Mean Vibration On Decel

[Phil's 71 Z]

Help Me !!!!
I have had a bad vibration only on decel. I have a s/b chevy bolted to a Doug Nash 5-speed and a 10 bolt with eaton posi. 1st and second have a bad vibe and the rest of the gears are not as bad. I pulled the tranny and had it looked at. I had a new drive shaft built. I removed and replaced the whole rear, gears and posi unit. I moved the exhaust around. I tried changing the pinion angle. I am stumped !!!!



I can jump on accel without a problem.

[sporter]

Sounds like you've taken the right steps. Next thing I would look at is the pinion angle under deceleration, your differential may be rotating just enough to get a bad pinion angle. How loose is your rear suspension and what kind of traction device do you use? We had South Side Machine bars on my son's '79 Z28 and they really helped to control the pinion angle under both acceleration and deceleration.

[GetMore]

I second the pinion angle on decel. That's what it sounds like to me.
One other thing to look at is the engine and transmission mounts. Maybe something is shifting over?

[minendrews68]

A lot of times overlooked but what about the U-Joints? Usuallly most things that are under a load (acceleration) things tend to be bound up and do not vibrate as much as with deceleration is probably why your noticing it more at that time. Be sure to look at all your mounts, even look at loose wheels, anything that rotates could possibly be the problem. I once had a vibration that you could barely feel, at idle you could not tell it but as you increased rpm you could. I changed rear end bearings, u-joints, clutch assy, just about everything I could think of when one day I was outside the car telling a friend of mine about the problem, started the car and you could feel it, not bad, but you could feel it after you revved the car up a little. He thought a while and told me to pull the fan belt off and start the car again. I did and no vibration at all, the culprit...I had used the stock fan on my Camaro (the clutch fan) and it was bad. Replace with a flex fan and never had any more vibration. I know clutch fan and flex fan but that was about 25 years ago..

[andrewb70]

How far is the slip yoke engaged on the output shaft? How much can you move your slip yoke up and down? Is the tailshaft bushing worn?

I am still chasing a vibration in my GTO with a Richmond 6 spd.

Andrew

[Phil's 71 Z]

I've changed the pinion angle up and down dozens of times, changed the driveshaft with a new one with new u-joints, tried long and short yokes, no play in output shaft, loosened exhaust and moved it around, does not have a clutch fan. I have a WS6 trans-am rear end brake set up. I am starting to wonder if it has something to do with the E-BRAKES ?? But thanks for all your imput guys.

[Paul_J]

I believe what several people are recommending is not changing the pinion angle but checking to see if the pinion angle is changing during deceleration. During acceleration the pinion tries to climb up the ring gear rotating the pinion up if it is not controlled by some type of suspension component. During deceleration it will rotate downward. Certain suspension devices work great limiting upward rotation but do little to limit downward rotation. So the question is, what type of rear suspension do you currently have? I hope I didn't oversimplify this or talk down. Just trying to clear up some things and get your car fixed.

Just thought of this test to do:

Apply the brakes and put the car in reverse. With your foot still on the brake load the suspension and have someone safely observe the pinion movement.

Also, some pictures of the rear end setup would help.

[BritishGreen68]

My car did the same thing... let off the gas at the end of the 1/4 mile at about 110 and holy ****...... vibrate you to death. it was the pinion angle for me, I know you checked it but you should do (more) research on it. My friend just had this problem only it was minor but noticeable, his was the output shaft on tranny was too short... next maybe check tires..? there are only so many things that spin.. you can narrow it down by determining how fast of a vibration it is, if its slow it could be tires if its very fast it could be drivline issue.. that sort of thing.. you could take it to a chassis dyno and have them help you figure it out, get it dynod at the same time..