Rear Axle Geometry Observation - Oddity or Not?

[Tomnistuff]

I have hung my rear axle from its mount and attached the trailing arms to the axle.  I have also read a lot here about, and can confirm myself, the difficulty of attaching the front of the trailing arms to the body mounts.

Having spent 34 years as an engineer (automotive type), I am naturally reluctant to "force" things into compliance just to be done with it, so I started taking measurements to find out where the problem lies.

With the axle shafts horizontal, the center-to-center distance between the two axle mounts for the trailing arms is 906 mm on my car.  The front of the trailing arms are almost exactly the same distance apart, so that's nice - nothing appears to be bent.

HOWEVER, the two body mount studs at the front of the trailing arms are exactly the same spacing as the span of the rear axle alignment tool that has been discussed here often - 877 mm.  That means that in order to fit the front of the trailing arms, I have to "squeeze" the front of the trailing arms toward one another by 29 mm to get them onto the body mount protrusions and studs.

After asking myself, "Why?", and analyzing for a while, I came to the conclusion that there is no neutral position for the rubber bushings in the Pagoda rear suspension.  With the car simply sitting still, the fronts of the trailing arms are forced toward one another by approximately 29 mm, however in full jounce or full rebound, the trailing arm mounts on the axle tubes are at least that much closer together, thus relieving the side-to-side squeeze.  But at the same time, in jounce or rebound, the trailing arms are "twisted" relative to the body mounts, imparting a different kind of stress into the bushings.  If one tried to design out one kind of bushing stress, he or she would end up designing in more of the other kind of bushing stress.

The more I work on this car, the more I appreciate the sophistication of the Mercedes engineering that went into it.  I can now install my "come-along" and force these parts into alignment for attachment without feeling like I've done something wrong.

If anyone disagrees with my analysis, please let me know your logic before I get too far along and break something.

Tom Kizer

[mbzse]

.../... The more I work on this car, the more I appreciate the sophistication of the Mercedes engineering that went into it.../...

Tom, are you familiar with this Forum posting from 2010?
/hans
http://www.sl113.org/forums/index.php?topic=12503.0

[Tomnistuff]

Hans, I had not seen that thread before, but I just read it and agree with your posting of Chris Johnson's write-up.  Since my body was recently restored on a rotisserie, the suspension was all out of the car.  While checking it out before reinstallation, I found that the "a" dimension between the pinion shaft flange and the front edge of the carrier just below the upper mount was 10 mm short of the specified 158 +/- 1 mm dimension, causing the pivot bolt axis to tilt up in the front.  It apparently worked for many years like that.  I bought the car, then neglected it for about 23 years before getting serious and starting a leisurely strip-to-the-bare-metal rotisserie restoration in 2010.

Regarding my statement that there is no neutral position for bushings, I have concluded that the reason we have to "pull" the axle rearward to attach the trailing arms at the front is to pre-stress the upper rear axle mount so that during jounce and rebound, the upper mount experiences movement toward its "neutral" position.  I think that is the case, but with the three dimensional geometry of the suspension, it's difficult for me to imagine the mount stresses without seeing a dynamic 3D model.  Actually, when I install the rear springs, I will lower the axle tubes to the end of the shock travel to make it easier to install the springs.  That will also have the effect of pulling the trailing arm in the aft direction, since the front will not yet be attached to the car.  I remember that when I took it apart, it did not seem to be pre-stressed.

If I have problems, I'll ask for advice.

Tom Kizer

PS:  The reason I now have to pull the axle rearward to install the front of the trailing arms is because the "a" dimension was apparently originally set up to make the suspension easier to assemble, but not necessarily to work right.  The axle was already pushed aft by the incorrect "a" dimension.  The car was 20 years old when I bought it.

[wwheeler]

Tom,

Having just installed my rear axle, I can relate to your pains. I am also an engineer and felt that forcing the arms inward wasn't correct. I checked everything I knew until I decided it must that way. Trying to figure out the rear axle geometry gave me a headache! It is possible to get the arms on the wrong side and there is a good picture in the BBB how to determine which arm is which. The front bushing also has a top and bottom. Stamped into the bushing should be "UNTEN" which means bottom in German. That side goes down.

One caution about the rear bushings. Because I had to force the arms inward, the rear bushings wanted to "pop out" from the arm on the OD. I had previously used some grease on the ID and OD. I went back and removed the rear bushings but this time left them BONE dry on the OD. This time when I installed the arms, the bushings did not pop out and are now perfectly uniform. Attached picture is what the bushing looked like the first time.Yikes!

[Tomnistuff]

It is possible to get the arms on the wrong side and there is a good picture in the BBB how to determine which arm is which.

Hi Wallace,

I notice that there is a left and right arm and I labeled them when I removed them, however, I have no way of knowing whether they were installed correctly when I bought the car in 1987.  The front bushing retainer lip is symetrically opposite on the two arms, but I don`t know which goes where.  I can`t find a photo in the BBB that shows the difference in the arms.  Can you remember the difference?

By the way, I have the front bushings in correctly and I used only a little talc to install the bushings in the front and rear, as well as the spring rubbers.  They all worked out fine.

