Cooley On Cars
Car Tech 101: Limited-slip differentials explainedYour car's drivetrain would be about as sophisticated as a garden cart if it didn't have some sort of differential. And increasingly, automakers are touting their use of the limited-slip variant. Brian Cooley shows you.
[MUSIC] If two wheels are locked on an axle so that they are not free to turn separately, one or the other has to slide. So engineers had to find a way to connect both rear wheels to the end, without sliding and slipping on turns. The device which makes this possible is a part of the rear axle. It is called the differential because it can drive the rear wheels at different speeds. Now before we talk about limited slip differentials, let's get a quick refresher on differentials. Here's one out of a vintage Alfa Romeo, good because it's fairly simple and can get us down on the basics. The idea behind a differential is to allow the two, in this case rear drive wheels, to turn at different speeds while going in the same direction. That's important in, let's say, turning a corner. Where the outside wheel is going to cover more distance in the same time than the inside wheel. It has to turn faster. If it didn't do that, either the inside wheel would have to slip, or, these gears in the middle would take a beating. A differential allows these two to operate somewhat independently. Power comes in here, from the car's drive shaft here, turns this, which operates a pinion gear, that turns the ring gear, which turns planetary gears, which then turns bevel drive gears, that turn the axles of the output shafts. Now here's where this whole assembly falls down, as miraculous as it is. If one of these wheels loses traction, the power, or the torque, goes to the path of least resistance and spins that wheel, the one that's not getting any grip, while the one that does have grip, since they're getting very little or no power, that's just not a good thing. So whether your car is slipping a wheel because of a poor surface, or because of lifting during cornering, or just because you're standing on it and one wheel's lighting up, a standard or open differential tends to put the power in the wrong place when there's a lack of grip. That's where a limited slip differential comes in. It limits the slip that is natural to a basic differential. If a drive wheel loses traction, the differential sends power to the opposite wheel or axle with the best grip, or the slower moving wheel or axle. Now front-wheel drives have this same function, but it's built into their transaxle, transmission assembly in the front of the car. Now, we could spend hours on the details of how these work internally, but basically there are several types. Geared differentials use spring loading and a more complex pack of gears than a standard or open differential like we have here. Clutch or plate limited slip differentials use those kinds of parts to engage one output shaft or the other based on wheel resistance. If a wheel spins, its clutch or pressure plates loosen to send it less torque. Viscous coupled limited slip differentials rely on discs in the differential spinning in silicone fluid that gets either grabbier or less so based on that friction of spinning. And electronic limited slip differentials are controlled by the car's computer. It reads wheel slip, and then commands clutches electronically to tighten or ease up their engagement on one wheel or the other. And within those four major methods of doing limited slip, there's also another layer. Some that will sense torque, and others will sense wheel speed to make their decision about what to alter. So now you know, when you get a car that has a limited slip differential. It's got some kind of technology built into this power split device that allows it to sense when a wheel's lost traction and send more power or torque to the one that has it. It helps you with performance, traction, and safety. [MUSIC]