Drive line vibration in my 1966 Cadillac

[Jerry Leighton]

Hello fellow Cadillac enthusiasts,
     I wanted to share some information with you that you may find helpful.  Several years ago a vibration developed in the drive line area of my 66 convertible (under acceleration).  The CV/u-joints also were making clinking noises so I had the drive line professionally rebut and balanced.  The clinking noise was now gone but the vibration was still there.  I took my car back to the mechanic and they shipped off the drive line to a specialist at a military shipyard that had an even more advanced tools with very precise tolerances and had it inspected.  All checked out as the drive shaft was indeed balanced.  Hence, the mechanic said there was noting else he could do.
     Fast forward 12 years, I recently had some other work done to my car by a different mechanic.  This one specializes in working on vintage Detroit iron.  He noticed the vibration on the test drive and asked if I wanted him to fix it.  I agreed and filled him in on the history of this ailment and he diagnosed the following:  The differential was not pointed directly at the transmission.  It was actually pointing down about one inch too far.  The result was that the CV joints were under constant mis-alignment which resulted in the vibration.  This is what likely caused the CV failure in the 1st place.  So it turns out my car was likely in a rear end collision many years ago that caused the frame to bend slightly in the area where the rear control arms attach to the axle. 
     The solution: The mechanic had custom manufactured upper rear control arms made that are adjustable.  The result is that he was able to tilt the differential up about 1 inch back into perfect alignment...problem solved: the vibration is now gone!

[Rockfish39]

Hi Jerry,
             I'm very glad that the mechanic that you took the car to solved your problem. I just would like to add a little to what you had written for the benefit of any out there that are planning, trans-rear swaps of any kind.
The drive shaft is not supposed to be in a straight line. In other words, not perpendicular at a measured 90 degs to the u-joint mounting saddle on the diff. There needs to be an angle there. The angle between the driveshaft center line and vertical machined surface on the trans slip yoke should equal the angle between the the driveshaft center line and the vertical machined surface on the pinion u-joint saddle. Inequalities here could easily cause a vibration due to a loss of rotational vibration phase cancellation. This becomes a major deal when talking about lifted trucks, but the idea is exactly the same.
In your case, the past impact may well have pushed that diff angle out of tolerance which an adjustable control link fixed, but not exactly for why you may have thought so.

[Scot Minesinger]

Very helpful - thanks for posting.  Often friends ask me to look at vibration issues on their Cadillacs and this is new one to add to my war chest of causes to check for.

["Cadillac Kid" Greg Surfas 15364]

Pinion angle and transmission output/differential input alignment are frequently overlooked as in the case above. That must have been quite a hit to have bent the frame that far forward.  I'm surprised the body doesn't rattle on the miss-aligned frame.
Greg Surfas

[Dave Shepherd]

Greg, seconded, oft overlooked  issue, especially  with some hot rods we work on in the shop, $10.00 angle meter is a cheap checking tool.

[Lowrider2905]

Hi,
I noticed since I own my 66, that the diff input is facing downwards. Eve replacing all 8 bushings didn´t change it. I have an old picture somewhere from a commercial chassis, this has also the same issue. Seems, that this was an issue since they where new.

I don´t know if this is correct, but a mechanic told me, that due to the duble cardan cv´s it´s not a must have to have tranny and axle in one line, cause you don´t have the stress in the joints as in single cardans. But I can imagine, that every smoothness makes the parts lasting longer.

BUT I also have vibrations under some driving conditions and during this winter I like to work on a solution. Maybe adjustable arms? Concentric bushings? Honestly I don´t know what I will do, but I am planning a change.

Greetings, Richard

[Dave Shepherd]

There are online diagrams for all types of driveshaft designs that show the proper angles, I use them all the time, then there is no misinformation nor guesswork .

[Big Fins]

I know of no pinion input that is in a direct line with the transmission tailshaft output. Even in Class 8 trucks the output and pinions are offset and not in plane with each other. Perfectly straight would shudder like crazy under a torque load.

You may want to check the phasing of the driveshaft. I'm not sure if this would apply to the 66 year. But unless you are the original owner, you never know what has been done before you. Those shafts were only good for 70k at best.

[The Tassie Devil(le)]

I will throw in this bit of information here in that no driveline should be straight inline with the transmission as the universals will wear out very quickly.

The Universal Joints are designed to require movement otherwise the needle roller bearings will not move, and eventually become destroyed as they are sitting in the one place.

Any straight-line drive will use a solid coupler, or have the needle rollers replaced with bushings if a universal joint is used.

Bruce.

[Dave Shepherd]

Correct, those slight angle differences allow the load on the joints to spread over a larger surface, internally .

[Stargazer]

Just a quick note - if I'm not mistaken I believe that the slightly out of line Driveline to differential also increases the torque ratio somewhat.