Frustration after frustration.  I must have a cut or flaw in my hydraulic jack O-ring so I have to run and buy a new one (jack, that is).  It's creeping down as I pump it up.  It's not worth disassembling it for an O-ring.  That's a messy job and I don't trust myself to do it and get it right.  Anyway, life is too short to repair inexpensive tools.

Tom Kizer

[wwheeler]

The retainer lip points toward the driveshaft on both arms, so they face each other. Seems like they cover about a 1/2 of the circle. I had my axle rebuilt and they took the arms off and I had no way of marking them. I also did confirm that retainer position with a known original car.

I will find the section in the BBB tonight and post.

[Tomnistuff]

Thanks, Wallace.  That`s the way mine were and the way I put them back.  Don`t bother with the BBB tonight.  I believe you.  It`s probably in section 35-4 which is not in my BBB.  I suspect that I have an early edition.

Tom Kizer

[wwheeler]

Ok, so lousy scan but I didn't want to break the BBB binder. You can see the "holder" 3 and that is the part that faces towards the center. The holder is only 180 degrees wide.The directions for placing the arm holder is just to the left from step #14 but it is partially blurred from the scan. But you get the idea.

You can also see the bushing note about UNTEN just below #13.

I hope this helps.

[Bob G ✝︎]

Are you talking about the trailing  arm bushing or  'the front King Pins?
Bob Geco

[wwheeler]

This is for the rear axle trailing arm and specifically the front bushing.

[Tomnistuff]

Well, it has been a while since I posted about my rear suspension reassembly progress.  The reasons are two-fold.  I realized from Wallace’s reply #7 above that I had assembled the front trailing arm bushings upside down, leading me into a period of inactivity due to frustration.  The second is that when I started back to work on it, I ran into problems which many of you have probably also experienced.

The draft angle on the front bushing (hockey puck) center hole is opposite the draft angle on the body mounting cone, making it really difficult to install without the mysterious and illusive “tapered assembly mandrel” mentioned in paragraph 14 of Wallace’s “Axle Arm Scan” in reply #7 above.

I solved that problem by forcing increasingly larger sockets into the hole from the bottom side before trying to fit it onto the tapered mounting cone.  I just blocked the trailing arm down away from the car body with a block of wood and jacked the sockets into the hole, each larger one pushing the smaller one out the top of the hole.  When the largest socket was as large as the body mounting cone lower diameter, I removed the block of wood and jacked the “hockey puck” onto the mounting cone with a short piece of PVC pipe between the jack and the “hockey puck”.  The body tapered cone just pushed the socket right back out the bottom of the “hockey puck” as it was replaced by the body tapered cone – a perfect fit.  The socket just dropped through the PVC pipe onto the seat of the jack.  Of coarse, before I started jacking, the arms and axle were strapped into the correct position with ratcheting tie-downs to align the “hockey pucks” with the body tapered cones.

NOW!  There is only one problem left – a big one, at least for me.  When I torqued the 14mm nuts onto the bolts protruding from the body tapered cones, one was perfect.  The other one stripped the thread on the bolt.  It was not cross-threaded. The beginning threads are still fine.   It just stripped in the middle of the bolt as I tried to torque it to 72 lbs-ft (pretty much standard for a 14mm thread).  I have no choice but to replace it somehow.  It`s not going to be easy since the box sitting over the body tapered cone and bolt is the main rear cross-member just behind the seats, so I`ve got to get through two walls to get to the captured bolt.

Somewhere on the web in the last few days I have read about replacing the bolt or the body tapered cone and bolt together, either on a Pagoda or perhaps on a 190SL.  I lost it because I didn`t need it.  Now that I need it, I can no longer find it.

Do any of you know where the repair process is described?  Or, have any of you done it before?

Tom Kizer

[Tomnistuff]

I just had another thought.  If the head of the existing bolt is welded in place, I might be able to cut off the bolt, drill through the head from the bottom and retap the head of the bolt.  I can then replace the original bolt shank and nut with a 14mm bolt installed from the bottom.

Any opinions?

Tom Kizer

[wwheeler]

Sorry to hear about the stripped bolt. Sounds like a giant PITA. Hopefully someone hear has done that so you know what you are getting into beforehand.

I made the tapered mandrel and it works very well. Having a machine shop is handy for old cars. Not sure how you would install a bushing without some sort of fixture. Sounds like your invention did the trick. You probably want some anti-seize on those body bolts to prevent corroding in the future. 

Good luck!

[Tomnistuff]

I checked with K&K and verified that 1) they make it, and 2) the bolt head is welded to the inside of the "tapered cone" then the cone is welded to the underbody.  K&K calls it the "bell" since it looks like one before it's welded in place.  Mine has a really pretty weld bead around the periphery of the upper edge.  See the photo.  The two colored photo is caused by the new replacement K&K floor panel.

I guess I'll have to grind it off and call a local "mobile welder" to install a new one.  Before I call him, I'll take him a sheet with the above sketch and photos of new parts and the car so he can give me a good quote before I commit to paying him to come to my garage.

