If you've read How Car Engines
Work, you understand how a car's power is generated; and
if you've read How
Manual Transmissions Work, you understand where the power
goes next. This article will explain differentials --
where the power, in most cars, makes its last stop before
spinning the wheels.
The differential has three jobs:
To aim the engine power at the wheels
To act as the final gear reduction in the vehicle,
slowing the rotational speed of the transmission one final
time before it hits the wheels
To transmit the power to the wheels while allowing them
to rotate at different speeds (This is the one that earned
the differential its name.)
In this edition of HowStuffWorks,
you'll learn why your car needs a differential, how it works
and what its shortcomings are. We'll also look at several
types of positraction, also known as limited slip
Why You Need a Differential Car wheels spin
at different speeds, especially when turning. You can see from
the animation below that each wheel travels a different
distance through the turn, and that the inside wheels 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
travel at a lower speed. Also note that 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 back 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 your car did not have a
differential, the wheels would have to be locked together,
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 output to spin at a different speed.
The differential is found on all modern cars and trucks,
and also in many all-wheel-drive (full-time four-wheel-drive)
vehicles. These all-wheel-drive vehicles need a differential
between each set of drive wheels, and they need one between
the front and the back wheels as well, because the front
wheels travel a different distance through a turn than the
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. This is why these vehicles are
hard to turn on concrete when the four-wheel-drive system is
Spinning at Different Speeds We will start
with the simplest type of differential, called an open
differential. First we'll need to explore some
terminology: The image below labels the components of an open
When a car is driving straight down the road, both drive
wheels are spinning at the same speed. The input pinion
is turning the ring gear and cage, and none of the pinions
within the cage are rotating -- both side gears are
effectively locked to the cage.
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. See How Gears
Work for more information on gear ratios.
When a car makes a turn, the wheels must spin at different
In the figure above, you can see that the pinions in
the cage start to spin as the car begins to turn, allowing the
wheels to move at different speeds. The inside wheel spins
slower than the cage, while the outside wheel spins faster.
Open Differential - Straight (600KB)
Open Differential - Turning (1.1MB)
Problem with Open Differentials The 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
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.
The solution to these problems is the limited slip
differential (LSD), sometimes called positraction.
Limited slip differentials use various mechanisms to allow
normal differential action when going around turns. When a
wheel slips, they allow more torque to be transferred to the
The next few sections will detail some of the different
types of limited slip differentials, including the clutch-type
LSD, the viscous coupling, locking differential and Torsen
Clutch-Type Limited Slip The clutch-type LSD
is probably the most common version of the limited slip
This type of LSD has all of the same components as an open
differential, but it adds a spring pack and a set of
clutches. Some of these have a cone clutch that is just
like the synchronizers in a manual
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.
Getting back to the situation in which 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.
Viscous Coupling 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.
When a car is turning, the difference in speed between the
wheels is not as large as when one wheel is 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 simple experiment with an egg will help explain the
behavior of the viscous coupling. If you set an egg on the
kitchen table, the shell and the yolk are both stationary. If
you suddenly spin the egg, the shell will be moving at a
faster speed than the yolk for a second, but the yolk will
quickly catch up. To prove that the yolk is spinning, once you
have the egg spinning quickly stop it and then let go -- the
egg will start to spin again (unless it is hard boiled). In
this experiment, we used the friction between the shell and
the yolk to apply force to the yolk, speeding it up. When we
stopped the shell, that friction -- between the still-moving
yolk and the shell -- applied force to the shell, causing it
to speed up. In a viscous coupling, the force is applied
between the fluid and the sets of plates in the same way as
between the yolk and the shell.
Locking and Torsen® The locking
differential is useful for serious off-road vehicles. 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.
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 good traction.
The HMMVV, or Hummer, uses Torsen® differentials
on the front and rear axles. The owner's manual for the
Hummer proposes a novel solution to the problem of one
wheel coming off the ground: Apply the brakes.
By applying the brakes, torque is applied to the wheel
that is in the air, and then five times that torque can
go to the wheel with good
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
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.
*TORSEN is a registered trademark of Zexel