WRC explained: Differentials

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In our wrc.com glossary we explain, in simple terms, some of the words and phrases that are commonplace in the World Rally Championship.

Some subjects, however, deserve a bit more space than a line in the glossary. So this season we'll bring you a series of video features, produced in conjunction with a WRC expert, which cover some of these areas in more detail.

So far this year we've looked at aerodynamics, time cards and dampers. This time Citroen Junior Team engineer Francois Xavier 'FX' Demaison explains why World Rally Cars need differentials.

Follow these links to watch our exclusive wrc.com video in English or French, or read on for more details.

The theory
The drivetrain of a World Rally Car is fitted with three differentials (or diffs): one at the front, one at the centre and one at the rear. Differentials allow the wheels to turn at different speeds (hence the name) so the car turns more easily.

"Without differentials a car would behave a bit like a tank with both tracks going at the same speed; it would never turn," explains Demaison. "To turn a tank you need one track to turn at a slower speed, and it's the same for a rally car."

The front and rear differentials are known as 'limited slip', which means if one wheel breaks traction more power will be transferred to the non-slipping wheel.

How it actually works

Front and rear differentials

Located between the wheels at the front and rear of the car, the front and rear diffs control the amount of power going to the wheels via the car's four driveshafts.

At their most basic level the front and rear differentials allow the wheel on the inside of a corner to turn more slowly than the one on the outside. But this is only part of the story.

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