Four wheel drivetrains used in rallying

Have you ever wondered why all recent FIA World Rally Championship since 1984 have been won by full time 4 wheel drive turbo charged cars? There must be something special about these vehicles that makes them unbeatable in world rallying. Well there is.

Before going further the reader should be warned on some manufacturer’s marketing policies. Not all cars advertised as 4 wheel drive are really 4WD. Volkswagen and Audi sell cars advertised as 4WD which, in fact, are not full time 4WD. Cars such as the VW Golf Synchro, 4Motion and Audi A3, S3, TT as well as the Seat Leon (all of which share the same platform and engines) have engine torque sent to the rear wheels only in case front wheel spin is detected. The above mentioned cars use a Haldex type clutch center differential which is only activated when the front wheels spin and always deactivated when braking. Why? Because it’s cheaper to manufacture as fitting electronics to a mass produced car is much cheaper than fitting additional mechanical parts. Note that higher class vehicles from the same manufacturer use “real” AWD drivetrains. If you’re in the market for a full time 4WD car you might want to avoid such models and go for the real thing.
Worst still are implementations using 3 real open differentials (hence are full time 4WD) to which manufacturers add “differential self-locking abilities” by individually braking the wheel that spins (read BMW X5 using a system called ADB-X, Audi Allroad using the company’s EDS along with a TorSen center differential and Mercedes-Benz using a similar electronic system). Fitting fancy electronics to a car is, as above, cheaper than using real self-locking or limited slip differentials. Additionally electronic devices prohibit any sporty driving by overriding driver incompetence, are far from being efficient and make a car handle “safely”, thus covering the manufacturer against potential law suits. You have been warned.

There have been many attempts to build full time 4WD cars for everyday use, cars that are neither trucks nor all-around vehicles but have a rather sporty character. The first we know of, the Jensen FF back in 1966 (only 280 cars where made), not only had a full time 4WD drivetrain but also antilock brakes! This car was a total commercial failure. Latter attempts were more successful.

The advantages of full time 4WD are straightforward. Since a car has 4 wheels why should power only be applied to 2 of them? Applying power to all 4 wheels not only distributes engine torque (thus avoiding wheel spin under heavy load) but also allows a car to handle more precisely. Why aren’t all cars made that way you might ask. Well, like always, it’s a question of price. 4WD drivetrains are costlier to implement than 2WD ones are. For instance one must use three differentials in a full time 4WD car (although you can get by with only two like in the Citroën BX 4×4 GroupB rally car which never saw the light of day). One differential between each opposing wheel and one between the front and rear axles. On a 2WD car, only one differential between the driven wheels is used. With the price argument cleared, car manufacturers reserve this kind of vehicle to niche and specialized markets. In a rally car engine power is useless if traction is insufficient. Naturally all major rally cars are 4WD nowadays. Not so long ago FIA forced all manufacturers to produce 2500 cars in order to get the necessary homologation so that they could race in the World Rally Championship. This enabled people like myself to get our hands on some of these very special homologation cars. Cars that were, in fact, made solely for racing purposes but had the looks of everyday sedans (well almost…), the famous homologation specials.

A full time 4WD performance car, as mentioned earlier, needs 3 differentials in order to operate properly. A differential is basically a mechanical, gear-based, device that allows engine-driven wheels to rotate at different speeds while still being driven by the same power source. In most instances the differential splits engine torque evenly between the driven wheels. Differentials are used on the axles that hold the wheels that are driven by the engine and, in the case of a 4WD car, between axles. If no differential is present then the driven wheels would spin at the same speed in any circumstances thus rendering the handling of the car very unpleasant. The differential’s inherent ability to allow driven wheels to rotate at different speeds has a drawback. In the case one wheel spins a lot faster than the other a classic differential will transfer all engine torque to the faster spinning wheel thus depriving the most adherent wheel of any torque and consequently traction. Self-locking differentials (a.k.a. Limited Slip Differentials) address this problem by adding to the classic, free  or open differential, described above the ability to lock (drive both wheels at the same speed i.e. simulate the differential’s absence) under certain conditions such as when wheel spin occurs. For instance by locking itself, the differential, allows to avoid the immobilization of the vehicle in situations such as when one wheel sits on snow while the other sits on dry tarmac. In this case, the absence of a locking device would send all engine torque to the wheel that spins faster (the one on the snow) and the car would not be able to extract itself. Locking the differential would split torque distribution on both wheels thus allowing the car to move forward.

All attempts to build 4WD cars (with the exception of the Citroën BX 4×4 GroupB car which only made it to three races in its career and had no central differential) involved 3 differentials. Now this is where things get a bit more complex. The differentials in these cars and their self locking or slip limiting abilities make all the difference. Their type and settings can make a car handle exceptionally well or incredibly bad. Comparably to a 2WD car, where if a wheel spins the engine tends to send all its power to that wheel, thus immobilizing the car, in a 4WD car the same one wheel spinning would also draw all engine power and immobilize the car. To avoid this phenomenon most 4WD vehicles use differential locking techniques.

