Driveline vibration help

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m99roadster

m99roadster

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Im looking for some help with a driveline vibration that's driving me crazy. I have a 68 Dart 270 4 door that I swapped the rear end in from a 7.25 to an 8.75. When it was done I had the correct shock pads for the 8.75 but was told they wouldn't fit so they used the pads from the 7.25. I have a vibration between 60 and 70 miles an hour. The car was put on 4 jack stands to try to locate the vibration, it looked to me that the slip yoke was bouncing up and down when it hits 60mph. The transmission was completely overhauled about 2k miles ago.The shop its in believes it was the bearing in the tail shaft that was causing the problem so they replaced it. Vibration is still there but not as bad but still irritating. Could it be the spring pads, would they change the pinion angles? Or do slip yokes wear out where they ride on the tail shaft. Im thinking about buying a new slip yoke and replacing the drive shaft with an aluminum one and hopefully cure the problem. Any suggestions would be very helpful and appreciated.
 
Shock mounts cannot change pinion angle. Further, I'm pretty sure 7 1/4" mounts do not fit any of the larger axles.

Check for certain you have the correct U joints. Check for certain they are assembled right, IE non having a pin laying down in the end of the cap.
 
If you didn't have the vibration before changing the rear end I would look there first.

Axel bearing, pinion bearing etc
 
m99roadster said:
The car was put on 4 jack stands to try to locate the vibration, it looked to me that the slip yoke was bouncing up and down when it hits 60mph.

Or do slip yokes wear out where they ride on the tail shaft.
Click to expand...

slip yokes can be worn, but usually that is in high HP applications. the splines can get twisty, then they dont slide in and out like they are supposed to

i'm thinkin if the slip yoke is bouncing it's the universal or somethin in the tail of that trans.

you could replace the slip yoke, and U joint then test it again. at that point if it's still there you KNOW it's the trans
 
If you changed from a 7 1/4 to a 8 3/4 what did you do about the driveshaft? sounds like an un-balanced shaft
 
I'd like to thank those of you that have replied to this thread so far.

67 Dart273 : When I purchased the 8.75 I also bought the shock pads because I was told the 7.25 pads would not fit. The shop that did the installation insists they used the 7.25 pads because the 8.75 pads would not line up. The U joints have been checked and are installed correctly.

GEO : I never drove the car before the rear end was changed so I don't know if the vibration was there before. Axel's and axel and pinion bearings are all new, everything in the reared was replaced before it was installed.

moparlover : I think I'll follow you advice and replace the slip yoke and the U joint once again to see if it corrects the problem. I'll also send the driveshaft out again to be re-balanced. The tail shaft bearing and seal were just replaced about 10 miles ago. Hopefully this will cure it.

MOrePowAR : The driveshaft had to be shortened and the U Joints front and rear were replaced. The driveshaft was sent to Driveshaft Unlimited here in town for the shortening and was also balanced at that time.

To All : Is it possible the tail shaft in the transmission is bent, does that happen? If it is bent wouldn't it be expected to have a vibration at all speeds? Thanks
 
How about your pinion angle? Did the shop check that during the install?

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So you ran an unloaded rear end on jackstands in effort to find an imbalance.? You saw the slip jopping up n down at approx 60. Was that under power with a light brake application? The brake apply would lightly simulate load ie. traveling on level ground at cruising speed. What you experienced may just be the driveline loading and unloading.
What did you mean by shock pads? The U-bolt caps? Or did they cut off the spring perches and relocate? Your pinion angle could be off if that had occurred.
Please be careful running your vehicle at speed on jackstands, **** can get sideways in the blink of an eye. Property damage would be the least of your concern then, loss of life becomes a real risk. A stone in a tire tread becomes a bullet, just sayin .... B safe
 
If you saw it moving, doesn't that answer your question? Our car had a vibration at the top end. We eventually chewed a gear in the trans, and it was stuck in fourth, but we finished the race (10 hour endurance race). Rebuilt another spare trans we had, driveshaft looked good, but we had the vibration with the new trans, but at a higher speed. It was the slip yoke. It looked good, but was evenly worn. Replaced tailshaft bearing, and new slip yoke, and it was very smooth at the last race.
 
And of course, Mopar used some weights on tailshafts as vibration dampers. U joints and yokes are going to have SOME vibration as they can't rotate at a constant speed, and you could just be exciting a resonance that needs damping out. Just keep that in the back of your mind as a possible solution.
 
Update:

Adamm : yes the shop did check the pinion angle

funwithfuel : yes I did apply a little break pressure while checking it. I never heard the shock pads called u bolt caps, I guess you can call them that. The spring perches were never cut the rearend was an a body rear. And lastly thanks for the concern about the car getting sideways while on jack stand, safety is always a big concern

375inStroke : Drive shaft has been removed and new slip yoke ordered along with seal and new u joints. I was able to remove the u joint from the slip yoke with just my fingers, they slid right out.

Supershafts : as soon as the shaft is back together i'm going to have it rechecked for balance before installation

nm9stheham : Thanks for your advice I will keep that in the back of my mind

jdetmer : engine and tranny mounts are new and are still good

Thanks to all, I'll give an update when the new joints and slip yoke have been installed
 
m99roadster said:
I was able to remove the u joint from the slip yoke with just my fingers, they slid right out.

Supershafts : as soon as the shaft is back together i'm going to have it rechecked for balance before installation
Click to expand...

There is the problem, that should have shown up when balancing the 1st time, if the shop didn't see the tell tale issue by looking at it.
There machine if it's a better one should have kept wanting to move the weight a little opposite where it was last upon balancing.

A good idea to re-balance it with the new slip.

.
 
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