Torsen (Audi Quattro)
Audi’s system that forms Quattro (the only exception to this rule is the TT, which uses Haldex, which is something entirely else) uses Torsen, which was developed by the Gleason company from the States. It’s a pretty clever, but pretty basic mechanism that’s proven to be on of the better AWD designs (hence Audi’s success). Torsen stands for “torque-sensing” and it is in essence a pure mechanical LSD-based device (if memory serves me correct).

Inside a Torsen differential is a pair of worm gears and a worm wheel which are attached to the drive axles on the rear wheels. Torque can be transferred from the worm wheel to the worm gears, but not the other way around which prevents the car from slipping. In a normal situation, power is distributed just like a normal axle, however when one wheel starts to slip (where it spins faster than the other) the gears lock up so that the wheels spin at the same time. Normally when wheels start slipping, torque is directed to the wheel that’s spinning – this prevents that, helping to restore control.

There are three main benefits to Torsen: because it’s all mechanical, it reacts instantly, so there’s no delay. Secondly, the characteristics of the differential’s lockup is linear, and thirdly is that it is a permanent AWD system. Usually Torsen cars are biased at 50:50 but other ratios can be applied (note how I said normally; Torsen cars vary when the wheels start to slip) depending on the angles of the worm gears. Although Audi isn’t the only manufacturer to use it though – the older Toyota Celica GT-Four used it and so does the Hummer. As does the VW Passat, I think.

Viscous Coupling Porsche (911), Lamborghini Diablo VT, Volvo 850 AWD
Another popular form of 4WD on road cars is delivered through a Viscous Coupling system. What a Viscous Coupling is, is… well, as the name says a place that’s sticky where things are coupled. In this case it’s the drive shafts for the front and rear of the car. Imagine if you will, a container with an input shaft, and an output shaft. Inside this box are circular plates positioned close to each other, with half the plates from the input shaft, and half from the output shaft. The box is filled with a viscous liquid and then sealed.

The idea of the system is that if there is any difference in the turning speeds between the two shafts, the viscous liquid will transfer the difference in speed limiting slip. Normally, when the car’s driving in stable conditions where there is no slip, the front and rear axles run at the same speed, however when there’s slip, power will be transmitted from the faster driveshaft to the slower driveshaft via the liquid.

Please note that this is NOT an AWD system, rather a part time 4WD system. In regular conditions where there is no slip, no torque will be sent to the opposite car. This has advantages and disadvantages. If you have a regular RWD car and add Viscous Coupling to it, it will act like a RWD car (which is good), until it starts to slip whereupon it will become 4WD (also good, which is why the Porsche 911 Carrera 4 is so successful). However, in an FWD car, it really serves no other purpose other than to provide traction as it cannot correct understeer. Also, because the system relies on a viscous fluid to pick up slack, the system doesn’t react as quick as Torsen or others.

Usually, you will find this sort of AWD on mass production cars as it’s really cheap. It’s kind of effective. Kind of.

Active Differential – Broad term. Read. (Benz 4Matic, Nissan Skyline GT-R, Porsche PSK
Active differential is probably the most complicated sort of AWD system available. You can get it simple or complicated, but it relies a lot on computer gadgetry to tell the car about tires lip, and other crap like that. (Not really crap, but … yeah. Some cars, such as Porsche 959’s sensors included the throttle, turbo boost, steering angle and g-force to determine when and what degree the car’s torque bias should be between front and rear wheels).

It all starts out with a multi-plate clutch (a key difference between it and most other AWD systems). It isn’t a part of the transmission; rather it’s something separate… a sort of a differential clutch. Within the clutch are several plates, depending on the complexity of the AWD system and other parameters such as the power of the car, or its purpose, and an actuator that uses hydraulic power.

We’ll use six plates as an example as it’s an easy number to work with. The driveshaft hooked up to the front wheels gets two plates, and the rear driveshaft gets two plates, while there are two friction plates which rotate freely of the front and back. The purpose of these two discs is to lock up, and transmit power. RWD example: lock up one plate, and a little bit of power goes to the front; 2 plates, a little more power; 3 plates, 4 plates… so on so forth until all plates are locked up, whereupon a 50:50 split is made. This is all decided upon by the computer. Also note that this split also goes on even in cases of NO SLIP so that traction on dry pavement is optimized.

If there is slip, it works just like any other AWD system, directing power to the axles that need it, thus getting keeping your **** right side up and out of the ditches.

With this sort of system on board, and modern technology, nearly anything can be sensed. I know that Nissan’s ATTESA E-TS PRO used in the Skyline GT-R (R34) had a system that monitored G-Force, Boost pressure, throttle position and the speed of the individual wheels (via an ABS sensor) to see whether or not the car was running within its cornering limits. This is what allows the R-34 GT-R to powerslide smoothly.

Benz has a similar system on their 4Matic cars. It’s probably the dumbest thing that I’ve ever heard of, simply because it really could have been replaced by something better. The thing is, it works only in part time, so in normal conditions, the clutches are disengaged making it RWD. However when the computer’s sensors detect slip in the rear wheels, the clutch plates engage progressively to transfer torque. Why’s it dumb? Because a Viscous coupling could do the same job and cost thousands less.

Haldex - Volkswagen 4Motion, Volvo AWD
So what is Haldex? Well, it’s an active differential, but it’s been simplified to be more reliable and less costly to manufacture. Developed by Swedish firm Haldex, and controlled by software from Steyr-Daimler-Puch, it became an immediate success for its many advantages and simplicity in the Volkswagen Group, eventually being taken up by Volvo.

The Haldex unit consists of a multi-plate clutch that sits in an oil bath. These clutch plates are actuated hydraulically and are used to distribute power equally to the front and rear end of the car in normal conditions, however if needs be up to 100% of the torque can be sent to either the front or rear axle.

Because of its less complicated drive and its less complex mechanics, it can be put in less expensive cars. It works just as quickly as an active differential or Torsen, so there’s no real disadvantage in terms of time. I know that it’s not as good at Torsen for dealing with slippery situations, but it’s a pretty good alternative considering the savings in weight.

Oh, and the other thing is that its torque-holding ability. We’re not how sure it goes, but it can go reasonably high, proved by the 236 lb-ft R32 Golf / Audi TT 3.2 DSG / Audi A3 3.2, and 258 lb-ft Volvo S60/V70 R