The Four-Ball Gyro and Motorcycle Countersteering

Most two-wheel motorcycle riders know that, at highway speeds, if you want to turn left you push on the left handlebar and pull on the right handlebar. This is called countersteering. Countersteering is counterintuitive since pushing left and pulling right when the front wheel is not spinning would turn the wheel to the right. All good motorcycle instructors teach countersteering but few understand the physics of why it works, even though there is considerable discussion about it among motorcycle riders. This paper gives a simplified explanation of gyroscopic precession and then applies this to the front wheel of a motorcycle using two steps: 1) explaining how the wheel's lean is initiated, and 2) explaining how the lean will cause the wheel to turn. To assist with this discussion and to demonstrate the conclusions, a "wheel" was constructed using copper pipe, a bicycle wheel hub, and one pound of lead in each of four "balls" at the end of the spokes (see Fig. 1). © 2017 American Association of Physics Teachers.