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Old 11-03-05, 02:58 PM
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Originally Posted by jrennie
Based on my weight? Whats the rule of thumb for that? My old tires said max 125 and I just ran 120psi, the 130 minimun just looked strange.
Tires perform at their best with about 5% of deflection. That means, when you get on the bike, the rims will drop slightly toward the pavement...5% is an amount so small that a person standing behind you, looking closely will JUST see a bit of downward movement.

The actual width of the tire (the "marked" width is usually 2mm or 3mm more than actual width) and the total weight load on the tire control deflection.

The most popular tire for "roadies" are the tires marked 23mm (which usually are a "true" 21mm). On that size tire, a 140 pound rider on a 20 pound bike should run at about 95 PSI. A 200 pound rider on a 25 pound bike should run at about 135 PSI.

And, many folks prefer to ride with a bit less air in the front, and a bit more in back. That is because most of your weight is on the rear tire. So, to get 5% deflection in front and back, that "light" rider could go with 90 PSI front, and 100 PSI back, rather than 95 PSI in both.

Many Pro riders adjust their PSI to compensate for the quality of the roads, and to compensate for wet pavement. Rough pavement and water on the roads might bring PSI down 10 pounds or so. And, an ultra-smooth road with dry pavement might allow boosting PSI 10 pounds or so.

The PSI "maximum" marked on any tire is a "safety rating" that has no relationship to the PSI that provides the best performance for a rider of a given weight. A light rider on a 23mm tire ought to be riding at about 95 PSI, regardless of whether the tire "maximum" is listed as 110, 130, 150, or 180 PSI. What a "safe" PSI and the PSI level that provides the optimum contact patch, handling, braking, and shock absorption are unrelated concepts.
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