In automobiles, differential gears, which have one input and two outputs, ensure that the wheels on the axle which is being driven can rotate at different speeds. That is particularly important when negotiating curves, since the wheel, which is negotiating the curve on the outer side must travel a greater distance, compared with the other wheel on the inner side. The gears are usually located between the wheels of the corresponding axle, in the case of front drive they are on the front axle, while in the case of rear drive they are on the back axle.
Nevertheless, when driving off-road, or on black ice and snow, differential gears can encounter problems, as some wheels are able to turn while others do not move. Differential locks have been devised for such cases: in fact, they prevent wheel spinning by providing a stable connection of the wheels of an axle. A distinction can be made between cross differential locks and long differential locks, according to which wheels are connected to which axles. Long differential locks are utilized only with four-wheel drive vehicles, while cross differential locks are devised for front-engine vehicles and vehicles with rear-drive.
Differential locks fulfil the function of deactivating the differential. That means that force is equally distributed between both wheels and the rotary speed is identical for both of them. This way, a motor vehicle is able to drive safely on the slippery ground created by packed snow, and spinning of the wheels can be avoided; yet, this system entails also a few disadvantages. As soon as the differentials are blocked, an increased tyre wear is generated as the result of the uniform attrition and of the tyres’ increased dragging. Moreover, locking differentials must be cared for with a regular change of oil, whereas that is not necessary with open differentials.
There are several designs of differential locks. The sort of locks equipped with special friction discs, which are pressed one on top of the other through pressure plates, thus allowing improved flexibility, are known as disc differential locks. They are especially utilized in motor sport. Torsen differentials do not block the wheel axle completely, rather, they propel the torque – or output speed– back and forth between the wheels. This way the differential is open and gets locked only when the wheel does not have any grip. If at any given point the (rotational) speed difference persists for some time, the wheels get locked through the action of Visco-loks (Visco loks are patented speed-sensing limited slip differentials). By using a special oil, which alters its viscosity, the differential gets locked to a variable degree. The disadvantage in this case, is represented by the fact that the locking occurs only after a brief time delay. Electronic differential locks are the least widespread. The locking action is operated through a software programme, which monitors all relevant elements such as speed, rotation speeds, and pedal position.
The price range for differential locks oscillates between 2,000 and 4,000 Dirhams.