Subject: Measuring the circumference of a wheel
From: Jobst Brandt
For accuracy, the speedometer wants to know how far the bicycle travels per wheel revolution (under normal load and inflation). Therefore, that is what must be measured, and it is commonly called the "rollout distance".
To make this measurement, sit on the bicycle in typical riding position next to a wall for support, and roll forward, starting with the valve stem exactly at the bottom at a mark on the floor. When the stem is again exactly at the bottom, measure the distance traveled. Typically this distance, for a 700-28 tire at 120 lbs pressure, can be as much as 30 mm shorter under load than rolling the unloaded wheel for one revolution.
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