Technical FAQ: Tire rolling resistance as a function of speed

Published November 15, 2022 12:00PM

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Something I’ve wondered about is why I seem to get uphill PRs every time I switch to bigger tires. It always feels like something for nothing. In this GCN video about 28 vs. 32mm tires, they talk about rolling resistance being a function of speed, with bigger tires having lower resistance at lower speeds (good for recreational riders like me), and narrower tires having lower resistance at higher speeds (good for racers). Comfort advantages aside, it’s just got me wondering if the wider tires are allowing me to climb faster because of lower resistance at climbing speeds? I can’t recall if any of your testing addressed this issue (you did sort of mention it in this story).

Until I watched the video, I thought you were saying that GCN was claiming that at higher speeds, the rolling resistance of a skinnier tire in relation to a wider tire is less than at lower speeds, and vice versa. My immediate reaction was that there is no research to back up such a claim. (The letter below my answer is from Wheel Energy Oy in Finland, the world’s leading independent bike-tire rolling-resistance test lab, and it addresses that question.)

However, in that video GCN was actually saying that the rolling resistance of a 32mm tire is less than that of a 28mm tire, independent of speed. What they are saying is that, with the same rim, the aerodynamic drag of the 32mm tire is greater than that of the 28mm tire, and, the higher the speed, the more that will matter. They hypothesized that the 32mm tire would be faster at 30kph, and the 28mm tire would be faster at 40kph, because the 32mm tires would roll faster at both speeds, but at 40kph, their greater aerodynamic drag would more than make up for the higher rolling friction.

The former hypothesis definitely did not pan out. They felt the latter hypothesis was verified, but given the test method, one could easily argue that there was insufficient data to make that claim.

One thing that did stand out for me in that video, however, was the randomness of their tire-pressure choice. You want to minimize the number of variables in order to make the kind of claim they were making of comparing just tire widths. For tire pressure, to minimize added variables, I believe you would want to ensure that the tires were of the same hardness, meaning that each tire casing was under the same hoop stress. I explain hoop stress of tires as a function of pressure and diameter here.

To maintain the same hoop stress (i.e., the same “hardness”) of the 28mm tire as the 32mm tire, you would use the P1D1 = P2D2 formula I derived in that post. Since GCN was using 60psi in the 28mm tire, then, for the 32mm tire, they would want to use:
(60psi * 28mm)/32mm = 52.5psi
In other words, to make a more apples-to-apples comparison, they would have wanted to use 52.5psi in the 32mm tire, rather than the 45.5psi they did use. Without lab testing at those pressures, I don’t think that the assumption that the 32mm tires would roll faster on that particular section of road is one that we ought to take as a certainty.

You can see that, in general, the wider of two identical tires of two different widths will roll faster at the same pressure. However, you can also see that if you compare those two same tires with different tire pressures in each, whether the wider tire will roll faster or not will depend on what those pressures are.

So, to answer your question, if the climbing speed of your wider tires is measurably higher than that of your narrower tires, then it indicates that the rolling resistance of your wider tires is enough lower than that of your narrower tires that it overwhelms the speed loss from their added weight of the wider tires. At climbing speeds, I feel confident that you can ignore the difference in aerodynamic drag between the two tires.

I recently have also had the perception that fatter tires roll faster than skinnier ones. For 12 years or so, I have been doing cyclocross group training sessions with the same group of friends. Until this season, I always used 700 X 33C cyclocross tires. I always seemed to coast on dirt at speed at almost the same speed as the rest of the riders.

This year, I have a new bike with 12-speed SRAM AXS with its Flattop chain, and I only have one set of wheels with a cassette that works with the Flattop chain. So instead of switching wheels for these cyclocross group training sessions, I have been using the same wheels with 700 X 40C Challenge Getaway tubeless gravel tires on them that I use for all riding on that bike. On the flat sections of singletrack dirt on this cyclocross course, this season it seems as though I can coast on those tires at 20psi at the same speed as riders around me who are pedaling their 33mm cyclocross tires (presumably at higher pressure, maybe by 10psi or so). I definitely have the sense that my tires are rolling faster than theirs are. Of course, the tires differ not only in width and pressure but also in brand, model, construction, etc., so there are too many variables to say anything quantitative.

Before ending this response, I do want to address the idea you had that skinnier tires have relatively lower rolling resistance to wider tires at higher speeds than they do at lower speeds. I asked Petri Hankiola, founder and president of chief tester at Wheel Energy, to address this question. This is what he said:

“When we change the load, tire pressure, inner tube, rim structure and rim width in addition to the speed, the only possibility to find out the rolling resistance of the tire is to perform measurements.

In addition to the above, one more example from the real world, a customer sent 10 tires of different composition to the rolling resistance test, so the results varied from 0 to 7.2 Watts and all tyres weight was 240 – 244 g and size same 25 mm.

The theory function also says the following: Rolling resistance decreases the smaller the tire and the higher the tire pressure. The
indoor tracks were driven [ridden] for a long time with 18 mm tires at 16 bar pressure, [but] the new one-hour world record [Ganna] was driven [ridden] with 25 mm tires based on press information. When I look at the photo showing the contact surface of the tire, I would estimate the tire pressure to be 10-12 bar.

Summary: The theoretical function gives a general direction for future values if there is only one variable, for example speed. Unfortunately, the information cannot be used for bicycle tires because there are many variables (load, speed, tire pressure, rim structure, rim width, inner tube butyl, latex, tubeless, tire frame [carcass] structure, time-running, puncture-resistance, wear resistance, friction…).

In the course of 19 years, I have made more than 20,000 rolling resistance measurements, and the results still often surprise me and the customer, because the result is completely different from what we expected. Measurement always tells the truth.
Tyres Laboratory Wheel Energy Oy
Petri Hankiola, President”

In short, I think that idea that skinnier tires have relatively lower rolling resistance to wider tires at higher speeds than they do at lower speeds (and vice versa) needs to be nipped in the bud. I don’t think any data exists to bear that out.