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Automotive Tech

Limited Slip Differentials

What is a limited slip differential?

The purpose of any differential is to allow the drive wheels to turn at different rates from each other. The main reason one wheel has to turn at a different speed than another is that when turning a corner, the inside wheel needs to spin at a slower rate than the outside wheel because the inside wheel follows a smaller arc. If the two wheels were locked at the same speed, one or both of the wheels would have to break traction in order to be able to go around a corner.

Most vehicles come with an "open" (aka "peg leg") differential that basically does its job of allowing the wheels to spin at different rates. The biggest problem with this is if one of the drive wheels slips, an open differential sends more power to the slipping wheel, not less (it is the path of least resistance). This is most noticeable when one is stuck in the snow: one drive wheel will spin, and the other one will just sit.

From a performance standpoint, this means when one is cornering and the wheel on the inside of the turn is being unweighted and begins to slip under power, much of the power will go to it, causing it to spin more, which may cause understeer (a front end push to the outside of the turn) in FWD cars. The wheel on the outside of the turn, which is being weighted and therefore has more traction, does not receive as much power, so the power is wasted on the spinning inside wheel. Also, when launching aggressively off the line, when one wheel begins to spin, it will receive the bulk of the power, causing it to spin more.

Now, limited slip differentials (LSDs)(they come in all different configurations, and not all are called LSDs [torsen diffs for example] but we will call them all LSDs to simplify things) use a mechanical, hydraulic or electronic mechanism to supply power to the wheels that grip, not just the wheels that slip (yes, just like in the Subaru ads). As the name implies, they provide only a limited amount of slip between the driven wheels. So, when cornering, the outside wheel will get its fair share of power, lessening understeer and allowing one to power out of corners better. Torque steer is also lessened, and wheelspin on aggressive launches is minimized.

But the key feature of LSDs is that they still allow the drive wheels on either side of the vehicle to turn at different speeds, which is why we have differentials in the first place. A drag racing trick is to weld an open peg leg differential together so that both wheels are locked and receive equal power. This makes for great drag launches, but trying to go around corners, even very slowly, is extremely difficult, and can break axles as the outside wheels fight the inside wheels.

So the end result with an LSD is better handling and accelleration. And a much lighter wallet if you are buying one for a car that does not have one already. The installation of an LSD involves basically dropping and opening up the transmission.

Quaife seems to be the most popular for FWD Honda/Acuras, but Kaaz and others make decent ones, and the OEM Integra and Civic Type R LSD's are also sought after.