Third major revision: March 2 1997
Last update: August 8 1997

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Introduction

This article was originally written in the fall of 1992. Back then as is now, there was a general lack of good information regarding all wheel drive vehicles and how they differ from traditional four wheel drive vehicles. I have updated the original article to mention current vehicles.

Definitions

It is important to get the definitions down first, since for any four wheeled vehicle, all wheel drive and four wheel drive literally mean the same thing. Generally speaking "all wheel drive" implies permanently engaged or automatically engaging four wheel drive and "four wheel drive" implies manually engaging, part time four wheel drive. The auto industry usually abide by these definitions but not in all cases. The now departed "all wheel drive" Ford Tempo and Subaru Justy were really part time manually engaging systems, like the older Subaru GLs. The term on demand four wheel drive means that it is a part time manually engaging system.

The automotive media shares a lot of the responsibility for the confusion. Factual errors are common, so is the careless use of the two terms interchangebly.

For this article I shall be using the terms loosely and will be more specific where necessary.

Differentials

A differential is a mechanical set of gears which takes input torque from a driveshaft and splits it evenly to two output axles, allowing them to rotate at different speeds. A differential in a front wheel drive or rear wheel drive car allows both wheels to apply power to the road and yet be able to rotate at different speeds so that the car can turn without resistance.

A permanently engaged four wheel drive system needs to have three differentials to enable it to apply power to four wheels and be able to turn without resistance: The front, rear and center diffs. (diff = short for differential) This is because the distance traveled by the turning front wheels is not the same as the distance traveled by the non articulating rear wheels.

Power leaving the gearbox first goes to the center diff, which then splits it via the driveshafts to the front and rear diffs. Manually engaging part time four wheel drive systems in most cases do not have a center diff, so they cannot be used in the dry. When four wheel drive is engaged in such a system, the front and rear axles are locked together and will rotate at exactly the same speeds. The difference in front and rear wheel speeds have to be scrubbed off by the tires.

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 special kind of 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. This 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 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)

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 78:22 are available. 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 "catch up". 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 World Rally Championship killer 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!

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 turn a bengal tiger into a pussycat. This 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.

Subaru deserves special 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. That much sophistication in a sub $30K car is truly impressive and by all accounts it works very well too.

The automatic version of the Audi V8 (and A8) 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.

Traction control

For all its technological glitz in the 1980s, all wheel drive cars eventually turned out to be a commercial failure with the exception of the niche brands like Audi and Subaru. In the late 1980s there were all wheel drive offerings from every major manufacturer as it was the latest fad. Many of these manufacturers have since dropped their all wheel drive models in favor of highly profitable truck based sport utility vehicles or SUVs in short. For the cars a more simple and inexpensive alternative was there waiting to be exploited.

Anti-lock braking systems have speed sensors on two to four wheels to detect differences in speed between wheels so that the computer could intervene and "pump" the brake on the locking wheel. By a few simple extensions to the system, it could be made to brake a spinning wheel, thus effectively transferring power to the one with grip. More sophisticated systems would reduce engine power to further slow the wheelspin, but generally speaking, traction control is merely an optimization of two wheel drive using ABS technology.

Current versions of Audi quattros (dubbed quattro IV) use all wheel drive in conjunction with 4 wheel traction control. Under no slip conditions, power is delivered 50-50 to the front and rear via a center torsen differential, which would take care of limiting slip between the axles. The traction control system would take care of limiting slip between each wheel of a given axle. Thus for the first time, the quattros have to lose traction on all 4 wheels before they become immobile.

The prior generation of quattros had center torsen differentials (except A8/V8) and manually lockable rear diffs which locked it solid. This featured automatic unlock at speeds exceeding 15 mph to aid the forgetful driver. The V8 quattro had a torsen diff in the rear and either a computer controlled clutch in the center (for automatic transmission) or a torsen (manual transmission).

Consumer considerations

Many potential buyers of all wheel drive cars wonder if the extra "stuff" would mean more problems or if the system would lead to heavy penalties in fuel consumption. Real world experience has shown that all wheel drive systems are not known for any kind of teething problems. The probability of an extra set of driveshafts failing has turned out to be as probable as a V8 engine failing because it has double the number of cylinders over an inline 4. This is a good analogy because with the power split over more wheels, the drivetrain is less stressed.

Those implementations that rely on ABS wheel sensors to lock differentials would be as likely to suffer from problems as any car with anti lock brakes. i.e. no greater than average.

In fact, many of the suspicions of all wheel drive come from the world of manually engaging part time systems where attempts were made to make four wheel drive engagement less cumbersome, with features such as automatically locking hubs and/or "shift on the fly four wheel drive". An all wheel drive system is always engaged and is actually simpler because it eliminates the need of these convenience "features" and their associated parts, which are the usual source of problems.

Accusations that four wheel drive wastes a lot of gas is only applicable to part time manually engaging systems. A full time system with a center diff has none of the tire scrubbing waste of the former. Furthermore, research by Audi showed that as tractive loads built up, the tire losses of two wheel drive exceeded the losses caused by the extra weight and inertia of a full time four wheel drive system. Tire losses were found to rise disproportionately with load. Consider the extreme case of the "burnout" or wheelspin scenario, where 100% of the tractive energy is converted to burning rubber rather than propelling the vehicle.

