Centering the rear end/thrust angle

[Craig 94 TA GT]

I got an adjustable panhard rod and have been taking countless measurements to try and center the rear end. The only "tutorials" I've seen is to center the rear based on the fenders. I don't think this works well, especially with a 9" since the pinion is not at the direct center. My basic goal is to have a thrust angle of 0. Putting the pinion at the center (based on the body) may not work either because the front of the driveshaft doesn't seem to be centered anyway.

Also, back in '99, I had an accident where the whole rear suspension was tweaked to the left slightly, so that's mainly why I don't trust centering the rear based on the fenders. So is there a way to make sure that the rear is 90 degrees in relation to the driveshaft? The only way I can think is by the picture attached, but I still can't picture if this is 100% correct?
Thanks in advance.

[Jeff_in_Atl]

Take it to an alignment shop. I thought I had mine dead on too. Turned out to be a bit off.

[Injuneer]

The goal is not to center the pinion under the body. The goal is to have the rear wheels directly behind and parallel to the front wheels. If your suspension was damaged in an earlier accident enough to move the body one way or the other, you should have had the subframe checked for squareness. The only piece of the suspension that would have moved it to the left would be bending the panhard bar, or bending the brackets that the bar mounts to. Beyond that its chassis damage.

You need to have the entire back end checked on a frame alignment machine to insure the subframe isn't damaged.

[Craig 94 TA GT]

So if the front and rear tires are parallel, but the pinion of the rear is slightly off, does that mean the thrust angle will be off? Or is the tranny output shaft offset along with the rear end?

Based on some measurements (that are probably only accurate to .25" or worse), it looks like the front and rear wheels are closely aligned now. I had to move the whole rear about an inch towards the passenger's side.

I guess I'll measure again and take it to an alignment place. I should probably also measure from front tire to rear tire. Last measurement I took (before I adjusted the rear), it showed the passenger side .25" longer than the driver's side. I'll see if I need adjustable LCAs.

So all in all, which is better, to have the thrust angle at 0, or have the rear to front tires exactly parallel? Or should the thrust angle be 0 if the rear tires are parallel, even with the 9" rear?

Thanks for your suggestions.

[Injuneer]

The thrust angle has nothing to do with the location of the pinion. It is the angle the axles make with the longitudinal centerline of the vehicle. It has to be a perfect 90-degrees, or the drive wheels will "thrust" the car to one side, rather than perfectly straight ahead. In the perfect set up, the front suspension places the front wheels equidistant from the centerline of the vehicle, and the rear suspension holds the axle perpendicular to the centerline of the vehicle, and the wheels equidistant from the centerline. Everything is symmetrical.

The panhard rod keeps the body centered over the rear axle assembly, and the LCA's control the angle the axles make with the longitudinal axis of the vehicle. Either the panhard rod or the LCA's being out of adjustment can cause the thrust angle to be off.

If you moved the body to one side, so that the offset pinion of the 9-inch rear was centered under the centerline of the vehicle, the body would be hanging over the end of the axle/wheel on one side, and the wheel would be out from under the body on the other side. When all 4 wheels were tracking straight down the road, the body would be on an angle to the direction of travel. It would be "crabbing", an easy way to spot a vehicle that has had severe frame damage.

[Wild Willy]

You call it 'crabbing', and old trucker would call it dog-tracking. When the rear axle's idea of straight ahead is different from the front axles- Not something you want on a street car- there *might* be an advantage to it if you were only circle racing-

I guess to change that angle you would need adjustable swing arms in the rear?

[Joe 1320]

Be se to make sure the differential housing and axle tubes are straight. I've seen instances where the rear wouldn't align properly because the axle tubes had shifted slightly and needed to be corrected.

[Craig 94 TA GT]

Thanks everyone for the clarification. For some reason, I was so caught up in thinking that the thrust angle was the driveshaft angle to the rear, but I was missing the point of what's more important - the actual drive wheels angle and position in correspondence to the front wheels.

I will take measurements again this weekend, just to see how it's looking. I have had to hold the steering wheel way more left now to keep the car straight, so I'm not sure how it's responding to move back to center based on my previous measurements. I'll just try to get it close and see if I need adjustable LCAs, then I'll probably take it to an alignment shop that can get real readings.

[Joe 1320]

Again........Make sure the axle tubes are in perfect alignment and 90º from the center section. Don't just check wheel alignment!

[Craig 94 TA GT]

Ok, I'll do that, but I thought that the 9" was just 1 piece housing between the axles and the back of the center section. I'll still check angles anyway, and i know that the axles themselves are just held on by pressed on bearings, so I wonder if they could shift too?

[Craig 94 TA GT]

I got a computer alignment printout and it is much closer now. The thrust angle is now -0.22. I believe prior to the adjustable panhard, it was at -0.55. They said it still needs to move to the right a bit more, but it's supposedly within spec now. I'll probably try a bit more on my own.

The other issue out of spec is the left front tire. Camber is 1.2 (range: 0.9 to -0.1) which is close, caster 3.2 (range: 4.9 to 3.9), toe 0.12 (range: -0.1 to 0.1). I wonder if this is causing the car to pull right. I don't think I can use the GM tool to adjust because the Spohn k-member doesn't have the holes that the stock k-member has. I can live with the right pull as long as the thrust angle is correct and the driveshaft is close to centered.