4x4 Explained

Introduction to All Wheel Drive systems - By Eliot Lim

Differential Locking

This is a core design issue in all wheel drive technology because these have a profound effect on the cars' road behavior. Consider the case of the simplest all wheel drive car with 3 "free" diffs. The car can be rendered immobile if any one of the four wheels lose traction because basic differentials equalize the torque output. A simplistic way to look at this is to think that the basic "free" differential sends power to the axle with the least grip, so if one wheel loses grip, all the power is sent there, leaving nothing for the remaining three. In reality, the differentials are equalizing the power distribution, so everything is equalized to zero in this case. Remember that a four wheel drive vehicle has twice as many wheels as a two wheel drive to lose grip and mobility on. And since four wheel drive vehicles would tend to be used more in bad conditions, it is quite important to have some form of differential locking. Every full-time four wheel drive car on the market today has some form of diff locking. A good way to understand this concept is to trace the evolution of the very early systems to the state of the art.

Audi was the first manufacturer to successfully sell high-performance, permanent four wheel drive with the quattro, released in Europe in 1981 and in the US in 1983. (The car goes by the more popular name turbo quattro coupe in the US and more recently, the Ur Quattro around the world). The cars were very successful in rallying , winning several world titles and it set the automotive world ablaze because four wheel drive was never previously associated with ultra high performance. Even though the 1966 Jensen FF was the first vehicle to have full time four wheel drive (and also anti lock brakes) the car was a commercial failure and it was left to Audi to break through the public consciousness and go into the history books for launching the full time four wheel drive revolution.

During the 1980s Audi decided to spin off four wheel drive and the quattro name to its entire range of cars. The first generation quattros had simple locks for the center and rear diffs, which locked one or both of them solid (no speed difference) to dig one out of deep trouble. When the center diff was locked, it meant that one had to lose grip on one rear and one front wheel to become immobile. When both the center and rear diffs were locked, one had to lose both rear wheels and one front wheel to get stuck. The locks on these Audis were manually engaged and were quite cumbersome since the driver already had to worry about shifting and steering in addition to this. Audi found that many drivers forgot to disengage the locks once they got going again.

Thus development went in the direction of automatically locking differentials. First on the scene was the viscous coupling (VC for short) which used a silicone liquid in a casing designed so that minor speed differences were allowed between the two axles but increased slip would lead to a rapid increase in the viscosity of the fluid which would then lock up the coupling. The viscous coupling was used in two radically different ways:

Some manufacturers used regular differentials in conjunction with the VC where the VC functioned as a diff lock that acted automatically when conditions needed it. The current Mitsubishi Eclipse GSX and current manual transmission all wheel drive Subarus use this scheme. The departed BMW 325ix and Toyota Celica turbo all-trac also used it.

Audi, during the development of the original quattro, also played with VCs and came up with a completely different way of using a VC. In this implementation, the VC is used as the center diff, resulting in a part-time, automatically engaging four wheel drive system. In this implementation, the car is basically front wheel drive, with the rear wheels coasting along and minor speed differences absorbed by the VC when the vehicle was turning. When front wheelspin occurred, the speed difference would increase to the point where the VC with its viscous liquid churning would start transferring some of the torque from the front to the rear wheels and thus the vehicle would become four wheel drive. Note the difference between this system and the former. The latter is auto-engaging part time four wheel drive, while the former is full-time auto-diff-locking four wheel drive.

The part time automatically engaging system was never put into production by Audi but was instead spun off to VW, which did put it to market as the syncro system. The simplicity of this implementation has drawn a very wide range of manufacturers to use it as well, from all the minivan implementations, many of the newer SUVs to exotics like the current Porsche 911 turbo and Carrera 4 and the Lamborghini Diablo VT (these have permanent drive on the rear wheels, of course). Volvo is a new player in this field and its latest all wheel drive offerings also use this scheme, with an unusual cocktail of limited slip devices thrown in, namely a traction control system at the front and a regular mechanical limited diff in the rear. Several magazines have found this system to be in need of further refinement.

