Boyle’s Law:
The One Thing Gravel Riders Should Know

by John Neugent

Tech articles | Commentary articles

John Neugent probably knows more about bicycle wheels than anyone else alive. Maybe more about bikes as well. He's spent his life in the bike business, at every level. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price. If you are in the market for a set of wheels, please, check out John's site. He really knows his stuff. —Chairman Bill

John Neugent

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John Neugent writes:

Gravel bikes are pretty much road bikes with big tires. There’s been a lot written about wider tires for road bikes and now we are moving into even wider tires with gravel bikes. While we all know car tires run at low PSIs and support a car, how is it that a low tire pressures can support more weight than higher pressures of a road bike tire?

To boil it all down (perhaps it should be Boyle it all down), the number of square inches in “pounds per square inch” matters. The basic formula is pressure X volume is a constant. The higher the volume, the lower the pressure to get to the same result. When you use a pump, you push the handle down thereby decreasing the volume and increasing the pressure to push air into the tire. Road bike pumps tend to have smaller diameter shafts to move less volume at higher pressures. Large tire pumps for kid's bikes and off-road riding tend to tend to have larger barrels to move more volume at lower pressures.

This is magnified by the fact that the volume of a tire goes up as a square to the radius. So slightly bigger tires have a disproportionate amount of volume. 28mm tires are 25% higher in volume than a 25mm tire. So in rough numbers, if you are running 100 psi in a 23mm tire, for a 38mm tire you can run about 43 psi to get the same result. Actually even lower because now you have more volume so there is less chance of pinch flats.

Road riders have a hard time grasping that 32mm tires can often be run at 30 psi. Much has been written about this understanding that using lower pressures is now popular so I won’t go into it.

Fatter tires on a gravel bike work best with lower pressures. Photo: Tim Reddington

Add to this the fact that studies have shown that as long as you have enough air to avoid pinch flats, there is little correlation between pressure and performance efficiency. However, too much pressure will affect both the ride quality and cornering ability, especially on bumpy roads. So the good old finger test to check your air pressure most likely has results similar to using a pressure gauge.

There has been much written about having the right tire pressure makes more of a difference in both ride quality and performance than buying more expensive equipment. And it costs nothing. And you can probably figure it out with the old finger test. Back in the early ’70s when I had a shop and ran 27-inch tires I never used a pressure gauge. As it turns out, that was probably a good idea.

John Neugent was was one of the first to establish quality hand building in Taiwan around the turn of the century. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price.

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