A bicycle is practically a rolling physics lab. From the forces you exert on the pedals to your energy output while climbing a steep road, there are hundreds of avenues for physics exploration. One investigation that works well in high school and college physics courses is investigating weight distribution of a rider on a bicycle.
Have you ever seen a cyclist get low and forward while diving into a turn? Or have you seen a mountain biker shift her weight backward when descending a steep pitch? How the rider is positioned on the bicycle is the dominant factor in determining the center of mass of the bicycle-rider system. (By comparison, a motorcycle typically weighs much more than its rider, thus the rider’s actions have a much smaller influence on the location of the center of mass.) By moving forward, a rider can place more weight over the front wheel, creating the extra friction and traction between it and the road that makes high-speed cornering possible. Likewise, a rider who moves her weight backwards when descending a steep incline keeps her center of mass between the wheels, making it much harder to “endo” (go “end over” the handlebars).
To study weight distribution quantitatively, we mounted a road bike in a stationary trainer perched atop two Force Plates. One Force Plate was for the front wheel and the other was for the rear. In the graph, you can see that without a rider, the weight was distributed fairly evenly between the two wheels. With a rider aboard, the center of mass moves rearward, with nearly 60% of the total load on the rear wheel. Once our rider started pedaling, the front and rear wheel loads oscillated by about 10%, although on average the total load was just about the same as it was while seated without pedaling.
The wheel load oscillations were about a quarter of a rotation out of phase with each other. This forward-and-backward loading and unloading of the bicycle is a result of the movement of the rider’s legs. By attaching a Goniometer to the rider’s right knee, we could gauge the position of the drive-side pedal and plot wheel load versus pedal position.
Have you ever used a Vernier sensor on a bike ride or done your own bicycle physics investigation? If so, we’d love to hear about it! Send us photos, write ups, and experiment files to firstname.lastname@example.org, and your experiment may be in the next Caliper.