What kinds of wheel imbalance are there?

Instrument for measuring wheel runout

What kinds of wheel imbalance are there?

Imbalance can be static or dynamic. In both cases the issue is a mismatch between the axis of rotation and the axis of inertia (the latter running through the wheel’s centre of gravity). Let’s try to decode these terms in plain language.

Static imbalance arises when the wheel’s axis of rotation is parallel to the axis of inertia, and the centre of gravity does not sit on the axis of rotation at all (the wheel’s mass is unevenly spread around the circumference). In this case, if you put the wheel on a freely spinning axle, you can watch it swing a few times like a pendulum and come to rest with the centre of gravity at the lowest point.

Dynamic imbalance arises when the centre of gravity sits on the axis of rotation, but the axis of inertia does not coincide with the axis of rotation, crossing it at a certain angle (the wheel’s mass is unevenly spread across its width). In this case imbalance can only be identified while the wheel is turning.

It is dynamic wheel imbalance that tyre-fitting technicians eliminate by fitting balance weights on both sides of the wheel’s rim at particular points.