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.
Air pressure significantly affects the measurement, as Brandt says. Rolling the bicycle by more than one turn of the wheel and dividing by the number of turns can increase the precision of the measurement. A long carpenter's tape measure can not only measure the rolling distance but also provide an optical guide to steer the wheel straight. The front wheel provides a more consistent reading when riding, as it does not "creep" due to pedaling or skid when braking. -- John Allen.
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