All About Differentials
Vehicles have differentials to allow wheels to operate at different rates of speed. Typical cars, vans, & trucks all have differentials that are called "open". These allow for no more torque to be supplied to a wheel with traction than to the wheel that is slipping. This works fine when conditions allow traction at both drive wheels, but when either drive wheel looses traction the trouble begins. A "2WD" vehicle with an open differential could arguably be characterized as a "one-wheel-drive" when either drive wheel loses traction.
2WD with open differential:
When one drive wheel loses traction the vehicle cannot move since no more torque can be supplied to the wheel with traction than to the slipping wheel.
4WD with two open differentials
Same as on 2WD except that each axle has a chance at getting traction. In this case if either front wheel AND either rear wheel loses traction, the car cannot move. This often happens in an off-road situation when the vehicle becomes supported diagonally on one front wheel and the opposite side rear wheel.
LIMITED SLIP Differentials
A small step up from the "open" differential is a "limited-slip" differential. In the case of the limited slip differential, some useful torque may be applied to the non-slipping wheel and motion is possible.
These are called limited slip because they allow a limited amount of slippage between the wheels. Typically the amount of slippage that they allow is quite high and are of little value in off-road or truly slippery conditions. Limited-slip differentials typically employ some form of clutch mechanism to create the limited-slip effect. The best use of a limited-slip differential is on pavement where one is attempting to accelerate very quickly and both tires have decent but slightly different traction. The most effective variety of limited-slip differential is the TORSEN (TORque SENsing) differential.
2WD with limited-slip differential
If one drive wheel completely loses traction the vehilce will not move. The amount of torque needed to be passed to the wheel with traction will exceed the differential's "limit" and you go nowhere. If one drive wheel has marginal traction, you might be able to move if the traction is sufficient enough as to not exceed the slip limit of the differential.
4WD with two limited-slip differential
This is a highly unusual arrangement. I believe that late model Audi Quattro’s use TORSEN differentials at both the front and rear (center also perhaps). If one wheel on each axle completely loses traction the vehicle will not move. However if you have no traction at one axle and marginal traction at the other, this arrangement can get you moving if the traction is sufficient enough.
In off-road or snow conditions, this is a BIG step up from an open differential. The locking mechanism "locks" both sides of the axle together and eliminates any side to side slippage. The down side to this is that on the street this is a real problem allowing you to turn corners since the left & right wheels MUST travel at different speeds around any corner. If the left & right wheels are locked together turning a corner on the street would be a traumatic experience. You would tend to be pushed straight ahead (instead of turning) until one wheel started skidding on the pavement. To alleviate this on-street problem, locking differentials must be un-locked. There are two primary ways this is done. In the 4X4 community this is most often done by using an automattically unlocking differential that will unlock when you trun a corner with traction on both wheels. Available from several manufacturers, the best known of this type of "locker" (as they are called) is the Detroit Locker. Many 4X4 enthusiasts have one of these installed in their rear differentials. These once unlocked & relocked with quite some drama requiring drivers to be careful turning corners on the street. Newer ones offer a nearly unnoticeable transition.
Another type of locker is one that will lock & unlock at the command of the driver. The aftermarket variety of this system is called the ARB Air Locker & uses compressed air and driver selector switches to lock & unlock either the front or rear differential. Typically the differentials are left unlocked during all street driving and are locked when one goes off-road or perhaps travels on a snowy street. The Vanagon Syncro uses a similar mechanism to lock & unlock it's differential(s). In the Vanagon's system, an
air vacuum (instead of compressed air) is used to lock & unlock the differential(s). In the US and Canada, Vanagons were only available with this locker in the rear differential. In some other countries (South Africa, Australia) a locker was also available in the front differential.
This explains the unused light & knob positions on your diff lock control panel. I have heard that the center position on the control panel was used to lock & unlock the connection between the rear axle and the viscous coupling.
2WD with a locking differential:
If either one of the two drive wheels has traction the vehicle will move. Both drive wheels always will rotate at the same speed.
4WD with two locking differentials:
If any one of the four drive wheels has traction the vehilce will move. All four drive wheels always will rotate at the same speed. You must loose traction on all four wheels to stop the vehicle.
Typical 4x4 vehicle (Jeep for instance) does not have a center differential. Instead the front & rear axles are connected directly by gears when 4WD is engaged. This direct connection is what prevents them form using 4WD on the street - again the going around corners problem & wheels needing to spin at different(ial) speeds. 4WD vehicles such as the Vanagon Syncro employ a center differential which allows for some degree of different(ial) axle speeds. These vehicles are often referred to as All Wheel Drive (AWD) to distinguish them from the 4X4 variety of 4WD vehicles. The Syncro uses a viscous coupling as its center differential which acts kind of like a limited-slip/locking differential. I use those normally separate terms together hear to indicate the nature of the viscous coupling operation.
While it allows for some slippage, the greater the slippage, the greater the degree of lock-up, to a point where the coupling is nearly locked, and transfering as much torque to the front wheels as possible. Things have gotten blurry in the 4WD / AWD market lately as many manufacturers are now installing center differentials in traditional 4X4 vehicles. This allows them to operate in 4WD all the time, yet most have the ability to lock the center differential for maximum off-road performance. The viscous coupling
in the Vanagon Syncro does not have an engagable 100% lockup feature.
We would rate drive & differential systems in the following order of traction performance (without regard for a center differential, or lack of):
2WD open differential ------ Typical Vanagon
2WD L/S differential
2WD locking differential
4WD open differential ------ Vanagon Syncro
4WD one L/S differentials
4WD rear locking differential ------ Vanagon Syncro with optional rear diff lock
4WD two locking differentials