Help , cannot diagnose vibration

Light vibration at 60 to 70 mph , none below 60, slightly better over 70. can drive but uncomfortable(2500 miles since construction), coasting in neutral makes no difference, braking no difference, turning no difference

Sorry to bring this up because I know there so many opinions and variables but i am desperate! Here goes:
1956 Buick Special 2 dr sedan: see pix
rebuilt stock front suspension
1965 401 motor rebuilt- stock rubber motor mounts on fabricated steel mounts
Muncie m20 4 speed, Wilwood hydraulic clutch linkage
Disc brake conversion/ Scarebird
87 GN rear rebuilt replaced axels- Four link rear suspension with stock springs and panard bar
custom built drive shaft - balanced and re-balanced twice- no difference
tried 4 sets of wheels and tires and combinations no difference/ has 1" adapters to change bolt pattern on rear axle. Have run with stock wheels and tires / no diff
slight play in yoke to transmission - rebuilt no difference
pinion angle checked at 4 wheel drive extreme vehicle shop (they think it is driveshaft) also checked the frame for straightness& track =ok
tried to put hose clamps on driveshaft, could not eliminate vibration
have not replaced brake drums
What is NEXT STEP? DESPERATE!

If was me,I would pull the drive shaft and have it checked. May also be the clutch, but that should be engine speed noticeable rather than road speed.

Nice car.

Ben

Did you test in 3rd gear?
If it was not a custom driveshaft, I would recommend swapping a known good one...you may have to have another fabricated.
I had that problem after replacing U-joints (originals were glued in). Apparently the driveshaft was damaged during removal, because a junkyard shaft cured the vibration.
Check the engine rpm when vibrating the most and then again (same rpm) sitting in neutral and then with the clutch depressed.
(hard to do safely: tow up to speed with the driveshaft removed.)

As you suspect, your brake drums could be way out, also axles can be a problem if they are bad enough. Try to ensure the problem isn't in the engine area as suggested.

Was the engine balanced? What did you do for a flywheel when coupling the M21 to the externally balanced 401?

Thanks . rear has been checked and rebuilt, I have run it on a lift with wheels up but at ride height... can see vibration but cannot find where it is coming from.. thanks for help. Getting closer to drive shaft replacement.

I will go in another direction. Are the tires in balance? Can you exchange wheels and tires from another car? What condition are the shocks in?

One more thing you may wanna check is your rear tail housing bushing. If that is worn it will allow the yoke on the driveshaft to move in a oblong manner feeling like a front u joint is going out. This does usually show up around the speed you're talking about.

yes , i had that suggestion so had the rebuilt trans rebuilt new bushing installed that.s what lead to this post...

Using the 87 GN rear end as a starting point, attached is the first part of the propeller shaft chapter out of the '87 Buick shop manual. Most of this looks like you have gone through it already. It does detail the U-joint angles, runout specs, and the hose clamp balancing procedure.

Just scanning the diagnosis chart, it points to the driveshaft being the cause of high speed vibration. Hope it may lead to some new ideas.

Hi newbie here and I just saw this thread. I don't know if you've solved the vibration issue yet but I had a similar situation in my '69 Cutlass that I added a 200-4R trans to. I also had to have a custom driveshaft built and ended up with a vibration that drove me and the guys at the trans shop nuts. It turned out the custom drive shaft I had made was about 1/8th of an inch too long and was stressing the u joints but only certain rpm's. There was a lot of trial and error trying to find an answer to this situation but it worked out in the end. I hope this helps if you haven't found an answer to the problem yet. - Bill

Quote:

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Originally Posted by oldfoneguy

Hi newbie here and I just saw this thread. I don't know if you've solved the vibration issue yet but I had a similar situation in my '69 Cutlass that I added a 200-4R trans to. I also had to have a custom driveshaft built and ended up with a vibration that drove me and the guys at the trans shop nuts. It turned out the custom drive shaft I had made was about 1/8th of an inch too long and was stressing the u joints but only certain rpm's. There was a lot of trial and error trying to find an answer to this situation but it worked out in the end. I hope this helps if you haven't found an answer to the problem yet. - Bill

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Thanks .... It looks like all opinions agree that driveshaft is most likely cause .... Thanks to all for input! Paul Cialone

Thanks , looks like all opinion is driveshaft. Thanks for all advise. Paul