Dealing With Wheelspin

Jake Venter explores the logic of torque, and how to use a differential lock properly.

- Jake Venter

When I was an apprentice, we sometimes played the fool with a mechanic when he was on the point of driving out of the shop. Three or four of us would lift the left or right rear wheel almost off the ground. If the car had a normal differential it would be unable to move.

The slightly raised wheel would spin and the other wheel would get practically no torque. A normal (open) differential divides the available torque evenly between the wheels, but this torque depends on the traction at the wheel with the least traction. You cannot apply a force with your hand unless it makes contact with something; equally, you cannot transmit a torque through a shaft unless there is some resistance at the other end. Try it with a pencil; you can only transmit torque if you grip the other end.

One day, our workshop trick failed. The car was fitted with a limited slip differential that employed friction to send some torque to the wheel on the ground, and the car pulled away from us.

LIMITED SLIP DIFFERENTIALS