I didn’t see this posted anywhere on the Forum: https://blog.flocycling.com/aero-wheels/when-the-rule-of-105-can-make-you-slower/
What are your thoughts?
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Total drag from all sources matter.
Looks like they are assuming tire pressures between a 25 mm and 28 mm tire are the same. Based on that assumption, their data is correct. But the optimum tire pressure for a 25 mm tire and 28 mm tire for the same road and wheel is going to be different. When you optimize tire pressure for the tire size, wheel width, and road condition, the rolling resistance is going to be roughly the same between the two different size tires.
Looks like they are assuming tire pressures between a 25 mm and 28 mm tire are the same. Based on that assumption, their data is correct. But the optimum tire pressure for a 25 mm tire and 28 mm tire for the same road and wheel is going to be different. When you optimize tire pressure for the tire size, wheel width, and road condition, the rolling resistance is going to be roughly the same between the two different size tires.
Yeah, folks always seem to forget this with the big tire stuff.
It’s one thing to go from old school 19mm tires up to 23’s or 25’s because the average amateur racer isn’t some 145 pound weight pro cyclist able to run decent tire pressures for roads on skinny tires.
A 23 or 25 on a modern width rim is usually plenty. HED for their rims state pretty low needed tire pressures for those modern sizes as people forget a bigger rim bed is more volume. The same that a new tire size is a new volume.
A 23mm on a 17 or 19mm internal isn’t the same volume as a 23mm on a 21mm internal. Necessitating less pressure.
28’s except for folks of a weight really needing them, are largely IMO a “rail trail” gravel tire size. Not a triathlon/road racing/time trial tire. Now, Paris Roubaix, yeah…that’s a LOT rougher than your average rail trail. So seeing a pro cyclist run a 28 there makes total sense. Even then I thought 25’s were more common.
Given ALL of that, I feel like just maybe they’ve got motivation to push that out given some other brands these days have even wider max rim width wheelsets where 25’s and 28’s “match up aero wise” better.
I’m only 175-180, but I love 28s racing on Michigan roads. I will take a marginal loss for taint comfort.
I also wondered about tire pressures and optimization in this post. I know the guys at Flo have also looked at vibration and damping effects in past and they should be pretty aware of Tom A. and the pressure break point issue with tire pressure (because they did an article on it already https://blog.flocycling.com/aero-wheels/optimizing-tire-pressure-using-vibration-to-measure-impedance/#60-80_psi) but for some reason they did not address that in the post. As always the net effect is what is important and there are a few items at play, rim width internal and max external, tire size, tire pressure are a few). Tom A. and Josh P. as well as a host of others could be great commentators on this.
but for some reason they did not address that in the post.
On the other hand, neither is anybody arguing the opposite side, at least not in depth. Absent from these discussions in general is analysis on how various factors (especially tire width) affect breakpoint pressure. There’s a BRR article that sometimes gets referenced, but it assumes that tire compliance is proportional to spring rate against a flat surface, while offering little to justify this assumption.
Seemed a little click-baitey? The largest difference was 0.43 watts, smallest was 0.08. I guess an extra 0.43 watts will make you slower, buuuuut…
My CeramicSpeed OSPW Aero, on the other hand, now we’re talking real savings.
Seemed a little click-baitey? The largest difference was 0.43 watts, smallest was 0.08. I guess an extra 0.43 watts will make you slower,
the sensitivity of A2 is > .43 watts but < 1w.
Looks like they are assuming tire pressures between a 25 mm and 28 mm tire are the same. Based on that assumption, their data is correct. But the optimum tire pressure for a 25 mm tire and 28 mm tire for the same road and wheel is going to be different. When you optimize tire pressure for the tire size, wheel width, and road condition, the rolling resistance is going to be roughly the same between the two different size tires.
That’s only true if the road is very smooth. If you are on chipseal or similar, then compliance and Crr will be lower for the larger tire at lower PSI.
Odd that they did not include these factors. The difference in any case would be less than they report and the 25mm tire would win.
Another important factor is handling in crosswinds, which relates directly to resistance and comfort. A smaller tire is usually more predictable IME, probably due to trailing and leading edges having similar properties, and side-force also behaving similarly on the front and rear parts of the rim.
