A limited slip differential is a mechanical form of traction control.
When you have a lot of power, minute differences in traction between tires allow one of them to slip more than the other. And because a differential is by design supposed to allow difference in speed between wheels, this results in runaway wheel spin since once the wheel breaks loose, its traction is less, and this makes it go faster, and this means it has even less traction and so on in a vicious cycle. This ultimately results in all of the power going to just one wheel – the wheel without traction. Thus, you get poor accleration.
Limited slip differentials are designed to allow difference in speed between wheels but only to a point so that that the wheel which still has traction will get some of the power. There are varying designs for doing this.
Lockers obviously just say, fuck it, you don’t get ANY difference in wheel speed. Their on a common axle now; deal with it. This is bad for cornering and road use though. Great for offloading however because eliminates all wheelspin due to traction disparity which is common in offroad scenarios.
Geared LSDs use complicated gearing to limit torque disparity between wheels. Good because they are very consistent and will redistribute power immediately when traction is lost. Bad because they are complicated, expensive, and somewhat rough.
Viscous LSDs, which you asked about, use a wet clutch between the two half axles. When there is a disparity between the wheel speeds, the discs rotate at dissimilar speeds causing the fluid to heat up and expand which forces the clutch plates together and makes a common axle. Viscous LSDs are good because they engage smoothly but bad because they require sustained periods of wheel speed disparity to work. And also, the fluid eventually heats and cools so many times it loses its properties and needs to be replaced.
So, why would you choose a viscous LSD? When you want some mechanical traction control that isn’t rough as nails and that won’t eat tires up.
Examples
- Audi has favored geared LSDs
- BMW and Subaru have favored viscous LSDs
- Lockers are seen a variety of 4 wheel and offroad vehicles from a variety manufacturers