The tests of the Compass Bon Jon Pass tires on
www.bicyclerollingresistance.com don’t come as a total surprise, because we knew all along that these drum tests doesn’t replicate the real world. In fact, drum testing was behind the now-discredited belief that narrow tires, run at ultra-high pressures, were faster than wider ones. Today even professional racers ride 25 mm tires at less than 100 psi - on smooth roads – and wider ones on rougher surfaces. If they believed the drum tests, they’d still be on 23 mm rubber inflated to 130 psi or more.
What is a little surprising is that the same tires (Bon Jon Pass with Standard casing) were tested by the respected German magazine TOUR as the fourth-fastest tire in the world. How can we reconcile these dramatically different results?
Drum tests as used by
www.bicyclerollingresistance.com suffer from an inherent problem: The convex drum surface digs into the tire. The more supple the tire, the more the drum digs into the tire. The tire is forced to flex more, which absorbs energy. That is why a stiff tire, like the Marathon, performs well on the drum (but not on real roads, which are flat rather than convex).
Increasing the tire pressure makes the tire harder, and so the drum will dig into the tire less. According to the recent drum test, inflating the Bon Jon Pass to 90 psi makes it almost twice as fast compared to 30 psi. They tested the narrower tires at 120 psi, so it’s not surprising that they scored better than the wider tires at 90 psi. Overinflating the Bon Jon Pass to 120 psi would have made it much faster – in their test.
On real roads, higher pressures don’t make tires faster, because the greater hysteretic losses (the tire deforms more) are countered by the lower suspension losses (the bike vibrates less). This is now widely accepted – it’s been shown in test after test, not just by Bicycle Quarterly, but also by Joshua Poertner and others.
The German tests by TOUR magazine didn’t use a drum, but a pendulum on a flat surface – imagine a rocking chair with the runners replaced by bicycle wheels. This eliminates the convex roller digging into the tire. They still don’t have a way to model the rider in their setup, so they don’t measure suspension losses. Even though the Bon Jon Pass scored well in TOUR’s test, I don’t think this totally reflects how it rolls in the real world. It gives an indication how much energy the casing absorbs – which is an important factor, but only half of the equation. On real roads, wider tires will roll faster than TOUR’s test suggests, as they transmit fewer vibrations.
The current ‘Wide Tire Revolution’ came out of real-road testing, with a rider on board. These tests showed that higher pressures don’t make tires faster and that supple, wide tires (which must run at lower pressures) don’t give up any speed to narrow, high-pressure tires. It’s no surprise that our tires, which were designed based on this real-road testing, don’t score well in a drum test.
What we care about is how well our tires roll on real roads, not how well they score in a test that doesn’t replicate the real world. We are excited that many races, on gravel and on paved roads, have been won on Compass tires. We think these results, as well as the real-road testing, give a better indication of their performance.
Jan Heine
Founder
Compass Cycles
www.compasscycle.com