Limited Slip Differentials
by Jim Kerr

Putting the power to the ground is always a challenge for drivers stuck in a snow bank, climbing a loose and slippery hill, or just trying for maximum performance from their high-powered sports vehicle. A limited slip differential may be the answer to help get that vehicle moving.

The General Motors Versatrak axle has clutches similar to a limited slip differential but uses gerotor oil pumps to apply the clutches when axle speed differs from the differential carrier (housing) speed. Click image to enlarge

Several different types of limited slip differentials are offered from the vehicle manufacturers. Cone type, clutch type, viscous, locker, Torsen, and even electronic are all terms you may encounter when looking at differentials. All of them have the same purpose: transfer the torque from the wheel that is spinning to the wheel on the other side of the vehicle that may have more traction.

There are advantages of using an "open" or non-limited slip differential. They are cheaper to build, they are lighter, they allow the vehicle to turn a corner easily, and they keep the vehicle stable when accelerating on ice because only one wheel spins. Some designs of limited slip differentials act like an open differential during cornering, but other designs keep both drive wheels coupled for maximum traction. When this happens, it is easy for one wheel to slide on a corner and the vehicle becomes unstable.

The most common limited slip differentials found in performance cars and trucks are of the cone or clutch type. Used in the rear differential, these units have a cone or a series of clutch plates connected to the side gears in the differential. The axle shafts are splined into the side gears, and when one wheel slips, the rotating side gear causes the cone or clutch plates to lock or apply. By locking one axle to the differential, the other axle is forced to turn. Many cone or clutch type differentials use preload springs to help keep some tension on the unit at all times. A special oil additive is often required in these types of differentials to prevent the clutches or cones from grabbing or chattering as they apply. Using regular gear oil can cause noise and rough operation that can damage parts.

Locking type differentials are used on some trucks. There are two popular types of locking differentials: the Eaton locker and the Detroit locker. The Detroit locker is a heavy-duty unit found in some 3/4 and 1 ton trucks. It provides positive engagement of both axles while driving. The easiest way of describing it is to say it operates like a ratchet wrench: teeth on the side gears are engaged by "dogs" that prevent the gears and axles from spinning. Detroit locker operation is accompanied by clunks, snaps, and bangs as the gears unlock and lock again after turning a corner. This is disconcerting, but normal.

Eaton locking differentials use small spring loaded weights that spin when one of the axles spins. The weights fly out and catch on a locking dog. This causes a cam plate located behind one side gear to turn and force the clutch plates together, causing both axles to turn at the same speed. Put too much torque through the clutch plates, and the teeth sheer off one of the plates and the differential then operates like an open unit. For this reason, the Eaton locker is designed for low speed operation and slow acceleration. If you want acceleration performance, you would be better to use a cone or clutch type unit. Locker type differentials use regular gear oils. Do not use limited slip additives or the clutches will not work properly.

The Torsen differential uses a combination of gears to split the torque between the axles rather than apply clutches. Click image to enlarge

The Torsen unit is a performance type of differential. Rather than using clutches, the Torsen design uses parallel helix gears to keep the torque balanced to each drive wheel. It acts like an open differential while cornering (unless you accelerate), and sends power to both rear wheels at any speed or slip condition. The Honda S2000 uses a factory installed Torsen differential, and other manufacturers ranging from Hummer to Toyota have used them in performance models. There are several aftermarket units available.

Viscous limited slip differentials use the properties of silicone fluid to apply torque to both drive wheels. When one wheel starts to spin, it causes half of the housing holding the silicone fluid to spin. When the fluid is forced to spin, it gets thick and transfers the motion to the other half of the housing. This takes place smoothly and quickly, and because it still allows some slip, it is suitable for front wheel drive vehicles too. The silicone housing in Viscous limited slip differentials is a sealed unit, so the differential uses regular gear oil for rear wheel drive applications, or transaxle oil for front wheel drive units.

The manufacturers are starting to build differentials that use oil pumps to apply clutch plates (GM's VersaTrak) or computer-controlled electromagnets to apply clutches (Acura's MDX). Limited slip differentials can offer superior traction, better hill climbing ability, and quicker acceleration. Given the slippery road conditions in Canada during a major portion of the year, I would recommend selecting a limited slip differential option whenever possible if getting another vehicle.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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