I have always had a vibration while driving my 71road runner. It really starts to become noticeable around 45 - 50 mph. At 65 - 75 it is quite pronounced and rather annoying. If I ever take it up to 80 -85 it feels like the car might come apart. I finally said what the hey and wanted to push the issue and ramped up from there. All the sudden at exactly 110mph the vibration totally goes away and becomes so smooth and quiet I thought the car shut off. (actually scared me) I never took it any higher but as soon as I back it down from 110mph Mr. vibration is back. I have changed the tires all new balance and such, but no change.
To get specific the vibration is much like a slow rolling drone that just repeats. Similar to the sound /feeling of flying in an old WWII bomber with those big 'ole radial engines.
Do any of you guys out there have a similar situation or possible know what might be causing this? I am not really sure if it comes from the motor, drive shaft balance, or what. However, as best I can tell I don't feel the vibration just holding the gas down to simulates revs at say 70 mph while sitting in the driveway. I gotta tell you this car is so smooth and fun to drive at 110 mph without the vibration but we all know that is not possible. Any help getting a solution to this problem would be greatly appreciated.

Get 2 jack stands for the rear axle and some help. Run it thru the gears while you watch the driveshaft/wheels from the side and back. Good Luck!

I had something similar, I replaced the u-joints and added a couple shims to correct a pinion angle issue and the drumming went away. How does the driveshaft yoke feel in the transmission (loose? up and down?).

Tom

Even though your wheels and tires will show a "Balanced" condition on the balance machine, you shouldn't rule them completely out. I have a similar situation I went through the entire drivetrain, driveshaft, axles, rotors, drums, etc. and found that the magnum 500 wheels were the problem. They would balance out just fine but when you spin them by hand on the balance machine, you could see them wobble side to side about half an inch. Put some "known" good wheels on it and see if it changes anything.

Dave

U joints/angle

so how do you go about correcting the angle? special tool?

Virtually identical symptoms on my RR for years. Checked tires and wheels and changed the transmission mount, which did improve it slightly.
The real problem turned out to be the U joints...what an amazing difference that made.

I agree with all the other replies. I believe it's something to do with the driveshaft. I had a similar vibration at the exact same speeds you mentioned. It was finally fixed when I had a slipyoke conversion done on my B&T trans and replaced the driveshaft. In my case, I believe the crosspin in the ball-n-trunnion joint was slightly bent from an earlier attempt to press it out.

Sounds to me like the ujoint rollers are piling up on or one of the joints is froze up and no longer 'ujointing' the way it should be. You cannot properly check a ujoint short of taking the drive shaft out and physically checking them. Either that or your drive shaft has a very slight bend to out of balance condition that after XXX speed it literally out-vibrates the vibration.

Hey thanks a lot guys this is great information! I will focus on the drive shaft/u-joints first and go from there. I will also swap out my wheels with my friends and see what happens. What degree is the generally correct pinion angle?

their might be something in the archives that will take care of you on this. EDIT my sugggestion would be to R&R the ujoints & I'd be surprised if that didn't take care of you. Be sure after you get em in to set the shaft down on the ground & whack each ear right behind the ujoint cap several good licks with a BFH flip it 180 & do the other side. When you are done you want to be able to by hand move each ujoint thru its range of travel in both planes & have it move FREELY, try it before & after you whack it & you'll feel the difference & know when you have it right. and you might have a driveline shop check the balance while you are at it, call them & see if the ujoints need to be in or out.

I remember reading something about the 71-72 RR/Sats having a resonance problem related to the floorboards. The article showed a weight attached to the floor to change the resonant frequency of the floor and reduce/eliminate the problem. Not sure about where to look for info on that article, but I'm pretty sure it was a Mopar Action article - deep tech, not just the garden variety. Mabye part of their Bold Beeper project?

Clair

sounds like a pinion angle problem to me...the reason it gets bad/smooths out is because the frequency has increased high enough to be above the resonant requency of the car...at 110, you're between the first and second harmonics.

ive got the same drone. Brand new u-joints. Im thinking pinion angle and need to check it. is this something I can do witrh a protracter and a level or do I nee a special tool?

Thank you, thank you, thank you to all. I am here to validate your expertise!!! It was indeed the U-joints. Primarily the front one. It looked like someone had been chewing on it. The car now drives slicker than snot on a glass door knob.