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Old 09-12-10, 11:17 AM
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brianogilvie 
Commuter & cyclotourist
 
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Location: Hadley, MA, USA
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Bikes: Boulder All Road, Surly Long Haul Trucker, Bike Friday New World Tourist, Breezer Uptown 8, Bike Friday Express Tikit, Trek MultiTrack 730 (Problem? No, I don't have a problem)

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This is a little off the main topic, but since it came up:

Originally Posted by LongIslandTom
Also, tire pressure makes a huge difference. Firmer tires have less rolling resistance. Soft tires balloon out at the bottom as they roll, and this flexing in the sidewalls eats up pedaling power. Typical narrow 700x23c road bike tires are inflated to around 120PSI. Hybrids with wider tires typically have lower tire pressures like 80PSI or so.
Jan Heine and Mark Vande Kamp measured the real-world performance of various tires, and they determined that a wider tire (28-30mm on 700C rims) with a supple casing, run at moderate pressure, was more efficient than narrower tires run at high pressure. The higher rolling resistance of the wider tires was more than balanced out by the lower suspension losses from road irregularities. See Jan Heine and Mark Vande Kamp, "The Performance of Tires," Bicycle Quarterly vol. 5, no. 1 (2006), p. 1. (Not available online, unfortunately.)

They also found enormous variation in the performance of different brands of tires, depending on how supple their casings are.

Finally, Frank Berto has calculated that the ideal pressure is that which results in the tire dropping 15% of its width under load. This article from Bicycle Quarterly explains the principle and provides a handy chart for calculating optimal pressure.
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