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 rear wheels.
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 engaged.
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
differential.
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. When a car makes a turn, the wheels must spin at
different speeds.
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.
We give our thanks to
HOW STUFF WORKS for allowing us to use their info and
graphics!
For more information on how engines
or anything else in this world works, visit
www.howstuffworks.com.
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