Had a discussion today with our oldest son regarding drive line angles. He's into trucks and getting his feet wet installing lift kits and so forth.
So the subject of correct drive line angles came up. Everybody knows that ideally the longitudinal center line of the rear differential should be parallel to the longitudinal center line of the engine/transmission. Otherwise the speedup and slowdown on each revolution of the drive shaft can create some nasty vibrations.
But wait, have you ever looked closely at the drive line angles on the C3 with the body off and the drive line in place? You will see that the center lines of the differential and transmission are anything but parallel. What's up with that?
The method of making the diff center line and transmission center line parallel to each other is often called "Type Z" because if you look at the angles from the side, the center lines and the driveshaft make kind of a "Z".
There is another method, not commonly used, called "Type W". In this case, the drive shaft center line becomes the reference point. If the differential is rotated so that the pinion points down towards the ground with respect to the drive shaft center line, AND the transmission main shaft ALSO points down towards the ground with respect to the drive shaft center line, then you have a "Type W" drive line angle. And that's exactly what the C3 uses.
On a "Type Z" setup, the task is to make the center lines PARALLEL to each other.
On a "Type W" setup, the task is to make the angle between the differential center line and the drive shaft center line EQUAL to the angle between the transmission center line and the drive shaft center line.
I was able to get mine pretty close. The differential center line points down about 3.8° and the transmission center line points down about 4.3°.
The pics show the measurements before I started the alignment. For simplicity you can subtract the reading you get from 90°, but you don't have to. The goal is for both measurements to be equal. It is important to jack up one end of your frame so the installed drive shaft is level (0°) OR measure the drive shaft center line angle and use that reading as the reference point. Either way works, but's it's easier if you position the drive shaft level.
If you want to study up on all this Google "Drive Line Basics Machine Service Inc" and look for the booklet "Drive Line Basics". I extracted a couple of pages and posted them here as jpegs, couldn't figure out how to post a pdf.
Hope this helps if you are working a drive line vibration problem.... Cheers!