Finding the true Centerline of the car

[MoparArmyVet]

Okay guys, I am trying to align the rear axle and despite countless measuring and double checking each several times prior to welding the new front spring perch mounting brackets on the new rear frame rails, the axle is not aligned. In my case, the passenger side is about 7/8" farther forward then the driver's side. This will make the car crab to the left for sure. Should I simply remove the rear axle assembly, cut the spot welds on the spring perch mounts, slide the rear axle into position, align it to the centerline of the vehicle (triangulation), the spot weld the spring perch and double check alignment?

Also, can anyone tell me the correct way to determine the true centerline of the vehicle? Mine is a 1973 Cuda if that matters. Many thanks in advance!

 

[moparleo]

vehicle centerline.

If you have had an alignment done recently on any car. You should have received a print out of the alignment. These printouts not only show the toe, camber and caster angles, it will also have a reading of the thrust angle of the car. The thrust angle is the difference between the true centerline of the vehicle and the rear axle centerline. Very few cars have zero thrust angle. The front end of the car will be aligned to the centerline of the rear axle. This will make the car track in a straight line. If you align the front end without measuring the rear thrust angle and it is out considerably, the steering wheel will never be straight, as the front of the car will be pushed in the opposite direction of the rear axle making you **** the steering wheel to go straight.

 

[Ray]

When I converted mine to IRS from an 07charger I took center to center measurements for front to back location, marked a locating point on the body and did cross measurements for tracking ..

 

[ramenth]

In this case -welding in a new spring mount - the best way to do it is by measuring squares.

A little math history for us all: Pythagorean Theorem. A(squared) + B(squared)= C(squared). You don't need to know the numbers of anything, it just explains how to get structural sheetmetal into square. The only numbers you're going to be worried about is "C", the diagonals, and you'll measure for those numbers.

You'll see control holes in the rear rails. These were where the car was set on the jig going through the assembly line. We use those in collision work to give us our measurements. Measure from these to a point on the spring mount, working opposite of each other. Right rail goes to left spring mount, left rail goes to right spring mount. You get the idea. Keep working with the spring mount until you get both sides to the same measurement.

If your rail replacement was fairly precise you'll be able to work with your measurements to get your spring mount into the position it should be in.

 

[MoparArmyVet]

Thanks Guys! Thanks to you, it now is correct!

One of my other concerns was the passenger side being higher than the driver's side. That was caused by the Super Stock springs and S/S front perches. (Duh, on my part.) When aligning body panels, returning the vehicle to stock, is key. Once I put the old springs on, everything lined up, and I was able to finish aligning the panels prior to welding everything in place.

 

[moparleo]

Great ! It does help though to know more details ( non stock springs) when trying to diagnose a problem. Show us some pictures when you can. I should have remembered the jig holes. Thanks Robert

 

[ramenth]

Good to see you got it!