What is "limited slip differential"?

Dear Car Talk

Dear Car Talk | Aug 01, 2002

Dear Tom and Ray:

I'm planning to buy a new Toyota RAV4, and I know I want all-wheel drive and an anti-lock braking system. We'll mostly use it on-road, but sometimes we have to drive in snow and ice in the winter and up a steep, rocky, sometimes-muddy driveway during the summer. One option Toyota offers is "limited slip differential." I haven't been able to find out anything about what this thing does or whether I need it. Can you tell me? -- David

RAY: Well, David, to give you an answer, we have to delve into the murky world of automotive differentials. Buckle up.

TOM: The differential is a magic box containing six gears that sits in the middle of the axle, between two driven wheels. And its job is to allow those two wheels to turn at different speeds -- while still delivering power.

RAY: Wheels have to be able to turn at different speeds, because when you're making a turn, the outside wheels always travel farther -- and therefore faster -- than the inside wheels.

TOM: So through a cosmic meshing of gears, which we can't possibly explain without using our hands, the differential adjusts the power that goes to the wheels. When the vehicle is going straight, each wheel is given 50 percent of the power. But as the vehicle turns and the outside wheel goes faster, the differential reduces power and velocity to the inside wheel by the exact same amount that it adds them to the outside wheel.

RAY: Why are we telling you all this nonsense? Well, stay with us, because here's the evil, dark side of this miraculous differential: If one of the wheels is on ice or hung up in the air, the differential sends 100 percent of the power to that spinning wheel. And if one wheel is getting 100 percent of the power from the differential, the other wheel must be getting what? 0 percent!

TOM: You can see how this can be a problem. If you're driving a car with two-wheel drive and one of those wheels gets stuck on ice, that's the only wheel that will get traction, so you're done for.

RAY: So what a limited slip differential does is it fixes that differential oversight. When one wheel is spinning, a set of clutches inside a limited slip differential shift some of the power to the other wheel -- so the wheel that's NOT stuck has power, too.

TOM: So the answer is yes, David, it is something you want. Even with all-wheel drive, you could get into a situation where both axles of your RAV4 had one wheel on ice. In that situation, without limited slip, you'd be stuck. With it, you'd be home-free. Which is the reason you're buying an all-wheel-drive vehicle in the first place, isn't it?

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