Originally Posted by
Bill Kapaun
People actually think the tire gets smaller when they sit on the bike?
I think of a tire like a rubber band that not under tension.
It doesn't matter what shape it's in. It's still the same length.
What does decrease with weight (on the wheel) is rolling radius. It's true that the tire's material doesn't get "shorter", but it does deform into a flat contact patch. A straight line is shorter than the distance around an arc, so some of the circumference is "lost" when measuring length traveled. We think of that rollout measurement as circumference, but it's really the distance traveled by the hub/bike with one revolution of the wheel/tire system, and more deformation (and a shorter rolling radius) can impact this some.
In any event, that's why I recommend checking any rollout measurement against a known route, and making % adjustments. In theory, after two or three rides, you should have the computer pretty much bang-on.