axle positioning question

[kiheimatt]

Please see attached.

Im trying to find the correct center for my axle. (measured at curb weight) Also... Im seeing a few different numbers for original tire diameter and ride height. I know there was some variability, but is 25" a pretty fair number to use for axle positioning? I noticed someone had mentioned that there were a lot of entries on ride height... but searching "ride height" I only saw his comment.

UPDATE: I found a thread where multiple people measured ground to fender lip at the top of arch... but it just confused me more. It seemed to me the average number for the rear was about 63 cm.... but thats less than the calculated diameter of the stock tire. When I look at side profile pics of the cars.. they seem to be have a few inches above the tire.

I will say people commented about having full tanks of gas and a trunk full of tools, etc... top on, top off... so I understand the variability... but generally speaking, parked at typical wet weight, how much gap above and in front of the tire is there to the nearest inch or so?

In my diagram I used:
- tire diameter 25"
- tire to fender distance at 9-oClock position 2.25"
- center of wheel to fender distance at 12-oClock position 16.5"  (I'm guessing this should be closer to 15.5" or so...)

[neelyrc]

kiheimatt
I measured the diameter of one of my newly mounted 195/75 tires a few weeks ago at 25 1/2 inches.  Manufacturer advertises 25.7 inches.  This morning I took quick (and rough) measurements from center of star on rear wheel cover to fender.  At 9 o'clock (front) I  have 14," at 12 o'clock I have 13 1/4," and at the rear I have 19 1/4."  I hope this is of some help.

[kiheimatt]

Thank you!

So if I'm reading that right, then the radius (half the diameter) of your tire is 12.75"

So...
if you are 14" in front, that means you have 1.25" of space space between fender and tire, and you have .5" between top of tire and fender.

Interesting. That is much closer top measurement... but corresponds well to what others have said.
If I adjust for my actual tire size now (24") your front measurement is identical.

(No special considerations... like a trunk full of lathe chucks or anything?

I will go with the attached... I think it looks and works well.

[neelyrc]

Yes, radius of tire is 12 3/4" and clearances are as you calculated.

Trunk is empty except for spare wheel w/tire, jack and standard tool kit.  Tank is about 3/4 full.   

[wwheeler]

If I get the subject of this post correctly, you are trying the position your rear axle on the chassis? If so, the best, most reliable tool is the one attached. This tool, that you can easily make at home, mounts on the two studs of the arm mount and the pointer aligns with the end of the axle pivot bolt. When it is aligned, the pin on the tool lines up with a centering point on the bolt. Lots of posts about that here. I would think there are too many variables to do it any other way. If you want, I can post more pictures of the tool. Hopefully that helps.

[Tomnistuff]

I'm not an expert on axle positioning by any means but I have done it once, both left/right and fore/aft so here are my observations.
If the results from using the alignment tool indicate that the axle is improperly positioned, the lateral positioning of the axle is done with the locating strut that ties the axle laterally to the car body.  If the results show that the axle is improperly positioned fore and aft (in other words, more or less than the 280 mm dimension of the alignment tool shown above), then there is method of moving the axle fore and aft while keeping the pinion shaft of the differential horizontal.  It is to adjust the "a" dimension shown in figure 8.17 on page 139 of the Haynes Mercedes-Benz 230, 250 & 280 1968 thru 1972 Owner's Workshop Manual, and is also allegedly shown in the Big Blue Book in Figure 35-4/21.  My BBB does not show that sketch and dimension (that section is missing from my BBB), but my Haynes manual does show it.  The dimension is given in the BBB on page 35-0/1 as, "Check distance "a" between surface of joint flange and support of rear axle suspension (see Fig. 35-4/21) .... "a" = 158 +/- 1 mm .  The joint flange is a ruler laid vertically on the pinion shaft flange surface that the driveshaft U-joint bolts to, and the "support of the rear axle suspension" is the front surface of the curved I-beam cross-section carrier that the axle hangs from, which is attached to the rubber mount in the trunk.  The measurement is taken from just below the rubber mount where the carrier I-beam is vertical.  The axle can be positioned fore and aft on the large pivot bolt around which the left axle and the differential "pivot" during jounce and rebound.  The procedure for adjustment is described in the Haynes Manual (but not well) and I assume that section 35-4 of the BBB, if you have it or can find it, also describes the setup of the "a" dimension.  Maybe someone in the group has and can provide Figure 35-4/21 and the BBB description of the "a" dimension adjustment.

