A Torsen limited-slip differential

A Torsen ("TORque SENsing") limited-slip differential isn't technically a limited slip differential. It's to call it just a Torsen differential or a helical differential.

The Torsen differential* is a purely mechanical device; it has no electronics, clutches or viscous fluids.

­The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.

These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs.

However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero.



Hummer!

The HMMWV, or Hummer, uses Torsen® differentials on the front and rear axles. The owner's manual for the Hummer proposes a novel solution to the problem of one wheel coming off the ground: Apply the brakes. By applying the brakes, torque is applied to the wheel that is in the air, and then five times that torque can go to the wheel with good traction.