Viscous Couplers

Ever wondered why all recent FIA World Rally Championships (since 1982) 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 I would like to warn you 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, Motion and Audi A3 and TT (which use 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. If you're in the market for a full time 4WD car you might want to avoid such models.

There have been many attempts to build full time 4WD cars for everyday use, cars that are neither trucks nor all-around vehicles. The first I know of, the Jensen FF back in 1966 (only 280 cars where made), not only had a full time 4WD drive train but also antilock brakes! This car was a total commercial failure. The advantages of full time 4WD are quite clear. Since a car has 4 wheels why should power be applied to only 2 of them? Applying power to all 4 wheels not only distributes engine torque (thus avoiding wheel spin) 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 cost much more to implement than do 2WD ones. 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 4x4 GroupB rally car which never saw the light of day). One between each opposing wheel and one between the front and rear axles. For a 2WD car, one differential between the driven wheels is enough. With the price argument cleared, car manufacturers reserve this kind of vehicle to niche and specialized markets. In rallying power is nothing without traction. Naturally all major rally cars are 4WD nowadays. Not a long time ago FIA forced all manufacturers to produce 2500 cars in order to get the necessary homologation so that they could race in World rally. 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 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 device that allows the wheel which sits on the outside (longer radius) of a road bend to spin faster than the wheel on the inside (shorter bend radius). These devices 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 a turn thus rendering the handling of the car very unpleasant. Self-locking differentials add to the classic "free" differential the ability to lock (drive both wheels at the same speed) 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 a 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 4x4 GroupB car which never made it to racing and had no central differential) involved 3 differentials. Now this is where things get complicated. The differentials in these cars and their self locking 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 torque to that wheel, thus immobilizing the car, in a 4WD car the same one wheel spinning would also draw all engine torque 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 viscous couplers. 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 and the other half to the outgoing one. The discs are pierced and the viscous fluid completely surrounds them. Minor speed differences are allowed between the two axles but increased slip leads to a rapid increase in the viscosity of the fluid which then locks up the coupling.

Viscous coupler

A Viscous Coupler

A typical viscous coupler (left part of the picture) mounted aside the rear differential Mitsubishi Lancer GSR EvolutionV)

Viscous couplers are convenient devices mainly because they are not very expensive. Their major drawbacks are:

Of course the efficiency of the Fergusson layout greatly depends on the characteristics of the viscous couplers (type of viscous fluid, design and spacing of the discs, 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 locking characteristic which varies with the square of the axle speed difference while the "normal" ones have a locking characteristic that varies more linearly with the axle speed difference.

A 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 company 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 differential has the advantage of being a fully mechanical device which guarantees its instantaneous response and progressiveness. Its main advantages therefore resume to:

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

Torsen differentials are expensive devices. They split torque in a 50:50 proportion in no-slip conditions and can manage slips up to 20:80 ratios between the wheels they drive. The main examples of Torsen differential applications are:

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.

There are so many details and technical stuff I could mention here that the information would probably be rendered unusable. I will therefore go into no 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 in those high performance full time 4WD turbo charged devils you have automatic differential locking and slip control, high output turbo engines exceptional road holding abilities and performance.

There are a number of problem areas with these components some of which are due to design and others to torque increases as a result of higher turbocharger boost pressures. Starting at the gearbox end, the first problem occurs with the spindles of the epicycle gears which often work loose due to being rather crudely staked in position. There is a series of ventilation holes drilled in the cover plate which precisely lines up with the axis of these spindles with the result that they are allowed to snag on these holes. We have come across instances where a spindle has been broken off with catastrophic results. There is a serious problem before this grenade effect though - when the spindles work loose, the epicyclic gears become misaligned resulting in end thrust forces which effectively turn the gears into milling cutters and they start to machine away the housing. The metallic swarf is carried around the gearbox and the only proper course of action is a complete strip down and clean of the gearbox components. Whenever we replace
a clutch, we now inspect the epicyclic splitter and make sure that the offending spindles will not move by TIG welding their ends. The situation has been improved on Evoluzione by a redesign of the ventilation holes in the cover plate and improved staking of the gear spindles.
The next problem area is associated with the pre-load on the differential and pinion bearings. When any torque is applied to the drive unit, the tendency is for the crown wheel and pinion to be forced into or out of mesh by the sliding contact. The amount of pre-load on the bearings determines how much torque can be transmitted without allowing end float which would cause the meshing of the gears to become incorrect. In theory, when the unit is first assembled, the pre-load on the bearings will be sufficient to handle more torque than the engine will produce. What happens in practice though is rather different. The nut retaining the rear drive output flange is prone to working loose because it is almost impossible to stake it properly. This allows the pre-load on the pinion bearings to be reduced so that the correct mesh of the gears is disrupted - result noise, followed by knackered diff. It is not sufficient just to re-tighten the loose nut as there is no way of knowing that the crown wheel and pinion are properly in mesh, or that there is correct pre-load on the pinion bearings. Equally, anyone who says that the oil seal on the output flange can be changed simply by undoing the nut and removing the flange is talking out of their proverbial orifice! When the differential is expected to transmit greatly increased torque as in situations where the engine has been 'chipped' these problems are bound to occur sooner if not immediately! To enable the use of more torque, the differential should be assembled to take account of this - such as increasing the pre-load on the bearings and the use of special lubricants.