I managed to get mine changed by trial and error and some careful plastic hammer work.  My car's previous owner had moved the axle substantially, but I don't remember which direction it was from where it is supposed to be.

Good luck if you need to move the axle fore or aft, and I really mean that.  It was not fun.

Tom Kizer

[mbzse]

.../... axle is improperly positioned fore and aft .../...

The matter of the longitudinal positioning of the upright support, i.e. where to clamp the rubber bushing... is specified quite close at +/-1mm, measurement "a" in drawing attached.
Discussed previously; see for example http://www.sl113.org/forums/index.php?topic=12503.0

Hard to measure, when axle is mounted in car.
Se for instance this thread http://www.sl113.org/forums/index.php?topic=19920.msg140662#msg140662
/Hans S

[wwheeler]

I made and used the tool and managed to align my rear axle a few years ago. Tom is right, this is no cake walk. It is complicated with many ways to go wrong and is hard work. The axle is heavy and the alignment check has to done with the car on the ground. No lifts allowed. But it is very precise like everything else on these cars. In fact MB had the tool many years ago and you might be lucky enough to find one. Do your homework and research before you start. The extra time spent beforehand will pay off.

Keep in mind you also have to adjust the driveshaft length if you change the position of the rear axle front to back.

[kiheimatt]

THANK YOU FOR THE TOOLS AND ADVICE!

[kiheimatt]

Would you mind posting more pictures of the tool? Very appreciated.

[wwheeler]

Here are some more. The second pic is the tool in use. The screw pointer is right at the dimple in the pivot shaft. Hopefully that helps.

[A Dalton]

All specs/data for my alignment jig are taken from the 113 Chassis Frame Measure Charts and body geometric schematic spec/data sheets from the factory. Along with factory alignment spec/data ..

 The reason the tool uses the step bearing mounts for relative measure is cuz of their geometric convienience to tool construction.

 The actual measure for the pivot offset is for the offset spec measurement to be that distance to the right of the actual chassis C/L.. This can  be found by using the Center Bore holes in the chassis.  The tool was just designed around the other mounts for ease of measure without having to deal with the C/L bores [ which would entail dropping lines , etc]. But , these center bores locations were used in the tools geometric design..the tool simply allows one to not be concerned with  C/L Bores, as the measures are taken from step bearing C/L to reach the same conclusion.
......remember too , the tol. for pivot pin location  has a +/- 2 mm .

A Dalton
BenzTechs

[GGR]

In my experience, all data and procedures in the BBB are essentially a starting point, that will then need some fine tuning once the axle is in place. Ride height is variable, as I believe US market Pagodas came with a higher ride height compared to the European market. So in the end, and within some limits, I guess ride height is more of a personal choice. Ride height in the front can be adjusted with different springs available and through the the thickness of the rubber shims on top of the springs. Same in the rear, plus the various positions of the lower spring support plates relative to the arm, and through various thicknesses of the shims on each side of the center spring.

For a given ride height, elements to be taken into consideration to properly position the rear axle will be the relation between front and rear ride height, neutral or slightly negative camber of the rear wheels, and the fact that transmission output flange and axle input flange need to rotate within parallel plans (meaning the shaft won't necessarily be fully aligned with rotation axis of both flanges - a slight angle is sometimes recommended so that U joints wear better and last longer). It is also good to check lateral positioning by measuring the distance between the rims and the inner fender at 9 o'clock on the left, and 3 o'clock on the right while facing the rear wheels. In the end, all this will need to be checked when adjusting alignement at a professional place equipped with the righ devices.