One of the less understood parts of a car is the rear differential mechanism. It is a geared contraption, that makes wheel motion possible, through differential transmission of engine torque.
What is a Differential?
The differential is a mechanism that transmits torque, generated in the engine, to the wheels, via the driveshaft. It controls the speed of wheel rotation, through a gear mechanism. While turning, both wheels rotate at different speeds and this differential speed distribution is made possible by the differential mechanism.
Most automobiles are rear drive type and the differential is placed at the rear, in connection with the driveshaft. The differential can allow for equal and different rotation speeds. Normally, the housing of the differential is known as 'pumpkin' due to its shape.
There are three main types of differentials; which are open, limited slip, and locking differential. Each of the types has different wheel spin control mechanisms. How much of torque will be needed by a wheel, depends on the traction or friction experienced by it.
One problem with conventional open rear differential becomes apparent, when one of the wheels slips and the entire power is transferred to it, leaving the other grounded wheel with unequal power and consequently, no forward car motion. The locking version of the differential was invented to take care of this problem.
What are Locking Rear Differentials?
To take care of unequal torque power distribution, the selectable (manual) and automatic locking rear differentials were developed. The wheels of a car, fitted with a such a differential, will have the same rotational speed as they are interlinked or locked in relative motion to each other. Regardless of the traction (friction) experienced by a wheel, the rotational speed is maintained to be the same for both. It offers better traction for wheels, compared to the open one. It is most advantageous to have a locking differential, when both wheels experience different amounts of traction.
The locking differential technology comes in two main types: 'Selectable (manual)' and 'Automatic'. Let us have a look at how these two differential mechanisms function.
An automatic locking differential locks and unlocks wheels entirely on its own, without any driver mediation. The mechanism is unlocked while turning or cornering, as it requires different rotational speeds. The advantage of this mechanism is that one doesn't need to stop, in order to unlock the differential. One problem that has been noticed is that tire wear can increase in a car, with locked rear differential. Drivers take time to get used to a locked differential, and tend to under steer, while cornering.
Another option is to have a selectable differential, which gives you an option to lock or unlock manually, when needed. This mechanism allows you to have the best of both, locking and open differential mechanisms. One can switch over to locking differentials, using a control mechanism, fitted near the driving seat.
The selective locking differential is more complex in its mechanism, with some of the types requiring the driver to stop, while making the change in differentials. It requires judgment on the part of the driver, who must make the switch, according to the kind of terrain he is driving on.
Increased tire wear and slipping on ice remain the two major problems of using these differential mechanisms. An alternative is to use the limited slip differential or using a traction control system.