Torsen LSD
Torsen stands for “torque sensing”. It is a purely mechanical LSD invented by the American company Gleason Corporation in the 1950s. Gleason has changed hands several times since then. The Torsen LSD is widely used in the center differential of four-wheel drive vehicles, most notably Audi’s quattro system. However, it is also used in many rear-wheel drive sports cars and even front-wheel drive hatchbacks.
The traditional Torsen LSD (now called Torsen A) uses 3 pairs of worm gears/worms to achieve the limited slip function. Their gear threads are specially shaped so that torque can be transferred from worm gear to worm gear, but not the other way around. Otherwise, they would be locked. It is this special feature that achieves the non-slip function.
In normal cornering, i.e. without tire slip at either wheel, the Torsen differential provides the same functionality as an open differential. The addition of a worm/gear pair does not affect the speed difference between the output shafts. The outer wheel’s half shaft turns slightly faster than the differential case, while the inner wheel’s half shaft turns slightly slower than the differential case. The speed difference between the left and right worm gears is exactly matched in the synchromesh gears. Note that the worm/gear pair does not lock, as torque is transferred from worm gear to worm gear.
When one wheel loses traction and slips, the differential tries to transfer more power to the slipping wheel. Now the worm/worm gear pair comes into play. The slipping wheel’s worm gear will drive its worm gear through the synchronizer, and drive the coupling worm gear. This worm gear tries to drive its matching worm gear, but as we said before, when a worm gear drives a worm gear, they are locked. As a result, the worm/worm gear pair is locked together so both wheels will spin at the same speed and get the car out of trouble. Since the Torsen is a permanent mechanical design, there is almost no delay in its response.
Torsen B
In recent years, many front-wheel drive and less powerful rear-wheel drive cars have switched to the Torsen B, a cheaper version of the original Torsen A. The Torsen B places the worm gear parallel to the worm gear. The construction is different, but the working principle is much the same. Its parallel gear design is said to be quieter in operation. The downside is that the Torsen B cannot transmit as much torque when locked as the Torsen A. From an engineering perspective, this means it has a lower torque bias ratio (TBR). No wonder most powerful rear-wheel drive cars continue to use the original Torsen A differential.
