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LJM97Z 12-08-2012 01:05 PM

Car shakes in rear?
Ever sence i installed my new 9" i'm getting a shaking that starts around 50-55 mph, I've installed new rear tires & checked "runout" on the axles/drums, U joints are new, I was told it might be a driveshaft problem, but why after installing a rearend, Any ideas would be greatly appreciated.. Its a 78 cutlass G body.

hcompton 12-08-2012 01:17 PM


Originally Posted by LJM97Z (Post 1620465)
Ever sence i installed my new 9" i'm getting a shaking that starts around 50-55 mph, I've installed new rear tires & checked "runout" on the axles/drums, U joints are new, I was told it might be a driveshaft problem, but why after installing a rearend, Any ideas would be greatly appreciated.. Its a 78 cutlass G body.

Ford 9" right?

Is it installed dead straight any crab walking or other issues. Does it do it on and off the gas. New tires are new with the rear or replaced after the rumble was noticed. New tires can be bad or have a bubble just like old one. That is why i ask.

Did you check the rotors and other parts for damage or wear. New does not guarentee right.

Where did you get the rear is it an aftermarket kit or just an old rear reused or rebuilt. A bad rear can vibrate and rumble and may need to be rebuilt may also need a new bearings which is normally the reason for vibration unless it has broken gears. Usally its the inside bearing that are bad. Since you can replace the outside ones alot easier. But replace them all if you go in.

Is it four link. How it is installed. Welded to the roll cage or the cars frame?

Sorry for all the questions just trying to get a handle on the problem so at least someone on here will be able to help.

scholman 12-08-2012 01:19 PM


I had this happen one time. I installed a new U-joint on a Dodge van and it cam back with a vibration like yours. It turned out that I had not centered the caps on the U-Joint in the rear end yoke. I slid it over and the vibration was gone. It is a place to start.

LJM97Z 12-08-2012 01:30 PM

Thanks guys, it is a new rearend (all new), just the houseing was fabbed up from a reputable shop in iowa, I'm still trying to determine where my pinion angle is at, The 3rd. member is -4, the driveshaft slopes upward 2*, I was told 2 different things on this, One shop said its at -6*, the other said -2*, I'm trying to cover all the basics, I have a good 1500.00 in this rearend, All the parts are from Quick Performance out of Iowa. The cars a 4 link control arms.

The one thing i have noticed is there is a 1/2" difference between the DS & PS front wheel well to the tires.

lmsport 12-08-2012 03:47 PM

Have the drums been balanced?
There should be some indication on the drum of how material was added or removed.

hcompton 12-08-2012 04:13 PM

Well with the four link you should be able to change the pinion angle pretty easily. I would give it a try in both directions and see which ones tunes out the problem.

With four links i like to use a Fabric tape measure the kind used for sewing just to double check things quickly. Measure all the control arms and alingments and make sure its in the car straight and adjusted straight with the front wheels.

If no love from pinion angle adjustments i would take the car and have it put on a four way alignment rack and make sure its square and properly aligned. Also check the front tires you never know.

If all that fails try a euro shop and see if they have a on car wheel balancer and have a custom drive shaft built. A good drive shaft shop may also be able to find the problem just by throwing the shaft on a balancer. Fabbed up drive should not be that expensive.

Richiehd 12-08-2012 04:58 PM

Who made the driveshaft? and are the trunnions timed properly? Driveshaft angle sounds good. Check wheek balance again, all 4. Does it shake in the seat (Rear) or the steering wheel (front)?

I had a Mustang once checked everything, trunnions were turning in the driveshaft yokes. Had a new shaft made, perfect!

LJM97Z 12-08-2012 05:07 PM

The driveshaft is factory, so this may be the problem, its very possible the problem was there with the old rearend but just wasn't as noticable. I'm going to have the driveshaft checked 1st.

LJM97Z 12-10-2012 11:16 AM

Had the driveshaft checked, it was out of balance quit a bit, so i hope i found it.

fordss 12-10-2012 12:35 PM

so did u get the wheel balanced it sounds like that Cheaper fix

LJM97Z 01-24-2013 10:26 PM

Well i changed out the front tires/wheels with another set i had, replaced the inner/outer tie rods/drag link & steering box, front end is much tighter, also changed the rear tires/wheels, The shacking is still there, it is no better/worse, it happens around 60-65, I'm really puzzled on this, I do feel it in the seat though, The tranny mount is good, Is it at all possible that the transmission could be causing it? Like a bent spline, The only other thing is it has to be something in the rearend, i just don't know what.

I've also currected the pinion angle & alligned the rear tracking.

What else should i look for, could it be axle/pinion/carrier bearing issues, all this stuff is brand new.

One thing it does do, when i'm on the throttle hard its not that bad, but gets worse off the throttle when its at those speeds.

Just to clarify, Its not a "vibration", more of a shacking/wobbling.

timothale 01-25-2013 09:17 AM

pinion angle
When getting on the throttle the rear usually squats, changing the pinion angle , Check pinion angle vs trans output shaft angle. adjust the pinion to get it correct.
A wrong length drive shaft from mixing parts ?, Go to inland empire drive shafts and check out their how to for drive shafts.

dklong 01-25-2013 10:17 PM

Like timothale, said pinion angles change under acceleration and decel, are you using the factory 4 link or aftermarket links, ( you can buy adjustiable top links , to replace the factory ones . to change the pinion angle) what rear springs are you using , get the car sitting at the right ride height,

prostreet6t9 01-26-2013 01:36 PM

Normally when you have a angle problem you will feel the shake during accel or decel. Have the D/L checked for straightness and balance and verify the rear joint is correct to the yoke.

S10xGN 01-27-2013 11:17 AM

Since it's a GM G-Body, you might also check and be sure the 4 link bars are OK. This happened to my son's MCSS one time and it was a broken lower link that allowed the rear axle to move around...


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