Just did some research and found this post by assassin10000:
Now, after grabbing my angle gauge and taking some measurements I found my angles were this in relation to the ground or rather 0 degrees:
Meaning that I was at the very outside limit of the supposed 1-3 degree operating range of the U-joints, and explaining why I was getting a light driveline vibration at 85+mph on the freeway.
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So with the car up in the air I figured it was time to try out a different pinion angle than what I had. First thing I needed to do was figure out what the current control arm lengths were set at and how much they needed to change. A few measurements later this is what the control arm lengths were on my AE86 (IE: stock lengths):
9 9/16" or 24.3 cm - Upper
18 5/8" or 47.3 cm - Lower
A quick measurement and calculation of the distance between the control arm mounting points on the differential gave me a rough estimate on how much I needed to change the length for each degree. That being said on the AE86 chassis:
3mm or 1/8" of control arm adjustment = about 1 degree. (ie: remove 1.5mm or 1/16" of length fromthe lower arms and add 1.5 to upper arms changes pinion angle 1 degree)
Knowing this I adjusted my control arms to the following lengths to give me a pinion angle of about 3.5 degrees. (Also, since I have traction brackets I get a little less than 1 degree of change per 3mm or 1/8" of adjustment.):
9 3/4" or 24.8cm - Upper
18 3/8" or 46.7cm - Lower
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Another reason I chose to go with 3.5 degrees as opposed to matching the 4 degrees that the engine/trans sits at is this:
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Due to the tendency of the pinion to rise under load, some angle must be present at rest.
Also, if you paid attention earlier you'd realize instead of having a negative pinion angle, I have a positive one. This is an unfortunate side effect of this particular chassis, suspension and the lowered ride height. The ride height is low enough that the differential pinion flange will not be low enough in relation to the engine/transmission to get the ideal negative pinion angle. While still having the ideal 'matching' angles for a one piece style driveshaft (see below). The most I was trying to do was get rid of the vibration.
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Ideally, the angles between the transmission output shaft and driveshaft, and between the driveshaft and the pinion will be equal and opposite.
After tightening everything back up, I took it on a quick test drive - vibration is gone, which will have to be good enough for now.
I'm going to go back and perhaps try a 2 degree pinion angle which will leave me a .5 degree negative pinion angle in relation to the driveshaft and see how that feels. Mostly because as an unfortunate side effect the more angle there is, the more torque it takes to overcome the difference. And we all know the smoking hot 100 whp that my car might have is just not enough and I need to conserve every last bit of power possible.
Ok. Forgot to give another update after doing the adjustment/changes.
I've found 4 degrees is working the best on my cars setup with solid heim 4-link suspension. It's currently set as close to 4 degrees as I can get and is working pretty well.
The 3.5 was still causing a vibration, just a lot higher up in the mph range than before. (so it was better, but not fixed)
Andrew