Spyker DefinitionSource (google.com.pk)
When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. This is accomplished with a differential. A differential allows one input shaft (e.g. - the driveshaft of car or truck) to drive two output shafts (e.g. - axles shafts that go from the differential to the wheel) independently with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the differential ring gear. Each powered axle requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third or 'center' differential can be used to distribute power between the front and the rear axles.
The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making slippage unlikely. Once it does slip, however, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel will spin faster than the other wheels due to the lower traction at that wheel. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles, because 4WD vehicles have twice as many wheels with which to lose traction, increasing the likelihood that it may happen. 4WD vehicles may also be more likely to drive on surfaces with reduced traction. However, since torque is divided amongst four wheels rather than two, each wheel receives approximately half the torque of a 2WD vehicle, reducing the potential for wheelslip.
Many differentials have no way of limiting the amount of engine power that gets sent to its attached output shafts. As a result, if a tire loses traction on acceleration, either because of a low-traction situation (e.g. - driving on gravel or ice) or the engine power overcomes available traction, the tire that isn't slipping receives little or no power from the engine. In very low traction situations, this can prevent the vehicle from moving at all. To overcome this, there are several designs of differentials that can either limit the amount of slip (these are called 'limited-slip' differentials) or temporarily lock the two output shafts together to ensure that engine power reaches all driven wheels equally.
Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, providing torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.
The two most common factory-installed locking differentials use either a computer-controlled multi-plate clutch or viscous coupling unit to join the shafts, while other differentials more commonly used on off-road vehicles generally use manually operated locking devices. In the multi-plate clutch the vehicle's computer senses slippage and locks the shafts, causing a small jolt when it activates, which can disturb the driver or cause additional traction loss. In the viscous coupling differentials the shear stress of high shaft speed differences causes a dilatant fluid in the differential to become solid, linking the two shafts. This design suffers from fluid degradation with age and from exponential locking behavior. Some designs use gearing to create a small rotational difference that hastens torque transfer.