[dataway]

I keep having a heck of a time trying to square up my 68 GTO to align the front end. Every measurement I make shows either the front wheels, or the rear wheels offset 1" total (meaning something needs to be moved 1/2")

I built a pretty accurate laser device the attaches to the rear wheel rim at the bead. Every measurement I take shows the front wheels offset 1" to the right (passenger side). So either they are both front wheels are offset, or the rear axle is not centered in the frame ( 1/2" off towards the driver side)

Everything is all stock, all matching code and date OEM wheels, all the same tires (I'm not using the tires for any measurement). Front end has same number of shims in the same places on both sides.

So, I have a couple of questions.

1. What is the best way to check if the rear axle is centered in the frame.

2. Can anything be done about it if it is off center?

3. Can it still be aligned if this can't be cured?

4. Is there any weird way one of the front wheels could be offset more than the other? (again, all OEM parts). One oddity, one front wheel has a 2 piece drum, the other is one piece, both were used in 68.

The rear wheel track is already about 1" less than the front on a 68 GTO as per factory spec ... so that doesn't make things any easier.

Any ideas?

[george kujanski]

Alignment machines can determine the thrust angle of the car, which is the forward thrust vector of the rear axle and generate alignment specs to correct for it. This info is from my time at Sun Electric.

George

[cmoc-bob]

On YouTube, Foothill Paint and Fabrication did a multi-part story on frame squaring. They are doing a '67 and it was quite a bit off - looking
like it was mfg'd that way too.

[VCho455]

Have you verified all the wheels have the same backspacing? I bought a Firebird once that had Chevy rally wheels with 3 different offsets.

Have you measured the rear brake backing plate to the frame on each side? That should verify if the axle is centered in the frame.

[graygator]

Probably not the same, but I've had rear axle 'alignment' problems on a 72 Lemans (GTO) and a 69 Lemans convertible. First the 72: I slid it into a curb on a rainy day and it not only bent the axle shaft/flange but also bent the rear four link arms. After the axle shaft was replaced, you could see the car 'dog walk' if you were in a following car. The rear axle was visibly offset. But not so incredibly bad that I didn't drive it like that for a long time.
The 69 Lemans convertible, on which I did a frame off restoration in my home garage, had both the frame and the rear axle bent. With the body off the frame, but with suspension, wheels and tires, I was trying to determine how bad the frame was out. I knew by marks on frame holes and other damage that it had been previously wrecked and the frame pulled back. But I couldn't get my measurements to agree with each other. Finally, I realized that one of the rear axle tubes wasn't straight. Don't assume anything is straight on these old cars, especially if you don't know the history.
The frame was still bent, so I had a shop straighten the frame and I built a 12 bolt rear axle to replace the bent 10 bolt BOP rear. I also installed some adjustable UMI upper controls arms. Additionally, don't assume the body is centered on the frame as you can move it a bit side to side. I'm still finishing up the 69 but it drives good and I've got 295/40R18 s on the back without any rubbing. Currently has my garage floor based alignment job and it drives well.

[DanC]

Originally Posted by dataway

One oddity, one front wheel has a 2 piece drum, the other is one piece, both were used in 68.

My '68 Firebird had the same odd setup before I changed over to front disc. Some things just makes one go "hummm"

[JLMounce]

From your original post you know you have a parts discrepancy with the front drums. That's the first place I'd start measuring. If there's a parts mis-match, I'd be looking to make sure the spindles are the same and the drum hubs are the same. I'd measure the different drums for any depth differences.

Also, as pointed out, if it still carries it's factory stock control arms, these are not robust pieces, can and do bend. Because they are not adjustable, any damage, or fatigue has the ability to move the axle itself. This is also a potential corrective path for you. If, based on your measurements, the frame is straight, there's no track width discrepancy in the front and you still can't figure out why the axel appears to be shifted, adjustable control arms will have the ability to correct that for you.

These cars didn't have fantastic tolerances off the show room floor. It's always possible your car left the factory as you are observing it.

[dataway]

Thanks folks for the excellent suggestions, I'll pull the wheels and do some measurements. I've owned the car for 50 years so it's boring history is well known to me No major accidents, but quite a few teenage burnouts on the one legged rear. I did a complete frame off restoration, but it's still possible some suspension components are tweaked from years of use. All new bushings etc. of course.

So among the various possibilities are that the axle is shifted directly sideways, or that the axle is "cocked" to one side slightly. I see the second issue as being more likely. It's also possible I hit curbs, or engaged in some other Dukes of Hazard behaviour that I don't recall so many years later.

I'll check all the drums again, but pretty sure I would have noticed any differences when restoring them, but, you know ...age.

Glad to hear there are options should I need to use them.

Couple of pics of the rear axle during a restoration mock up, YES I did flip the lower arms over to the correct orientation before I installed them

[Delight]

Hello, any new developments? I'm very interested in your project.

[AROWHED]

Noticed a similar issue with my 65 GP. Dropped a plumb bob from three points: The dead center of each rear axle flange, and the hole in center of the frame, under the oil pan. Measured from hole to flanges for triangulation. Using adjustable upper rear control arms, set rear axle to make both distances identical while painstakingly keeping pinion angle correct. Then used a laser mounted to rims without tires on rear axle to determine front tire in\out position relative to rears, and also to set the toe. The front wheel's in\out offset based off the rears were not the same when comparing left side to right, but the minor difference wasn't worth messing with, 1/2" if I recall correctly.

[dataway]

Exactly my situation. And about the same measurement ... 1/2", which means a 1/4" movement would equalize it.

Sidetracked by other things, but the car is still on the quick lift waiting for me to get to it.