PLATED DIFF
A plated-type LSD is probably the harshest of all the ‘slippers’ available, because in simple terms it is either ‘open’ or ‘locked’, and there’s not much in between. Like all diffs, power is applied from the pinion shaft to the crown wheel, which then rotates the diff assembly giving you drive.
How it allows slip, or rather doesn’t allow wheel slip, differs from other LSDs. As the name suggests, inside a plated diff are a number of clutch plates which are held under tension by a sprung plate. This tension determines the amount of static pre-load the diff has, or in other words the amount of torque needed to break the friction between the plates to allow one wheel to rotate faster than the other. This static preload can be tuned to suit the application, by altering the arrangement of the plates and by adding or removing spacer washers.
The static pre-load is just one aspect of a plated diff, though. The slightly more confusing part is the dynamic or ‘PowerLock’ effect. When power is applied, the cross-shaft holding the planet gears tries to force the two halves of the diff apart. This increases the amount of friction between the clutch plates and encourages both wheels to rotate at the same speed, effectively ‘locking’ the diff.
To do this a plate diff is designed with what are known as ramp angles in the cross-shaft; the angle cut into the body which the cross-shaft runs up against to force the two halves of the diff apart. Also, you will notice there is another cut out on the opposing side of the diff body too. This is another ramp angle, but rather than forcing the clutch plates together when power is applied this does the same thing when lifting off. These ramp angles can be altered to effectively tune the diff to suit a specific application. A steep ramp angle will mean more force is applied to the clutch plates more quickly, therefore making for a more aggressive locking action, whereas a shallower ramp angle will have the opposite effect.
Plate diffs are very well suited to fast-road and race RWD applications because they apply equal torque to both rear wheels when you stand on the accelerator, and can induce controllable power-slides. They are perhaps best suited to older cars with leaf sprung rear suspension like the Mk1 and Mk2 Escorts, because these types of cars often tend to lift the inside rear wheel during cornering. In these situations, a plate diff is ideal because it will apply equal torque to both rear wheels, meaning you don’t lose drive.
Traditionally, plated diffs haven’t been popular with front-wheel-drive cars, especially fast-road applications, but in recent years modern versions from the likes of Kaaz have started to gain popularity, especially with big-power modern FWD Fords.