Any change in weight in the vehicle will change the ride height and the drivetrain component angles. I believe it is optimal to have the car as close to race/street ready as possible when setting the pinion angle. I would have fuel in tank and driver weight in seat. My opinion.

Extra weight will change things, but the amount of the extra weight makes the difference. Someone sitting in the driver seat will have that weight split between the front and rear suspension and won't have as much effect as 10 gallons of gas hanging off the rear end. The only reason any of that would make any difference is if it changed the relationship between how parallel the rear axle pinion shaft was to the output shaft of the transmission.

The purpose of the 3 degrees down on the pinion shaft center line angle (as compared to the parallel transmission output shaft center line) is to aid in the prevention of damaging parts under the load of acceleration. That is the only purpose for the difference from the two angles being parallel: To prevent damage to parts on acceleration only. No other reason. None! If its off a degree or two, it won't likely be a life changing event. Four or five degrees off (the 3 degree standard) could be a different story, bad vibrations could occur. If its the wrong way, it could be disastrous. Drive shafts don't like to be bent in the middle.

The 3 degrees has always been the preferred goal for a duel purpose machine. A strictly drag racing machine may benefit from more degrees, but in my mind that is questionable.

Pinion angle confuses a lot of people because it is described as pinion down. The pinion angle has nothing to do with how the rear end sits in the car.. Nothing to do with that.

The center line of the transmission's output shaft has to be parallel with the center line of the pinion shaft for the drive shaft to function correctly. Those two shafts don't have to be in alignment with each other, on the same plain, or level with anything, but they do have to be parallel with each other. The universal joints in the drive shaft take care of any out of alignment (with in reason) as long as the shafts are parallel. As the drive shaft spins, the
U joints cancel out the vibrations caused by any misalignment of the two shafts.

Problems happen when the trans output shaft and the pinion shaft get out of parallel. It creates a different angle for the u joints to deal with and then vibrations happen because the drive shaft tries to bend with each 1/2 revolution.

So in your car, the transmission is fairly firmly mounted and doesn't move very much. The rear end is a different story. It moves up and down with the suspension. It also rotates with acceleration (going forward the axle pinion wants to roll upward), and with deceleration (stopping or going in reverse, the axle pinion wants to roll down). Under normal driving, these things usually happen under a very low load, so any damage takes a long time to show up.

Hard acceleration is a different story. From a standing start, the tires want to resist turning The motor is twisting the driveshaft, the rear end, mounted on springs tries to get the tires rolling, so the whole rear axle assembly starts twisting the rear end upward to help get the tires to turn. That axle rotation is the problem child. As the axle rolls upward, it radically changes the angle of the pinion shaft from the parallel position the drive shaft likes. Everything is under great pressure and the weak point shows up faster.

Eons ago some smart drag racers discovered that if you rotated the axle down (from being parallel with the transmission) when everything is under the big load of leaving the starting line and it starts to twist upward, less damage happens. The problem is, when the car is not drag racing, that axle rotated down from being parallel with the transmission caused a vibration in normal driving. trial and error arrived at a a 3 degree down from being parallel with the transmission was the happy point most of the time.

Notice none of that had anything to do with ground level. It was all about being parallel with the transmission output shaft.

Here's where I get confused. Let's say the trans. output shaft happens to be 1 degree down (from level).As I've read. you would then tip the pinion up 1 degree (from level) to make them parallel then drop the pinion down 2 degrees to account for the pinion rolling up under acceleration. So the pinion is actually 1 degree down sitting still. I get this. But, following that logic, let's say the output shaft is 3 degrees down. To make the pinion parallel, you roll the pinion up to +3 degrees. Now, if you drop it down the 2 degrees the pinion is still angled1 degree up. Is that correct?

Do your self a favor, always draw yourself a picture!

You are correct. If the trans output shaft angle 3 degrees down at the measuring point (back of the trans) (from earth level). The pinion shaft needs to be 3 degrees up at the measuring point (front of the pinion) (from earth level) to be parallel. If you want to roll the pinion down 2 degrees for the performance advantage, the pinion angle will still be 1 degree up at the measuring point (from earth level)

If you choose to drop the pinion angle the recommend 3 degrees, at the measuring point the pinion angle would be zero degrees, or at earth level.

Where it really gets fun is when you have a trans output shaft that is 6 or 7 degrees down from earth level, and even more fun when the trans output shaft is 5 or 6 degrees up from earth level (like a car with the rear jacked way up and the front dropped way down).
Hope the attachment works.

You don't mention anything about the height of the rear assembly in the car and how that effects the calculations. As the relationship between the front and rear u-joint C/L heights in the car change, everything else changes as well.

Most OE cars have the engine/trans assembly pointing down in the rear around two to three degrees. With the tire sizes and ride heights used, the rear is typically placed in the car far enough out of the chassis so the rear u-joint C/L is lower than the front u-joint C/L. This results in the driveshaft running up hill to the yoke, the pinion being near zero, and the rear operating angle (what we now call pinion angle) being in the one to three degrees down or negative (like a V) area. Works fine. Can be fine tuned with shims.

Now someone decides to lower the car, put big tires on it or even backhalve the car. This moves the housing up into the car, sometimes a lot, without moving the engine off the stock mounts. Now the rear u-joint C-L is substantially higher than the C/L of the front yoke. Now your "keep the output shaft & pinion parallel" rule flies out the window as doing so creates an awful positive (like a A) pinion angle situation. And positive pinion angle in a performance application is to be avoided, always.

Oftentimes the compromise results in a "broken back" configuration with both u-joints being made to operate at equal but opposite angles. We've been able to pick the back of the trans up and/or lower the front of the engine a bit to reduce the angle, match that angle down on the pinion the opposite way, and end up with the d-shaft level in the car (zero) with a degree or degree and a half of operating angle on both joints.


Back in the old Super Stock springs days, when gaining a foot a separation to plant the tire was the goal, this stuff got goofy. Now, with Cal-Tracs and other leaf spring systems, the separation and spring deflection is reduced or eliminated. And dealing with driveline angles getting way out of whack is not an issue. There are some folks who still use certain amounts of negative pinion angle as a tuning aid however.

Nothing to do with it. Locations, ride heights and angles.

You could do this to determine and measure ride height, i.e. how far the rear sits up into the chassis.

I prefer to decide where and how I want the car to sit when done, build the car to those dimensions, and then adjust the springs to get it there.

Save yourself some headache. Jack up the car with stands under the front end and the rear axle. Adjust the rear height with squares of plywood or flooring tile under the stands until the trans is level. Now you can turn the pinion down however many degrees you want of pinion angle. Done, No calculations. No second guessing is it up or down? It's down.

If a guy digs deep enough, he can usually find an exception to any "rule". I covered most of the exceptions in the last paragraph, except when the rear axle is higher in the car then the transmission. A vehicle with a back-half job or a "Z" framed vehicle falls into that category. I also didn't cover the race cars with the solid mounted axles, or the other modern drag racing suspension improvements simply because most of those systems are not on many street driven cars. I have seen the "broken back" drive shaft set up cause more damage to street cars then any other form of "custom" designed suspension modifications.

If you choose to set up your pinion angle on a "normal" street driven vehicle, I stand by what I wrote. If you choose to set up the pinion angle on a radically modified vehicle, more information regarding your set up would be required and I would do that through PM, so it doesn't confuse an already confusing issue.