Next came the torsen (stands for TORque SENsing) differential, which was embraced by Audi in its second generation quattro system. Audi was approached by FF development (owners of the VC patent) during development of the original quattro back in the late 1970's but the VC was rejected for reasons that will become clear shortly. The torsen diff was invented by an American company (Gleason corp.) and had all the advantages of the VC and none of its disadvantages. It is a fully mechanical device of worm gears and a worm wheel whose workings are quite difficult to describe with words and probably beyond the scope of this article. However, the torsen's characteristics is the issue that is of interest here. The torsen differential will split torque 50:50 in a no-slip condition. However, when one axle slips, the torsen diff will send more torque to the axle with more grip, in other words, it works in an exactly opposite way to a conventional diff. Torque splits of up to 80:20 are available, depending on the pitch of the worm gears. And since it is a completely mechanical device, the locking action is instantaneous and progressive as opposed to the VC, which has a very slight lag for the viscous fluid to heat up and suddenly lock. The torque sensing characteristics of the torsen also allows it to be proactive in preventing wheel spin rather than reactive, in correcting a wheel spin situation. The torsen diff is thus "more sensitive" to slip than the VC. Its locking action is also more progressive. (Porsche also rejected the VC in the 964 Carrera 4 because they felt that the VC was too difficult to control and that it had exponential rather than linear locking characteristics.)

More importantly, the torsen does not lock or inhibit speed differences under braking, thus allowing all 4 wheels to rotate independently at their own speeds when no power is applied. The torsen diff only locks in a power application situation while the VC locks both during acceleration and braking. The torsen has a torque sensing characteristic while the VC has a rotational sensing characteristic.

The VC's rotational sensing characteristic initially caused lots of problems for the engineers. Anti lock braking systems rely almost entirely on speed differences between the 4 wheels to detect a locking wheel. Thus, when the transmission tries to force 4 wheels to rotate at the same speed, it creates serious difficulties for the ABS system.

The engineers had to use a variety of hacks to get around this problem. Mitsubishi delayed ABS for a while for its first generation GSX, then finally decided to make ABS and rear VC limited slip mutually exclusive options. The VW syncro system simply disconnected four wheel drive the moment the brake pedal was stepped on via a secondary clutch. Most other vehicles using this implementation of VC have a very similar disengage feature. The very successful World Rally Championship Lancia Delta Integrale even went as far as to apply a little bit of power (via the engine computer) to reduce the drag of the VC when the brakes were applied! Some very crude systems used a overrun device that is conceptually similar to the bicycle crank. This meant that while four wheel drive was disengaged during braking it was also inoperative when reverse was engaged!

The easiest hack was to reduce the effective viscosity of the fluid in the coupling, so that the drag was reduced. This also meant that the VC's locking effectiveness was reduced, which is probably quite acceptable for a vehicle intended primarily for paved roads. The VC's attraction is its simplicity and cheapness, not its sophistication.

In the late 1980s Porsche and Mercedes were treading slowly and came out with all wheel drive vehicles of unparalleled complexity. Mercedes' 4Matic system used the ABS sensors to determine wheelspin. In the dry, the Benz was a rear wheel drive car. When the wheel sensors determined that the rear wheels were spinning, a signal was sent to the computer to start engaging a hydraulically actuated multi plate clutch to send power to the front wheels. Clutch engagement was progressively altered by the computer. When the computer determined that even more traction was needed, a second clutch would start locking the rear diff. When the brake pedal was pushed, both clutches disengaged instantly to allow ABS to work without interference.

