8 1/4 Rear "Whomp Whomp" Vibrations at High Speeds

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Dodge72

Dodge72

Odd one out
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So recently I changed a lot underneath my Dart. New front disc brakes (and with that, new upper control arms and bearings and the like), a 8 1/4 open diff with 3.23 gears, and with that I changed to BBP with aftermarket Rallyes and new tires. Got the driveshaft shortened and balanced, with new U-Joints and U-Bolts. Got a good alignment afterwards to properly fit the bigger tires. Drives straight by itself, little if any wandering. If I get on the highway and past 65 MPH, I start hearing in the rear this 'whomp whomp whomp' sound and feel a vibration. It's a very droning sound and gets worse really fast if I past 70. If I go 75+, the noise gets so damn loud and the vibrations make the entire car shake! Scares the crap outta me, so I really try to stay put at 60 MPH, because even the noise at 65 is irritating. I've tried looking around and people are saying lots of things; axle bearings, wheel bearing, tire rotation needed, tires balanced, ect. ect. My car is absolutely fine at anything under 60 MPH; no noise, no vibration, just a really smooth ride. I haven't been able to crawl under there (it's rained inches and inches within the last few weeks), but what should I check for in the upcoming drier days?
 
First thing I would check is the pinion and drive shaft angles.
 
At 65mph with 3.23s, the driveshaft is doing about 2650 rpm. That's pretty fast: about 44 cycles per second
The tires(say 27 inchers) are doing about 817rpm, or 13.6 cycles per second .
It seems to me if you are able to count the whoomps, it would almost have to be neither.
So that takes us back to post #2. Better check that first.
So if you don't find it there,I think I would be looking for some foreign thing tied on under there, maybe caught in the brake cables, or the shackles,etc., that is flailing about in the wind .
I will say though that whomp is very descriptive of an out-of-round tire, or a bent axle, or two rear tires of differing circumferences, forcing the differential into full-time action.

After proving the rear tires are twins,I would put the car on axle-stands and bring it up to speed and have a helper observing the wheels and tires. Keep one foot hovering over the brake pedal, and do this in the driveway with plenty of room in front of the car, in case it hops off the stands. With an observer tho, you should not need to really get into it. Both a bent axle and an out-of-round tire will manifest almost as soon as the assembly begins to rotate. To see a bent axle the observer needs to be behind the car, so be sure the tire-treads are clean, and eye-protection is recommended. To see an OOR tire the observer needs to be off to the side of the car. If you suspect the tires, pull the wheels off, secure the drums and retest. This test may be difficult with an open diff. You may have to do one side at a time;just remember that if one wheel is stationary, the other one will be traveling at two times the indicated speedo speed!
By process of elimination, if it's not in the tires and/or wheels, you will need to check the axle flanges for runout. If they are good, it must be in the driveshaft. With both tires off and the axle flanges running true, a remaining vibration must be in the D-shaft.
Unless your pinion angle is way out to lunch;So it's back to Rusty's post.
 
My bet is the drive shaft if all else checks out. Did you put the C-clips on the U-joint that centers the u-joint in the rear end yoke?
 
What sound does a flat tire on an Italian car make???


Wop, wop, wop....
 
SGBARRACUDA said:
My bet is the drive shaft if all else checks out. Did you put the C-clips on the U-joint that centers the u-joint in the rear end yoke?
Click to expand...

Same here, and just because a driveshaft or whatever is spinning at speeds much higher than the sound seems doesn't mean much to me.
Harmonics can match up at intervals much slower than the driveline actually spins.

In my experience driveline balancers (the people) really are not all that good at it, or the equipment they use isn't.
I had mine cut and rebalanced at a race shop and it's still out of balance.
Not near as much as it was, but still out.
 
I let my truck sit for almost a year after the rearend went. When I finally go it fixed it made a very similar womp womp sound. turned out the old tires had developed un-even wear from me not rotating them and then a slight flat spot after sitting.

I diagnosed it by moving the tires around to different axles. The sound and vibration would change a little so i was pretty sure that it was the tires, as I had tried to control for any other variable. Got new tires and it cured the problem.
 
Thanks for all the suggestions, I'll have to break out the stands and get my dad to come help me out with it once we have a day off. The rain actually stops for a couple days so that should be the time to do it. I'll go get me a couple tools to measure any runout and the pinion axle too, it'll be an afternoon!

