What's A Limited Slip Differential?

We hear this term every now and then, usually with performance cars - but what does it actually mean?

We're not talking about brake-based torque vectoring. We're not talking about the electronic illusion of traction - we're talking about the proper, full monty: a limited slip differential. To understand why they're so great, we first need to understand what a differential is and why cars have one.

The majority of cars on the road come with something known as an open differential. What it does is it sits between the left and right driven wheels of an axle - or between the front and rear driven axles. The function of a differential is to allow the wheels to turn at different speeds, which is something you need when you want to go around corners. Naturally the outside wheels have to spin faster than the inside wheels as they turn in a larger radius, and the differential allows the wheels to slip against each other.

Without a differential, you get chatter as one wheel gets dragged along to keep up with the other. You also have issues with traction and control, which is why they're a necessary item in every single car you see on the road. But they do have their problems, especially when you're right on the limit of grip.

For you see, when an open differential loses grip on one of the wheels connected to it, it will automatically dump all of the torque into that wheel. The other wheel basically loses any form of drive, and the wheel that's already losing grip will spin uselessly. In a way it's slightly safer for most applications as the car will automatically slow down until it regains traction, but when you're driving hard this is far from ideal.

Which is why the limited-slip differential (LSD) was invented - and in its simplest form, the LSD is there to make sure both wheels keep getting drive even when one has far less grip. The name is its function: it limits the slip between the driven wheels, so that as one wheel slips the torque will be spread across both driven wheels. 

There are multiple benefits to an LSD. You can accelerate harder out of a corner because you have more torque getting to the wheels, and you can even use the LSD to pull a car's front end into a corner or kick the tail out (fighting understeer and inducing oversteer respectively). On the very edge of grip, a well-set-up LSD can even let you enter a corner more quickly. But among limited-slip differentials, there are so many different flavours you can choose from with their unique strengths and drawbacks.

Helical LSDs

Also known as a torsen (torque sensing), or torque-biasing differential, the helical LSD consists of a clever arrangement of gears inside the differential casing. The helical LSD constantly adjusts and "senses" the amount of grip at each wheel to deliver torque to the wheel with the most available grip. Cars like the Toyota GR Yaris and Honda Civic FD2 Type R come with this type of LSD from factory, and they were a factory-option on some older BMW models.

These are incredibly easy to live with and fairly low maintenance in nature, with none of the discomfort or roughness you might get with other LSDs. The downsides, however, are that they can be slightly slow to react and they are absolutely useless if one wheel lifts completely off the road surface - it will ultimately spin uselessly and take up all the torque.

Clutch-type LSDs

This is the most serious type of LSD, whether it comes from factory or aftermarket. The clutch-type LSD uses a series of clutch plates inside the differential casing to limit the amount of slip between the wheels and transfer torque when necessary. The actual operation can be a little tricky to understand, but the nature of clutch-type LSDs also allows them to be tweaked and tuned with the amount of locking force and the threshold of slip between the wheels before they engage.

Due to the nature of their construction, they can react extremely quickly as well as being more predictable in nature as they can be tuned to a driver's preferences. The problem is they require specific differential fluids and additives to keep them working smoothly, as well as being chattery in operation when taking tight turns even at very low speeds. They can be slightly uncomfortable at times, but when you need them to work they deliver as necessary.

Viscous-type LSDs

We only mention this type of LSD in passing, because they were around for a very brief period of time and phased out soon after. This type of LSD is far quieter than a clutch-type and smoother as well, but it has nowhere near the aggression of other LSD types, nor is it as responsive. It uses hydraulic fluid to transfer torque, a little like a torque converter in an automatic gearbox, and they can be prone to wearing out fairly quickly. Avoid them or replace them with something else if necessary.

Electromechanical LSDs

This is perhaps the single largest step in differential development since the invention of the car. Electromechanical controls will, as you guessed it, electronically control and limit the slip of a differential as and when you need it. Most of the time, they will function like an open differential being easy to use and get around with - but when you push it until a wheel starts to slip, it can limit the slip and transfer torque to the wheels with grip.

Electronic control also allows the systems to decide when best to activate or deactivate - for the reason that when an LSD locks it can also provoke a car into understeer if it's at the wrong point in the corner. With electronic control it can leave the LSD unlocked until the exact moment you need it. There's a reason that Lotus models only came with factory LSDs when electronic control systems were offered in tandem. Programmed properly, they can even allow a car to gain speed while in a drift - something really unheard of with traditional systems.