I know that there are a bunch of threads on links on this but I have found some that aren't consistent and thought I'd double check. I just switched over to a UMI adjustable torque arm.

The car is level, with the weight on the tires.

Basically, I have the pinion angle set as poorly illustrated in the attached picture. The driveshaft slopes down towards the rear at a 1.5* angle. The base of the torque arm slopes down towards the front of the car at a 0.5* angle, creating a V shape.

My understanding is that I want to have the pinion pointed down 2* more so that if it raises up 2* under load, I would have offsetting 1.5* angles.

 

First of all, the easiest way to write the angle is to put the arrow at the front or rear of the number to indicate which end is going in which direction. You have a down arrow in front of the driveshaft's 1.5* so to me that looks like the front is lower than the back. You do however say in the text that the back is lower than the front so the down arrow should be behind the number. You're always taking your driveline angles in reference from front to back. You diff is pointing down at the front 0.5* but actually the back is up 0.5*. The way you wrote it is correct.

Now, your driveshaft is pointing down to the back at 1.5*. The diff is pointed down at the front 0.5*. That gives the u-joint a working angle of 2*. That is not the pinion angle. The angle of the pinion is in relation to the crankshaft angle. Since you don't have an engine/crankshaft/transmission angle in the above picture, you don't know what your pinion angle is, only the working angle and only the working angle of the rear u-joint, not the front one.

Torque arm suspension doesn't have a huge amount of rotation under acceleration unlike leaf springs which can easily wrap up and change the driveline angle.

You pinion should be on the same plane as the crankshaft and maybe about 1* down from there. The diff could be pointed up at the front 2* in relation to the ground and still have a 1* down pinion angle in relation to the engine. With the pinion and crankshaft on the same plane, the working angle should be close to zero. It can be changed by putting on a different height tire, changing the ride height or even changing the transmission mount to raise or lower the back of the transmission.

The best way to set up driveline angles is to point the transmission output shaft directly at the diff pinion yoke. In most production cars, this is already very close but once you start changing things, angles get changed. Put on some lowering springs and the body of the car including the engine and tranny will sit lower than stock. The diff pinion is still at the same height so you change a bunch of angles just by lowering the car. The pinion angle may change very little but the working angles will probably change.

My car is a very good example. With 32" tires on the back, the pinion yoke is moved a long way from the ground compared to stock however the body still sits relatively low like stock. I have the back of the transmission pushed up as high as will go and the tranny yoke is still not pointed at the diff yoke. My driveshaft goes upward towards the rear and my diff yoke is angled upward at the front to match the angle of the engine/tranny. Although my working angles are higher than I would like, the pinion/engine angles are close to the same plane and that keeps the u-joints happy and vibrations to a minimum.

 

Thanks Stephen.

I measured ay the balancer and the engine is sloped to the rear by just over 1*.

 

Spohn Racing have told me to set my road race car at -1 degrees.