I don’t think you can make any conclusion here… what’s completely missing is how they chose optimum tire pressure… and what it was?? 🤷ðŸ¾â€â™‚ï¸
Velonews did a test for Paris-Roubaix tires and used an air shock to try to simulate the impedance/vibration losses a floppy human would add (vibrations making it past the tire go into your body vibrating it and that energy will then be lost as heat)… and something I don’t think many noticed was that there seem to be NO CORRELATION between optimum pressure and tire width even same brand/casing in different sizes… every single tire seems to have an inherent optimum pressure (could there be resonant frequencies for each tire in each size??)… and likely an optimum pressure on a given surface…
Interesting, but from a practical point of view… not sure what to make of this analysis actually…
another factor that could contribute to above… could be the the subject of this analysis… the vibration characteristic of tire when it’s on a narrower rim and light bulb shaped or on a wider rim and more arc shaped are likely different… so maybe they ARE on to something… but don’t think it’s fully hashed out… too many variables not held constant to flesh out I think…
Take a close look at the data from VeloNews/Wheel Energy analysis on tires used in Roubaix 2021… where the used an air shock to simulate a human being’s contribution of vibrations losses… there didn’t seem to be a correlation between tire size and optimum pressure as many assume… many may have missed that… but it demonstrates that since this is a major input of all tire pressure calculators I’m familiar with, tire calculators can lead to wrong optimum pressures. It would seem to indicate that each individual tire, inherently has properties that lead to it’s own optimum pressure…
…and don’t forget watts to spin:
https://drive.google.com/file/d/1pkTgyw0g_lzS2AP8WFt45xPxw8LqFF7J/view?usp=sharing
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Looks like they are assuming tire pressures between a 25 mm and 28 mm tire are the same. Based on that assumption, their data is correct. But the optimum tire pressure for a 25 mm tire and 28 mm tire for the same road and wheel is going to be different. When you optimize tire pressure for the tire size, wheel width, and road condition, the rolling resistance is going to be roughly the same between the two different size tires.
Not true…
a) from a practical perspective… on rough roads the 25mm likely won’t allow you to use the optimal pressure because of risk of pinch flats…
b) bending of tire carcass is what causes the rr losses in heat… the fatter tire with a larger diameter (think, closer to flat) also has to deform its carcass less to produce its contact patch AND the contact patch for thinner tires are closer to sidewalls meaning the arc tire carcass needs to be bent to produce the Edit is more for the thinner tire in this respect as well…
When you optimize tire pressure for the tire size, wheel width, and road condition, the rolling resistance is going to be roughly the same between the two different size tires.
why do you think so?
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison#drop45
Not relevant, not what is observed in real world since it doesn’t take into account impedance/vibration loses… the assumption that an air spring, is an air spring, is an air spring show to be false…
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison#drop45
Glad to see BRR breaking it down, as I’ve known this for a couple of years. At the same pressure, the larger tires have lower resistance, but in the real world no one rides the larger tires at the same pressures as a smaller tire. All of this brings us to the point of, once you’ve settled on a tire, pick the size that is most aero for smoother roads or is beefier for rough roads.
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison#drop45
questions. do you believe that the higher pressure is the fastest pressure, in every instance? and, do you believe the wider tire is the fastest tire if you pump it up to the highest pressure?
my guess is, no and no. and BRR doesn’t think this either. but its testing says that the answer is yes and yes. i think there’s a lot of validity to BRR, but where BRR’s protocol seems (to me) to fall down is in pressures. it can tell you (perhaps) what tire is fastest, but it can’t tell you much about what pressure to ride that tire at. and because it doesn’t have a handle on pressures, to me that says it doesn’t have a handle on (so called) breakpoint, which is to say, it can’t well factor vibration.
there’s some data i’ll be writing about in a couple of weeks - pretty rigorous field testing - that contradicts this notion that 25mm rolls faster than 28mm. but getting back to the “whole system” or “drag from all sources” thesis, the 28mm tire may not be the fastest tire. or it may be, depending on whether the wheel was built with 28mm in mind.
i’m not trying to be cagey or mysterious, the data is under embargo and i can’t publish until the embargo lifts. suffice it to say, i think 28mm is going to put up a good fight against 25mm, even in timed racing.
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison#drop45
Glad to see BRR breaking it down, as I’ve known this for a couple of years. At the same pressure, the larger tires have lower resistance, but in the real world no one rides the larger tires at the same pressures as a smaller tire. All of this brings us to the point of, once you’ve settled on a tire, pick the size that is most aero for smoother roads or is beefier for rough roads.
b) cannot for the life of me understand why these guys are still doing their tests like this when we know that this only accounts for ~half of the resistance of tires interfacing with road and the other half, the vibration loses are not directly correlated with width, rr etc …don’t get it 🤷ðŸ¾â€â™‚ï¸
a) think the most important thing this shows is that manufactures are more than likely still not making their rims wide enough… especially for gravel to benefit from vibration losses AND still be aero…