I just swapped out my rear axle for a 4th gen axle with better gears and disk brakes (almost free so cant complain). At the same time I also did while I was in there stuff and realized my control arms, panhard bar and torque bar looked crusty and had bad bushings. So I picked up all matching BMR stuff front and rear including 1.3" drop springs front and 1" drop rear. I did keep the stock WS6 front and rear sway bars just upated the links and bushings. Everything went together great, I did my final bolt torque with the suspension under load and adjusted my panhard bar to square up my rear axle. My concern now is now getting the pinion angle correct. I did match the adjustable BMR torque arm to the stock one as a base measurement to install it. I understand the geometry of the set up and that the engine/trans tilt down and the rear end has to be at the same angle pointing up. I want to know the math part of it? I found a couple things about the degree splits but nothing really on 3rd gens in particular. Anyone have or know what the correct setup should be? I have both an analog needle gauge and a digital meter.

 

The GM service manual (shop repair manual) has a writeup on it with specs. Do you have a GM shop manual?

 

I unfortunetly do not have a factory service manual.

 

There are two things to pay attention to and they both have to do with U-joints.

One is the parallel-ness (that's probably not a real word) of the front and rear U-joint operating angles. They need to be within a certain amount of each other so the drive shaft doesn't vibrate at speed.

The other is the operating angle of each u-joint. The angle cannot exceed a certain amount for the health of the U-joint bearings.

Normally I would explain more but I just don't have time right now . If you search for an old post by me with the word u-joint then you'll probably find it.

 

here is a decent write up and video by qa1 for your further explanation ... i have a very detailed one somewhere but couldn't find it quickly

https://qa1.net/resources/driveline-...MFntczE_CWkWvn

 

After some digging and more research, I found that the pinion just needs to -2 degrees of whatever the crankshaft or tailshaft is at

 

There really isn't any. It's much simpler than that.

There really isn't any such thing. It's just a car, same as any other kind of car in this regard. A little simpler than many others actually; a 4-link for example can change the pinion angle pretty drastically as the rear articulates, which isn't so much of an issue in our cars. The rule is, the rotating output shaft of the transmission and the shaft of the pinion gear need to be approximately parallel (you can measure their angles with respect to "level" to get that) when under load. I.e., both U-joints articulate about the same amount as they rotate. Not exactly parallel necessarily, just, close. Within a half degree or so under max load would be great. The reason you want them close is because if the angles at the 2 joints are different, the drive shaft shuffles from front to rear as it rotates, which will vibe like all hell obviously. The reason you don't want them exactly parallel is so that the U-joints articulate at least a little bit, which keeps the needle bearings from sitting perfectly still and digging into the trunnions & caps.

As far as their parallelism when NOT under load, i.e. just sitting there, it wants to be such that the pinion is below parallel, since it torques itself upward when loaded. The stiffer your whole setup is, the less of that will happen. A stock TA bends a good 2° under load, so with that, you'd want to start out at 2 - 2½° or so of difference. With a better TA it won't torque upwards as much, so maybe 1 - 1½° or so would be more appropriate. Even if the suspension members were perfectly rigid, the floor pan and whatnot, will still bend some, so there'll always be a bit of motion in it.

It's VERY rare that the pinion will point EXACTLY at the tailshaft. If it did, and the angle between the shafts was zero, then the U-joints would never articulate at all. In virtually all cars, the transmission will point to somewhere below the pinion, and the pinion will need to point a little bit less than that (by the 1° or whatever you decide on) above the transmission tailshaft.

Start out at 1° of difference between them if you have the BMR TA & X-member, and adjust from there if it vibes noticeably. Start out at 2° or so if the TA is still stock. That will look something like, the transmission pointing downward at 4°, and the pinion pointing upward at 2° or whatever, to get the difference you want.

 

Post #6,
https://www.thirdgen.org/forums/tran...ml#post6533245

 

Simple tip for measuring pinion angle. Take a measurement on the harmonic balancer and then the yoke flat on the rear. Car should be on it's tires and ideally with the driver in the seat or equivalent weight. The absolute best/easiest way to not be confused is remove the driveshaft and lock it in the shed, it just causes confusion.