Diff'rent strokes

Racecar looks at the different types of mechanical differential, their benefits and limitations

-  JAHEE CAMPBELL-BRENNAN

Whatever the axle configuration, or number of axles, between any driven wheels of an automotive powertrain (with a caveat for some EVs and hybrids) you will find a differential gear set.

The technology was conceived as a solution to two problems of the early, combustion-engined automobile associated with transmission of power from the engine to the road and manoeuvrability.

As a combustion engine has one rotational output and the requirement to power two driven wheels, the differential firstly serves as a mechanism of splitting the rotation of the crankshaft to drive the left and right wheels of an axle. The second, and more interesting, function it serves us as motorsport engineers is to provide a method of introducing a speed differential between opposing wheels.

To understand the requirement for this speed differential, we need to look to the foundations of vehicle dynamics.

As a vehicle travels around a corner at a given cornering speed, the inside and outside wheels are logically travelling on different radii due to their distances from the vehicle's centre of rotation. The inside wheel is closer to the centre of rotation than the outside wheel and therefore must have a relatively lower forward velocity to maintain a free rolling condition, without slip.

In the early days of automobiles featuring rudimentary solid axle configurations, the physics that enable these free rolling conditions are blocked by the fact the wheels are mechanically coupled to each other. This mismatch of speeds stimulates the inside and outside tyres to generate longitudinal forces in opposition of the yawing moment requested by the driver's steering inputs.