Part time, manually engaging versus full time/part time auto engaging

Part time, manually engaging four wheel drive systems also make it extremely difficult to have a decent suspension set up. For cars with front-wheel steering the front wheels have to travel a greater distance than the rears in a turn. Because there is no center differential, the rears would therefore have to scrub its excess speed and in doing so, lose some adhesion for cornering. With less grip in the rear, the vehicle becomes oversteery which to an average driver does not constitute safe behavior. The result is that a lot of positive camber is applied to the front, making the front wheels resemble an upright "V". The effect is that the front wheels now have a smaller contact patch and thus less grip in a turn. Remember that all this tweaking is to make sure that the vehicle is semi neutral in four wheel drive mode. When four wheel drive is not engaged, which is typically the majority of the time, one is left with a hopelessly understeery vehicle, because the front still assumes a rear end that has to scrub off excess speed. Anti lock braking, if offered will also be inoperative in four wheel drive mode, just when it is needed the most.

It does not take much to see that this is a very suboptimal implementation compared to all wheel drive systems, which are on the other extreme of being able to dynamically alter the division of power to each axle depending on which end is sliding. The behavior of a full time or part time auto engaging system is completely predictable and is therefore optimizable to dramatic effect.

Average consumers also tend to dismiss the need for good handling. The line "I am not going to race this vehicle" is repeated often enough. However, even if we were to judge vehicles solely as appliances, good handling does enter the equation. A good handling vehicle, such as the many excellent all wheel drive examples mentioned, will hide the difficulty of negotiating a turn, making it seem more effortless. The average driver would then feel more comfortable and confident and will therefore shed less speed entering a curve, leading to less momentum being lost, which in turn means that the vehicle does not have to consume energy reaccelerating back to its original speed. In other words, it would be a more energy efficient appliance. This point is hardly ever raised when discussing the appliance value of vehicles.

It is unfortunate that old fashioned part time, manually engaging systems are still being sold on many SUVs today with high prices that are a match for their mediocrity. There is no reason, from a conceptual point of view that these vehicles should not have an all wheel drive system. It is this author's opinion that consumer ignorance and a uncritical media are the main reasons for the lack of progress in the truck/SUV market.

It is false that a permanently engaged system is incapable of handling the rigors of off roading as well as the antiquated part time system. The Range Rover has been on the market since 1976 and it has had a full time system with a center differential since the very first one rolled off the production line. Likewise, the ultimate off roader, the Hummer uses a permanently engaged system with torsen differentials rather than solidly locked axles and part time manual engagement. Both of these vehicles are held in the highest regard with respect to their off roading capabilities.

4WD/AWD vehicles today

Audi and Subaru continue to successfully market all wheel drive vehicles and both marques actively compete in motorsports to show the worth of their technology. The Audi A4 quattro has been particularly successful in demolishing its two wheel drive competition in seven major touring car series last year despite severe weight handicaps. The Subaru Impreza turbo has been extremely successful in World Championship Rallying. The Mitsubishi Eclipse GSX has been less successful in the marketplace, with the vast majority of buyers opting for the front wheel drive version. Die hard Porsche enthusiasts prefer the rear wheel drive version of the 911 to the all wheel drive version. Four wheel drive is increasingly outlawed in competition because they tend to do too well.

With the success of sport utility vehicles the market for high performance all wheel drive vehicles will remain tiny. One could only hope that competition among the makers will eventually force all the SUV makers to bring their technology up to all wheel drive levels. This is starting to happen but at a very slow pace.

VW recently redesigned the Passat and based it on Audi A4 mechanicals. Since it was using a stretched version of the A4's floorpan, it made economic sense to use Audi's quattro system for the all wheel drive model, rather than to create a unique all wheel drive floorplan using the syncro system. Thus the "syncro" moniker becomes merely a generic term to distinguish an all wheel drive VW from a two wheel drive variant. This is not the first time that VW has used Audi quattro mechanicals. In the mid 1980's, Americans could buy the VW Quantum Syncro, which not only used the Audi 4000CS quattro's floorpan, but also the trademark inline 5 cylinder engine, mounted longitudinally ahead of the front axle.

To make matters worse, Audi has decided to market a rebadged VW Golf syncro as the Audi A3 quattro. This car, despite the name, uses the viscous coupling based, part time, automatically enganging four wheel drive system. The quattro name, which used to have special significance is now being diluted by marketing expediency. Considerable confusion has arisen from the naming corruption committed by the two companies.

In yet another twist, Subaru has for many years been quietly offering radically different AWD systems in the same car, depending on the transmission choice. The manual transmission Legacies and Imprezas use a full time system that is split 50-50 with viscous couplings for limiting slip. In the automatic transmission versions, however, the system is a part time, computer controlled, automatically engaging system.

How to shop for a 4WD/AWD system

My recommendations:
• Avoid part time, manually engaging systems, regardless of whether it has manual hubs, auto hubs or any four wheel drive "convenience" features.

• Avoid hybrid manually switchable full time/part time systems

• Full time or automatically engaging part time systems are superior from an engineering point of view and gets my vote

• A limited slip differential on the rear axle is a plus, alternatively 4 wheel traction control is also a big plus.

Bibliography

1. All wheel drive high performance handbook, Jay Lamm, Motorbooks International, 1990

2. "Four-father" Interview with Jorg Benzinger (Audi chief chassis engineer) Performance Car magazine March 1986, AGB Specialist Publications

3. "Four sight" Interview with Friedrich Bezner (Porsche 964 project engineer) Performance Car magazine March 1989, AGB Specialist Publications

4. "Inner vision" Interview with Fritz Naumann (Audi head of general vehicle development) Performance Car magazine March 1989, AGB Specialist Publications

Links to related sites

Not all the information contained in these sites are 100% correct!
• Audi home page
• Subaru 4WD tech page
• Traction adding devices FAQ
• Zexel Torsen home page
• Description of Hummer
• 4X4 guide

Comments? Questions?