It seems to be generally accepted but I don't know why. I personally don't subscribe to it as I don't think we have definitive data on it. We have sites like BRR that show a 28mm rolling faster than a 25mm at the same pressure but that doesn't translate to the real world. I think this confuses many and it leads to a false narrative. Lets assume 80 psi is the break point pressure for the 25mm tire, you wouldn't run the 28mm at 80psi. You would run it at say 75psi. What is the crr at the breakpoint pressure of the 28mm tire vs the 25mm tire at 80psi? This has been discussed several times on here and I forget who it was but someone mentioned that the crr would be roughly the same.

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I agree. And I also believe in between that resin covered wood and Paris Roubaix there is a spectrum. I don't believe you will see a huge difference in RR on a 28 and 25 tire at optimal pressure on many roads used in tri and TT.

It's interesting "oh, the aero penalty is measured and so small it can be ignored". "We didn't measure the RR gain, but trust us, it's worth it"

Measure the whole thing, then we can talk.

Summer projects.....:-)

I wasn't disagreeing with you. When casing tension is normalized, on a roller 23, 25, 28 all seem to roll the same. The higher volume tires seem to lose less energy from rolling over rough roads though. However, this is super specific to your road and tire pressure, which makes it hard to make a nice chart showing this. My point was that its very difficult to visualize this data in a way that allows for easy comparisons.

Also, cornering on 28mm tires feels much better IMO than 25mm tires, which is very important for them.

I have a mad scientist type question... Is it true that rolling resistance generally decreases as tire width increases, assuming we set each tire at its optimal pressure? And this depends on the road surface. But, there must be a point at which rolling resistance starts to increase again as width increases. So, do you think that there is some normal-like curve that illustrates the fastest width tire for a given surface, like this? Or, do you think rolling resistance does decrease indefinitely as width increases? If the former is true, then we will eventually arrive on an ideal tire width for various surface types. Then it is merely a matter of optimizing the wheel for max aero at that optimal width and pressure.

Certainly there is going to be a point where rolling resistance increases with additional width. The biggest toroidal tire that you can fit is ~350mm wide. It would literally be the shape of a donut with a tiny little hub through the middle. I don't see any reason to suggest that such a large amount of flexible casing would produce lower resistance on anything but the harshest of surfaces. Remember on a theoretical perfectly smooth surface the fastest theoretical tire is completely rigid. Of course this provides no grip and isn't a real world case, but it's a good spot to ground your thinking. Everything past that is a balance of absorbing surface imperfections with minimal flexing losses in the casing.

It needs to be tested, but anyone who's ridden a MTB or fat-tire bike on a road can feel the difference in rolling resistance between those tires and any normal road tire. I'm sure the knobs have an impact on that.

PS - Did you make that graph? I'd be curious to see the article it came from if you found it online.

I made it in Excel to illustrate my question. The data are nonsense - just illustrative to fall in the shape of an inverse bell curve.

if you lowered the pressure, going from 25c to 28c, by only 5psi then the 28c would well outperform 25c on a drum roller. it's more like 10 psi or more.

here's why i think teams are moving to 28c with the proviso that some of this is a guess and some is what i've actually heard.

1. handles better, corners better.
2. more comfortable.
3. less chance of puncture.
4. is faster for this reason: the wider tire with lesser pressure, while rolling equally well as or slightly better than the thinner, higher pressured tire, does much better at sucking up bad or irregular patches of asphalt, and the speed delta between 25c and 28c is much bigger over those bad patches, ruts, jumps, curbs, etc.

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Dan Empfield
aka Slowman

years back, when everyone was running 23mm tires, the enve 7.8 front was found to magically become a fast wheel when fitted with a 25mm tire which was considered wide at the time. i think it was Jim@ERO that identified this. so there is certainly precedent for wider tires reducing aero drag with the right rim shape.

as for RR, it was even longer ago determined that for a given casing tension and load, a wider tire has the same contact patch as a narrower one but it is a short, wide contact patch rather than a long narrow one. as well as giving better grip, this rolls better as there is less deformation in the direction of movement.

of course most things improve until they reach a threshold point and then start going the other way. where that point lies is dependant on many things and currently unclear

Thanks for the answer. To close the loop on if wider is at all better on decent roads. It would be really interesting to see the optimal setup for the same model of 25/28/30mm tires on the same wheel compared (combination of rr and aero) on decent road surfaces with pictures of the road surface. I'm very skeptical that 28mm is going to be faster than 25mm, and I assume 30mm will be the slowest.

Then add an accelerometer glued to the seat post also, and we can get data on comfort and hysteresis loss in the rider to some extent. If you need help analyzing the vibration data, I can help as I do some of that at work.




I think this is the crux of the matter, the manufactures claim that wider is faster but refuse to share any data making their claims look suspicious. If they had the data and it was as clear cut as they make it seem, why don't they release it. It would help to convince everyone especially hold outs like me to switch to wider tires/wheels. The fact that they haven't implies: the data isn't as clear cut as they make it seem, they haven't collected any or it's very noisy, or they've tested it and wider isn't faster so it's just marketing speak to convince people to buy new equipment. I hope it's not the last scenario. But with their handling of hookless to help their bottom line, I wouldn't be suprised.

I was using 5 psi delta as an example. Whether it’s 5, 8, 10, or 12 psi delta, the question still remains.

What are teams doing during TT’s? Are they running 25s or 28s? This seems to be a better comparison for tri. I’m not sure the criteria for selecting tire size is the same for a typical road stage (assume no cobbles and relatively smooth surface) vs a TT stage. It seems to me based on pictures I’ve seen that the top guys are running 25mm tires during TT stages but maybe I missed something. Perhaps Marc can chime in here.

I agree with #1 and #2 above. Not sure I understand #3. Could you elaborate?

#4 - so do 28mm tires roll equally as well as 25mm or do the 28mm tires roll faster? Marketing speak says they roll faster but then you just said something different and different from the original post I quoted.

Disclaimer: I run 28mm tires on my road bike on 19mm internal width wheels. I like them and will continue to ride them on my road bike. I run 25mm tires on my tri bike on 23mm internal width rims (Aeolus rsl 75s) as my training setup. I can see/understand the hype with 28mm tires and feel my tri bike setup could really use them for training purposes. What I don’t know is if I go to 28mm tires, am I gaining or giving up something? If so, how much? I don’t care about giving up something for training purposes So 28mm is a no brainer for my next set of tires on my training setup. But for racing purposes and getting every last .1 watts that I can, I’m not sure yet.

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I am going to try and make this happen. I have a trip scheduled and I can easily get 3 identical sets of wheels setup with 25, 28 and 30.

The riders have blacklisted me since i made them do 3 days of 100km/day of CRR testing at pretty hefty pace on crappy roads. I may have to do the tests myself :-)

If I can make it work out, we can align on how to collect the accelerometer data.

If it's at all near San Francisco. I'll be happy to do some of the test haha

Indeed! Faster is Faster, after all. ;-)

It's been a while since I've tried to measure these sorts of things, but something I always thought wasn't discussed much back in the day in terms of wheel/tire stuff was weight distribution front/rear.

"Fastest" just might be rider positioning such that more weight is on the rear wheel and dialed in aerodynamics/rolling resistance on the front wheel. Looking at the recent trends in the industry, though, we might not be able to test this until the pendulum swings the other way.

...me and my wired SRM Pro/PCV will be waiting... ;-)

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Kraig Willett
http://www.biketechreview.com - check out our reduced report pricing
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