Vibration in vehicle above 66 mph. Then oscillating then somewhat smooths out.

[Eric F]

1970 Road Runner. The vehicle has what feels like a driveline type vibration that comes in about 65-66 mph. It turns into an oscillating type vibration around 68mph to about 72 then the oscillation tapers out. About 2800 -3200 engine rpm. I bought the car in 2019 and was a rolling body. It came with the original engine rebuilt. A rebuilt 727. When I installed the engine and transmission I installed a Hughes unbalanced converter as the 383 is internally balanced. I used all new engine and transmission mounts. When I went to install the original driveshaft I noticed there were hose clamps on the shaft near the front joint . That tells me that someone was chasing a vibration before the car was taken apart.(car sat apart for 26 years) I took the driveshaft to a local guy who installed new joints and balanced the original shaft. He stated it was really close. I got the car running and experienced the vibration. I checked my driveline angles and found my drive shaft is at 1.8 degrees. Well within the .5 to 3.0 all internet car builders and guys who do custom engine swaps set there’s at. I had my local guy build me a driveshaft actually measuring from the transmission to differential. Guess what length as stock. I changed the complete center diff assembly from 3.55 to 3.23 all that did was change the speed the vibration is at. Still,same rpm. I changed the axle bearings to Green bearing from mopar. NO change. I swapped the axle from another set I had and on these I used DR diff’s bearing(updated green bearings with snap rings) no change. Again checked the driveline angle and came up 1.8 well within tolerance. I’ve tried different rims and tires from another car just to rule out my rims and tires no change. I had an oil leak from the front crank seal so at the time I installed a fluid damper of course NO change. I did replace the transmission tail housing with another one. I made sure I replaced the seal and bushing. The bushings was worn. The output bearing is ok. I recently replaced the rear leave springs bushing as the car had urethane bushing no change for the vibration . Just a little softer ride.
Driving the car this weekend I did notice that as the ride height changes due to road conditions. I.e suspension loads it’s seems to get more pronounced.
The suspension unloads it diminishes.
I did some research and found out the transmission that was in the car when it taken apart years ago had a cracked case. The shop that did the transmission swapped the parts from that transmission to a different case.
Please make to read the complete story before answering it’s this or that when this or that have already been replaced with NO Change.

 

[pnora]

Does it vibrate at all with the car sitting still? Seems like it is rpm related. I would check the crank to see what is in there.

 

[RemCharger]

By driveline angle do you mean the angle of the driveshaft?

If it's not engine vibration as mentioned, I'd try throwing in some 2 degree ( nose down) axle shims and see what it's like

 

[#41]

Mild vibrations can be tricky. You have sure covered the bases chasing this down. A few ideas:

You didn't say if the vibration is specific to 2800-3200 rpm in all gears, but with everything you have been through I would (1) wonder if the engine was performance balanced?

(2) I once had an issue with a bent 727 tailshaft itself. Turns out the trans had been used in a demo car and it was just the slightest bit tweaked. It took forever to find this!

(3) Your comment about it getting worse under a loaded suspension makes me want to say to look at the driveshaft and angles again. It's not clear - are the angles both 1.8 degrees and the correct orientation, or are they within 1.8 degrees of each other and correct orientation? You seem to be saying they are within 1.8 degrees of each other. If so, I would get some axle shims and shoot for zero degree difference between the rear and front angles.

(4) On one of my cars I had a brand new driveshaft that the shop swore was well balanced, but in eliminating possibilities I temporarily swapped in an aluminum driveshaft from another car and of course, the vibration went away.

Good luck.

 

[Fran Blacker]

Just in case you driveshaft is to short. Here are 2 photos of the distance from tail housing to center of u-loin weigh on wheels.

. If you have a larger length it's a problem, L'mm 3/4" or more. Had a to short driveshaft to out trans bushing. That had same symptoms as yours, while your under car check. If moves .010 it's worn,mine was .013.

 

[Eric F]

Does it vibrate at all with the car sitting still? Seems like it is rpm related. I would check the crank to see what is in there.

Answer to the question does it vibrate sitting still no. It’s smooth I even ran the rpm up to 3200 rpm and it’s smooth.

I have tried installing 2 degrees shims between the diff and spring. Pointing the nose of the diff up car gets worse. Pointing the nose of the diff down makes NO difference.

The 2 degrees is the difference from measuring at the crank and at the diff as viewed by the YouTube builders on how they measured driveline angles.

Thanks for the photo of the driveline. I’ll measure mine. If it’s ok then I’m guessing I’m pulling the trans .

 

[4speed68rt]

Did the local guy that built the driveshaft balance it after he built it, or just got it trued up, ready to balance?

 

[Eric F]

Did the local guy that built the driveshaft balance it after he built it, or just got it trued up, ready to balance?

Yes he did balance it. And he did it a 2nd time after I stated I still have a vibration. He stated that the balance if correct.

 

[pnora]

What motor and transmission mounts are you using?

 

[Eric F]

What motor and transmission mounts are you using?

Stock replacements from Napa.

 

[pnora]

Stock replacements from Napa.

Have you run it on jack stands with the suspension loaded?

 

[Ski 61701]

Can you swap the tires back to front ? I wonder if you might have a belt starting to separate or something of that nature with the tire?

 

[Geoff 2]

Balanced the DS & it was 'really close'. But was it bent? You cannot balance a bent DS.

 

[DPeltz]

Are you sure your damper, crankshaft, and flywheel/converter are correct/ balanced? As a teen 40+ years ago I bought a 74 Roadrunner 360-727.
I was working at a junk yard part time and picked up a 4speed conversion for next to nothing. I didn’t know about the counter weight on the converter and used a neutral balance flywheel.

Couldn’t detect the vibration in neutral, but I had the same 5-8 mph range where she vibrated. Above, and below that, was fine. Too many cans of liquid courage, and snow covered roads put me, and my buds upside down and eliminated the problem