Wheel Rotational Inertia

It's easy to calculate a wheel's rotational inertia using a kitchen scale, a stopwatch, and a tape measure. Weigh the wheel, time the pedulum, measure the length of the pedulum from the point of rotation of the pendulum to the center of the wheel, and enter the data in the form to the right. The rotational inertia of the wheel will be returned.

Most of the error of the method comes from measuring the period. Timing 100 swings and dividing by 100 gives a good estimate. This minimizes the error of starting and stopping a stopwatch by hand. A pendulum has the property that its period is constant as it slows down. Take care that the wheel swings in the same plane at all times. The method will be invalid if it does not.

See Performance and Wheels Concepts, Wheel Rotational Inertia for a complete explanation.

Data for complete wheels are shown in Table of Wheel Inertias and Mass. The wheels had tires, tubes, rim strips, rims, spokes, hub, skewers, free wheels, just like they would be ridden. As individual components, rims lend themselves to calculation of rotational inertias; tires and tubes don't. There is a large variation between advertised weights and actual weights as manufactured. More real-world, meaningful results come, in my opinion, from measuring wheels in an "as ridden" state. Hence the values here are for fully rideable wheels, just like the ones handed to you from your support vehicle.