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Old 05-12-2002, 04:29 PM   #31
ivymike1031
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If you look at just the differentials, then there are about 3 main type that you'll find, with a few sub-types crammed into those:

* Open differential (commonly used on front axles of 4wd vehicles): there is no slip-limiting mechanism. Torque to each wheel is equal, and speed of each wheel is completely independent of the others. The downside is that when one wheel starts to slip, the non-slipping wheel gets the same amount of torque as the slipping wheel, which means that the torque to the non-slipping wheel is severely restricted.

* Limited slip differential (several varieties): there is a slip limiting mechanism that works counter to a speed difference between the wheels, effectively transfering torque from a slipping wheel to a non-slipping wheel.
- viscous limited slip uses a fluid coupling between the axle shafts. The torque difference between the shafts is roughly proportional to the square of the speed difference between them
- frictional limited slip uses a clutch mechanism between the axle shafts. When one starts to slip, the torque difference between the shafts is sometimes (there are also schemes that alter this behavior) a fixed quantity independent of slip speed.

* Locked differential (there are many varieties, usually they are equipped with some sort of a mechanism to unlock the differential for driving situations where traction is available): These differentials force the tires to spin at exactly the same speed. For straight-line driving, if one tire is not slipping, then the other one is prevented from slipping. Torque is split based on available traction at each wheel


If you look at the whole 4wd/AWD system, then the biggest differences besides the above are:
- whether or not the transfer case includes a differential, and what type, if any, is included (open, limited slip, lockable, etc)
- how the differential is controlled, if there is one (manually mechanically shifted, manual air or electronic control, automatic electronic control, etc)


more info:
http://www.howstuffworks.com/four-wheel-drive.htm
http://www.howstuffworks.com/differential.htm

blah.
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Old 05-13-2002, 12:33 AM   #32
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Quote:
If you look at the whole 4wd/AWD system, then the biggest differences besides the above are:
- whether or not the transfer case includes a differential, and what type, if any, is included (open, limited slip, lockable, etc)
- how the differential is controlled, if there is one (manually mechanically shifted, manual air or electronic control, automatic electronic control, etc)
I would also add the qualifier of whether or not the system is intended to be used full time or part time. That's one of the biggest functional differences present in this symantic argument.
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Old 05-13-2002, 08:11 AM   #33
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Whether or not they're going to be used full time is the primary factor driving the choice for a "center" differential, right? (the one in the transfer case) Part time systems can get away with not having a center differential, while full-time systems absolutely have to allow slip between the front and rear driveshafts?

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