Have an oscillating vibration between 80 and 100

[Paul G]

From the beginning,

I had a driveline vibration that started to come in around 45 mph and just got worse with increasing speed, to the point where the rear view mirror was visibly shaking at 80 mph.

Got new tires on all four corners. That made it a little better.

Had the drive shaft balanced, it was slightly bent and out of balance. That changed everything. It is so much better now, smooth all the way to 80 mph. But, I still have that oscillating vibration, kind of a wrrrr....wrrrr....wrrr.... etc., in the floor and seat of the pants between 80 and 100 MPH. (professional driver on closed course    )

Checked pinion angles. Have about ½ a degree trans to driveshaft (driveshaft running down hill toward diff). About 1 degree driveshaft to diff. (diff pointing down). Picked up a 4 degree wedge pair to put under the axle. No help, still have that oscillating vibe around 80 to 100.

Trans mount... tight!

Engine mounts... tight!

Axle U-bolts.... Tight!

FYI. Last month while installing the Bouchillon trans kick down cable kit I noticed the trans mount bushing was sitting way to the driver's side. Bad. I had to shim the passenger side engine mount forward 3 washer thickness to get the trans mount bushing to sit square in the mount. The Bouchillon trans kick down cable bracket would rub on the trans tunnel the way it was. Needed the trans to be in the center of the tunnel so the cable bracket, which mounts on the driver's side of the trans, would not touch the tunnel.

I have been eliminating vibrations, one by one, for some time now. This is just the latest.  

[tan top]

cant suggest anything really  ,  only  just to eliminate  more stuff  , pull all wheel bearing  & check clean & re pack with grease install to correct specs    , check to see if the factory balance weights are still on the drums  
see you have road wheels / magnums in your aviator picture , check to see if the  any of the lug nut holes are elongated  or have been over torqued & the tip of the lug nut has actually not gone trough the wheel & hitting drum/ hub    also would of thought  the dude who balanced your new tires would have picked up on any wheel with excessive run out or buckle  
keep coming back to the drive shaft being slightly bent & then re balanced  ,
bent drive shaft & even if it is only slight  !! don't sound good ,  
also when the vibration his at your indicated speed !!  make a note of the revs & see if it happens  at the same revs in another gear , if it does , think i would be looking at may be something going on in the gearbox ?? comverter ??   

[RECHRGD]

I would still look at the driveshaft balance.  I was chasing a vib like that for over a year after installing a Geat Vendors OD.  I had the shaft balanced several times and even had a whole new one made.  I finally "balanced" the shaft on the car while on jack stands using two pipe clamps.  It took hours to get it right.  Bob

[flyinlow]

I had a cyclic vibration caused by loose trans to engine bolts. Had a badly out of balance Alt. that sounded like drivetrain noise.


Does the vibration change if you speed up to 100 then shift to neutral and coast? (closed course, pro driver)

Try fliping the driveshaft 180* in the companion flange.    Just a couple ideas, good luck.

[Paul G]

Does the vibration change if you speed up to 100 then shift to neutral and coast? (closed course, pro driver)

When rolling down the road well over 80 mph popping it in neutral made it go completely away only one time, most other times not. If I am on the gas then off the gas during the vibration episode I can make it worse by getting on the gas, lessen it by getting off the gas. I just replicated this with the car sitting on four jackstands in my garage, running the car up through this vibration zone it worsens when on the gas, lessens when off the gas.

Trans output shaft bushings? The driveshaft was bent and so far out of balance for who knows how long. He had to add weight to the front of the driveshaft to balance it. Could that have damaged the trans output shaft bearing? The driveshaft is off the car right now. There is some play, side to side, up and down, even in and out, in the trans output shaft moving it by hand. Is this normal? 

How does a drive shaft get bent anyway?   

[flyinlow]

Increasing the thottle at 80mph would increase the engine/torqueconverter speed a few 100 rpms ,make the drivers side of the engine raise slightly,while the passenger side of the engine would torque down, the front of the rear axle pinion would raise slightly pushing the driveshaft yolk slightly deeper into the trans and the exhuast pitch would change due too more exhuast flow. What in that list would make the vibration worse?


The drive shaft yoke should slide in and out of the trans. It rides on a bronze bushing in the tail shaft. There is a ballbearing farther forward in the tailshaft that suports the transmission output shaft that the driveshaft yolk splines over. The yolk can be pushed up and down some in the transmissions I have had. Maybe John or RD can give you the max. play.

I would think, running over something, high centering the car, or mishandling while the shaft is out of the car could bend It. I put a discount u joint in my RT 30 years ago. Snapped at 2-3 rd shift , bent the shaft 45* broke the tailshaft,ripped the companion flange ears off and took out the left exhuast sytem in about 1/2 second at 80 mph. I was very lucky the car did not pole vault . The junk yard driveshaft I got to repair the car needed balanced twice.

[Patronus]

Sounds like tires to me. Did you have them balanced at the hub or the lugs?