Bicycle Quarterly rolling resistance tests: Spring 2013

[youngs_modulus]

I can't speak to Jan Heine's comments because I don't have access to the BQ article in question. I would be interested to know why he thinks rollers somehow penalize tires with supple casings. Tom Anhalt's roller-based method seems to pick some supple tires as the fastest.

Jan Heine makes a lot of claims in your quote, but doesn't explain why he believes them. I assume his reasons are elaborated upon in the article in question.

Cheers,

Jason

[Marin]

Jan thinks that supple casings save energy because there's less kinetic energy transmitted to the bike and rider, which dissipate the vibrations as heat. His tests on rough surfaces seem to support this theory. I also tend to believe him.

[rchung]

IIRC, Heine changed his rolling resistance test protocol a couple of years ago. He used to use roll downs on a hill, which have horrendous precision. I think now he uses either constant speed or constant power runs on a smooth road. This, approach, too, can have variable precision.

Field tests I and others (including Andy Coggan and Tom Anhalt) have done on real roads show that the ordering from roller or pendulum tests is almost always preserved when on a real surface (that is, when Tire A > Tire B on roller or pendulum tests, Tire A > Tire B on the road). The exception may be that on smooth rollers RR always decreases with increasing pressure but there's some evidence that on some surfaces you can reach a point at which increased pressure can result in increased RR.

Bottom line, Heine seems to dislike "lab" tests in favor of field tests but in general we've found field and lab results to be consistent.

[Marin]

Good to know since lab test data is easier to find for interested consumers like me. Thanks to all who do tests, lab and field.

[kgt]

youngs_modulus post was really well written from an engineer's point of view. It was not that informative or useful from a cyclist's point of view (I am an engineer myself BTW).
The question, at the end, is if lower rr equals a faster tire in practice i.e. in a race. The equation is not obvious. An example: Formula one tires have really high rr in comparison to many passenger car tires. But these are the fastest tires one can use on a formula one car.

[istigatrice]

Formula 1 cars require grip, not low rolling resistance. Hence the RR is compromised to obtain more grip. I'm not suggesting grippy tyres need a high RR, but I can't imagine tyres having both low RR and high grip.

Those cars aren't very aero either, they're not about fuel economy or straight line speed. It's all about cornering speed, you've got to go around the corners quickly.

On a bike it's much more simple than an F1 car. Bike tyres need low rolling resistance for good "fuel economy", the fastest bike tyres will have a low RR.

Sorry about the OT. But you can't really compare bikes to F1 cars.

[math]

The question, at the end, is if lower rr equals a faster tire in practice i.e. in a race. The equation is not obvious. An example: Formula one tires have really high rr in comparison to many passenger car tires. But these are the fastest tires one can use on a formula one car.

I don't think this is difficult to understand. Formula-1 cars have plenty of power. Their main problem is how to transfer this huge power into speed. For this you need the tires to have grip on the surface. High Crr means high grip. So tires with the highest Crr combined with high vertical load are fastest. They even heat up their tires before the start of the race to get a surface of sticky rubber.
Bicycle racers have limited power. The rolling resistance of a bicycle tire is more than enough to transfer this power into speed. For them a high Crr is a loss of power, which would otherwise be used to overcome the aerodynamic drag.
Have you ever seen a bicyclist accelerating with spinning wheels?

[rchung]

The question, at the end, is if lower rr equals a faster tire in practice i.e. in a race. The equation is not obvious.

Well, is the hour record a race? I recently did some analysis prior to a recent attempt at the hour record. The Crr and CdA I calculated from prep runs exactly predicted the speed for power during the attempt, and the Crr from the prep runs was consistent with earlier roller tests.

[youngs_modulus]

Field tests I and others (including Andy Coggan and Tom Anhalt) have done on real roads show that the ordering from roller or pendulum tests is almost always preserved when on a real surface (that is, when Tire A > Tire B on roller or pendulum tests, Tire A > Tire B on the road). The exception may be that on smooth rollers RR always decreases with increasing pressure but there's some evidence that on some surfaces you can reach a point at which increased pressure can result in increased RR.

That preserved ordering is exactly what I would expect, and strongly suggests that the field tests are validating the lab results. That validation seems to be exactly what Heine claims he hasn't seen.

Bottom line, Heine seems to dislike "lab" tests in favor of field tests but in general we've found field and lab results to be consistent.

And this is what's raising a red flag for me. Lab tests and field tests go hand-in-hand in the service of testing falsifiable hypotheses. If Heine dislikes lab tests in favor of field tests, he's setting up a false dichotomy. His claims (in a blog post) about how flexible frames "plane" (maybe operate at resonance? It's hard to tell) are vague, as are the methods he mentions. If Marin has accurately relayed Heine's theory about supple tires being penalized on smooth rollers, I'm doubly dubious. The information I've seen thus far smacks of what Richard Feynman called "cargo cult science."

http://en.wikipedia.org/wiki/Cargo_cult_science

The information I've seen thus far makes me nervous, but I must emphasize that it's all second-hand information. That's not enough to evaluate Heine's claims. I don't have access to either his rolling resistance articles or his frame flex article. It's not fair for me to reach a conclusion without reading the full articulation of Heine's arguments.

