Bicycle Tires and Efficiency / Rolling Resistance

Introduction

I’ve seen a lot of articles and videos about how “wider tires are faster”. This is incorrect. Yet like many commonly believed falsehoods, it springs from a thread of truth. It’s a misleading interpretation of how tires are tested for rolling resistance.

I’ll describe the testing and the truth of what it really means.

The Testing

This site has excellent info about bicycle tires: https://www.bicyclerollingresistance.com

Equal Pressure Testing

In many of the tests, wider tires have lower rolling resistance. However, in these tests they inflate the tires (both wide & skinny) to the same pressure. This is unrealistic and misleading, since nobody actually runs wide and skinny tires at the same pressure.

If you inflate a wide tire to the same operating pressure as a skinny tire:

  • You exceed the wide tire’s maximum pressure, which is unsafe.
  • The wide tire will be much more rigid and uncomfortable to ride, with poor traction.

If you inflate a skinny tire to the same operating pressure as a wide tire:

  • The skinny tire will be soft and sloppy, with poor traction.
  • You will get pinch flats when you hit bumps.

So equal pressure testing is purely a theoretical educational exercise, completely useless for pragmatic purposes.

Equal Comfort Testing

Often, the tests inflate the tires to pressures that give the same yield or squish measured in absolute terms. That is, under the same load, the wide tire and the skinny tire both flatten or squish by the same fixed distance (say, 5 mm). This should make the tires feel the same when riding, hence the name “equal comfort”.

When tested this way, tires of different widths tend to measure the same rolling resistance. That is, all else equal (same brand/make of tire, same load, etc.). So one could say that width doesn’t matter.

However, in order to make the yield distances the same, the skinny tire must be underinflated, and the wide tire overinflated, relative to each other. This is a variation of the same mistake that the equal pressure test makes: inflating the tires to pressures you would not actually use when riding.

So, like the equal pressure test, it is interesting and educational, but impractical.

Proportional Displacement Testing

Another way to test tires is to inflate them so that each tire yields or squishes a distance proportional to its width, under the same load. 15% is a typical value, so a skinny 23 mm tire squishes .15 * 23 = 3.45 mm, while a wide 32 mm tire squishes 4.8 mm.

Not coincidentally, the pressures that give this result are near or equal to each tire’s recommended operating pressures. This makes the test representative of actual real-world conditions. And not surprisingly, this testing shows that skinny tires have less rolling resistance than wide tires (all else equal).

But Wait, There’s More!

So when we inflate tires to the recommended pressures, what exactly are the characteristics that depend on tire width?

Wider tires…

  • Have more comfort
  • Have more traction
  • Have higher weight
  • Have higher rolling resistance
Conclusion

Key take-aways for cyclists:

  • Articles & videos saying wide tires are faster, are incorrect and misleading.
  • However, while wide tires are slower, the penalty may be smaller than you think, which could make it worth paying for increased comfort and traction.
  • For maximum speed, choose the skinniest tire that provides the comfort and traction that you need.