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Torsen Differential (Chassis Sys 3)

This Topic is intended for Systems 3 Students yet it is of interest to earlier levels.

How it Works:- 

Torsen differential - Audi Quattro system*

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  
E: Synchromeshes  
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 4WD

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.

However, Torsen-equipped 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.


Quick response, permanent 4WD


Pricey, torque split not variable

Who use it ?

All non-Golf-based Audi quattro models, Toyota Celica GT4, Hummer etc.

Another Topic will be published soon.

More interesting topics will be added in the next up-date.