Drive a car in a straight line and all four wheels turn at the same speed and travel the same distance for any given time. Turn a corner though, and the situation changes. Through a bend the outer wheels have to travel further, and hence faster, than those on the inside of the bend.
As any car has the driven wheels connected through the drivetrain, this ‘difference’ in rotational speed would cause the inner wheel to slip as it tries to catch up with the outer wheel – a problem which would not only lead to loss of control, but would put massive stresses on the transmission components. The problem is increased in 4x4s as all four wheels are trying to rotate at varying speeds in certain conditions.
Wheel slip doesn’t happen though, because cars are fitted with a differential. By use of a set of planet gears placed in between the input shaft from the engine and the driveshafts or halfshafts (and front-to-rear too in 4x4s), the driven wheels can rotate at differing speeds through a corner. A simple differential, working this way is known as ‘open’.
The trouble with open diffs however, comes when one wheel loses traction – on ice, snow or mud, for example, or on track when cornering hard and the weight of the car shifts strongly on to one side. In such conditions all the power (the torque, in fact if we’re being correct) starts spinning the wheel with least resistance and nothing goes to the tyre that’s still got grip – not what’s needed to get out of a muddy field or round a lap quickly. The solution is a limited-slip differential (LSD).
There are several types of LSD you’ll hear about with the main ones being: viscous – a form of which was on the Escort RST and Sierra Cossie; plated – such as found in the Atlas axle of the late Capris; and ATB-type (Automatic Torque Biasing) – most famously the Quaife ATB, which was standard-fit for the Focus RS and is a popular upgrade for plenty of fast road and track cars around the world. These diffs all work in slightly different ways, but the basic principal is that when one wheel starts to lose traction and spin faster, the LSD distributes the torque so there’s a driving force at the wheel that’s still gripping.
In massively simplified terms, viscous LSDs use a viscous fluid inside with vaned discs which transmit the torque to a slower moving wheel by the internal friction to speed up its rotation (in much the same way as a torque converter transmits power to an auto gearbox). These are relatively gentle in action, usually cheaper and long lasting. Plated diffs use clutches and springs to shift the torque between the wheels. They can be harsher in use, but have the advantage of being adjustable for a driver’s style and will still drive one wheel, even when a halfshaft or driveshaft on one side snaps – so are favoured by rally cars and extreme track racers, especially in RWD set-ups. The ATB uses gears to distribute power to the wheels, are fit-and-forget, and unless driving to the limit are a good compromise between cost, performance and usability. ATBs are suitable for all drive types, especially FWD.
And then there’s the cheap option of welding your open diff to permanently ‘lock’ it. We would advise against this! You may think you’re a driving god, but unless your name’s Loeb, Button or Ken Block, then there will come a time, a damp road surface and a tighter-than-you-thought bend that all combine to spit you in to the scenery.