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Back to Cars
What is a differential?
A differential is a device that sends power from a
driveshaft to both sides of an axle. The
rotational torque from a driveshaft is
generally laid out horizontally down the length of the vehicle (typical
RWD). But the wheels need to be turned at a 90-degree angle from the
position of the driveshaft. The axle
driving the wheels is split into two parts and the inner ends of both sides
are connected to the differential. The differential is made up
of a series of gears that can direct the rotational power from the
driveshaft to 90-degree angles and turn both
sides of the axle.
A differential can also be used between the front and rear axles on a
four-wheel-drive or all-wheel-system by splitting power between both axles
and ultimately driving all four wheels.
There are three major types of differentials:
Open Differential
Also called a slipping differential, this is the most common
differential available on stock cars. An open differential allows
for free slippage between the two sides of the axle. When a car is
turning, the inner and outer wheels must rotate at different speeds.
If they are forced to rotate at the same speed, there will be increased
wear on the entire drivetrain and tires as the slippage needs to "rubbed
off" somewhere, normally the tires. An open differential allows for
some slippage to occur between the two sides of the axle so that the tires
do not rub the ground. With both tires left off the ground,
put the transmission in neutral and with your hand turn one of the wheels.
The other wheel should spin in the opposite direction with an open
differential. If the opposite wheel does not spin, the differential
may be damaged.
The downside to an open differential is that it offers the least amount
of traction. In a low-traction setting (dirt, sand, snow, ice,
gravel), attempting to accelerate beyond the traction capabilities of the
tires causes only a single tire to spin while the opposite tire generally
does not spin at all or very little. A 4WD or AWD vehicle with open
differentials on the front and rear axles will spin one tire on the front
and one tire on the rear in this situation.
Limited-Slip Differential (LSD)
To solve the problem of the low traction of an open differential, car
manufacturers typically offer a limited-slip differential (LSD) on
off-road, luxury, and high-performance vehicles. A LSD is similar to
an open differential in that it allows some slippage to occur, but it
differs from an open differential by limiting the slippage, usually to a
percentage difference. Under normal conditions, a LSD will allow
minor slippage between the wheels. But when the slippage passes
beyond a pre-determined difference, after a slight delay, the axle sides
lock together somewhat and send part of the rotational torque to the
opposite wheel of the slipping wheel. There are several types
of LSD's available from manufacturers all over the world, each offering a
different mechanical solution. With both wheels lifted up and the
car in neutral, turn one wheel with your hand. The other wheel
typically won't move at all until about 1/2 of a full turn, then it will
start to spin.
Locking Differential
A locking differential is unlike an open differential or LSD in that it
allows no wheel slippage to occur. The axle sides are ultimately
"locked" together with a series of toothed gears, similar to the operation
of a manual transmission. A locking differential causes excessive
wear on the drivetrain and tires and therefore is not used on street
applications. It's normally reserved for off-road use where the
slippage can be innocently rubbed off by the tires on the low-traction
ground, snow, or ice. A 4WD's transfer case is a type of locking
differential as it locks the front and rear axles together for superior
traction.
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