Originally Posted by Vengeance Race Engines
That's not entirely true. You don't want to jack the car up as you need to check it on level ground at ride height with suspension loaded with driver. There should be no need to add/multiply/subtract/divide etc...it helps to have the back of the driveshaft disconnected from the yoke, and use an angle finder on the flats of the pinion yoke. Whatever angle that is, that is what the pinion angle is. At that time, if you have a girdle diff cover with a flat spot on the back, check that angle and see if it is the same or if it is off, note how much it is off by so you can check it from there instead of having to crawl under the car and disconnect the shaft everytime. Angle of the driveshaft means nothing. What negative angle you start with depends on the setup. Stock type suspension can be anywhere from -6 to -2, ladder bar and 4-links can be anywhere from -2 to 0 as they do not move. There is no mathematical formula to know how much it will move, so you get a starting point depending on what parts are used, and creep up on it. We also make sure the engine has just enough of an angle for proper oil drain back, but no more than 3-3.5 degrees. Knowing where the driveshaft is and adding and dividing by 2 or whatever people do, does nothing, the rear influences the driveshaft on launch, not the other way around. Optimally, you want the rear inline with the driveshaft and transmission on launch. There is no formula to know where that will be because you won't know how much it moves, other than track performance so adding and dividing doesn't do much nor will it give you a number to be at that will make it "inline" when launching the car. If you don't have enough negative angle, the pinion will go into positive, meaning past that center, or "inline" and will not run as good, also leading to broken u-joints. If you need a baseline on where to start, fill us in on what parts are used, but doing it that way won't help you much. 2cents
Beware of what people post on the internet, kids.....
The angle of the driveshaft is CRITICAL!!! The pinion angle is not just the pinion angle, as stated above. The angle of the pinion in relationship to the ground, or earth, or gravity is completely irrelevant. What is critical is the relationship of the tranmission output shaft, the driveshaft, and the pinion gear.
When setting up the driveline, the critical measurements are the working, or sometimes called, the operating angles of the front and rear u-joints. The front working angle is defined as the angle between the transmission output shaft and the driveshaft, while the rear working angle is defined as the angle between the pinion gear and the driveshaft. The front and rear working angle need to be equal, but opposite, and for smooth operation, they need to be as small as possible (preferable under 2 degrees), without being zero, under load. Here is a good diagram:
So the OP is correct, if you don't want to do any math, then position the car, with the suspension loaded, so that the driveshaft is level with respect to gravity, earth, aka "level." Then you can measure the pinion angle and the transmission angle, and those will be your working angles, because the driveshaft angle is zero.
Alternatively, you can have the car in any position, with the suspension loaded, and do some simple math, and arrive at the same results.