I'll check the tires as well, but these tires and wheels have only 2 months (maybe 600 miles?) of wear and tear on them, and most of it was just in and around town. Can't sure to rotate them and go back on the highway to see if it changes. Thanks again, I'll update it once I have time to work on it.
 
I've been fighting this same problem. The rear tires aren't that old, but they did sit for a while with the car on them at one point. I've changed axles, pinion angle, driveshaft re-balanced on a 3500 rpm machine, had the gears set up twice, I've driven it with my slicks, and I still cannot find the problem. The whomp whomp whomp noise and vibration does not vary with speed. It just gets louder and vibrates around 60 - 65 mph and varies with intensity whether I'm changing speed or not. If you find out what your issue is, let me know!
 
If it is tires, put the car up on stands and slowly rotate the rear tires to look for flat spots or excess out-of-roundness. Then put about 7-8 psi in them for a short time to see if it changes anything.

While you have the car up on stands and are checking the rear for motion and vibrations, also take a look at the back of the transmission, in case it is resonance between the engine/trans + driveshaft working against the body. A rear damper weight hanging off the back of the trans was not all that uncommon on B-bodies.

Ditto for the exhaust pipes and mufflers and their hangers; these can really shake the body if the hangers are too stiff.

When up in the air, put the stands under the axle housing ends to keep the weight on the springs, and put the front up on blocks to keep the car as level as possible.

Good luck and be persistent....I had this on an old Econoline van that I never found....

BTW, was this vibration and noise not there with the old rear axle?
 
Anytime you run car on jack stands, especially when said car has a strong vibration at speed, BE CAREFUL that car does not shake off the stands. Good luck. Bent axle is my thought.
 
The only time I could think of where it sat is when the front discs were going in, the rear end was in an needed the new wheels and tires so it could be rolled around for about 2 1/2 weeks. It is possible that they could have gained flat spots in that short of time?

Before I changed the axle I had the stock 7 3/4 on stock steelies and newer tires. I didn't drive on the highway much (and still don't), but I can't remember hearing the 'whomp whomp' sound. There might have been some vibrations though at higher speeds, but not nearly as much.

Now when I put it on the jack stands, do I have to put it up to 65+ MPH to see if anything is out of whack? That's a really high speed on jack stands...
 
Did you try pulling the driveshaft off the rear yoke and turn it 180 and bolt back on?
 
Is it RPM or gear related?

Take it out and see at what RPM it starts.

Try that RPM in different gears to see if it's engine RPM related or vehicle speed related...

Most likely something out of balance like suggested above...
 
I have not tried anything yet, I got off of work later and the sun was already setting.

Krazykuda: It's definitely speed related. I can tell exactly what my speed is when the noise and vibrations start. And no difference if I floor it to reach the speed, or if I cruise down a hill on the freeway and let up on the gas; the vibrations are still there if the speed climbs above 65+.

As far as the exhaust goes....that's an interesting point. I once had an awful vibration when idling and taking off and found out it was a loose bracket that was just hanging off the exhaust pipe that was in between the pipe and the floor. The thing had to be cut off since the bolts broke off. But that was at idling, not at high speeds. Way different noise.

I have a day off tomorrow to jack it up and take a look underneath there at the very least. Maybe rotate my tires too. Tomorrow I should be able to update. Love all the help!
 
Best bet is just to jack up that rear and check out those tires / drive shaft. I suspect the drive shaft make sure the u joint has all the clips .if u got some old drums in the back maybe the drums gotta get balanced also?
 
Let us know how it goes . My buddy and I are trying to figure his vibration problem too but he's got the ball and trunion. Sorry to ask a question on ur thread but can a torque converter also cause this vibration?
 
I'm chasing a similar noise. Does the noise go away at speed when you put the transmission in neutral and shut off the motor? If yes, likely engine/torque converter/trans. If no, likely drive line.

When the car is up on jackstands and level front and rear does the noise/vibration occur at speed? (be careful to sit in the car with foot over the brake and SLOWLY increase the speed ONLY if everything is feeling stable enough to do so.) If no sound vibration or significantly reduced, it's likely the rear end/wheels/brakes.

