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Gear Guy

Are slick bike tires THAT much faster than knobbies?

Could you please tell me what you figure the efficiency of slicks versus knobby biking tires is? A local dealer says it is insignificant, and I figure it is ten to 15 percent. Bill Invermere, British Columbia

A: Your local dealer has been filling his tubes with something other than air, then sucking on the valves. At face value, the idea that something smooth has lower rolling resistance than something rough is so obvious that to deny there's a difference puts you in the world-is-flat camp. But, a simple empirical test I have conducted here at Gear Guy Laboratories concludes that your estimate regarding the efficiency difference is likely very accurate.

Here's the test: Near Port Townsend, Washington, where Gear Guy Enterprises has a satellite office, we have county roads and state roads. The state roads are all asphalt, nice and smooth. The county roads are "chip seal"—basically, sprayed with oil then covered with a layer of crushed rock. Passing cars eventually pound the rock into the oil, creating a semblance of a modern road, albeit a very rough one. I will go so far as to say that the difference between the asphalt and thechip seal is about that between a slick tire and a knobby, in terms of the"bumps."

On to our experiment: If I am going 18 mph on the smooth asphalt and then roll onto chip seal, my speed instantly drops to 16 mph. I'm also forced to drop a gear if I want to maintain pedaling cadence—a 2-mph difference is roughly 11.5 percent.

My own estimate is that given that knobbies are wider than most road tires and run at lower air pressure, the actual friction difference is more in the 15-20 percent range.

So roll on, brother, you're right and the dealer is wrong.

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