Driveline Angle and Pinion Angle

Yep, all the angles, tranny, rear, 'pinion' are all relative to each other. The ground has nothing to do with the measurements...

 

The pinion needs to angle downwards. Ideally, setting pinion angle will allow the driveshaft and pinion to be straight under load. The pinion will rise upwards under load, and bring the driveshaft with it. Under deceleration with a manual transmission in gear, the pinion will angle downwards. You need to find the right setup for your car which will keep deflection of the joints under 3 degrees under deceleration and around zero while accelerating. This may involve getting excessive torsional movement out of the suspension by replacing bushings, etc. I usually shoot for 2 degrees pinion angle for a street driven car. A drag only car can see between 1 and 3 degrees, adjusted as necessary. Leaf spring cars are a nightmare if you don't know how much the spring "wraps" under load. Traction bars have a duality of purpose on a leaf spring car, since they both apply force to lift the car under load, and limit rotation of the differential housing to prevent pinion angle from going beyond the operational limits of the u joints.

 

I used to be confused about this too, until I did a lot of research. As SRGN points out, these "ideal" pinion angles you read about are dynamic not static, i.e. "under load". Most people leave out that part because they don't know how to quantify it - very hard for the average person to measure angles while accelerating/cruising. There is a very good post on here somewhere about someone measuring angles with a camera while driving.

With all the high performance, aftermarket parts we put on these Camaros, angles can be all over the place. On my last build before this one, I spent a lot of time playing with different shims to get my pinion angle in a good spot...