How Differentials Work

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How Differentials Work

If you've read How Car Engines Work, you understand how a car's power isgenerated; and if you've read How Manual Transmissions Work, youunderstand where the power goes next. This article will explaindifferentials -- where the power, in most cars, makes its last stop beforespinning the wheels.
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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 isnot an issue. There is no connection between them, so they spin

independently. But the driven wheels are linked together so that asingle engine and transmission can turn both wheels. If your car didnot have a differential, the wheels would have to be locked together,forced to spin at the same speed. This would make turning difficultand 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 offorce is required to make a tire slip. That force would have to betransmitted through the axle from one wheel to another, putting aheavy strain on the axle components.
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What is a Differential?

The differential is a device that splits the engine torque two ways, allowingeach output to spin at a different speed.
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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-drivevehicles need a differential between each set of drive wheels, and theyneed one between the front and the back wheels as well, because the frontwheels travel a different distance through a turn than the rear wheels.

Part-time four-wheel-drive systems don't have a differential between the frontand rear wheels; instead, they are locked together so that the front and rearwheels have to turn at the same average speed. This is why these vehiclesare hard to turn on concrete when the four-wheel-drive system is engaged.
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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 orfinal drive ratio. These referto the gear ratio in the differential. If the final drive ratio is4.10, then the ring gear has 4.10 times as many teeth as theinput pinion gear. See How Gears Workfor moreinformation on gear ratios.

When a car makes a turn, the wheels must spin at differentspeeds.
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On Thin IceIf 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 ofthe gearing in the transmission you will have lesstorque available to the wheels. This will make it easierto accelerate without spinning the wheels.

Now what happens if one of the drive wheels has goodtraction, 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 maximumamount of torque is limited to the greatest amountthat will not make the wheels slip. It doesn't takemuch torque to make a tire slip on ice. And when the

wheel with good traction is only getting the very smallamount of torque that can be applied to the wheelwith less traction, your car isn't going to move verymuch.
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Off RoadAnother time open differentials might get you into

trouble is when you are driving off-road. If you have afour-wheel drive truck, or an SUV, with an opendifferential on both the front and the back, you couldget stuck. Now, remember -- as we mentioned on theprevious page, the open differential always applies thesame torque to both wheels. If one of the front tiresand one of the back tires comes off the ground, theywill 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 toallow normal differential action when going aroundturns. When a wheel slips, they allow more torque tobe transferred to the non-slipping wheel.

The next few sections will detail some of the differenttypes of limited slip differentials, including the clutch-type LSD, the viscous coupling, locking differential and

Torsen differential.


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Clutch-type Limited SlipDifferential

The clutch-type LSD is probably the most common version of the limited slip

differential.

This type of LSD has all of the same components as an open differential, butit adds a spring packand a set ofclutches. Some of these have a coneclutch that is just like the synchronizers in a manual transmission.
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The spring pack pushes the side gears against theclutches, which are attached to the cage. Both sidegears spin with the cage when both wheels aremoving at the same speed, and the clutches aren'treally needed -- the only time the clutches step in iswhen something happens to make one wheel spinfaster than the other, as in a turn. The clutches fightthis behavior, wanting both wheels to go the samespeed. If one wheel wants to spin faster than theother, it must first overpower the clutch. The stiffness

of the springs combined with the friction of the clutchdetermine how much torque it takes to overpower it.

Getting back to the situation in which one drive wheel ison the ice and the other one has good traction: Withthis limited slip differential, even though the wheel onthe ice is not able to transmit much torque to theground, the other wheel will still get the torque itneeds to move. The torque supplied to the wheel noton the ice is equal to the amount of torque it takes tooverpower the clutches. The result is that you canmove forward, although still not with the full power ofyour car.
http://www.howstuffworks.com/clutch.htmhttp://www.howstuffworks.com/clutch.htm


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Viscous Coupling

The viscous coupling is often found in all-wheel-drivevehicles. It is commonly used to link the back wheelsto the front wheels so that when one set of wheelsstarts to slip, torque will be transferred to the otherset.

The viscous coupling has two sets of plates inside asealed housing that is filled with a thick fluid, asshown in below. One set of plates is connected to eachoutput shaft. Under normal conditions, both sets ofplates 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 correspondingto those wheels spins faster than the other. Theviscous fluid, stuck between the plates, tries to catchup with the faster disks, dragging the slower disksalong. This transfers more torque to the slower movingwheels -- the wheels that are not slipping.


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When a car is turning, the difference in speed between the wheels is not aslarge as when one wheel is slipping. The faster the plates are spinningrelative to each other, the more torque the viscous coupling transfers. Thecoupling does not interfere with turns because the amount of torque

transferred during a turn is so small. However, this also highlights adisadvantage of the viscous coupling: No torque transfer will occur until awheel actually starts slipping.


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A simple experiment with an egg will help explain the behavior of the viscouscoupling. 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 afaster 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 quicklystop 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 theyolk 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 forceto 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 betweenthe yolk and the shell.


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Hummer!

The HMMWV, or Hummer, uses Torsen differentials on the

front and rear axles. The owner's manual for the Hummerproposes a novel solution to the problem of one wheelcoming off the ground: Apply thebrakes. By applying thebrakes, torque is applied to the wheel that is in the air, and

then five times that torque can go to the wheel with goodtraction.
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How Differentials Work