Technical FAQ: Tire size and rolling resistance

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Dear Lennard,
With regards to sizing tires to rims, You said, “A general guideline is to take the inner rim width and multiply by 1.25 for the minimum tire size and by 2.5 for the maximum tire width. So, a 22mm inner rim width should not have less than a 27mm-width tire on it, and no bigger than a 55mm tire.”

Can you address if this is the “printed” value (which assumes a given rim chosen by the manufacturer which results in some 23s by one manufacturer being wider than 25s of another manufacturer when installed on the same rim) or is this the measured value when installed on the rim in question? (In which case Don’s 23s measuring out to his frame’s 26 limit would seem to be on the cusp of meeting the “general guideline.”)
— Nick

Dear Nick,
It’s the size indicated on the side of the tire.
― Lennard

Dear Lennard,
Thanks for the thoughtful discussion regarding all things and in this case rolling resistance. As is sometimes the case, going for a ride brings me some issues to ponder and think about:

Efficiency or drag caused by tires comes from tire hysteresis and aerodynamic drag. A nicely done study of a single tire with various sizes, tubes (butyl & latex), and tire pressures and the resulting drag from these permutations was done by Bicycle Rolling Resistance. For 23, 25, 28, and 32 mm widths inflated to 100 psi, the difference in drag was 10.6 watts for a 23mm tire and 9.7 watts for a 32mm. However, if the 32mm tire is run at 80 psi, the drag is 11.0 watts.

To me, the most relevant number that comes from this test is that the efficiency gain or loss is on the order of magnitude of 1 watt per wheel. While this test does not seem to consider aerodynamic losses, the larger casing should result in greater drag. I feel that larger-width tires do not provide a significant advantage with greater efficiency, but a smoother and more comfortable ride quality. On rough terrain, the greater protection from pinch flats, rider comfort, and better traction in most cases are where the real benefit lies, even if it costs a couple of watts, which it seems it would after you lower the pressure in a larger tire.

The tests were done on a metal roller that may only replicate a smooth pavement and not account for bumps or surface variations encountered on the road. As tire pressure is increased, the bumps encountered result in greater vertical force being transmitted to the bicycle and rider that can only represent an energy loss as it would not provide any forward propulsion. However, as a surface imperfection is traversed by a bicycle tire, the potential to transmit this vertical force is likely fairly similar irrespective of tire pressure. If the lower pressure tire is less efficient at transmitting this force into the bicycle and rider, it likely is absorbed by the tire and is not going to result in any forward propulsion either. In other words, if the tire is better at absorbing the bumps and converts that energy into heat rather than transmitting it to the bicycle, it may feel less disruptive, but I am not sure that it rolled over the imperfection with less energy loss. More comfortable, certainly which is where wider tires earn their money.

My two cents, humbly presented as I know a lot of attention has been paid to this topic by people that specialize in this field.

Thanks again for all the valuable information that you bring to many subjects.
— Tim

Dear Tim,
I think your analysis is mistaken that, “the potential to transmit this vertical force is likely fairly similar irrespective of tire pressure.” There is a huge difference in energy loss between deflecting the entire mass of the bike and rider up and over each bump vs. deflecting only the localized amount of tire tread and casing when the bump is fully absorbed by the tire.

This has been proven time and time again in lab tests as well as by cyclocross riders and mountain bike racers, and I explain it in detail. In suspension terminology, the harder the tires, the greater the “unsprung weight” that moves up and down the full height of each bump the tires encounter, and the greater the energy loss. It’s why full-suspension motorcycles are faster in motocross than hardtail motorcycles.

As for the study you cite, the fact that Bicycle Rolling Resistance’s test setup has no damping means that its results must be taken with a huge grain of salt. They are simply inaccurate at higher pressures on rough surfaces. I explain here why damping in the test fixture matters. Bicycle Rolling Resistance’s tests always show an ever-decreasing rolling resistance with increased tire pressure, because there is a static load on its test wheel.

In real life, however, once the tire is hard enough that it bounces along, the damping of the rider causes the rolling resistance to increase steeply due to impedance. Our Paris-Roubaix rolling resistance tests at Wheel Energy Oy in Finland showed this, and you’ll see more of that in my upcoming test of gravel-tire rolling resistance. Without damping, the tire can skip across the diamond plate on Bicycle Rolling Resistance’s test drum, thus not revealing the losses due to impedance.

Bicycle Rolling Resistance’s test does show that wider tires roll faster than skinnier ones at the same pressures as well as at pressures where the tire drop under load is the same. In our Paris-Roubaix rolling-resistance test, where the load is applied on the wheel through a pneumatic shock with damping and hence more accurately simulating the damping of the human body, we also saw the speed advantage of wider tires. Whenever there were two tires of the same model in different sizes in our test (we had three instances of this), the larger one rolled faster on the simulated cobblestone surface.

As for aerodynamic drag, that depends on the rims. With wider rims, the drag of a wide tire is reduced, and rims are becoming wider for that very reason. Also, by not requiring parallel braking surfaces on the rim walls, disc brakes allow the toroidal rim shape that allows the air to reattach to the rim after passing the tire that is narrower than the rim.
― Lennard

Dear Lennard,
Kudos for responding to the comment about your tire rolling resistance column and becoming part of the paid aspect of VN. This was nicely explained.

Stubbornly, I was hell-bent that I wasn’t going to pay/join, but a few months ago I joined the overall Outside/VN thing. I really like Ben Delaney’s gravel stuff, so that nudged me along. I have really been enjoying it, as has my wife. I think it is just how things have shifted. Within the last few weeks, CyclingNews and CyclingTips have both put up paywalls too. Kind of like mobile phones to us old-timers; it still seems like I should get a new one for free every two years or so, but clearly that has changed!
— Scott

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.