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Old 11-09-2006, 10:52 AM   #17
Scooby Newbie
Member#: 80430
Join Date: Jan 2005
Location: loosening gas-caps on RX8s
1997 Eagle Talon
(plus a Legacy beater)


I have already posted links to the detailed primers that I have written on both differentials and limited-slip devices. Stop whining and do your homework.

As to Torsens and clutch-packs: you are correct that in both cases, no resisting torque is produced until some slippage occurs, but that misses the deeper difference between the two. Type-2 Torsens don't even start to lock until there's an imbalance in the amounts of reflected torques from the two outputs. Until that moment, the helicals are happily sitting in the middles of their pockets, with no (serious) force against the end walls. In contrast, modified clutch-packs (aka Salisburys) add clamping force on the plates as a function of input torque. This means that they are often already locked before either output starts slipping. It also means that they are often clamped solid when you have no wheel slip, which is a serious negative when powering out of a corner.

This is a huge difference. It causes a helical to be almost invisible until the moment that it is needed. Yes, a helical can't handle situations where one output reflects no torque at all (since helicals are only capable of a certain ratio of output torques and anything times zero is still zero), but the fact that a helical stays out of the picture until it is needed makes it ten times better for the front of a car (and if you really are lifting a wheel, then your problem is suspension, not drivetrain).

In contrast, because a clutch-pack is already locked when it needs to be (and doesn't snap open when you lift a wheel), it is a much better option in the rear (or anywhere in the system if all you do is drag-race). It takes less than a tenth of a second to snap-spin an AWD car. You can't afford a rear diff that is only going lock when a problem arises; you'll already be looking back at where you just were. You want the rear locked in advance. A little under-steer due to it being locked when no wheel slip is happening is a small (and affordable) price to pay.

- Jtoby
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