All cars require a differential unit. A differential unit (sometimes called the diff) is a very special mechanism designed to feed the engine’s torque to the driving wheels. In most cars, the differential is where the engine torque goes through its last gearing mechanism before spinning the wheels.

A differential’s purpose can be broken down into three functions. Firstly, a differential takes the power from the engine and connects it with the driving wheels. Secondly, a differental is the last gear reduction that will occur in the vehicle. Thirdly, the differential transmits the power from the engine to the wheels while allowing the wheels to rotate at different speeds.

A motor vehicle needs a differential because a vehicle’s wheels spin at different speeds, especially during a cornering manoeuvre. If the car did not have a differential, the driving wheels would have to be locked together and forced to spin at the same speed. Ouch! This would not only make turning a nightmare and increase the chance of losing control, but it is also hard on the vehicle. For the car to turn with the wheels locked, one tyre would have to slip. The large force needed to make a tyre slip on tarmac is great and the force would have to be transmitted through the axle from one wheel to the other, which would put a huge strain on the axle componentry – not to mention the rubber!

An open differential is the simplest type of diff and always applies the same amount of torque to each wheel. The problem is that as soon as a driving wheel loses traction, the torque gets thrown to that wheel – a bit like water always finding the easiest way down a hill.

With a limited slip differential, the diff mechanism can transfer the torque away from the wheel that starts slipping and shove it onto the wheel that still has traction. There are a number of ways of offering the limited slip differential feature. For serious four-wheel-drive vehicles, you can’t go past the Locking and Torsen option, which is simple but very effective. Even the mighty, massive Hummer has this feature. It’s a must for serious 4×4 trucks: if one wheel ends up off the ground, because the differential is locked into position, the driving wheels will continue to spin at the same speed.

The viscous coupling limited slip differential (what a mouthful!) is often found in all-wheel drive vehicles and is commonly used to lock the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.

The clutch-type limited slip differential is perhaps the most common version of limited slip differential. The clutches only work if one wheel wants to spin faster than the other, so as to keep as much torque to the wheel that has traction.

I love differentials! Where would we be without its inventors? Probably stuck in a ditch beside a sharp corner or something like that.

We hope that helps answer the question ‘What is What is a Differential?’!

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