What is a differential?

Vehicle transmissions are often shrouded in mystery. Yet, they are relatively simple and a good basic understanding of how they work can help you look after your car better, make it more reliable and less costly to run.

What does a differential do?

A differential allows driven wheels on the same axle to rotate at different speeds. This is important in cornering especially, not only because the passenger and driver's side wheels cover different distances and, therefore, rotate at different speeds but there is also a speed difference between the front and rear axles. If the wheels were forced to rotate at the same speed, the tyres could lose grip and cause a skid.

An 'open differential' allows the wheels to rotate at different speeds and is the most common type of differential found on road cars. A downside is that all of the engine power sent to the differential is directed to the wheel with the least grip.

This could mean that a wheel with no grip is left spinning, while the opposing wheel is not fed sufficient power to move the vehicle. This is why open differentials are not especially useful in an off-road situation.

The solution is to 'lock' the differential(s) so that the wheels turn at the same speed, regardless of grip levels. Locking differentials can be either driver-activated or automatic.

At higher speeds on tarmac roads, however, a locked differential will cause excessive torsional forces (‘wind-up’) to build within the transmission that can court mechanical failure. It could also cause a skid, where the driver could experience a loss of control.

What is a limited-slip differential?

Some modern cars with open differentials use their traction control systems to apply the brakes to a spinning wheel, allowing some power to be diverted to the opposing wheel. The Limited Slip Differential (LSD) is an alternative method. As its name implies, it allows speed to differ between road wheels but by only a prescribed amount. The LSD is, therefore, a halfway house between an open and locking differential.

Most types operate mechanically to impose a limit on the difference in speed between the wheels. The clutches within LSDs are bathed in a dedicated LSD oil. Torsen (Torque-Sensing) LSDs have become more common and rely on the interaction between different gears to distribute power between a pair of driven wheels.

Therefore, in a typical 4x4, (or all-wheel-drive) vehicle, you may find that it possesses three differentials, not all of the same type, one for the front wheels, another in the centre (possibly housed within the transfer box) and another for the rear wheels.

What maintenance do differentials need?

Most differentials, whether utilising gears, or clutch plates, are immersed in oil. Maintenance tends to be restricted to either maintaining the correct lubricant level, and/or renewing it at the prescribed time/mileage interval. The correct oil is essential. Using the wrong type can cause gears to wear and clutch friction materials to fail.

When driving, listen for any rumbling noises from the transmission, especially if the note does not change while cornering. This might indicate failing bearings and is something worth reporting to your garage for investigation.

We are also hearing that racing starts, especially on high-performance cars, are causing premature differential failures, due to the lack of mechanical empathy that results from 'dumping' the clutch.

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