How it Works:-
Torsen differential - Audi Quattro
Being the master
of 4-wheel drive, Audi always insists to use the most effective system regardless of the price. The Quattro 4WD system*
uses a pure mechanical LSD, Torsen differential.Torsen, means "torque-sensing".
It was invented by an American company calls Gleason Corporation. Its slip-limiting ability is implemented by cleverly
using worm gears / worm wheel pair. This pair has special characteristics: driving torque can be transfered from worm wheel
to worm gear, but not reversed. Otherwise, they will be locked up. It is such characteristic that it 'limits slip'.
A: Differential housing
B: Out axle
C: Worm wheel
D: Worm gears
F: Hypoid wheel (from engine)
G: Out axle
The above picture
explains how a Torsen differential works. In normal cornering, i.e., no tyre slip in any wheel, Torsen differential provides
the same function as a normal differential. The addition of worm wheel / worm gear pair does not affect speed difference between
output shafts. For instance, if the car turns left, the driveshaft to right wheel runs faster than the differential housing,
while the driveshaft to left wheel runs slower than the differential housing. The speed difference between left and right
worm wheels can be exactly matched in the synchromesh gears. Note that the worm gears / worm wheels pair do not lock up because
torque is transfered from worm wheels to worm gears.
When one of the wheels, say the right wheel,
loses traction due to poor road surface or whatever reason, the worm gear / worm wheel pair get into effect. At the instant
just before they become effective, one must know that by the basic differential theory no torque will be sent to the left
wheel, which is with traction. Instead, all the torque will be sent to the spinning right wheel. Then, the fast-rotating right
worm wheel will drive its worm gear, through the synchromesh and drive the left worm gear.
Now, you must still remember the basic characteristic
of worm gear / wheel pair. Well, when a worm gear drives worm wheel, they will be locked up. As a result, the left worm gear
and right worm gear are actually locked together, thus wheels on both sides will rotate at the same speed and get the car
out of the loss of traction.
Characteristic of Torsen-equipped
Except the first
generation Quattro system that appeared in the early Quattro coupe, most of the subsequent Quattro systems used Torsen differential
in center and rear axles. This is rather expensive.
4WD has many advantages. First of all, its pure mechanical parts react almost instantly to tyre slip. Secondly, it provides
linear lock-up characteristic. Thirdly, it is a strictly permanent 4WD system. In normal condition, torque split between front
and rear wheels is 50:50 (other ratios are possible, depends on the pitch of worm gears).
Apart from Audi, few other car makers adopted
Torsen LSD, mainly because of cost reason. Toyota's rally ace, Celica GT4 was one of the few exception. It used Torsen in
the rear axle. This might be part of the reason why it was so expensive over competitors.