LSD -- Keeping a better grip on reality

By T. Uncle

THE limited-slip differential is a device fundamentally designed to do just that, help prevent a vehicle's wheels from spinning in low-traction situations.

In varied forms, limited-slip differentials have been around for a long time, but their function in a motor vehicle has often been misunderstood.

To understand what a limited-slip differential (LSD) does, it is necessary to look at the function of a regular differential and understand why it is a necessary part of just about any vehicle where two wheels do the driving.

Basically, what the differential does is allow each driving wheel to travel at different speeds when necessary, such as when rounding a corner, while still sending power through to both.

Cornering without this ability to vary inner and outer wheels speeds would be virtually impossible.

The effect can be understood by considering what a line of soldiers does while executing a turn on the parade ground -- the soldiers at the "pivot" end of the turn mark time as those on the outer end march faster to complete the arc.

Automotively, this effect is achieved by an ingenious piece of mechanical trickery that allows the inside wheel to turn slower than the outside wheel as a turn is being executed.

The downside is that this independence of each wheel from the other can leave the vehicle with virtually no traction if one side happens to have grip and the other doesn't.

In straight-line situations, the drive transmitted to the differential is essentially even on each side, but under slippery conditions it always goes to the wheel with least resistance.

The effect is all too familiar: one wheel firmly planted and unmoving on the bitumen, while the other spins uselessly in the mud on the side of the road.

Devising a differential that enabled some of the power to get through to the wheel with more traction while still allowing the differential action in corners involved some compromise.

It would be fair say the common application was in cars likely to be put to extreme use, such as circuit racing or rallying where power levels are high and the need for traction is greater than normal.

Using a system of friction clutches built into the housing to send drive to the wheel with more traction, competition-style limited-slip differentials put a good share of the engine's power through to the road, but can be tricky to handle on corners as the pronounced limited-slip, in which both wheels try to spin at close to the same speed, means the car wants to go straight ahead when attempting to round a corner.

Less dramatic LSDs are commonly used on the more powerful road cars, but usually the degree of slip is not greatly different to a standard differential, so that cornering behavior will be predictable.

LSD design has become more user-friendly in recent years with designs such as the Australian Hydratrack system used by Holden Special Vehicles and in Ford's Outback utility.

Rather than clutches, this system uses viscous silicone fluid couplings to regulate the power supply and is consequently smoother in operation, with less friction in straight-ahead situations where no differential action is required.

Another system proving to be popular in various applications -- on and off-road -- is the Torsen positive-locking LSD that "reads" torque in order to decide which wheel should be receiving drive.

The Torsen-style LSD can be found in various off-road vehicles and high-performance road cars.

It is worth noting here that the "lock-up" center differential found in most off-road vehicles with full-time four-wheel drive, such as the Toyota LandCruiser, does not automatically lock up all four wheels when activated.

Because a constant four-wheel drive vehicle requires a center differential to deal with the changes in front and rear axle speeds that occur when driving on sealed roads, its effectiveness off-road can be compromised by the same limitations that affect a regular differential on a two-wheel drive car.

That is, if either front or rear axle is encountering slippery conditions, the center differential will simply allow the torque to follow the path of least resistance, meaning that without a center differential lock, even a four-wheel drive can find itself in a situation where only one wheel is spinning.

The locking differential allows power to go to both front and rear wheels.

Conventional four-wheel drives do not have this problem because they use a direct lock-up between the front and rear axles when off-road, and only operate as two-wheel drives on sealed surfaces.

It might be comforting for most car owners to know that LSD is rarely needed under regular driving conditions.

Only if you drive a car with massive torque, or venture off- road regularly, are the terms LSD, Hydratrack and Torsen likely to have any relevance.