Why would thrust angle change?
 

I've had my 84SD on the alignment machine 3 times & the thrust angle has changed.

I changed shocks, front brakes, rotors, hoses & passenger outer tie rod when I bought the car. The local alignment shop, not MBZ (or likely otherwise) intelligent aligned it. It steered OK but steering wheel was cocked.

Specs: Front Camber L: -.1* ; R .1*
Front Caster L: 10.4*; R 10.3"
Toe L: .22* ; .22*

Rear Camber L -.1*; R -.8*
Toe L .23* ; R -.07*

Thrust Angle .15*

I took it back to straighten the steering wheel and the pull is more pronounced.

Last time on machine:

Front Camber L .2* ; R .2*
Caster L 11.3*; R 10.4*
Toe L .21* ; R .21*

Rear Camber L -.9* ; R -.12*
Toe L .26*; R -.29*

Thrust Angle .28*

Now the rear wheels are tilted in more & the car pulls more to the right. I'm going to give them 1 more chance to make it steer straight then take it to a MBZ shop.

Right caster can increase and camber can be adjusted. How much to correct the pull? Will rear bushings in the rear suspension likely correct? (Summer project)

Eh?

What are you doing to the rear suspension on a first generation W126 to change any of the settings? Isn't it the same set up as a W123?

(I thought so)

Are you fiddling about with after market trailing arm bushings or spring pads?

More information needed please!

The car will pull to the side with the least caster. ~.5* less caster on the L is a good start. Even caster will cause a pull to the right on just about any chassis, but if the tech put less caster on the R, then that's just ignernt.

Subframe bushings can cause a shift in thrust line, but that's not what I see here. This is mostly a change on the RR. Could be bushings, or the target/sensor of the alignment machine. The camera based machines with targets are easy to mess up. The targets that mount to the wheels can shift during compensation for a couple of reasons, making any measurement on that wheel invalid. The ones with sensors on the wheels clamp to the wheels pretty tight and rarely shift. Another possibility is that the rear slip plates were locked on the lift. Some have pins to remove, others are locked in by other means. If the rear slip plates are locked, you can't trust the rear axle measurements.

A possible cause for the rear axle camber change could be a change in rear ride height.

Nothing was done to the rear that I know of except my installing shocks all around. Shocks were installed before the 1st alignment. I can't account for the change in rear settings.

I have recently installed a differential mount but that was after the alignment and not reflected in the specs listed above.

The car pulls right. My thinking is it needs more caster on the Right front. I don't know how much. Caster is not a wear setting and difference is acceptable &/or desired if stacked in the correct direction.

Camber pulls to the most positive but can wear so should be used judiciously. Front Camber is currently the same and therefore not affecting steer direction.

My understanding is that the rear is generally not adjustable unless special parts are installed. I'm thinking that rear bushings are likely worn and causing negative rear camber and perhaps the uneven rear toe. There is 166,000 mi on the odometer.

Oil change records indicate that odo was working although I had to knurl the odometer gear shaft.

I had similar problems with mine. Worn trailing arm bushings can be hard to detect without unbolting and lowering the arm. One of mine was so worn that it wouldn't have been long till the rubber disintegrated and gone to metal-to-metal. From the "outside" it didn't appear worn. The pull was very significant with any change in direction as well as even slight variations in road surface.

I've attached a copy of the specifications from the FSM. Your numbers don't appear to be that far off but I would be leery of any shop that does an alignment and lets it go out the door with a canted steering wheel. That, IMHO, is basic alignment 101.

If the original rubber parts are still on the car then I would expect them to be on their last legs just from an age perspective.

Even so I get the feeling that the biggest trouble you are having is believing in numbers - I can't tell for sure but it could very well be a case that the operator of the machine should RTFM.

I reckon you need to do some checks of your own so you can judge whether they are doing the job properly or not. Measurement of camber is quite a simple thing to do - either with a "bought at a shop" camber gauge - or with some bits of stuff that you are likely to have in your workshop =>

http://www.peachparts.com/shopforum/2539510-post9.html

You might be interested in this thread where it turns out that some of these fancy alignment machines need to be "swept" before they give the right number =>

http://www.peachparts.com/shopforum/2716547-post21.html

Sorry wrong terminology - you need to sweep the wheels - I don't know what that means. I do my own alignment and suspension adjustments.

Agreed on the RTFM. The pull to the right is not unexpected given the settings. I think I've confirmed that rear bushings need to be replaced when the weather improves. I'll do the front also just because then all will be new.

For now, I'll have them align it again and not give it back with obviously incorrect specs especially when it drives like the specs indicate. They did say that the machine is calibrated every 6 months.

I didn't know you couldn't do a live caster adjustment on a Merc, but then I never do it on any vehicle. I always do a caster sweep after a caster adjustment because I do not trust the machine to accurately infer caster for live adjustments. I say infer because it can't "see" caster like it does toe and camber. It measures the change in camber when you turn the wheels left and right to infer caster. Measuring caster during a live adjustment adds another dimension to be measured and I've verified that it's only a ballpark value.

As the alignment machines get easier to use, the knowledge required of the tech to get by can be reduced. Since it's not the machine that aligns your car, don't judge a shop by it's equipment. You can do badass alignments with less than $400 in equipment.

Wow I didn't realise that the machines also measure castor in the same way I do it "by hand".

I think I should get out more and go and see what these fancy machines actually do!

I doubt very much if they would agree to do this but I'd be interested to see their numbers if they were to measure and adjust the vehicle - take it off the machine - and then put it back on and measure it again.

If they can't get the same numbers then flying the boys in from NASA to calibrate the machine won't make any difference if Mungo doesn't know how to press buttons...

I'll take it in, get the numbers. If it doesn't drive right, I'll ask them to do over then get those numbers. If it drives OK, I'll let them off the hook.