At Compass Bicycles, we often get the question: “I am riding 23 mm tires right now, and I wonder whether a 25 mm-wide tire would make a noticeable difference.” The graph below, taken from the Spring 2013 issue of Bicycle Quarterly, shows the increase in cross-section, and thus air volume, as tires get wider.
Two millimeters may not seem like much, until you calculate the air volume of the tire. The air volume roughly is proportional to the cross-section of the tire. You can see above that a 25 mm tire has 18% more air than a 23 mm tire. Will you notice that difference? You probably will!
Going even wider, from a 25 mm tire to a 31 mm tire, you increase the air volume by more than half. That is not just noticeable, it’s huge. And even the difference between a 38 mm tire and a 41 mm tire still is 16%. No wonder the Grand Bois Hetres feel so much more comfortable than the Lierre and Pari-Moto tires, even though they use the same casing.
When you compare your average 23 mm-wide racing tire with the Grand Bois Hetre (above), you see why we love these tires so much. They roll as fast as a good racing tire. They weigh only a little more (the difference for two tires is less than half a water bottle). And they have more than three times as much air volume.
At which point does it no longer make sense to make the tires wider? I think there are two limiting factors:
- Frame design: 42 mm is about the widest tire you can fit into a performance frame without going to wide mtb-style cranks.
- Riding out of the saddle: For a rider my weight, a tire at less than 2.5 bar (37 psi) starts feeling squishy when riding out of the saddle. This means that beyond a 42 mm tire, there is a trade-off in feel. On a racing bike, I probably would prefer 38 mm-wide tires for their more positive feel, whereas on a randonneur bike (which doesn’t engage in sprints very often), the extra comfort of 42 mm tires are worth the small price in road feel during those city limit sprints.
For me, tires narrower than 30 mm are hardly worth considering. And even 30 mm tires feel compromised on many roads. 38 mm tires offer most of what I need, but I prefer 42 mm tires for the added comfort and safety they provide. If I were to go on a round-the-world tour, I’d probably use wider tires yet.