Car Shakes at 45-70 mph

[81CutlassDsl]

I have an 85 Cutlass that i can not find the source of a shake. It has done this for several years now (car got parked from 2010-2012 because of this). I feel it in the seat and not in the wheel. The shake seems to diminish when the car is on a road with concrete expansion joints.

I have tried
A. New Shocks
B. New U-Joints
C. Balancing the Tires
D. My 81 does not have the shake so i swapped tires between the 2 cars.
The 85 still shakes even with the 81 tires on it.
The 81 did not shake with the 85 tires on it.
E. New Tie Rods
F. Ball joint has some slop in it and will be replaced shortly.

Kind of at a loss. I know the rear springs on the 85 are getting weak. Anyone know what could be causing this?

 

[joesregalproject]

Are you sure it might not be body bushings?

 

[DRIVEN]

Mine did that when it had a bent axle.

 

[seawolf18]

Take the drive shaft out and get the balance checked on it. If its out of balance it will cause a shake you fell in your butt\seat. Could also be a bend axle like the other poster said.

 

[tc1959]

Separated tire possibly, Flat spot from sitting . I would find someplace to get it up on a lift and have a look at everything underneath while it is in gear.

 

[JohnS]

I had the same issue but my shake started at 55. I had 2 bad motor mounts and a bad trans mount, once replaced no more shake. This was on an 87 Cullass with a 3.8.

 

[gp02a0083]

check the drums as well to see if the counter weight has fallen off

 

[ed209]

Drive shaft centered my bros caprice did a rear end swap put the driveshaft in off center and it viberated like crazy

 

[81CutlassDsl]

The Car is still at it. Getting ready to just part it out and find another one.

Additional
G: New Rear Springs
H: All body Mounts are in place
I: New Rear Drums
J: Force Balanced the Tires
K: New Rear Axle Bearings and shaft runout checked.
L: Drive shaft removed and balanced.

The shake now starts at around 55 and lasts to 65. Also it seems to get worse the longer you drive it.

Bumps still seem to disrupt the shake. If i throw the car in neutral while the shake is present... the Shake is still there. I assume that rules out any engine issues. I am totally at a loss at this point. Thrown more parts and money at the car than it is worth. Any other ideas?

 

[CWPottenger]

Have seen it occur with a bad rear control arm bushing or a "tweaked" control arm.