Can you Diagnose this Vibe??

[TripleT]

Hey folks,

Sorry I have not been active in a while but I'm trying to change that.  I thought I'd throw this one out there and see what you thought.  My 70 Cuda with a 727 runs great, but when in 3rd and at a steady speed, I can feel a slight repeating vibration through the seat.  If I had to describe it, it woud be like "Thrummmmmmm  thrummmmmmmm thrmmmmmmm" over and over.  No other symptoms or problems, just a repeating cyclical vibration.  Something in the tranny out of balance?  Drive shaft issue?  Thanks! 

 

[tactransman]

It sounds like a bent drive shaft or bad/uncentered u joints. Is the transmission mount OK?

[FJ5_440]

It sounds like a bent drive shaft or bad/uncentered u joints. Is the transmission mount OK?

Take a good look at your U joints.  I felt the same thing for a little while before my old 64 Sport Fury tossed the U joint.  Without the drive shaft loop I would have had a big problem.

[TripleT]

Thanks fellas, I will start there.  I really appreciate the feedback.  Thanks.

[boydsdodge]

I have had that in a few mopars, it's a real pain in the ass to figure out.
I have changed everything I could think of in my small block swinger and it's still there but not so strong but there.
Changed the drive shaft twice, trans mount, engine mounts, torque converter, harmonic balancer, rims, tires.
I think it might be a Mopar thing, i feel the same droning in my dodge 318 van on the highway. Next engine is getting a balance.

[MEK-Dangerfield]

I have it.   

It is slight, and I don't feel it until I get to about 1800RPM. It is speed dependant. If I go faster, so does the vibration, but it seems to get lost a bit if I suddenly nail it, then it finds me and starts the shivers again.    I never really tried to figure it out, but someday when I have figured out all of the other problems...


  Mike

[tactransman]

Also,on my '73 Chally,I had a lady back into my rear wheel (Cragar S/S) when I was looking for a parking spot along time ago. It bent my wheel and when I finally put another wheel on,I found out it bent my axle flange too! 

[Chryco Psycho]

Cheap tires will do it too , I have found 1/4" steps in the tread of brand new BFG tires for example

[Aussie Challenger]

  Not sure but could the rear axle pinion angle be wrong.   

[twist]

I have had that problem in my Charger for 10+ years, no idea what causes it. I have replaced U joints, balanced the drive shaft, balanced tires, replaced the trans mount, nothing seems to get rid of it. I just learned to live with it. I only hear it at highway speeds

[TripleT]

Thanks very much, it does only seem to be noticable at steady highway speeds.  I'm going to be doing a complete overhaul sometime in the next 3 years, during which I will be fiddling with everything.  If I can't see anything obvious, which I really can't, I think I will live with it until then.  Thanks guys.

[Ck[FIN]]

  Not sure but could the rear axle pinion angle be wrong.   

I have also heard that this will cause vibrations.

[Chryco Psycho]

  if the pinion is low or high it will vibrate

[ntstlgl1970]

What engine RPM does it happen at?

Note the RPM and see if you can duplicate the vibration sitting still with the engine at the same RPM. Drivetrain/Wheels/tires vs. Engine Vibration

If you let off the gas at the same speed does the vibration change? Mounts/U-Joints/Driveline angles etc.

Do you have flowmaster mufflers? - Some have a resonance that is annoying

or

turn downs at the rear axle - can cause the body to vibrate at a certain frequency

Do you have any tires with a leak that need air somewhat often? If so, do you get your air from a gas station? Water in tires

If the vibration is in the back, swap tires (if possible) front to back and see if the vibration changes location.

Run the car on jackstands, with the jackstands under the axle and front lower control arms to load the suspension. Have a friend simulate the vibration and look for issues (this is how I found out I had water in one of my tires on my car, you could see the tire deform when the vibration occured)

[MEK-Dangerfield]

What engine RPM does it happen at?

Note the RPM and see if you can duplicate the vibration sitting still with the engine at the same RPM. Drivetrain/Wheels/tires vs. Engine Vibration

If you let off the gas at the same speed does the vibration change? Mounts/U-Joints/Driveline angles etc.

Do you have flowmaster mufflers? - Some have a resonance that is annoying

or

turn downs at the rear axle - can cause the body to vibrate at a certain frequency

Do you have any tires with a leak that need air somewhat often? If so, do you get your air from a gas station? Water in tires

If the vibration is in the back, swap tires (if possible) front to back and see if the vibration changes location.

Run the car on jackstands, with the jackstands under the axle and front lower control arms to load the suspension. Have a friend simulate the vibration and look for issues (this is how I found out I had water in one of my tires on my car, you could see the tire deform when the vibration occured)

Holy crap!!   

  Water in the tire NEVER occured to me!!