Bicycle-Wheel Gyro

• Two handles (plastic handles from a hardware store work perfectly and are cheap; get the kind that’s designed to screw onto a file)
• A bicycle wheel (cheap or free from a thrift shop, bike store, or friend)
• A low-friction rotating stool or platform (typing or computer chairs often work well)
• A partner
• Optional: plastic spoke guards, screw eye (also known as an eyebolt), drill, hook, chain or rope suspended from a large stand or a ceiling

1. Screw the handles onto each side of the wheel’s axle. You may have to remove the outer nuts to clear enough axle for the handles. You may want to put plastic spoke guards on the hubs first to protect your fingers from the spinning wheel.

2. If you have the screw eye, drill a hole in the end of one handle and mount the screw eye in the hole. (See diagram below for assembly; click to enlarge.)

Sitting on the stool or chair, hold the wheel by the handles while another person gets it spinning as fast as possible. Lift your feet off the floor and tilt the wheel. If the stool has sufficiently low friction, it should start to turn. Tilt the wheel in the other direction and see what happens.

Get the wheel spinning, then use the screw eye in the end of the handle to hang the wheel from a hook mounted to the free end of a chain or rope. Hold the wheel so that the axle is horizontal, then release it. The axle will remain more or less horizontal while it moves slowly in a circle. (See diagram; click to enlarge.)

If you don’t have a chain or rope, rest the screw eye on your fingertips. Be sure to practice this before you try a demonstration. You will have to move with the wheel as it slowly turns in a circle.

A rotating bicycle wheel has angular momentum, which is a property involving the speed of rotation, the mass of the wheel, and how the mass is distributed. For example, most of a bicycle wheel’s mass is concentrated along the wheel’s rim, rather than at the center, and this causes a larger angular momentum at a given speed. Angular momentum is characterized by both size and direction.

The bicycle wheel, you, and the chair form a system that obeys the principle of conservation of angular momentum. This means that any change in angular momentum within the system must be accompanied by an equal and opposite change, so the net effect is zero.

Suppose you are now sitting on the stool with the bicycle wheel spinning. One way to change the angular momentum of the bicycle wheel is to change its direction. To do this, you must exert a twisting force, called a torque, on the wheel. The bicycle wheel will then exert an equal and opposite torque on you. (That’s because for every action there is an equal and opposite reaction.) Thus, when you twist the bicycle wheel in space, the bicycle wheel will twist you the opposite way. If you are sitting on a low-friction pivot, the twisting force of the bicycle wheel will cause you to turn. The change your angular momentum compensates for the change in angular momentum of the wheel. The system as a whole ends up obeying the principle of conservation of angular momentum.

Unfortunately, the gyroscopic precession of the wheel hanging from the rope is not explainable in as straightforward a manner as the rotating stool effect. However, the effect itself is well worth experiencing, even though its explanation is too difficult to undertake here. For more information, consult any college physics text on the subject of precession.

Astronauts have experimented with toy gyroscopes in the weightless environment of the space shuttle. Even when an astronaut gives the spinning gyro a shove, the toy’s axle stubbornly resists changing direction.

Spacecraft use small gyroscopes to sense their orientation in space. When the spacecraft turns, a freely floating gyroscope will not turn with it. Larger gyroscopes are also used to change the orientation of the spacecraft. The spacecraft can exert a torque on a rapidly spinning, massive gyroscope, which will in turn exert an equal and opposite torque on the spacecraft, causing it to turn.