Differential Basics

In an automobile and other wheeled vehicles, a differential is a device, usually consisting of gears, for allowing each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them.

A vehicle's wheels rotate at different speeds, especially when turning corners. The differential is designed to drive a pair of wheels with equal force, while allowing them to rotate at different speeds. In vehicles without a differential, such as karts, both driving wheels are forced to rotate at the same speed, usually on a common axle driven by a simple chain-drive mechanism. When cornering, the inner wheel travels a shorter distance than the outer wheel, resulting in the inner wheel spinning and/or the outer wheel dragging. This results in difficult and unpredictable handling, damage to tires and roads and strain on the entire drive train.

The following description of a differential applies to a "traditional" rear-wheel-drive car or truck: Power is supplied from the engine, via the gearbox, to a propeller shaft, which runs to the rear axle. A pinion gear at the end of the propeller shaft is encased within the differential itself, and it engages with the large ring gear. The ring gear is attached to a carrier, which holds a set of small planetary gears. The three planetary gears are set up in such a way that the two outer gears (the side gears), can rotate in opposite directions relative to each other. The pair of side gears drive the axle shafts to each of the wheels. The entire carrier rotates in the same direction as the ring gear, but within that motion, the side gears can counter-rotate relative to each other.

Thus, for example, if the car is making a turn to the right, the main ring gear may make 10 full revolutions, and during that time, the left wheel will speed up because it has further to travel, and the right wheel will slow down correspondingly, as it has less distance to travel. The side gears will turn in opposite directions relative to each other by, say, 2 full turns, resulting in the left wheel making 12 revolutions, and the right wheel making 8 revolutions.

When the vehicle is travelling in a straight line, there will be almost no movement of the planetary system of gears, other than the minute movements necessary to compensate for slight differences in wheel diameter, undulations in the road (which make for a longer or shorter wheel path) etc.

Problem: Loss of Traction

One undesirable side effect of a differential is that it can reduce overall torque - the rotational force which propels the vehicle. The amount of torque required to propel the vehicle at any given moment depends on the load at that instant - how heavy the vehicle is, how much drag and friction there is, the gradient of the road, the vehicle's momentum, etc. For the purpose of this article, we will refer to this amount of torque as the "threshold torque".

The torque on each wheel is a result of the engine and transmission applying torsion, a "twisting force" against the resistance of the traction at that wheel. Unless the load is exceptionally high, the engine and transmission can usually supply as much torque as necessary, so the limiting factor is usually the traction under each wheel. It is therefore convenient to "measure" traction in terms of how much torque can be generated between the tire and the ground before the wheel starts to slip. If the total traction under all the driven wheels exceeds the threshold torque, the vehicle will be driven forward; if not, then one or more wheels will simply spin.

Traction-Adding Devices

There are various devices for getting more traction from vehicles with differentials. One solution is the limited slip differential (LSD), the most well known of which is the clutch-type LSD. With this differential, each of the side gears has a clutch, which limits the speed difference between the two wheels.

Another solution is the locking differential, which employs a mechanism for allowing the planetary gears to be locked relative to each other, causing both wheels to turn at the same speed regardless of which has more traction; this is equivalent to removing the differential entirely.

Electronic traction control systems usually use the ABS system to detect a spinning wheel and apply the brake to it. This progressively raises the reaction torque at that wheel, and the differential compensates by transmitting more torque through the other wheel - the one with better traction.

A Viscous Coupling replaces the differential entirely. It works on the principle of allowing the two output shafts to counter-rotate relative to each other within a viscous fluid. The fluid allows slow relative movements of the shafts, such as those caused by cornering, but will strongly resist high-speed movements, such as those caused by a single wheel spinning.

A four-wheel-drive vehicle will have at least two differentials (one for each pair of wheels) and possibly a center differential to apportion power between the front and rear axles. Vehicles without a center differential should not be driven on dry, paved roads in all wheel drive mode, as small differences in rotational speed between the front and rear of the vehicle cause a torque to be applied across the transmission. This phenomenon is known as "wind-up" and can cause damage to the transmission. On loose surfaces these differences are absorbed by the slippage on the road surface.

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