Car wheels spin at different speeds, especially when turning. As a vehicle goes
through a turn, each wheel travels a different distance through the turn. The
inside wheels (for example, the right wheels during a right hand turn) travel
a shorter distance than the outside wheels. Since speed is equal to the
distance traveled divided by the time it takes to go that distance, the wheels
that travel a shorter distance must travel at a lower speed. Also, the
front wheels travel a different distance than the rear wheels.
For the non-driven wheels on your car - the front wheels on a rear-wheel
drive car, the rear wheels on a front-wheel drive car - this is not an issue.
There is no connection between them, so they spin independently. But the
driven wheels are linked together so that a single engine and transmission
can turn both wheels. If the car did not have a differential, the wheels
would have to be locked together, and therefore forced to spin at the same
speed. This would make turning difficult and hard on your car: For the car
to be able to turn, one tire would have to slip. With modern tires and
concrete roads, a great deal of force is required to make a tire slip.
That force would have to be transmitted through the axle from one wheel
to another, putting a heavy strain on the axle components.
The differential is a device that splits the engine torque two ways,
allowing each wheel to spin at a different speed so that the car can
make turns. The differential is located between the two wheels, and
is attached to each wheel by a half-shaft rotated through a bevel gear.
Four-wheel drive cars have a separate differential for each pair of wheels,
and many have a differential between the front and rear wheels (for a total of 3
differentials) to facilitate turning on pavement. Part-time four-wheel-drive
systems don't have a differential between the front and rear wheels; instead,
they are locked together so that the front and rear wheels have to turn at
the same average speed, which makes these vehicles are hard to turn on
concrete when the four-wheel-drive system is engaged.
A grooved, or splined, axle side gear is positioned on the splined end
of each axle. The side gears are driven by "spider" gears, which are
little gears mounted on a shaft attached to the differential
case. As it is supported by the differential case, the side gear can turn
inside the case.
The differential case can be turned, revolving around the axle gears. The
differential pinion (a pinion is a small gear that either drives a larger
gear or is driven by one) shaft turns the ring gear, which is fastened to
the differential case. The propeller shaft (drive shaft) connects the
transmission output shaft to the differential pinion shaft. The turning
differential case is mounted on two large bearing holders. These bearings
are called carrier bearings.
The propeller shaft rotates the ring gear pinion, and the pinion turns
the ring gear. The ring gear then turns the differential case and pinion
shaft, but the axle side gears will not turn. Note that the input pinion
is a smaller gear than the ring gear; this is the last gear reduction in
the car. You may have heard terms like rear axle ratio or final drive
ratio. These refer to the gear ratio in the differential. If the final
drive ratio is 4.10, then the ring gear has 4.10 times as many teeth
as the input pinion gear. By passing the differential pinion shaft
through two differential pinion gears that mesh with the side
gears, the case will turn and the axle side gears will turn with it.
During turns, the side gears turn at rates dictated by the radius of the
turns, and the spider gears then turn to allow the outer wheel to turn
faster than the inner one.
For lubrication fluid, a very heavy oil, must be used in rear axle housings.
Special hypoid oils are used in the differential case. Even another type of
fluid, or oil must be used in a positraction type differential.
The oil is circulated by the ring gear, and flung all over all the parts.
Special troughs, or gullies are used to bring the oil back to certain spots,
like the ring and pinion area and the piston bearings. The fluid is kept
in with gaskets and oil seals. The bottom of the housing has a drain plug,
and another filler plug is located part way up the housing. The housing
must never be filled above this plug.
The housing fluid lubricates some of the outer bearings,
but others have lubrication fittings for the injection of wheel bearing
grease. A hand gun, not a pressure grease gun must be used to grease these
bearings (sparingly). A pressure grease gun could inject grease into the
brakes-- greasy brakes are inefficient at best!
Finally, some bearings are filled with grease at the factory and are sealed.
These never require attention unless they are defective.
Types of Differentials
The most common type of differential found on cars and trucks are known as Open
Differentials. An open differential always applies the same amount of torque
to each wheel. There are two factors that determine how much torque can be
applied to the wheels: equipment and traction. In dry conditions, when
there is plenty of traction, the amount of torque applied to the wheels is
limited by the engine and gearing; in a low traction situation, such as
when driving on ice, the amount of torque is limited to the greatest
amount that will not cause a wheel to slip under those conditions. So,
even though a car may be able to produce more torque, there needs to be
enough traction to transmit that torque to the ground. If you give the
car more gas after the wheels start to slip, the wheels will just spin faster.
If you've ever driven on ice, you may know of a trick that makes acceleration
easier: If you start out in second gear, or even third gear, instead of
first, because of the gearing in the transmission you will have less
torque available to the wheels. This will make it easier to accelerate
without spinning the wheels.
Now what happens if one of the drive wheels has good traction, and the other
one is on ice? This is where the problem with open differentials comes in.
Remember that the open differential always applies the same torque to both
wheels, and the maximum amount of torque is limited to the greatest amount
that will not make the wheels slip. It doesn't take much torque to make
a tire slip on ice. And when the wheel with good traction is only getting
the very small amount of torque that can be applied to the wheel with
less traction, your car isn't going to move very much.
Another time open differentials might get you into trouble is when you are
driving off-road. If you have a four-wheel drive truck, or an SUV, with an
open differential on both the front and the back, you could get stuck. If
one of the front tires and one of the back tires comes off the ground,
they will just spin helplessly in the air, and you won't be able to
move at all.
