The Home Lab

by François Grignon

What do an electronic fish scale, a set of Volkswagen valve adjustment shims, a pail of water, an old bicycle fork and a stopwatch have in common? They were all part of the experimental apparatus for this test.

No fancy equipment in this basement. We just used what was handy. A little ingenuity can go a long way.

Wheel weight, of course, is a simple measurement. The kitchen's scale, with a resolution of 10 grams and a proven accuracy (against my butcher's certified scale) is ideal for weighing bike components.

Moment of inertia, however, cannot be measured directly. It has to be inferred. To obtain its value, we clamped the wheel in a fork and attached a small point mass of known value (the stack of Volkswagen shims) to the outer periphery of the tire.

This imbalance mass, when raised from the bottom position, causes the wheel to oscillate like a pendulum. The period of oscillation can be related mathematically to the moment of inertia of the system. Using a stopwatch, we measured the period of oscillation over four cycles (three times to average out errors) and then calculated the moment of inertia.

To obtain values of the drag induced retarding torque on the spinning wheel, we made use of the fact that this torque is equal to the product of the moment of inertia times the rate of change of angular velocity.A complete bike was used as a test stand. It was held steady by a home trainer clamping the rear axle and the front wheel was propped up by placing a short post under the bottom bracket. The steering was immobilized with the use of... a coat hanger.

The front wheel was spun to 60 km/h with the help of an electric drill driving a 5 inch rubber sanding disc whose edge was held firmly against the tire. The drill was removed and the spinning wheel was allowed to decelerate on its own while a close watch was kept over the speed reading on the cycle computer. The stopwatch was started when 45 km/h showed on the speedometer and stopped when 35 km/h appeared. This allowed to calculate a close approximation of the deceleration rate at 40 km/h. Elapsed time was also measured as speed went from 8 km/h to 5 km/h.

At low speeds, aerodynamic drag is practically nil and the deceleration could all be attributed to bearing friction. At the higher speed, drag is a combination of aerodynamic forces and bearing friction. Bearing friction being practically independent of speed, its now known value could be subtracted from the total drag calculated from the higher velocity deceleration time.

To calculate wheel stiffness, we measured deflections with a dial gauge under the application of a known force. For lateral stiffness, we used 10 lb. (a pail of water, as weighed by the electronic fish scale). The gauge we used has a resolution of one thousandth of an inch

For radial stiffness, we loaded the wheel with a weight of 100 lb. How? That was tricky. The wheel was mounted in a Park truing stand. The dial gauge was supported by an arm clamped onto the sturdy stand and its wand was rested against the inside of the rim at the twelve o'clock position. The whole setup was put on a bathroom scale. Yours truly then placed his hands firmly against the top of the wheel and leaned his whole body over the wheel until the desired pressured was applied. In this position, both the scale's dial and the dial gauge could be read. It looked like a circus act, but it worked.

Copyright © 1998, François Grignon

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