Limited Slip Differential – Its Role and Operation

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Limited Slip Differential - Its Role and Operation




Renault Mégane RS (2020)


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Renault Mégane RS (2020)

The self-locking device has become common in sports cars. You will soon understand why.

For sports cars, the limited slip differential has become almost indispensable and those who drive such cars know it very well. Its role is to prevent a wheel from slipping due to a slippery surface or too high power. Explanations.

Its principle

The differential is the organ which distributes the torque between the two driving wheels of an axle (it can also distribute the torque between two axles). It is basically designed to allow the wheels to spin at different speeds while optimizing their grip. It intervenes for example in bends, when the inner wheel travels a shorter path than the outer wheel (in this case, the inner wheel turns less quickly than the outer wheel). It does this through a system of gears mounted on a central frame anchored to the ring gear which receives the movement of the transmission shaft.

When cornering, the transmission system balances the speed difference of the two wheels, so that the transmission does not deform, but this has a flaw: if the ground has little grip or if the power is too high , the wheel with the least grip starts to slip (to turn faster than the other).

This is why, on sports cars (but not only), it can be useful to replace the common differential, called “open differential”, with a differential which allows the wheels to turn at different speeds. but only up to a certain degree of freedom, hence the term “limited slip”.

There are several

The self-locking differential can take different forms. It usually consists of helical gears or more complex hydraulic or electromechanical systems. The self-locking does not leave one wheel stationary while the other unloads all the power. It therefore forces it to turn a total of turns (expressed as a percentage) in relation to the wheel which turns faster.. This ratio is specified because it makes it possible to evaluate the degree of performance of the component.

You did not understand ? Well, let’s make it simple. For example, a limited slip differential of 35% will make sure that for every 100 revolutions made by the wheel that slips, the other (the one with the most grip) makes 35.

This explanation is at least true for mechanical systems, which 20 or 30 years ago were the only ones possible in sports cars (like the Alfa Romeo 75 Turbo Evolution) and on all-wheel drive cars as well as many All Terrain.



Self-locking differential


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Self-locking differential

With the arrival of ESP and traction control which act on the brakes to “correct” the behavior of the car, the slip limitation function has also been delegated to the brakes, with programs often “dedicated” such as ‘Electronic-Q2 des Alfa Romeo 159 and Brera (unlike the 147 Q2 or GT Q2 which had a rather mechanical Torsen).

Here, when the system’s sensors detect a loss of grip, the system slows down the spinning wheel, so that the differential sends driving force to the wheel that is most anchored to the ground. This device costs less than mechanical systems and requires less maintenance, even if the effect on sports cars is not always completely satisfactory

The return of mechanical systems

In recent years we have seen the return of mechanical limited-slip differentials which, without excluding electronics, have become an integral part of the equipment of high performance cars, such as the Peugeot 308 GTi, the Toyota GT86 and the Subaru BRZ, the Renault Megane RS, the Ford Focus RS. And the next BMW 128ti.

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