I make rubbings of tire sidewalls and I like to doodle — hence this very stylized tracing of the Schwalbe Durano tire used on my Valkyrie Tour. You can see from this that the maximum recommended inflation pressure for this tire is 115 psi and the minimum is 85 psi.
Do you think it’s always easiest to pedal your bicycle when the tires are pumped up to the maximum inflation pressure? That’s not necessarily correct. What?!? Well, unless you’re riding on a road as smooth as glass or you weigh quite a bit, a lower pressure might actually make you a faster rider.
There’s a “magic land” somewhere between tires that are so hard that they jump all over the road and tires that are soft and deform too much, losing the ability to “push back” on the road, helping move the bicycle forward. The idea behind the perfect pressure is that the tire is allowed to become a shock absorber — sucking up the irregularities of the road before they get to the rider and tire her out. A recent article in Bicycle Quarterly estimated that even on smooth roads, you exert 10% of your power just overcoming these “suspension losses”.
So what’s the optimum pressure? Well, some use the 15% “tire drop” formula. Very simply, the height of the tire is measured without the rider on the bike. With the rider on the bike, the tire pressure is reduced until the height drops by 15%. Voila! Uh, except for the fact that you really do want to stay within the recommended max and min pressures and this might not be possible… unless you want to play around with tires of varying widths.
Vittoria Tires has another method that relies on rider “feel”. Using a chart, the rider starts with the recommended inflation pressure and then reduces it by 5 psi at a time until the tire “wallows”. This is the least amount of pressure for this rider. Next, the rider increases the pressure by 5psi until the bike bounces. This is the maximum pressure for this rider. Back down we go, 5 psi at a time until things feel just right.
The long and short of it is that rarely does the maximum tire pressure provide the best ride. It may sound really fast, as you go humming down the road, but the power meter will show that it’s usually not as efficient as a lower tire pressure.
“Hey Mister. Your tires need some air.” That’s what someone standing by the side of the road might have said to cyclist Jan Heine. Indeed, Jan was running low pressures in these 41mm tires during a tour of gravel roads in the Cascades. The result was an efficient but comfortable ride. Looks can be deceiving.
So — go forth and let some air out of your tires!
PSI RX by Jan Heine, Adventure Cyclist, March 2009, pp 34 – 35.
Comfort Equals Speed by Jan Heine and Mark Vande Kamp, Bicycle Quarterly, Autumn 2009