If I tried to do what I suggested two posts back, I would probably just screw it up and still have to grind it off and replace it.

Oh well, when I finish this job, I can work on the broken door hinge grease fittings.  Every one that I have tried to remove has very easily snapped off.  I have ordered new grease fittings from my "NEW" local Mercedes dealership.  They are only about $3.00 each.

I am at the point in my restoration where every time I start a job, I find something else broken or break something else myself.  For about a month, it has been one-step-forward followed by one-step-back.

Thanks for your sympathy.  I'm still looking for one of you who has had this trailing arm cone problem and solved it.

Tom Kizer

[Benz Dr.]

My only advice is to make sure you have it welded on straight and in the right spot. Small deviations of a mm or two probably won't hurt you too much.

[stickandrudderman]

There is a drawing on this site that shows exactly how this "bell" should be mounted. Look in the TM under "chassis".

[jameshoward]

Can't you cut new threads? You may very to go fractional smaller or imperial, but surely the hassle and risks of cutting off the bell are far greater? Isn't that the easier route?

[George Des]

Another thought here would be to build up the stripped area with a weld bead using the welding rod they use to build up shafts. You may then be able to grind this to the point you can run a die over it to rethread. You probably wouldneed to consult with a machine shop to ensure this wouldn't affect the hardness of the bolt.

[Benz Dr.]

I would be more inclined to use a coat hanger or similar rod and oxy/acl.  This method will give you material that is roughly the same hardness as the base metal.

 I have a cone here if you end up going that way.

[Tomnistuff]

Wow, with all your suggestions, there seem to be lots of options.

My problem now is to choose the options that are within my abilities and resources.  I`m not a welder and the car is immobile.  I know that the absolute right way to do it is to buy a new cone, grind off the weld bead of the old one and call the mobile welder to reweld on the new one.  To that end, I found a mobile welder only one mile away, so I ordered a new cone from SLS.  Then I found out that the nearby mobile welder is out of business.  The only other mobile welder in this area is about 25 miles away and very expensive, according to another welder who is not mobile.

One method within my capabilities is to cut off the stud, drill it out to accommodate a 12 mm tap, tap it to 12 mm x 1.5 or 1.75 and replace the nut and stud with a bolt.  The only question is whether the newly tapped head of the original weld-stud is thick enough to act as a weld-nut for the 12mm bolt (The original stud was 14 mm).

Before I make a decision, I`ll contact the far-away mobile welder to see if he will do it for a reasonable price (and maybe I can teach the king`s horse to talk).  Pardon the old joke punch-line.

Thanks much for all your analysis and ideas.  In the end, I`ve already spent so much on this restoration that I'm tempted to ask, "What's another two or three hundred dollars?"

Tom Kizer

[Tomnistuff]

Can't you cut new threads? You may very to go fractional smaller or imperial, but surely the hassle and risks of cutting off the bell are far greater? Isn't that the easier route?

After all the good suggestions, I decided (while waiting for the replacement bell to arrive from SLS) to evaluate the question that jameshoward posted (quoted).

The maximum root diameter of the original (stripped) thread, M14x1.5, is 12.344 mm.  The nominal O.D. of an M12x1.5 bolt or stud is 12 mm.  So I carefully hand filed the stripped M14 weld stud down until the thread marks disappeared (12.3 mm O.D.).  I kept filing the weld stud, while measuring the diameter in many places, until it was 12 mm diameter.  Finally I bought a good M12x1.5 die-nut and threaded the stud.

Everything installed properly and the only difference is that the driver`s side is 14 mm and the passenger side is 12 mm.  With the 12 mm nut torqued to 44 lbs-ft and the 14 mm nut torqued to 77 lbs-ft, the clamp loads are approximately 5,300 lbs on the passenger side and 10,700 lbs on the driver`s side.

Since, as far as I can see, the primary loading on the joint is in the horizontal plane and is applied to the welded-on "bell" through the bushing.  Any tensile loads on the stud must come through the trailing arm front bushing as it is flexed by movement of the rear axle.  I don't expect any problems.  I have calculated that the tightened 12 mm stud still has about 20,000 lbs of tensile load margin left.

So I still have the new bell with the welded stud if I'm wrong.

If anyone sees that I have committed a major error, please let me know.

Tom Kizer

[jameshoward]

I think you'll be ok!!

I imagine parts of the space shuttle didn't get that considered level of detailed work.

[garymand]

Do you have length on the stud to add a locknut?  My only concern is the thread strength, so I would use as much nut as possible.  I think you made a very good choice.

[Tomnistuff]

I used a nylok nut so it is quite thick with lots of threads.  There really wasn`t room for a second nut, even if I had gone with normal nuts.

Tom Kizer

[wwheeler]

The new bushing kits come with a nylon lock nut. My only concern with these is that the nut looks really tall. But in reality, a fair portion of the nut's height is the nylon ring that acts as a lock. So you don't get as much thread engagment as you might think. Gary is right that you want to use every thread on the stud that you can.

Either way, I think you are fine and made a great choice. I can only imagine that filing the stud was absolute drudgery!