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
The torque on each wheel is a result of the engine and
transmission applying 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
define traction as the amount of torque that 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,
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 N-m of torque to move forward (i.e. the threshold
torque). Let us further assume that the non-spinning traction on the ice
equates to 400 N-m, and the asphalt to 3000 N-m.
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 N-m), 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 forward.
With a differential, however, as soon as the "ice
wheel" reaches 400 N-m, it will start to spin, and then develop less
traction ~300 N-m. The planetary gears inside the differential carrier will
start to rotate because the "asphalt wheel" encounters greater
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 the stopped wheel by extra speed of the spinning
ice wheel. The torque on both wheels will be the same - limited to the lesser
traction of 300 N-m each. Since 600 N-m is less than the required threshold
torque of 2000 N-m, the vehicle will not be able to move.
An observer simply sees 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).
A traction control system (TCS), in German known
as Antriebsschlupfregelung (ASR), is typically (but not
necessarily) a secondary function of the anti-lock braking system (ABS) on
production motor vehicles, designed to prevent loss of traction of driven road
wheels. TCS is activated when throttle input and engine torque are mismatched
to road surface conditions.
Intervention consists of one or
more of the following:
Brake force applied to one or more wheels
Reduction or suppression of spark sequence to one
or more cylinders
Reduction of fuel supply to one or more cylinders
Closing the throttle, if the vehicle is fitted
with drive by wire throttle
In turbocharged vehicles, a boost control solenoid
is actuated to reduce boost and therefore engine power.
Typically, traction control systems share the electro
hydraulic brake actuator (which does not use the conventional master cylinder
and servo) and wheel speed sensors with ABS.