Limited Slip Differential - How it Works

How a Limited Slip Differential (LSD) Works. The information required for this article is from the Jeff Daniels article in EVO Magazine, Issue 81, July 05

see also:
limited slip diff manufacturers
transmission manufacturers

A Standard limited slip diff, as fitted to the
majority of cars allows the two driven wheels to rotate at different speeds while delivering the same torque to each wheel. The wheels need to travel at different speeds when the car is cornering as the outside wheel has to travel a greater distance than the inside wheel.

This system works fine in most daily driving situations but this system has faults which will become more apparent on slippery surfaces. For example when driving on snow, one wheel may have little or no traction and hence the torque created is negligible and as the diff
transmits the torque 50/50 between the two wheels, the other wheel gets 50% of next to nothing which is why the car will not move forward.

This is where a limited slip diff can be useful, on slippery surfaces and high speed driving.
During high speed cornering there is more force placed on the outside wheel which can
lead to the inside wheel breaking traction, resulting in no forward drive. This can lead to
massive instability and is potentially very dangerous. A limited slip diff limits the speed
differential between the two wheels, when it senses that the speed difference has become
to great it applies a force (normally this force is applied by the opposite wheel when it has grip) from which the driven wheel can use to drive
forward.

The force that is applied can be created using a variety of methods. The ZF developed unit and Torsen Diff are mechanical, there are units which use a viscous coupling and electronic units which work by means of a multi plate clutch.

In most of these units the amount of torque distributed between the wheels is limited. If 100% of force were able to be distributed to both wheels then this would be a locked unit with no differential effect. This would mean that both wheels would always want to travel the same distance even when going around corners. This would lead to terminal understeer and worn tyres (where the inside wheel is spinning faster than the speed of the vehicle)! This system also means that the drive shafts on either side have to be strong enough to take 100% of the torque which leads to more expensive and heavier components.

Torque distribution is usually limited to between 30 and 50%. ZF type diffs are popular in competition as the torque transfer builds as speed increases, other simply switch on when the set speed differential is detected.

Modern systems use stability control to give the effect of a limited slip diff, they do this by applying a braking force to the wheel that it sense is slipping (creating torque) and therefore enabling more torque to be distributed to the other wheel. Therefore if your car has ABS then the building blocks of a stability system is already in place. However this system is not ideal for competition usage as it entails using the brakes and therefore slowing down forward motion. Which is why there is no substitute for a genuine limited slip diff in motorsport applications.

As a side note, if you are using a front wheel drive car for track days and you find that you are finding it hard to get traction out of slow corners, dragging on the brake while coming out of the corner will give better traction until you can get the wheels pointing in a straight line.

The same method (ie applying a slight force on the brakes) can also be used when driving a car on slippery surfaces such as mud and snow. Applying a slight force on the brakes while driving forward will stop the one wheel from spinning and allow the both wheels to drive the car forward. This technique can be remarkably effective and allow a 2wd car to have traction in some very slippery conditions.

Limited Slip Differentials (LSD) Types

1 Way

Only applied during acceleration, primarily designed for front wheel front engined cars.


1.5 Way

Primarily applied during acceleration but also applied slightly during deceleration. Improves drive out of the corner and increase stability under heavy braking. The benefits are easily harnessed by anyone and this is perhaps the most effective type of LSD.


2 Way

Increases traction under acceleration and braking but also makes the car less willing to turn. Requires a chassis set up which promotes easy steering and requires a good driving technique.


Variable LSD

It is possible to adjust the strength of the LSD effect under acceleration and deceleration. It is possible to make big changes to the handling balance of the car.


Active Yaw Control

Adjusts the amount of torque distributed between the left and right side. By increasing the effect of the AYC a turning moment i produced enabling the car to turn more effectively. However if the effect is too much the car can become very oversteery.


Adjusting the Variable LSD

Increasing the strength of the diff under acceleration increase the forward drive out of the corners.

Increasing the strength of the LSD under deceleration will make the car more stable under heavy braking but it will also make the car harder to steer. For cars which have a tendency to understeer anyway this adjustment is not recommended. However for a driving style where the braking is left very deep into the corner then this adjustment is desirable.