Most implementations use the classic Ferguson layout  which consists of 3 differentials 2 of which, the central and rear, are coupled to the wheels they drive through “free” differentials which use viscous couplers as locking devices. A viscous coupler can be seen as a tube containing a pressurized viscous fluid in which discs are rotating. Half of the discs are attached to the incoming axle while the other half to the outgoing one to the tube’s walls. Each pair of discs faces each other i.e. a disc attached to the incoming axle faces its counterpart attached to the outgoing axle. The discs are pierced and the viscous fluid completely surrounds them. Minor speed differences are allowed between discs. Increased slip (i.e. rotational speed difference between discs) leads to a rapid increase in the viscosity of the fluid which, in turn, locks up the coupling.

Schematic view of a Viscous Coupler

View of a Viscous Coupler

Viscous Coupler mounted on a differential
A typical viscous coupler (right part of the picture) acting as the locking device aside a center differential

Viscous couplers are convenient devices mainly because they are not very expensive and do not require extensive maintenance. Their major drawbacks are:

  • An exponential increase of their locking to speed difference curve (their are not very progressive)
  • A delay in their locking ability induced by the time the viscous fluid needs to increase its viscosity
  • Are difficult to handle under braking (they lock in braking situations)
  • The close relation of their locking abilities to that of the viscous fluid temperature (its viscosity decreases as temperature increases)

Some of the more known examples of this 4WD layout are:

Of course the efficiency of the Ferguson layout greatly depends on the characteristics of the viscous couplers (type of viscous fluid, design and spacing of the discs, fluid pressure, etc). Additionally consider that there are mainly two types of viscous couplers, the “cheap” ones and the more expensive ones. The “cheap” viscous coupler has one part of its discs fixed on the differential’s housing while the other on the outgoing axle. These devices cost roughly half the price of a “normal” viscous coupler which has half its discs fixed on the incoming axle and the other half on the outgoing one. The “cheap” versions have a slip limiting characteristic which varies with the square of the axle speed difference while the “normal” ones have a slip limiting characteristic that varies more linearly with the axle speed difference.

A far more efficient (and expensive) 4WD layout is the one involving a TorSen ( which stands for TORque SENsing) differential. This extraordinary device, invented by the American Gleasman (patent pending, 1958) and manufactured by the Gleason corporation, is based on the non-reversibility of worm gears and worm wheels (i.e. when you turn the worm wheel the worm gear turns but not vice versa). The TorSen is the only mechanism which acts like a differential and locking device at the same time. It has the advantage of being fully mechanical which guarantees its instantaneous response and progressiveness. Its main advantages therefore resume to:

  • Instantaneous response
  • The linear character of its locking to speed difference curve (smoothness)
  • No locking or speed difference inhibition under braking (it acts only when power is applied to it)
  • Integrates a “free” differential and a locking device in one part
  • Its compactness, the TorSen has only 8 moving parts
  • No wear as opposed to more traditional self-locking differentials based on friction plates

TorSen differential
The Torsen differential

    • A: Differential housing
    • B: Out axle
    • C: Worm wheel
    • D: Worm gears
    • E: Synchromeshes
    • F: Hypoid wheel (from engine)
    • G: Out axle


There are mainly four drawbacks in Torsen differentials:

  • They are expensive devices
  • They tend to generate more heat under heavy use than open  or other types of Limited Slip Differentials differential which of course means more power losses as some of the kinetic energy is transformed into heat rather than transmitted to the wheels
  • Due to its inherent design if one wheel is completely off the ground or completely loses traction the TorSen will act as an open differential
  • Is extremely difficult to assemble. Although it only holds 8 moving parts there is only one way to fit them together

The TorSen splits torque in a 50:50 proportion in no-slip conditions and can manage torque distribution differences of up to 20:80 ratios between the wheels it drives. Some examples of commercial TorSen differential applications are:

  • The Audi Quattro Turbo (the earliest series used a manually locking rear differential and a TorSen center differential)
  • The Honda Integra Type R (front wheel drive)

The most important difference between TorSen differentials and viscous couplers is that the TorSen has a torque sensing characteristic while the VC has a rotation sensing characteristic. That’s why TorSen differentials only lock when power is applied to them whereas viscous couplers lock both when power is applied and while braking.

More information on the TorSen differential and its derivatives can be found here.

All 4WD implementations using three differentials two of which are self-locking, usually the center and rear, are able to send engine torque to at least one wheel that has more traction than the others. Implementations using three self-locking differentials, such as the Mitsubishi Lancer Evolutions and some Subaru Impreza WRX STI, are able to send torque to at least two wheels with better traction than the others.

There are many details, technicalities and other types of self-locking differentials we could cover here but they would probably render the information unusable. We will therefore not go into more detail (unless you suggest otherwise).

When you add a turbo charged, fire-spitting engine to a 4WD car the mixture becomes explosive. To sum up the situation, high performance full time 4WD turbo charged vehicles have automatic differential locking and slip control, high output turbo engines and exceptional road holding abilities and performance. Read on…