Viscous Limited Slip

The best way to transmit power to the rear wheels of a vehicle is with a solid axle between the wheels. Since the wheels are mechanically connected they will always turn at the same rpm. When traveling in a straight line 50% of the drive torque goes to each wheel. If one wheel loses traction it takes less torque to rotate and the torque is automatically applied to the wheel with the traction. If one wheel is lifted off the ground it will be taking 0% of the torque and the other wheel will get 100%. Of course a solid axle does not work very well if we want the vehicle to make a turn. In a turn the wheels on the outside of the turn have to rotate faster than the wheels on the inside of the turn.

Through the use of a differential we are able to apply power to both wheels while allowing one wheel to turn faster than the other. The TR6 uses what is called an “open” differential. There is nothing to resist the difference in rpm between the wheels. The torque on one wheel will always be the same as the torque on the other. As long as both wheels have enough traction to sustain the applied torque the result will be the same as the solid axle. However when one wheel loses traction and requires less torque to rotate, the torque applied to the other wheel will drop also. If one wheel lifts off the ground and spins freely, requiring virtually no torque, the engine cannot apply any torque the other wheel to move the car. A number of “limited slip” differentials have been designed to attempt to solve this problem.

The ideal purpose of a limited slip is to allow one wheel to turn slightly faster than the other to allow the car to steer but to not allow difference in RPM to exceed what is required for the car to steer. One design, called a “locker”, allows one wheel to spin faster than the other up to a certain difference in rpm when a device locks the two together. This may be acceptable in a full race application but it is a bit abrupt for street. Another type uses friction between clutch discs to resist difference in rpm. Tension on the discs can be internally adjusted but does not increase as the difference in rpm increases. Generally a setting that provides enough friction to be effective under full power will cause difficulty in steering. A “gear type” limited slip allows a difference in rpm when no torque is applied but will lock when torque is applied. This is good when accelerating in a straight line but will cause difficulty in steering during acceleration.

In my opinion, the “viscous limited slip” comes the closest to achieving the ideal purpose of a limited slip. It has a viscous coupling connecting the two axles together. This is a sealed unit with a series of discs encased in a viscous fluid. Alternate discs are connected to each axle. When both wheels are turning the same speed the entire viscous coupling rotates together and no action is taking place. One wheel can turn slightly faster than the other with moderate restriction allowing the car to steer. However if one wheel loses traction and the difference in speed starts to increase the restriction to rpm difference increases along with it. As the action in the viscous coupling increases, the fluid heats up and becomes more viscous causing even more restriction. This type of limited slip accomplishes both goals of allowing free steering and effectively applying power to the wheel with the traction.