No car will go round a corner without them, no 4x4 is a true 4x4 without a couple of them. What we are talking about are diffs. Diffs are short for differentials. They are the cornerstone to any transmission system and the key to development of the 4x4.
Without them cars wouldn't be able to corner. If you are driving down a road then both left and right wheels are travelling at the same speed. But, if you started to turn into a corner then the inside wheels would have a shorter path than the outside wheels and this would mean that they must travel at a slower speed.
So there is a difference in speed and if the wheels were both connected to the gearbox via the prop shafts then the outer wheel would have to 'scrub off' its extra speed as there would be no allowance for any speed difference. As the wheel would rather rotate freely than slip or skid, the vehicle would resist turning the corner.
A diff absorbs any difference in speed between the two sides allowing the vehicle to corner. It does this by a series of bevel gears. The diff is mounted in a cage which is connected to the crown wheel of the vehicle. At the ends of the half shafts, inside the diff, there are two gears. At 90Ã¯Â¿Â½ to these gears, there are four bevel gears and it is these that hold the key to the differential.
In ordinary conditions, travelling along a straight road, the differential is, in effect, not working as there is no speed difference between either side. The diff cage will still rotate as it is connected to the crown wheel.
When the vehicle starts to turn, there is a speed difference and the diff comes into action. The internal pinions start to rotate in opposite directions. If you imagine that the two side gears are travelling in the same direction, absorbing the speed and allowing the two sides to travel at different speeds. The crown wheel will rotate at the average of the two.
This is good for normal road conditions but for one problem. This is when, for example, you have one wheel sat on the road and one on a sheet of ice, one side has grip and the other doesn't. The wheel on the ice will spin, the diff will kick in and do its job, and the wheel on the road will remain still, your vehicle will go nowhere. In this instance a Limited Slip Diff (LSD) would work better.
An LSD minimises the effect in such situations and ensures that power is transmitted to the road. LSD's are the same as normal diffs but with two extra parts, the pressure rings and disc clutches.
The pressure rings are attached to the diff case so rotate when it does. However, they can move axially, meeting the disc clutches which are located between the pressure rings and diff case. The clutches are alternately drive and driven discs, the drive discs have extensions which locate the grooves in the diff case, whilst the driven discs have engaging teeth which meet with the side gears of the differential.
The LSD works by increasing the torque needed to turn the spinning wheel. When it is divided equally by the differential, greater torque is offered to the wheel that has grip, and no more is delivered to the road. It does this by forcing the pressure rings apart when a difference in rotational speed of the two sides is detected. This means that the clutches are forced together, causing friction and resisting the motion of the side pinions, in effect locking the axel so the wheel with grip can be driven.
A viscous diff sounds in principle like it wouldn't work but does the job brilliantly. Instead of using a mechanical locking system the viscous diff utilises the shearing forces of a liquid to provide lock. The viscous unit is mounted onto the output shafts and takes the form of a drum, inside which are many rows of steel discs. The unit is sealed and filled with silicone fluid.
It is the shearing forces of the liquid against the discs which locks the differential case. When the diff is in operation, one set of discs will rotate faster relative to the other. The silicone fluid will create a dragging force against the discs which resist any 'out of sync' movement of the discs. Therefore, the faster set of discs will be slowed to restore uniform movement and the diff will be locked.
In a 4x4 the front and rear diffs are the same as that of a car but they must have another diff in the center as they drive all the wheels. The center diff is used to solve the same sort of problems as an ordinary diff.
If the front and rear wheels, both transmitting power, were connected without a diff then any differences in rotational speed would result in axle windup and even damage.
However, if one wheel of a 4x4 is on a patch of ice then when the driver accelerates then the central diff will transfer all the power to the wheel that is spinning and the car would go nowhere. For this reason, the central diff will always have some sort of limited slip action. The most common type is viscous as it is one of the most cost effective and easy to apply solutions.