The Mercedes 4Matic was a part time, automatically engaging four wheel drive system. The reason given by Mercedes why they went to great pains to design a part-time four wheel drive was that they did not want to upset their loyal clientele with a full-time four wheel drive, which because of the driven front wheels, would "change the traditional feel of a Mercedes". One could also speculate that they were too proud to use anything less complicated than Audi, which in the marketplace is considered "lower". In practice, the 4Matic system worked no better and no worse than the other crop of full-time four wheel drives, but its cost and complexity made it look bad. This original 4Matic system has been ditched and the latest 4WD Mercedes is now a full time system, including the system to be used in the "M" class SUV. The Nissan Skyline GTR uses a system that is conceptually similar to the original 4Matic.

Porsche used a similar system of locking clutches (though they are implemented quite differently) as the Mercedes in the limited production, state of the art 959, but the center diff (which is actually just a hydraulic clutch) was engaged at all times except when parking so that the steering would be easier to turn. Torque split in the 959 varied with load and conditions. (via the progressive locking of the clutch). Unlike all other implementations of all wheel drive, the 959's torque split varied under no slip conditions. i.e. In every other all wheel drive system, the split is fixed until slip occurred, after which the various limited slip devices would begin to alter the split. In the 959, the all wheel drive computer is fed information from many sources, including the throttle position, steering angle, g force accelerometers and even the turbo boost gauge. In a straight line, under maximum acceleration, the system will send up to 80% of the power (from a normal 40 front/60 rear split) to the rear wheels, even if all 4 wheels are turning at exactly the same speed. This was by far the most complex and sophisticated all wheel drive system ever built.

The 959 was followed by the 964 which was first introduced in 1989 as the 911 Carerra 4. Porsche claimed that this was an evolution of the system used in the 959 and is even more advanced. However, this was a fixed split system like all the others, with computer controlled clutches acting as limited slip devices. The 964's trump card, however was that the speed sensors and accelerometers were used with the computer controlled locking rear differential to cure the 911's natural tendency to oversteer if the throttle was suddenly lifted off in a turn. The rear diff would start locking when the computer detected that oversteer was imminent. A locked rear diff would induce understeer, which in turn countered the oversteer. Through the use of all wheel drive and smart differentials, Porsche was able to tame a formerly unruly beast into a much more docile animal. This, according to their chief engineer was their main reason for implementing all wheel drive in the 911, as the 911 with its rear biased weight distribution is not in a real need of extra traction.

In 1993 Porsche updated the 911 with a brand new rear suspension. Even the rear wheel drive version was tamed and thus the justification of using a highly complex computer controlled all wheel drive system disappeared. The four wheel drive version of this 911 (alias the 993), has a much simpler, lighter and cheaper part time automatically engaging VC system such as those found in the Golf syncro and most minivans. However, the smart rear differential that fought the deadly oversteer was retained to quell any remaining tendency to oversteer. The new watercooled 911 (aka 996) C4 uses essentially the same system as the 993 C4, but with additional computer controlled stability assistance tweaks. It is somewhat disappointing to see that Porsche, once the clear technological leader in this field retaining the viscous coupling in its latest AWD offering while many other new AWD offerings such as the new VW Golf 4Motion and 1999 Jeep Grand Cherokee resort to more sophisticated designs.

Subaru deserves mention here because in the automatic version of the Legacy and Impreza (including the Outback variants), it uses a computer controlled system much like those found in the Mercedes 4Matic, automatic Audi A8/V8 and the earlier Porsches. Subaru has been offering this sophisticated system for a long time in a relatively inexpensive car. Much more recently other makers have started offering conceptually similar systems. The Honda CR-V, the 1999 VW Golf 4Motion and its derivatives such as the Audi TT now use a system that is conceptually similar.

The Audi A8 (as well as the automatic version of the Audi V8) also used a computer controlled clutch to lock the center differential, in a manner similar to the systems just described. The automatic transmission supplied a ready source of hydraulic pressure to lock a pack of clutches, so it was tapped. This system represented Audi's first successful mating of automatic transmission with quattro all wheel drive. Current quattro models with automatic transmission use a center torsen differential with the exception of the A8.

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