I have witnessed a tiny adjustment in the "parallel-ness",  between front and rear cure such issues. I was surprised to find that when connecting a shaft between the transmission and the rear diff, a tiny variance in angle can cause that kind of vibe. Out of ignorance, I kind of figured that's what the u-joints took care of. Not so. Transmission and differential must be exactly parallel. an appropriately placed wedge, or wedges can make all the difference. I'm sure you all know this, but I found that surprising.

L

Last edited by Lance (2/12/2021 4:44 PM)

I've been watching this thread off and on so I'm not up on the whole conversation. So if this suggestion has been made just ignore it. Back in the old days tires were balanced on the car. The drive tires were balanced by jacking it up then running the car in gear to find the balance. I remember seeing speeds over 100 MPH while helping to do that. The car was held up by the jack only on the differential or in the case of the Morris Mini it was the oil sump.

I mentioned my truck having issues I rotated the driveshaft 180 and cleaned up the mounting flange with a file.  Smooth as can be now.

HudginJ3 wrote:

I put my car in the air and ran it at 70 and 80 MPH. Found the left rear tire out of round by 1/4 inch. I could only spin the front tires by hand. Putting a stick next to the side wall and next to the tread revealed they were round. BUT I haven't done anything about it yet.

wow , you really lift it and run 80 mph ??  seems very unsafe . and what about rear hangin floating at 80 mph 

Impressive.

HudginJ3 wrote:

Alessandro wrote:

HudginJ3 wrote:

I put my car in the air and ran it at 70 and 80 MPH. Found the left rear tire out of round by 1/4 inch. I could only spin the front tires by hand. Putting a stick next to the side wall and next to the tread revealed they were round. BUT I haven't done anything about it yet.

wow , you really lift it and run 80 mph ??  seems very unsafe . and what about rear hangin floating at 80 mph 

 
I don't do it with the rear hanging. I jack up the whole car, level. I place the jack stands and under the springs. If you let the differential hang it causes to much vibration. When I start I do it at idle in gear and look for anything not turning absolutely true. Anything out of true round is a vibration or flutter. At about 80 MPH is the speed at which a vibration max's out that can be fixed by balancing. Beyond that most of the time it will be a mechanical issue. You don't go 0 to 80 on the get go. Something could get hurt. Do it in increments and with a helper. If your tires are out of round or out of balance take them off and bolt or nut on your drums and do it again to check your drive shaft and axels. If your tires are out of balance you won't need to go to up to 80. Some times even your drums can be out of balance or round. Then you will need to take them off and let everything coast to a stop. Iv'e never had anything fall off a jack or jack stand without me being serious about what I was doing in my younger days.

 
I have done the same just after I swapped the AOD and new universal joints in my car. I only did it long enough to see how it shifted.