# Gyroscopic precession of a bicycle wheel

• Cibek
In summary, gyroscopic precession occurs when a spinning object experiences an external torque, causing it to rotate around a different axis. This phenomenon can be observed in a bicycle wheel with a rope attached to it, where the wheel will stay upright and rotate horizontally when spun. The precession is not caused by a change in alignment or gravitational force, but rather by the torque applied to the spinning object. This can be visualized by considering a wagon wheel with rigid spokes, where the sections of the wheel will be pushed in different directions, resulting in overall precession.
Cibek
Hello!

I have recently been looking into the phenomenon of gyroscopic precession in my free time, and there is a scenario which I am not sure I have fully understood. Here it is:

Imagine a bicycle wheel with a shaft running through it's center. The shaft is attached to a rope at one of it's sides, and is held upright before it is spun up. If the wheel was not spinning, the force of gravity would simply cause the wheel to start swinging around the attachment point of the rope. However, when the wheel is spinning gyroscopic precession will cause the wheel to stay upright and instead start rotating around the rope, in the horizontal direction. This is supposedly because a force applied to a spinning object acts 90 degrees from the direction it was applied, which in this case causes the wheel to start rotating around the rope.

My questions are:
Is the precession occurring because the wheel is starting to change alignment when it starts swinging, and therefore the gravitational force is applied at an angle? In that case, am I right in assuming that the rotation would not start while the wheel is completely verticly upright? Or can a completely upright wheel still precess and rotate?

Cibek said:
Is the precession occurring because the wheel is starting to change alignment when it starts swinging, and therefore the gravitational force is applied at an angle?
No, the precession occurs because, and at the same instant that, the torque is applied. There does not have to be (and in the idealised case there is not) any rotation in the direction you indicate.
Cibek said:
Or can a completely upright wheel still precess and rotate?
Yes.

Cibek said:
This is supposedly because a force applied to a spinning object acts 90 degrees from the direction it was applied,
The effect of an external torque on the spin axis is: Rotation of the spin axis around an axis that is perpendicular to both: the current spin axis and the external torque.

Cibek said:
Is the precession occurring because the wheel is starting to change alignment when it starts swinging, and therefore the gravitational force is applied at an angle?
No, its occurring because an external torque is applied, which changes the angular momentum vector:
http://hyperphysics.phy-astr.gsu.edu/hbase/rotv2.html

Last edited:
Thanks a lot for the answers, they made it a lot clearer.

There's a simple way to get a handle on why precession occurs.

Take a wagon wheel because it has rigid spokes, and cut the wheel between each spoke so that it's free from it's neighbors. You can focus attention at the sections that are above, below, and horizontally displaced from the hub at time zero.

If you refer to your drawing, as a section of rim passes over the top, it's being pushed, or accelerated to the left. This means that as the wheel section reaches a position horizontal with the hub it will tend to be displaced to the left. The section of wheel at the bottom is pushed to the right.

The forces on the horizontally displaced sections, for the most part, only twist the section about the axis of the spoke.

All this adds up to overall precession.

## What is gyroscopic precession?

Gyroscopic precession is the phenomenon that occurs when a spinning object, such as a bicycle wheel, experiences a change in its orientation in response to an external force acting on it.

## How does gyroscopic precession affect a bicycle wheel?

When a bicycle is in motion, the spinning of the wheels creates a gyroscopic effect, which helps to keep the bike upright and stable. As the bike turns, the force from the ground on the front wheel causes the front of the bike to lean, and the gyroscopic effect causes the front wheel to turn in the direction of the lean, helping to keep the bike balanced.

## Can gyroscopic precession be used to steer a bicycle?

While the gyroscopic effect does help to keep a bicycle upright and stable, it is not the primary mechanism for steering. The main ways a bicycle is steered are through the handlebars and the rider's body movements.

## Does the size of the bicycle wheel affect gyroscopic precession?

Yes, the size and weight of the bicycle wheel can affect the magnitude of the gyroscopic effect. Generally, larger and heavier wheels will have a stronger gyroscopic effect, making the bike more stable and less prone to tipping over.

## Are there any other factors that can affect gyroscopic precession on a bicycle wheel?

Other factors that can affect gyroscopic precession on a bicycle wheel include the speed of the bike, the angle at which it is turning, and the distribution of weight on the bike. These factors can impact the strength and direction of the gyroscopic effect, and therefore, the stability of the bike.

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