which one?
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Member
Joined: Mar 2004
Posts: 349
Likes: 1
From: CO
Car: 1990 Firebird
Engine: LS3
Transmission: 4L60E
which one?
ok I'm wondering, everyone it seems like has or is planning on getting a posi rear end. if I understand that correctly that means both of the drive tires actually drive the car where as the stock open end only drives the rear left tire. (is that correct is my first question). I haven't seen anyone that has a limited slip rear end, if I understand that correctly its a combo of both of the above. so which would be better then? a posi or limited slip? thanks in advance
Supreme Member
Joined: Oct 2003
Posts: 1,083
Likes: 4
From: DFW
Car: 1992 Z28
Engine: 5.7 TPI
Transmission: 700R4
Axle/Gears: G80 3.23
I think the passenger side is the one that drives the wheel.
And I think if you have a G80 installed, it only spins both wheels up until 25mph. If you need both wheels for traction 25+ then damn.
And then I believe a true posi is a full locker. Kind of expensive.
And I think if you have a G80 installed, it only spins both wheels up until 25mph. If you need both wheels for traction 25+ then damn.
And then I believe a true posi is a full locker. Kind of expensive.
Supreme Member
Joined: Jul 1999
Posts: 18,457
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From: Loveland, OH, US
Car: 4
Engine: 6
Transmission: 5
"PosiTraction"= Limited Slip
It's a trade name for it, that GM used back in the 60s.
The 2 terms can be used interchangeably as far as describing the function at hand.
There are also "lockers", which "lock" the wheels together always; great for straight-line traction but not so good for the street. There are also "ratchets" (lots of names for that) like the NASCAR guys use.
Your understanding of what happens is close to right, but not entirely. What an "open" or non-posi or "unlimited slip" rear does, is to allow the 2 wheels to turn at different rates, such that the average speed of the 2 is equal to what both of their speeds would be if they were traveling in a straight line. If a wheel loses traction and begins to spin, it's possible for that wheel to spin at twice the speed it should, and the other to sit absolutely still, receiving virtually no drive power. It could happen on either side, since in theory the drive is applied equally. In practice though, because the torque of the drive shaft (rotating clockwise viewed from the front) tends to lift the right rear off the ground, thereby removing its ability to maintain traction.
A "limited slip" or "posi" or whatever you want to call it, simply puts a clutch system in there, such that in order for a wheel to spin at some different rate, the clutch friction has to be overcome first. In these cars it takes about 50 ft-lbs of torque to do that. That means that 50 ft-lbs is the minimum drive torque that will ever be available. You could put one wheel on ice and one on asphalt, and the one on asphalt will still get its 50 ft-lbs no matter what. So at least you can go somewhere. There are other systems (the Zexel Torsen for example) that don't use clutches, but the end result is the same. And there are some (the Eaton Gov-Lock, for instance) that act like open rears until the RPM difference between the 2 exceeds some value, indicating that one of them must be spinning; it then applies the clutches. But the majority of them for these cars are the pure clutch type as described above; no RPM or speed dependency of any kind, they act the same at all times.
It's a trade name for it, that GM used back in the 60s.
The 2 terms can be used interchangeably as far as describing the function at hand.
There are also "lockers", which "lock" the wheels together always; great for straight-line traction but not so good for the street. There are also "ratchets" (lots of names for that) like the NASCAR guys use.
Your understanding of what happens is close to right, but not entirely. What an "open" or non-posi or "unlimited slip" rear does, is to allow the 2 wheels to turn at different rates, such that the average speed of the 2 is equal to what both of their speeds would be if they were traveling in a straight line. If a wheel loses traction and begins to spin, it's possible for that wheel to spin at twice the speed it should, and the other to sit absolutely still, receiving virtually no drive power. It could happen on either side, since in theory the drive is applied equally. In practice though, because the torque of the drive shaft (rotating clockwise viewed from the front) tends to lift the right rear off the ground, thereby removing its ability to maintain traction.
A "limited slip" or "posi" or whatever you want to call it, simply puts a clutch system in there, such that in order for a wheel to spin at some different rate, the clutch friction has to be overcome first. In these cars it takes about 50 ft-lbs of torque to do that. That means that 50 ft-lbs is the minimum drive torque that will ever be available. You could put one wheel on ice and one on asphalt, and the one on asphalt will still get its 50 ft-lbs no matter what. So at least you can go somewhere. There are other systems (the Zexel Torsen for example) that don't use clutches, but the end result is the same. And there are some (the Eaton Gov-Lock, for instance) that act like open rears until the RPM difference between the 2 exceeds some value, indicating that one of them must be spinning; it then applies the clutches. But the majority of them for these cars are the pure clutch type as described above; no RPM or speed dependency of any kind, they act the same at all times.
