I don't know if this alignment discrepancy is enough to cause the violent shaking but this is what I have.
Looking from the rear of the car, these two plumb bobs show the alignment of the pinion shaft of the diff. The string on the floor is the alignment of the engine/trans centerline. The engine/trans is offset slightly such that it intersects a point between the two plumb bobs, approximately under the center of the diff.
I put a light behind the vertical string of the rearmost plum bob on the diff and adjusted it so the shadow is aligned with the front plumb bob on the diff. This casts the shadow most of the way to the front. You can see the shadow diverges from the string on the floor. In order to adjust this, I would need to change the center line of the engine trans and the best I could do is make them parallel.
At this point, I could measure this angle but it's not going to tell me much. I still won't know if it's enough to cause the violent shaking. So the next step in this will be to adjust the engine/trans centerline and then roll the car outside and run it on jack stands to see if I can replicate the issue.
Transmission: .8 down
Driveshaft: 5.5 down
Diff/Pinion: .8 down
Looking at the diff/pinion from the wheel well side, is the nose of the diff tilted slightly up .8 or is the nose of the diff tilted slightly down .8?
The answer to this is important and I am not clear on it from what you posted.
It comes down to this, the definition of what you meant by saying the diff/pinion is .8 "down".
Thanks, Robert
In reply to robert777 :
Pinion at the driveshaft is tilted up. So "down" is moving from front to back. All of the components are higher in the front than the back.
Thanks for letting me know. it is a mystery as this is how it should be, so I don't see an obvious cause for the vibration.
The different in alignment from side to side from the trans to the diff does not seem like it should be enough to cause this vibration.
Thanks, Robert
In reply to robert777 :
Thanks for the input. I guess I'll straighten it up and see. After that, the only thing different is the transmission itself and I just don't see anything there having the kind of inertia to cause as much vibration as I was feeling. When I test it on stands, I'll do one run with the wheels on and if it's still vibrating, I'll take the rears off and try again.
Here are 2 videos, one short and one longer that explain a lot on driveshafts and a vary peculiar characteristic they have that vary few people know or realize concerning how a drive shaft whips when it turns, they accelerate and decelerate on every 360 degree rotation depending on how much angle the front and rear u-joints operate on.
I'm betting Jim has done research and knows what is in these videos, posting these just in case they might help and for those who don't know much about how a drive line and u-joints operate check these out, Pretty interesting.
This guy who did this video below is a bit out there but its worth watching:
I hope these might be helpful so posting them.
Robert
In reply to robert777 :
Thanks again, Robert. I've definitely watched a bunch of videos on this subject including the first one. The second is great and hilarious! One thing I've noticed is that none of them mention aligning the axis parallel both vertically and horizontally. Maybe it should be obvious. Or maybe on most cars there's no adjustment horizontally.
The lateral offset is for sure not going to cause any undue vibration. Lots of cars have come this way from the factory--I have even added to the lateral offset on a certain Corvette chassis. There's also a certain crazy Cougar-ish track car with a giant Jon Kaase engine mounted pretty much where a passenger's feet would go--solid axle. At any rate, are you sure the driveshaft is actually your problem? Sounds more like tire or bent rim . . .
In reply to rustomatic :
Definitely not a bent rim but the tires could be flat-spotted. But I've felt the vibration from flat-spotted tires and this is far more violent.
From all the info I've found, the engine/trans and pinion need to be close to parallel. Seems like that should be true no matter which plane is considered but I'll keep it in mind.
Thanks!
