Different types of 4WD

1) Torsen differential - Audi Quattro system*

Being the master of 4-wheel drive, Audi always insists to use the most effective system despite regardless of price. Its Quattro 4WD system* uses a pure mechanical LSD, Torsen differential.

Torsen, means "torque-sensing", 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 a special characteristic: driving torque can be transfered from worm wheel to worm gear, but not reverse. Otherwise, they will be locked up. It is such characteristic that limit 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 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, 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.

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.

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.

Note: * the "Quattro" mentioned here is the traditional Torsen system marketing in the name Quattro. That includes all Quattro models until the arrival of Audi TT (which uses the Haldex system). Since then the name Quattro becomes a marketing trade mark rather than indicating the actual mechanism. At the time of writing, all Audi Quattro models, excluding the Golf-based A3, S3 and TT, still employs the traditional Torsen system.

2) Viscous-Coupling differential

Viscous Coupling center LSD is commonly used in many simple 4WD systems. One of the earliest examples was Volkswagen's Syncro system.

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.

Characteristic of Viscous Coupling center differential

Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.

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 effective 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.

3) Viscous Coupling Differential Lock

While Torsen 4WD is too expensive, Viscous-Coupling LSD is part-time only, most 4WD cars, including the rally ace Celica GT4, Subaru Impreza, Mitsubishi Lancer and Ford Escort RS Cosworth adopted another kind of center differential - basically it has a regular center differential which distribute torque to front and rear under normal condition, an additional Viscous Coupling Differential Lock provides anti-slip function when needed.

Characteristic of this system

The Viscous-Coupling Differential Lock is virtually the same as what we have learned earlier, therefore it also has slight delay and non-linear characteristic. However, in reality this might not be as serious as we thought, otherwise it would have been impossible that all the top rally cars use it. Moreover, Viscous-Coupling Differential Lock system is lighter and cheaper than Torsen system, while having superior effectiveness over the part-time Viscous-Coupling LSD system.

Advantage:	Good balance between price and effectiveness
Disadvantage:	No special flaw
Who use it ?	Lancia Delta Integrale (with Torsen in rear axle) Ford Escort RS Cosworth (with Viscous-Coupling LSD in rear axle) Mitsubishi Lancer GSR, 3000 GT VR4. (with Viscous-Coupling LSD in rear axle) Subaru Impreza and Legacy manual versions (with Viscous-Coupling LSD in rear axle) Toyota Celica GT4 (with Torsen in rear axle) Bugatti EB110 (set to 70% rear bias)

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.

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.