Rolling Resistance and Tire Pressure
Skinny high pressure tubulars may not be the fastest tires you can use. The latest RoadBikeRider newsletter has some interesting info on tire pressure and rolling resistance.
We heard the buzz about a surprising new tire test in Bicycle Quarterly, a nifty magazine published in Seattle and edited by Jan Heine. The name was Vintage Bicycle Quarterly until recently, but “vintage” has been axed because it implied the mag was about old, collectible bikes and equipment.
Well, plenty of pages are devoted to arcane and interesting gear, history and randonneur-style riding, but Bicycle Quarterly also publishes cutting-edge material. The tire test is testimony, appearing in the Autumn 2006 issue.
Heine gave RBR permission to summarize several major findings. Interestingly, they confirm lots of what Uncle Al has been ranting about for years regarding tire width and inflation pressure.
Some test conclusions will be particularly enlightening if you’re riding on narrow, high-pressure clinchers seeking more speed via lower rolling resistance. Your skinny tires may not be as fast as you think.
For the full eight-page report on tire performance, order the Vol. 5 No. 1 issue from http://www.bicyclequarterly.com. The test included nine 700C tires, seven 650B tires and two tubulars. The protocol and results were reviewed by industry experts. These are eight findings:
—With roughly the same power output, the rider’s speed can vary by as much as 20% depending on tire choice. For example, the rider on the fastest tire [in this roll-down test] moved down the road at approximately 16.4 mph (26.2 kph) while the same rider on the slowest tire went approximately 13.6 mph (21.7 kph).
—Many longtime riders believe tires with a cotton casing are faster than modern casings made from nylon. Testing seems to confirm this. The best-performing tire in the test, the Deda Tre Giro d’Italia 700x23C (actual width 24.5 mm), has a cotton casing.
—Tire pressure has only a small effect on the rolling resistance of most tires. Narrow 23-mm tires seem to roll fastest at pressures of 105 psi (7.2 bar) or more. However, running these tires at 85 psi (5.8 bar) for improved comfort increased the test times only 2%. Wider 28-mm tires are as fast at 85 psi as they are at higher pressures.
—Tubular tires perform worse at very high pressure. At 130 psi (9 bar), the narrow Clement Criterium rolled slower than it did at a more comfortable 105 psi. The wider Clement Campione del Mundo rolled slightly faster at 85 psi than at 105 psi.
—Wide tires do not roll slower at lower pressures. In fact, testing indicated that a wide tire at lower pressures rolls faster than a narrow tire at high pressures, if all other factors remain the same. Even narrow tires can be ridden at comfortable pressures with only very small concessions to performance.
—Tires rolled slightly slower with Michelin’s relatively thick latex tubes than with butyl tubes. Thinner latex tubes, like used in tubular tires, may offer better performance, but when used in clinchers they are more prone to punctures caused by friction between tire and tube. Latex tubes do improve comfort.
—Perhaps the most important result of the test is that tire pressure does not significantly affect rolling resistance. Wide tires in particular do not need high pressures to roll fast. But because many current wide tires are designed to handle high pressure, they have strong casings that lack suppleness. This results in higher rolling resistance than necessary.
—The test’s findings point to a new direction for performance bicycles. For most cyclists, wide, supple tires at low pressures offer more speed, better comfort, increased versatility and improved safety than today’s narrow high-pressure tires. However, this type of wide, fast tire currently is not available. Hopefully, these test results will help persuade manufacturers to produce them.