Put the car in gear and let the driveline spin at idle speed. Do your tires and rims spin true and round?( keep long hair, loose clothes and body parts away from anything moving.) Wheels/ brakes/ axles.

Are engine and tranny mounts good? Have driveshaft and u-joints checked for balance. Switch tires and rims to see if the sound changes. Pull wheels, bolt the drums or rotors on and re-spin looking for runout.

Post your findings in case I'm missing something. I'm troubleshooting mine further this morning. I have it narrowed down to the rear end, as I had my new driveline checked and replaced the rear trans bushing that holds the output shaft for the driveshaft. My engine and trans being in neutral doesn't affect the sound/vibration. I have new tires and rims out back, but that doesn't mean they are not out of round.
 

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Nice to meet you and your Dad at the AAM Christmas party!

A couple of suggestions. Since you didn't have the vibration before you got the new tires and wheels, I would take your Dart back to the place where you got the tires and ask them to check the balance. Since they are pretty new, they should do it for free. When they spin them, if there is a flat spot or a belt separating or a bad wheel it should show up so ask them to look.

#2 Pull the new driveshaft and take it back to the place that made the shaft and ask them to test the balance - again, new parts mean they should be willing to check it for free.

If you are still stumped, I'd suggest that you put the car on a lift and hold the rear axle up with high stands (so the rear isn't hanging from the springs, but rather acting like the wheels are on the ground) rather than putting it on jack stands - I'd hate for that sweet 72 to end up in a junk yard or you getting hurt. If you are still hunting problems when I get done with the wiring stuff on my Dart, you could bring it over to Vashon and put it on my lift for the cost of a ferry ticket. I'll PM ya when I get my wiring back together. It would also let you checkout the bottom of the car thoroughly and look through my extra hood trim.
 
Had a similar issue on my 72. I had my wife drive behind me and observe. Everything is new, including custom rims by wheel Vintiques. Turns out the tire with Wobble was a bent rim issue. Straightened and spin balanced..no issues.
 
jbc426 said:
I'm chasing a similar noise. Does the noise go away at speed when you put the transmission in neutral and shut off the motor? If yes, likely engine/torque converter/trans. If no, likely drive line.

When the car is up on jackstands and level front and rear does the noise/vibration occur at speed? (be careful to sit in the car with foot over the brake and SLOWLY increase the speed ONLY if everything is feeling stable enough to do so.) If no sound vibration or significantly reduced, it's likely the rear end/wheels/brakes.

Put the car in gear and let the driveline spin at idle speed. Do your tires and rims spin true and round?( keep long hair, loose clothes and body parts away from anything moving.) Wheels/ brakes/ axles.

Are engine and tranny mounts good? Have driveshaft and u-joints checked for balance. Switch tires and rims to see if the sound changes. Pull wheels, bolt the drums or rotors on and re-spin looking for runout.

Post your findings in case I'm missing something. I'm troubleshooting mine further this morning. I have it narrowed down to the rear end, as I had my new driveline checked and replaced the rear trans bushing that holds the output shaft for the driveshaft. My engine and trans being in neutral doesn't affect the sound/vibration. I have new tires and rims out back, but that doesn't mean they are not out of round.
Click to expand...

UPDATE: Found and fixed it! My noise was transmitting itself so that it sounded as if it was coming from the transmission output shaft. It never sounded like it was actually the passenger side rear wheel bearing. No wonder these knocking noises are so frustrating!

It turns out I had gear oil get past the inner axle seal on the passenger side rear, and ruin one of my Green Strip bearings. The bearings had <15K miles on them and I regularly drive the car hard in the corners. The driver's side bearing was good and felt smooth when spun. The passenger side had a bunch of ugly gear oil in it's cavity and had significant rough feel to it when spun. I replaced both inner seals, had fresh Green Stripe bearings pressed on and finished the re-assembly this morning. The test drive was awesome. What a relief!!

This was one of the final bugs that I was fighting to find after doing a 408"Magnum EFI stroker and 200R4 w/ lock-up converter swap last year. It is always such a pleasure when these elusive bugs are finally found and eradicated. To celebrate, I'm going to drive over to the coast on one of my favorite roads (Highway 17), look at some pretty girls and grab some breakfast.
 
Good deal! Yes, very frustrating and great to fix. Hmmmm.... that is a negative to the Green bearings that I did not realize.
 
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