Limited Slip Differentials
A Limited Slip Differential (also known as a LSD) attempts to address the
problems of an Open Differential. A Limited Slip Differential is very similar
to an Open Differential, but it adds a spring pack and a set of clutches.
Some of these have a cone clutch that is similar to the synchronizers in a
manual transmission. The spring pack pushes the side gears against the
clutches, which are attached to the cage. Both side gears spin with the
cage when both wheels are moving at the same speed, and the clutches
aren't really needed - the only time the clutches step in is when something
happens to make one wheel spin faster than the other, as in a turn. The
clutches fight this behavior, wanting both wheels to go the same speed.
If one wheel wants to spin faster than the other, it must first overpower
the clutch. The stiffness of the springs combined with the friction of the
clutch determine how much torque it takes to overpower it.
Therefore, in the situation where one drive wheel is on the ice and the
other one has good traction, With this limited slip differential, even
though the wheel on the ice is not able to transmit much torque to
the ground, the other wheel will still get the torque it needs to move.
The torque supplied to the wheel not on the ice is equal to the
amount of torque it takes to overpower the clutches. The result is
that you can move forward, although still not with the full power
of your car. This is why Limited Slip Differentials are popular in
Drag Racing - they minimize wasteful wheel spin on a hard launch.
The locking differential is useful for serious off-road vehicles and for drag
racing. This type of differential has the same parts as an open differential,
but adds an electric, pneumatic or hydraulic mechanism to lock the two output
pinions together. This mechanism is usually activated manually by switch,
and when activated, both wheels will spin at the same speed. If one wheel
ends up off the ground, the other wheel won't know or care. Both wheels
will continue to spin at the same speed as if nothing had changed. This
maximizes the amount of forward motion, irregardless of wheel slippage -
perfect for drag racing.
There are several types of locking differentials. An ARB Air Locker is a
unique differential because it acts like an open differential until an
on-board air compressor is activated by a switch. The air pressure is
used to lock the differential. This allows a very high breakaway torque
for racing but no compromises for daily driving.
A Detroit Locker, popular on muscle cars and some off-road trucks, is
a ratcheting type of locking differential. It is very strong and
will almost always provide equal torque application to each axle,
but it is noticeable when cornering.
Finally there is the spool, which solidly connects the left and right axles
with no slipping allowed. It is used for drag-racing applications only, since
it maximizes forward acceleration, but makes the vehicle very difficult to turn
and is very hard on the axles.
The Torsen differential is a purely mechanical device; it has no electronics,
clutches or viscous fluids. The Torsen (from Torque Sensing) works as an
open differential when the amount of torque going to each wheel is equal.
As soon as one wheel starts to lose traction, the difference in torque causes
the gears in the Torsen differential to bind together. The design of the
gears in the differential determines the torque bias ratio. For instance,
if a particular Torsen differential is designed with a 5:1 bias ratio, it is
capable of applying up to five times more torque to the wheel that has
These devices are often used in high-performance all-wheel-drive vehicles.
Like the viscous coupling, they are often used to transfer power between
the front and rear wheels. In this application, the Torsen is superior
to the viscous coupling because it transfers torque to the stable wheels
before the actual slipping occurs.
However, if one set of wheels loses traction completely, the Torsen
differential will be unable to supply any torque to the other set of
wheels. The bias ratio determines how much torque can be transferred,
and five times zero is zero. One novel solution is to apply the brakes and
the gas at the same time. This will create a level of "traction" on the
spining wheel, and allow the Torsen differential to shift power to the
Vicious Coupling Differentials
The viscous coupling is often found in all-wheel-drive vehicles. It is
commonly used to link the back wheels to the front wheels so that when
one set of wheels starts to slip, torque will be transferred to the other set.
The viscous coupling has two sets of plates inside a sealed housing that
is filled with a thick fluid, as shown in below. One set of plates is
connected to each output shaft. Under normal conditions, both sets of
plates and the viscous fluid spin at the same speed. When one set of
wheels tries to spin faster, perhaps because it is slipping, the set
of plates corresponding to those wheels spins faster than the other.
The viscous fluid, stuck between the plates, tries to catch up with
the faster disks, dragging the slower disks along. This transfers
more torque to the slower moving wheels -- the wheels that are not
slipping. The faster the plates are spinning relative to each other,
the more torque the viscous coupling transfers. The coupling does
not interfere with turns because the amount of torque transferred
during a turn is so small. However, this also highlights a
disadvantage of the viscous coupling: No torque transfer will occur
until a wheel actually starts slipping.
A positraction differential is a special traction differential.
Its purpose is to improve the way your differential performs under adverse
conditions. When one wheel starts to slip, these differentials transfer
the torque to the wheel that is not slipping. The car can then continue
to go forward. There are several different kinds of positraction differentials,
but all of them are based on a friction device to provide resistance to
normal differential operation.
A positraction differential provides better traction, which is handy when roads
are slippery. It also lends itself to fast acceleration.
One type uses four differential pinions instead of two,
with two pinion shafts. It also uses a series of four clutch discs. The
differential pinions run into resistance when they try to turn the axle
side gears. The resistance gets transferred to the pinion shafts driving
the pinions. The shafts are forced to slide up little ramps. This action
moves both shafts outward. The pinions cause the clutches to lock.
Other types use cone clutches, or disc clutches under
pressure from coil springs. By restricting the differential action, torque
is delivered to the slipping wheel.