Rear end questions
Thread Starter
Instructor
Joined: Apr 2021
Posts: 133
Likes: 5
Rear end questions
Ok so I I’m confused I thought all 1973 corvettes had posi rear ends, I had my car on the lift and tried to turn the back tire in neutral and also tried in gear and when I turned it the other back tire didn’t even move in any direction so does that mean I don’t have a posi? Another problem is my wheel turns easily but I get to this one point and it gets really hard to turn then once I’m past it it’s easy again until I get to that one point. This happens with both wheels.
Race Director
Joined: Mar 2017
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From: Virginia
AW is 3.08:1 in 1968 thru 1975 Corvettes. Posi was in all C3 diffs from 71 to 82.
But who knows what's in there now.
If that is a working posi diff, on a lift, in neutral, both wheels should turn forward. In gear, (manual?), you shouldn't be able to turn the wheels by hand. In the case of a car with no posi, which seems likely, in Park (auto) or in gear (manual), the wheels should spin in opposite directions, while the driveshaft stays put. The hard to turn spot you encounter is normal, and is likely the U-joints partially binding (which is why you must never turn the wheels with the engine while the rear suspension is drooping).
I don't know what is going on inside your diff, but I'd start by pulling the drain plug, and doing some investigation of the oil, and fishing with a magnet.
EDIT: With the car in neutral, does the driveshaft turn when you turn one wheel? Does this happen for each wheel, or only one of them?
But who knows what's in there now.
If that is a working posi diff, on a lift, in neutral, both wheels should turn forward. In gear, (manual?), you shouldn't be able to turn the wheels by hand. In the case of a car with no posi, which seems likely, in Park (auto) or in gear (manual), the wheels should spin in opposite directions, while the driveshaft stays put. The hard to turn spot you encounter is normal, and is likely the U-joints partially binding (which is why you must never turn the wheels with the engine while the rear suspension is drooping).
I don't know what is going on inside your diff, but I'd start by pulling the drain plug, and doing some investigation of the oil, and fishing with a magnet.
EDIT: With the car in neutral, does the driveshaft turn when you turn one wheel? Does this happen for each wheel, or only one of them?
Last edited by Bikespace; Sep 11, 2021 at 03:19 AM.
Thread Starter
Instructor
Joined: Apr 2021
Posts: 133
Likes: 5
AW is 3.08:1 in 1968 thru 1975 Corvettes. Posi was in all C3 diffs from 71 to 82.
But who knows what's in there now.
If that is a working posi diff, on a lift, in neutral, both wheels should turn forward. In gear, (manual?), you shouldn't be able to turn the wheels by hand. In the case of a car with no posi, which seems likely, in Park (auto) or in gear (manual), the wheels should spin in opposite directions, while the driveshaft stays put. The hard to turn spot you encounter is normal, and is likely the U-joints partially binding (which is why you must never turn the wheels with the engine while the rear suspension is drooping).
I don't know what is going on inside your diff, but I'd start by pulling the drain plug, and doing some investigation of the oil, and fishing with a magnet.
EDIT: With the car in neutral, does the driveshaft turn when you turn one wheel? Does this happen for each wheel, or only one of them?
But who knows what's in there now.
If that is a working posi diff, on a lift, in neutral, both wheels should turn forward. In gear, (manual?), you shouldn't be able to turn the wheels by hand. In the case of a car with no posi, which seems likely, in Park (auto) or in gear (manual), the wheels should spin in opposite directions, while the driveshaft stays put. The hard to turn spot you encounter is normal, and is likely the U-joints partially binding (which is why you must never turn the wheels with the engine while the rear suspension is drooping).
I don't know what is going on inside your diff, but I'd start by pulling the drain plug, and doing some investigation of the oil, and fishing with a magnet.
EDIT: With the car in neutral, does the driveshaft turn when you turn one wheel? Does this happen for each wheel, or only one of them?
-yes the drive shaft turns when I turn one wheel I would have to check the back right wheel I think it does also, how else am I supposed to tune the wheels to check without the bag suspension drooped, when it’s jacked up the back is gonna droop???
Tech Contributor
Joined: Aug 1999
Posts: 15,173
Likes: 3,982
From: Connecticut, USA
As mentioned above, if the car is on a lift with the suspension hanging then yes it will bind in one spot since the u-joints are at a severe angle. 73 would have had posi as standard. Open diff's were standard from 53-69 then posi was standard.
With the wheels at ride height turning them at one wheel should move both in the same direction. Open diff's they would turn opposite unless you had someone hold a wheel.
The posi break away torque is 40 ft/lb. That is how much force should be needed to break a wheel loose while the other wheel is secure. So you might have weak posi springs or may you have tuned posi which works better then a stock spring posi but looks like and open diff when checking on a lift like you did. Tuned posi's are not stock, were never offered by GM, were developed by some Ca hot rod guys 60 years ago and if done correctly last longer, lock up fine, and typically don't chatter. The key to building them is knowing there is more to it then leaving out the spring pack - that's where many fail trying to tune them.
With the wheels at ride height turning them at one wheel should move both in the same direction. Open diff's they would turn opposite unless you had someone hold a wheel.
The posi break away torque is 40 ft/lb. That is how much force should be needed to break a wheel loose while the other wheel is secure. So you might have weak posi springs or may you have tuned posi which works better then a stock spring posi but looks like and open diff when checking on a lift like you did. Tuned posi's are not stock, were never offered by GM, were developed by some Ca hot rod guys 60 years ago and if done correctly last longer, lock up fine, and typically don't chatter. The key to building them is knowing there is more to it then leaving out the spring pack - that's where many fail trying to tune them.
Thread Starter
Instructor
Joined: Apr 2021
Posts: 133
Likes: 5
As mentioned above, if the car is on a lift with the suspension hanging then yes it will bind in one spot since the u-joints are at a severe angle. 73 would have had posi as standard. Open diff's were standard from 53-69 then posi was standard.
With the wheels at ride height turning them at one wheel should move both in the same direction. Open diff's they would turn opposite unless you had someone hold a wheel.
The posi break away torque is 40 ft/lb. That is how much force should be needed to break a wheel loose while the other wheel is secure. So you might have weak posi springs or may you have tuned posi which works better then a stock spring posi but looks like and open diff when checking on a lift like you did. Tuned posi's are not stock, were never offered by GM, were developed by some Ca hot rod guys 60 years ago and if done correctly last longer, lock up fine, and typically don't chatter. The key to building them is knowing there is more to it then leaving out the spring pack - that's where many fail trying to tune them.
With the wheels at ride height turning them at one wheel should move both in the same direction. Open diff's they would turn opposite unless you had someone hold a wheel.
The posi break away torque is 40 ft/lb. That is how much force should be needed to break a wheel loose while the other wheel is secure. So you might have weak posi springs or may you have tuned posi which works better then a stock spring posi but looks like and open diff when checking on a lift like you did. Tuned posi's are not stock, were never offered by GM, were developed by some Ca hot rod guys 60 years ago and if done correctly last longer, lock up fine, and typically don't chatter. The key to building them is knowing there is more to it then leaving out the spring pack - that's where many fail trying to tune them.
Tech Contributor
Joined: Aug 1999
Posts: 15,173
Likes: 3,982
From: Connecticut, USA
No, I have not yet wrote all the details on how to do a correct posi tune and I am not sure I will.
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Tech Contributor
Joined: Aug 1999
Posts: 15,173
Likes: 3,982
From: Connecticut, USA