was invented by an American company calls Gleason Corporation. Its
ability is implemented by cleverly using worm gears / worm wheel pair.
This pair has a special characteristic: driving torque can be
from worm wheel to worm gear, but not reverse. Otherwise, they will be
locked up. It is such characteristic that limit slip.
B: Out axle
C: Worm wheel
D: Worm gears
F: Hypoid wheel (from engine)
G: Out axle
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, do you still remember the basic characteristic of worm gear / wheel pair ? Well, when 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 side will rotate at the same speed and get the car out of the lose of traction.
Apart from Audi, few
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
rear axle. This might be part of the reason why it was so expensive
|Advantage:||Quick response, permanent 4WD|
|Disadvantage:||Pricey, torque split not variable|
|Who use it ?||All non-Golf-based Audi quattro models, Toyota Celica GT4, Hummer etc.|
Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together.
In normal condition, front and rear axles run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyre slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented.
Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.
The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero.
To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.
It might be less
than Torsen system, but it is certainly the cheapest, so we can find it
in many mass production 4WD cars.
|Advantage:||Cheap and compact|
|Disadvantage:||Part-time 4WD only. Normally feels like 2WD.|
|Who use it ?||VW Syncro, Lamborghini Diablo VT, Porsche 993/996 Carrera 4 and Turbo, Volvo 850 AWD etc.|
|Advantage:||Good balance between price and effectiveness|
|Disadvantage:||No special flaw|
|Who use it ?||