[punkin]

Hello all,

I'm working on my 68 firebird. Replacing the rear end and doing some other suspension related upgrades. I want to be sure the rear is aligned and square and while at I want to verify it's centered. I figured while I'm at it I would make sure the sub-frames are square as well.

I need to ensure the car is level (front/rear and left/right). I've got some drawings showing where/how to measure sub-frame and suspension alignment but it isn't clear to me where to place a level to make sure it's sitting level.

Maybe I'm making this too complicated but I would like to minimize the hard lessons learned. The car is off the ground on cribs and have made shims to get it flat.

Thoughts? Experiences? I thank you all for taking the time to consider my questions.

[JLMounce]

ride heights from the factory are measured inboard of the front tires at the rocker and inboard of the rear tires at the rocker by a couple inches. I don't have the stock specs however. If you're doing upgrades you are probably lowering the car a slight bit anyway, which in that case, the stance you are after is more important.

If you want the car to sit completely level, the front needs to come down a bit more than the rear as the cars have a reverse rake in them to begin with.

The F-body "lean" is pretty characteristic in these cars with a tendency to lean either to the driver or passenger side by about 3/4". This is most often caused by the springs not being installed in the front suspension correctly. Contrary to common belief, the spring does not clock into the lower control arm, but instead is clocked into position in the upper spring mount. The top pigtails also rotate in different directions side to side. Looking from the top of the frame you will have two small holes in the upper spring perch. The driver side will have the hole positioned towards the front of the car. The passenger side that hole is positioned towards the rear of the car. The spring pigtail's position should put the end in the middle of those inspection holes...or as close as you can get them.

For positioning the height of the rear of the car it's best to use lowering blocks to fine tune. If you're using an aftermarket spring, opt for the spring in the rate you want/need with the least amount of de-arching to achieve lowering. Then use blocks to get it the rear end down where you want it. Leaf's with a lot of de-arching to achieve lowering beyond 1.5" can have a tendency to hyperextend on big bumps.

As far as getting the rear end positioned correctly, there's not really a ton of adjustment there. If the spring perches are installed on the axle correctly aligned, the alignment of the housing in the car should be pretty close, unless you've got deformation in the front and rear spring perches, since the leaf spring locates the rear end. You may have a tiny amount of left/right adjustment just in the tolerance between the perch on the axle and the width of the spring. That said, there is a stud and receiver on the spring and perch that locks things in place fairly snugly. Along with your lowering blocks (if applicable) you'll want some various triangle shims to set your pinion angle.

[Sirrotica]

I used to own a Snap On portable alignment unit which had four stands to sit under each wheel as you let the lift down. The tech that set the unit up leveled each stands from side to side and cross corners with a string level. He also emphasized that the inflation pressure in all the tires must be the same before starting an alignment. These stands had adjustable legs on them to level them.

You also had to bounce the front suspension upon sitting it on the stands to get it to ride height.

I was able to turn out good alignments using the leveled equipment, and following his instructions for many years.

Hopefully this will be of some help to your process.

One other thing, my 73 T/A was wrecked when I purchased it, by using tape measures we were able to determine that the sub frame was diamonded. One frame rail was pushed back from collision damage, necessitating a used replacement. Good luck on your Firebird.....

[punkin]

I thank you both! Very helpful. Plum bobs hanging from all over and little squares of tape on the floor trying to map the various alignment locations. I believe I have all the triangulation points identified but maybe over thinking it...with all these lines hanging straight down off the chassis, it occurred to me that if all things aren't level then those vertical lines won't be perpendicular to the underside of the car which could translate into incorrect measurements.

Jason, thanks for the advice on the ride height locations. I'll shim my supports to that they are at least level from side to side. I believe this would get me where I need to be.

Wish me luck...if the transplant goes well, the old BOP housing/posi/axles and brakes will be looking for a new home.