300, and Veloflexes, although I wish there was an easy change to an 11 speed hub as it's painful to have cassettes sent away to have that 1.8mm shaved off the back of the carrier...