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What is a Differential?
id you know that your wheels turn at different
speeds? During a turn, the outer wheels drive farther than the inner
wheels, and this is an important function of the differential.
Differentials have three main functions.
To begin with, they lower the overall drivetrain gear ratio, usually
somewhere between 2-to-l and 3-to-l. This allows the transmission to
operate in and around direct drive and reduces the load on the
differentials rotate the power flow by 90 degrees. That's because
most rear-wheel drive cars have lateral transmissions — transmissions
that run down the length of the car. The power flow leaving the
transmission is going the wrong way. The differential uses a
ring-and-pinion gear set to turn the power flow 90 degrees so the
drivetrain can drive the car properly.
One of the most
important functions of a differential is to allow the two drive
wheels to turn at different speeds. That difference is necessary for
turning corners. Here's why: When you're driving straight down the
road, the wheels on both sides of the car are traveling the same
distance, so they turn at the same speed. However, that changes when
you turn a corner.
Think of it this way:
When a car turns, the wheels form an arc, or part of a circle.
Because of that, the inner wheel — the wheel on the side closer to
the center of the "circle" — doesn't travel as far as
the wheel on the outside of the turn. To make up this difference,
the outer wheel has to turn faster than the inner wheel.
If the drive wheels
were locked together directly, the tires would skip or slide across
the roadway when you made a turn. That's where the differential
comes in. The differential connects the two wheels through four
two side gears and two spider gears. These
gears transmit power to the wheels, while allowing them to rotate at
different speeds when necessary.
While the differential
is necessary, it can cause another problem. Imagine a situation
where one drive wheel is sitting on dry roadway, while the other is
on a patch of ice. Since both drive wheels can turn at different
speeds, the differential will allow the wheel sitting on ice to
spin, while the one on dry road just sits there.
To combat this
condition, some manufacturers offered a limited slip differential.
Limited slip means there can be only so much difference between the
speeds of the wheels on either side of the car. There's plenty of
slip for going around a turn, but a limited slip differential won't
allow one wheel to spin while the other just sits there.
However, limited slip
is an option on most cars and trucks. So, if you drive in areas
where ice and snow are likely, consider this option when buying a
Differential Service Procedures
If your car has a
standard differential, the only service procedure necessary is to
have the fluid level checked regularly. Since this involves raising
the car and removing a side plug, it's probably best left to the
repair shop. Unless it becomes contaminated, there's no reason to
ever change the oil in a standard differential. As long as the level
is OK, it's fine. In general, you should have the differential fluid
level checked when you have the oil changed. Most shops will do that
automatically, but it won't hurt to ask for it.
If your car has
front-wheel drive, the differential probably is part of the
transaxle. In most cases, the transmission and differential will
share a common fluid collection pan, commonly known as a sump, so if
the transmission fluid level is OK, the differential fluid level
differentials do require service beyond simply checking the fluid
level. If your car has a limited slip differential, the fluid will
have to be changed on occasion. Forgetting to service the
differential could cause it to skip and make loud groaning noises on
What's important about this is the fluid
used. The wrong fluid in a limited-slip differential also will cause
it to skip and groan on turns. So if your
differential starts making noise right after being serviced, take it
back to the repair shop and ask them to make sure they used the
right type of fluid.
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