My 89 LX 5.0 convert has a vibration that I can feel between 40 and 50 mph. Sometimes it's in the seat and sometimes I can see it in the windshield frame. It put new tires on it. I replaced the rear shocks but not the quad shocks. I replaced outer rear wheel bearings and diff fluid. Replaced trans fluid (5 speed car). No metal in trans fluid or rear diff fluid. I haven't taken it to any shops just trying to figure it out on my own. Everything is stock. Car has 60,000 miles on it.

 

Being that the vibration occurs with speed 40/50.
Than it must be in the tires wheel or brake drums/discs.

Locating the bad component can be quite a challenge for an individual. But here goes any way. Raise one at a time a rear wheel on a jack stand and using the motor spin it up to about 50 MPH on the speedometer. Note the amount of vibration from each wheel. You will of course have to rotate tires from to rear to test the front.

Just a guess but look and see which tire has the most balance weights. the one with the most weight is the most likely guilty party. It may be O/R or out of round?

 

John, I'm just wondering if that technique will work with a posi-traction rearend. Seems you would need both rear wheels off the ground.

Racedaddy, did the vibration exist before you installed the new tires or did it begin afterward? If it was there before, then it likely isn't the tires. Other possibilities could be a bent or tweaked wheel or possibly a bad driveshaft.

Paul, any suggestions?

 

The vibration was there before the new tires. It had an odd size on there and the owner told me his daughter had a blow out. I bought the car and immediatley replaced the tires. Driveshaft doesn't have any dents or anything. One wheel on the front has some curb rash and the wheel weight is right on top of it. Maybe I can find someone local to let me try their wheels out to see if the problem goes away.

 

You would be correct in the case of some but not all posi-traction rear ends
Most or maybe I should say many posi-traction are of the limited slip type.

You can test with both wheels off the ground and the car in neutral not running.

Spin the one rear wheel by hand. In a non posi-traction the other wheel should go the opposite way if at all. If they go the same way when one is spun by hand than you have a posi-traction

The reason for a limited slip posi-traction is said to be safety.
Especially when one wheel gets on ice.

A true 100% posi-traction should only be used at the track IMO

When ever my shop had to do an on the car spin balance on a true 100% possi traction we would remove one wheel and place all the lugs nuts on upside down to hold the drum on while balancing the other wheel. Than reverse for the other side
We would lift the car and put a rim under the not to be balanced rear tire.
It was a real eye opener when we came upon a true possi-traction
For the rim would shoot out of there real fast. BOOM CLANG a LANG

racedaddy
Another thing to look for on both the Rear Brake drums or the drive shaft
is the possibility that balance weights may have fallen off of them.

However, there are NO such weights on Disc Brake ROTORS
These are drilled with holes to balance them. The one with most holes would be the first suspect.

I once had over sized lug nuts that required a couple washers to use
correctly. In my haste I once torque down one lug with out the washers.
This ruined the brake drum.

I discovered it by spinning the drum fast like by hand with no wheel on it.
It spun well. But, it decided to change directions with out my intervention.

Apparently the stretched lug wanted to always be on the bottom.
Thus, showing me I had a high point on my drum.

Should this be it?
Than when purchasing a drum or Disc you might want to do some cherry picking at the parts counter.

Look for the the least corrections

With a drum that means smaller weights
With a disc that means less holes drilled in it.

I once had 4 disc opened and on the counter just to find the 2 that balanced the easiest.

The parts guy thought I was crazy.

I once did on the car spin balancing years ago.
I don't know if that kind of wheel balancing is around anymore
But I think it better than the Computer balance of today IMO.

 

go to a shop and get your wheels balanced first, get the simple easy stuff out of the way first.


100% limited slip is still limited slip, you're thinking of a locker where you never get the one wheel deal, posi-traction is a chevy word, we have traction-lok!