More generally, though: Chung and Coggan (both solid field experimentalists) are comfortable with the lab/field result correlation and Heine is not. Heine's assertions about tires (e.g., slicks vs. treaded tires) also contradict Jobst Brandt's work. (Brandt is often abrasive but he's not often wrong). Heine may be making some extraordinary claims. It's a cliche, but it's true: extraordinary claims require extraordinary proof.

I'd love to read Heine's articles so I can evaluate his claims on their merits.



Cheers,

Jason

[kgt]

@istigatrice and math
Imagine a 80-90 km/h downhill on a road bike. Grip is crucial. Bikes do not just follow a straight line on a flat surface. A top tire is expected to track the tarmac perfectly so one can safely ride on his limits. That is not crr related.

[djconnel]

I really liked the youngs_modulus post but there's a few points:

• I do reject the concept of CRR. Resistance won't be truly linear with normal load since normal load changes the shape of the tire in a nonlinear way. I believe going from 10 newtons to 10.2 newtons has twice the effect as going from 10 newtons to 10.1 newtons to decent precision, but I don't believe 10 to 10.1 is 1% going from 0 to 10.
• Heine is talking about something different than what's measured in the roller tests. Roller tests measure energy dissipated in the tire and only the tire. Ride on a bare rim and you'll get essentially zero (by the standards of rolling resistance tests) rolling resistance. Try coasting a bike with bare rims on a rough surface and it won't roll very far. He's talking about an effective rolling resistance of a complete system, which includes a suspension effect on the load itself. The physics of "rolling resistance" typically don't include this.
• temperature may change order if we're talking about tubulars (which dissipate energy in glue) versus clinchers (which have no glue). However, I think this has been checked and not observed to be significant.
• It's not too crazy to think the relative rolling resistance ranking of 19 mm versus 26 mm tires might be different on a 19 mm versus 26 mm rim, especially for tubulars which are glued directly to the rim, but additionally for clinchers for which the relative sidewall deflection will be a lot different if matched versus mismatched.

[rchung]

Well, although the lab tests may measure something different than the field tests that Heine prefers, the two are almost always consistent with each other. Your bare rim example is exactly why I mentioned the one possible area of disagreement: in a pneumatic tire on real roads, there is evidence that you can overinflate tires to a point where rolling resistance increases (which is something you don't observe on smooth rollers). However, that happens at pretty high pressures, and Heine doesn't test that high.

[Marin]

One thing I'd like to see tested is the real-world problem riders face: Rolling resistance at comparable level of comfort.

Everyone knows that wide tires have lower RR at the same pressure as narrower ones. However, most people don't know that wider tires are also harder than narrower ones at the same pressure.

Setting tire pressure is always a trade-off between comfort and speed, and what interests me most is which tire and tire width is the best compromise.

[youngs_modulus]

Well, although the lab tests may measure something different than the field tests that Heine prefers, the two are almost always consistent with each other.

Again, this seems to be a confused approach on Heine's part. If his field tests are indeed measuring something the lab tests are not, and that mysterious thing matters at all, then the field tests should disagree with the roller tests.

Heine claims they do, but multiple independent researchers say they don't. In other words, Heine's results have not been replicated (as far as I can tell). This is a big problem.

If Heine has articulated a falsifiable hypothesis and rigorously demonstrated it to be true, then I'm interested. Thus far, I haven't seen anything remotely like that.

Cheers,

Jason

P.S. This is in no way meant to be an ad hominem attack on Heine. I like his line of tires quite a bit (though I wish he'd offer a slick tread, which he never will). He seems to be a force for good in the cycling world. I disagree with his ideas about rolling resistance, but that's all.

[math]

Heine claims they do, but multiple independent researchers say they don't. In other words, Heine's results have not been replicated (as far as I can tell). This is a big problem.

Heine's claims are particularly made for very rough hard surfaces, like chip-sealed asphalt, cobblestones and rumble strips. On these surfaces the wheels are not merely rolling but also making significant vertical amplitudes, where the spring-mass-damping properties of the wheels, especially the tires, become important and also, as Heine claims, the cyclist body in absorbing vibrational energy. I am not allowed to attach weblinks, but you can read his story by googling <heine bicycle suspension losses>.

As far as I know, the rolling resistance on such surfaces has not been measured often, so it is too strong to say that multiple independent studies are contradicting Heine. In fact, such surfaces would be hard to reproduce indoors, although at the University of Sherbrooke, Canada, they try to do so with shakers, but so far they haven't published about Crr of tires. The coasting-down method outdoors will also become problematic. In my view, this is a terrain where the pendulum roller could do a good job.