Differential is a device in
automobiles, usually consisting of gears, for allowing each of the driving
wheels to rotate at different speeds, while supplying equal torque to each of
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 driveshaft (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 (crownwheel). 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 traveling 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.
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 and so on. 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.
To illustrate how a differential can limit overall torque, let us imagine a
simple rear-wheel-drive vehicle, with one rear wheel on asphalt with good grip,
and the other on a patch of slippery ice. With the load, gradient, etc., the
vehicle requires, say, 2000 Nm of torque to move forward (i.e. the threshold
torque). Let us further assume that the traction on the ice equates to 400 Nm,
and the asphalt to 3000 Nm.
If the two wheels were driven without a differential, each wheel would "push"
against the ground as hard as possible. The wheel on ice would quickly reach the
limit of traction (400 Nm), but would be unable to spin because the other wheel
has good traction. The traction of the asphalt plus the small extra traction
from the ice exceeds the threshold requirement, so the vehicle will be propelled
With a differential, however, as soon as the "ice wheel" reaches 400 Nm, it will
start to spin. The planetary gears inside the differential carrier will start to
rotate because the "asphalt wheel" encounters greater resistance. Instead of
driving the asphalt wheel with more force, the differential will allow the ice
wheel to spin faster, and the asphalt wheel to remain stationary, compensating
for extra speed of the spinning ice wheel. The torque on both wheels will be the
same - limited to the lesser traction of 400 Nm each. Since 800 Nm is less than
the required threshold of 2000 Nm, the vehicle will not be able to move.
Note that an observer will simply see one stationary wheel and one spinning
wheel. It will not be obvious that both wheels are generating the same torque
(i.e. both wheels are in fact "pushing" equally, despite the difference in
rotational speed). This has led to a widely held misconception that a vehicle
with a differential is really only "one-wheel-drive". In fact, a normal
differential always provides equal torque to both driven wheels (unless it is a
locking, torque-biasing, or limited slip type).
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, the side gears are coupled to the
carrier via a stack of clutch plates 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 four wheel drive mode, as small differences in
rotational speed between the front and rear wheels 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.
A relatively new technology is the electronically-controlled active
differential. A computer uses inputs from multiple sensors, including yaw rate,
steering angle, and lateral acceleration and adjusts the distribution of torque
to compensate for undesirable handling behaviors like understeer. Active
differentials used to play a large role in the World Rally Championship, but in
the 2006 season the FIA has limited the use of active differentials only to
those drivers who have not competed in the World Rally Championship in the last
The first use of this technology on a production automobile was in the 1995
Nissan Skyline GT-R Vspec. The lockup of the rear differential and center
coupling were controlled as part of the ATTESA-ETS Pro system. Fully integrated
active differentials are used on the 2005 MR Ferrari F430 and on rear wheels in
the Acura RL.
The second constraint of the differential is passive – it is actuated by the
friction kinematics chain through the ground. The difference in torque on the
tires (caused by turns or bumpy ground) drives the second degree of freedom,
(overcoming the torque of inner friction) to equalize the driving torque on the
tires. The sensitivity of the differential depends on the inner friction through
the second degree of freedom. All of the differentials (so called “active” and
“passive”) use clutches and brakes for restricting the second degree of freedom,
so all suffer from the same disadvantage – decreased sensitivity to a
dynamically changing environment. The sensitivity of the computer controlled
differential is also limited by the time delay caused by sensors and the
response time of the actuators.