NOT good vibrations

[TKMSavoy]

This concerns by 1962 Plymouth Savoy. Four-door 318/727. The car has been in the family since new I have been the sole owner and caretaker For the last 45 years. She has slowly been developing a mild vibration on acceleration, close to a shuddering, that disappears around 15 to 17 mph. Between 37 and 52 miles an hour it develops a vibration/low frequency drone that then disappears. The shuddering at low speed and the drone at higher speed both immediately disappear if I lift the accelerator or put the car in neutral. I have run the car up to 85 miles an hour and there’s no further vibrations or drones beyond 52 mph.
The transmission has been recently rebuilt (Thanks DVW, the trans is like new). The drive shaft has been removed, the ball and trunnion joint has been rebuilt and the U joint has been replaced. The single balancing weight on the driveshaft is in place as it has always been. I had the differential serviced at a shop that deals only in differentials, and there are no issues to be found. Contact points were perfect. The lash was as it should be. The rear wheel bearings were slightly dry and regreased. Otherwise, no issues with the differential.
No, the story gets a little more interesting. Wondering if it was the pinion angle, I loaded approximately 200 pounds in the trunk and had my wife sit in the backseat and things changed. The shuttering at acceleration low speed became noticeably more prominent and yet at the same time, the vibration/drone between 37 and 52 mph completely disappeared. I mean completely. Would love to hear your thoughts on this. My instinct is to shim the rear end to change the pinion angle. It’s so I don’t know which direction it should be ashamed and also wondering what size shims to start with. Of course I recognize my limited skills and I could be completely wrong, but my research is pointing in this direction. I checked the angle of the differential drive shaft and transmission. Found the following. Differential up 1.2° trans down 1.5° centerline of the driveshaft 2.5°. I don’t think I missed anything. What do you all think??

 

[sam dupont]

front u-joint, unless the driveshaft is a two piece unit, then it may be the hanger, that I don't know

 

[khryslerkid]

OPERATING ANGLE​

Operating angles in a driveshaft are the angles between the pinion, driveshaft and transmission centerlines. The optimal angle for any driveshaft to run at is 1/2 degree, where many vibrational and frictional problems are non-existent. In order to minimize power loss and vibration in an offset configuration, the pinion centerline and the transmission centerline need to be parallel. In general, the largest angle for racing applications should 2 degrees and the centerlines should be parallel within 1/2 degree. With suspension movement the operating angle will increase, but should not exceed 15 degrees. If the centerlines are off too far, the u-joints travel at uneven operating velocities, causing vibration (this is the same problem induced by poorly phased end yokes). This vibration is hard to distinguish from an unbalanced driveshaft.

 

[dvw]

I'd try more downward pinion angle. Try 1- 2 degrees. .3 total is very little. With leaf spring wind up you may be getting to the point where both the pinion and the trans centerlines are pointing in the same direction rather than parallel under load.
Doug

 

[TKMSavoy]

Thanks khryslerkid and Doug — seems like the pinion angle is the primary culprit. The next step is to order some wedges and to tilt differential more downward. Never done this before. Is there any preferred provider of wedges?

 

[TKMSavoy]

Or do I have that backwards? Should diff be tilted more upwards with wedges? Should have paid more attention in geometry class.

 

[493 Mike]

Are the springs OEM? They do sag with time. All motor mounts in good shape?
Mike

 

[TKMSavoy]

Are the springs OEM? They do sag with time. All motor mounts in good shape?
Mike

Motor mounts are good, yes OEM springs. I was wondering about cause of vibrations — thought is that gradual weakening can cause pinion angle to eventually cross the line of workable angles? Makes sense

 

[sam dupont]

under acceleration: front u-joint, under deceleration: rear u-joint. Have you crawled under it and put a gorilla shake to the driveshaft?

 

[dvw]

Big portion of the wedge in the rear will tilt the pinion nose down. This will increase the C/L difference. When under accelleration
the two C/L's will come together being close to parallel.
Doug

 

[Geoff 2]

Why would the pinion angle suddenly change????

I would be checking the front [ Detroit uni ] joint that you had replaced. If the job was done badly [ pin not centered, housing not replaced? ] that will cause vibration.

 

[TKMSavoy]

Sam, the driveshaft is solid as can be — I (and others) have tried to find some play but there is none — and ball and trunion have been rebuilt and rear u joint replaced.
Geoff, ball and trunion functions smooth and easy, can find nothing amiss.
Doug, I ordered 2* shims from Summit and will let you all know how it goes — now I get what you are saying — if, for example, the rear leafs have been slowly sagging over the decades the pinion would have been slowly rising until, under acceleration it moves up even further, exceeding the workable angle, so I shim it down so that under acceleration it stays in an acceptable angle. Thanks again.

 

[Darter6]

so I shim it down so that under acceleration it stays in an acceptable angle. Thanks again.

Not sure on this . Did the shims help or are you waiting for them to arrive ?
How about the transmission mount ? I have seen posts that the 62 transmission mount is unique and has a spring loaded mount. I have no idea if it was a early 62 or mid year change. Just shooting in the dark for ideas to help you.

 

[TKMSavoy]

Thanks Darter6, waiting for the shims to arrive, hope to get to project next weekend. Trans mount is good per recent rebuild. Thanks for response, always appreciated.

 

[sam dupont]

slowly processing and worse with weight added along with confidence in the driveshaft suggests the springs might have sagged
jack the frame an additional 1 inch away from where the axle sits now and cut two blocks of wood to fit. Then drive it and see what it does.

 

[Geoff 2]

Before spending anu more money.....

Jack up rear wheels so they are off the ground.

Start engine & put into gear. Watch front of driveshaft to make sure it is running true.....if not, front u-joint has been incorrectly installed.

 

[TKMSavoy]

Thanks Sam, I think you are probably correct about the sagging springs. It’s the only thing that really makes sense. Going to go ahead and put the 2° wedges in. Will let y’all know how it turns out. Probably won’t be for a couple of weeks.

 

[TKMSavoy]

Before spending anu more money.....

Jack up rear wheels so they are off the ground.

Start engine & put into gear. Watch front of driveshaft to make sure it is running true.....if not, front u-joint has been incorrectly installed.

Geoff, thanks for the suggestion, I tried that after I rebuilt the ball and trunnion joint, didn’t run it at a high speed, but did not see any wobble or feel any vibration —

 

[Geoff 2]

Soooooooooooo, what would be the difference in drive line angle of:
[1] sagging rear springs
[2] three adults in the back seat dropping the rear of the car.
The only difference I can think of: Chrys does not supply 2* wedges to fit in case you load up the rear of the car...