Limited Slip Differentials

In "Fear and Loathing in Las Vegas," Hunter S. Thompson rattled off an in-car drug inventory that included five sheets of high-powered blotter acid, or lysergic acid diethylamide (LSD). Don't freak out, but your vehicle may boast LSD as well -- in the form of a limited slip differential. It won't send you on a hallucinatory journey through bat country, but it will help solve some of the problems posed by the more basic open differential.

When an open differential does its job, it ensures the same amount of torque travels to each wheel. The amount of torque actually applied to the wheels is limited by only two factors: equipment and traction. On the equipment side of the equation, you need an engine capable of producing the power and gears up to the task of transferring it. As far as traction goes, the wheel needs to be able to grip the ground without slipping, which can occur on ice, wet roads or even on dry ones if you floor the vehicle at a standstill. But what if only one wheel is on ice or in the mud? You wind up with one wheel spinning freely over the slippery substance, while the other wheel is reduced to the same torque. In other words, you'll have one wheel spinning in the mud and one wheel not rotating at all.

Limited slip differentials use various mechanisms to allow normal differential action during turns, while also solving the problem of slippage. When one powered tire slips, the LSD transfers more torque to the nonslipping wheel. As you might imagine, this makes all the difference between escaping a ditch and having to get out and push. The more common LSDs achieve this with clutches or a fluid-filled housing. To learn all about clutch-type LSDs, viscous couplings and other differential varieties, read How Differentials Work. As with open differentials, some work between two wheels on an axle, while others work along the driveshaft between rear and front sets of powered wheels.

What do you get when you add that lowercase "e" to "LSD?" Well, the adjective "electronic" has signaled the improvement of, among other things, our music, mail and toothbrushes. It has managed the same for our limited slip differentials.