Rear axle alignment

[Klaus Pieper]

After having eventually completed making the "special tool" to align the rear axle I am now ready to start the alignment process.  I have found much advice on this forum on how to tackle this job, but did not find any mention if the large center hex head screw on the rubber mounting in the trunk that holds the vertical support for the rear axle as well as the suspension thrust arm nuts need to be loosened before attempting the alignment.

Your advice would be much appreciated.

Regards,
Klaus

[dirkbalter]

Klaus,
I did the same thing, fabricated the alignment tool based on the tech manual info. I believe you are asking about the aluminum crossmember you see in the picture. By loosening and re-tightening the left or right nut, these will move the axle to achieve the alignment. Not sure I understand your question about the center hex nut?

[Pawel66]

The BBB instruction on rear axle alignment vis screwing in/out the strut shown on the picture does not mention loosening anything. The practice by experts, however, may suggest otherwise - we would need to hear more from them.

I have just been doing it, by BBB, so hope I did it right...

[dirkbalter]

Pawel,
Can you share to part of the BBB you are referring to? Happy to correct my statement and re-check my alignment if that's not the way to do it.
I know its in German, but attached is my take on the BBB.

[dirkbalter]

One more

[Shvegel]

The large nut in the trunk stays tight.  It has nothing to do with side to side alignment.  It is simply the "coat hanger" the center section hangs on.  The thrust arms (Trailing arms) are also left tight.  Although not mentioned the pinch bolts at the bottom of the axle stay tight as well.  All you are moving is the link between the center section at the side of the center to the body.  The one with the big aluminum casting on the end.  You are simply tweaking the axle from side to side and since all the mounting points are encased in rubber they will deflect a fraction of a degree accordingly.

[Pawel66]

Pawel,
Can you share to part of the BBB you are referring to? Happy to correct my statement and re-check my alignment if that's not the way to do it.
I know its in German, but attached is my take on the BBB.

Sure, enclosed. This is from the /8 BBB. I did this alignment while checking overall wheel alignment after replacing the steering box. The BBB is quite specific on what to loosen in the front end for caster alignment, nothing here. I think it is assuming some fine-tuning here, after the main alignment is done vie the use of the alignment tool.

My adjustment here was just a couple of minutes of angle of toe (where the tolerance is quite high anyway, as per the data table). I had one wheel having negative toe (left), the other positive (right), which meant to me, basically, that the rear axle goes left when the car wants to go straight, forcing the car to go right. With the adjustment I brought rear toe practically to zero.

[MarkCan]

Some good information here👍
I plan on pulling the complete rear axle out in the near future. (Full reconditioning) I’ll make sure I revisit when the time comes.

[dirkbalter]

Pawel, based on the question asked, I was strictly referring to the rear center alignment utilizing the "special tool" and believe the BBB confirms my statement. The nuts on the strut of the crossmember will allow you to achieve (center) alignment.
Prior to installing the rear end the so called dimensions "a" as well as the perpendicularity of the vertical crossmember needs to be set of course.
     
On a side note, coming off a recent heated discussion on the German form, it was surprising to see how often the rubber donuts in the trailing arms are installed the wrong way.         

[Benz Dr.]

Pawel, based on the question asked, I was strictly referring to the rear center alignment utilizing the "special tool" and believe the BBB confirms my statement. The nuts on the strut of the crossmember will allow you to achieve (center) alignment.
Prior to installing the rear end the so called dimensions "a" as well as the perpendicularity of the vertical crossmember needs to be set of course.
     
On a side note, coming off a recent heated discussion on the German form, it was surprising to see how often the rubber donuts in the trailing arms are installed the wrong way.       

Which way is the right way? I assume the ridge or ring points upward.

[dirkbalter]

Yes.
If the donuts are purchased from Mercedes, the will have a marking "unten" meaning down on them, indicating the face going downwards. Aftermarket ones don't have that.  Due to their shape, they are actually easier to fit upside down, especially if done without the cone alignment tool.
The upper edge of the donut acts as a seal, preventing moisture and other crap accumulating and damaging the mount ......

[WRe]

Hi,
here some photos what Dirk mentioned.
...WRe

[Klaus Pieper]

Thank you all very much for your valuable replies and information.
As advised initially by Shvegel, I did not loosen the trailing arms nor the mounting located in the trunk.  The "special tool" indicated that the rear axle was +- 4mm off center looking towards the diff.  I loosened the nuts on the locating strut and turned the strut anti-clock wise which moved the strut even more off center.  Turning the strut clock wise returned it again to the initial 4mm off center, but I could not improve on this value as the strut became too tight to turn.  I will give it another go tomorrow.

[lgarret]

dirkbaltr/benz dr,WRe....so you are all saying that the ridged side, which is the side that says unten faces the ground when installed?? and the smooth side faces up....it appears the internal hole is tapered in a way the seems that the unten/ridge faced side goes up, internal hole taper shaped to fit the fitting on the car???
its amazing what it takes at times to find the answers to what the factory did and why..
thanks for any clarity you can provide. I am in the middle of my diff refresh and want to get it right.

[dirkbalter]

Yes, the face that says unten “down” should face the (garage) floor.

[stickandrudderman]

Indeed, the installation of those doughnuts is the opposite way to how they look like they should be installed. Incorrect installation gives very vague handling.
Pawel, the "toe" is fixed on the rear axle and cannot be adjusted (although in race car set-up people have been known to put an axle in a press to get some toe in). The only angle that can be adjsuted is the "thrust angle" which should be as close to zero as possible (unless you're looking for best times around an oval circuit!) and is done via the adjustment being discussed here.
Lastly, the rear axle alignment should be done with the vehicles weight on the wheels.

[Pawel66]

Thank you. I think I used the wrong term... the rest I understand.