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Limited Slip Differentials

Limited Slip Differentials (LSD for short) are an attempt to remedy the problems that an open differential has when the drive wheels do not have the same traction. This applies not only during launches, but also in cornering. The two types of mechanical LSDs are Clutch Based and Gear Based. There is a fluid based LSD known as a Viscous Coupling, and there are Electronically controlled LSDs, but neither will be covered as there are none for available for our applications.

Clutch Based

These are the more common types of LSDs and were produced much earlier than the gear based type. Different manufacturers marketed them differently, but Chevrolet’s label of Positraction became the most famous and so clutch based are often affectionately referred to as “posi.” During typical operation, posi performs like an open differential and is similar in construction. But there are a few important differences.

Within the differential there are two clutches, one for each side gear, and a mechanism to detect slip and apply the gears to the clutches. When one wheel begins to slip, the spring pack applies force on the side gear of the wheel with traction and it is pressed against the clutch. The clutch is connected to the cage, and the friction generated increases the torque applied to the wheel with traction. The extra torque applied is proportional to the force needed to overcome the clutch, which in turn depends on the clutch and the stiffness of the spring. But the main limitation of the clutch differential is that you are still losing torque in this situation, but fortunately not nearly as much as with an open differential. Additionally, the clutch operation may not always be seamless, which could lead to some suspense around corners. Lastly, another limitation is that the clutches will eventually wear out.

You may have heard of 1, 1.5, or 2 way differentials. In a 1 way differential, the center cam is designed in such a way that it can apply the gear to the clutch only under acceleration. In a 1.5 way, it can apply in both acceleration and deceleration, but takes twice as much slip to apply during deceleration. A 2 way provides the same limited slip effect in acceleration and deceleration (although typically it is a little less during deceleration).

Eaton and Auburn make popular clutch based differentials that tend to be very durable. One of the reason clutch based units are still produced is that there are a wide variety of units rated to take the torque in high power applications.

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