[W126 Coupe] Rear suspension mods.

[Divov]

Firstly, it is extremely presumptuous of me to be commenting on design done by highly qualified German Engineers when, as only an enthusiastic amateur, my knowledge would barely fill a post-card. The following is merely my observations and experience based on trial & error under racing conditions and logical application.
However, the Merc Engineer's brief must have been to design a set-up which was perfect to keep the over-weight Industrialist alive after he may have had a few too many at the local beerhall!!
It is a very safe and forgiving car. They are marvellously engineered cars.
The SEC is a fast, luxurious freeway cruiser but no super agile cornerer - viewed by many as an "old man's car". 
When provoked fast in to corners, it's a overweight understeering bitch.
My objective is to alter that set-up to one giving sharper responses and faster cornering ability - even at the expense of the cars normal benign nature.

If you followed the thread on my aluminium billet anti roll bar mounting modification, I rigidly located the rear anti roll bar which definitely controls toe-in / toe-out on the back wheels. On my first effort, I tried to locate the roll bar mounting exactly where the designers planned with a stationary car - midway between the 2 mounting bolts.
With the stock rubber half moon mounts, these will compress under acceleration as the wheels attempt to overtake the body thereby inducing some toe in on the rear wheels. Conversely, under braking the body tries to rip off the wheels and those bushes compress the other way giving toe out as the anti roll bar pulls the wheels aft.
Looking at a Fiat Dino (Ferrari powered super Fiat) set up, this is really nifty arrangement. It works as follows:
The rear wheel toe-in is controlled by two rods (one on each side), each attached to the rear hubs aft of their pivot point and the other end these rods attach to the diff and they are also adjustable length. (the diff is mounted to the body, similar to the SEC)  On a Fiat Dino, static rear toe in is adjustable unlike the Benz. 
The clever part is that when the car is standing parked, these rods are at about a 7 degree angle relative to the ground. (visualise they run parallel to the drive shafts)
When the car squats under acceleration, these rods become more parallel to the road and hence become effectively longer and therefore push out on the hubs, swivelling them to give toe-in. Under braking, with weight transfer forward, the tail lifts & the rods create a greater angle, become effectively become shorter and therefore the wheels toe out.
What a neat set-up!  This is a infinitely better arrangement than that which the Merc uses.
Based on what both set-ups strive to achieve, I deduce that toe-in under acceleration and toe out under braking are desirable design objectives.
However, the SEC standard factory static alignment spec is _________ degree rear toe in.
I assume that they were terrified to have toe out (giving a whip-lash oversteer character) therefore they rather settled on a conservative toe-in setting.

My first attempt at a rigid mounting was not totally successful as I ended with this permanent toe-in set-up and I was somewhat disappointed with the cars turn-in in the corners.
Understeer definitely improved (because the anti roll bar was more efficient) but, turn in remained soggy and the car still tended to plough into corners on the outside front wheel if I attempted the classical racing "late entry turn in" cornering technique. The plough on effect was so bad that I would miss the corners apex completely.
Rear toe-in will counter the front wheels effort to turn in to a corner and cause the plough on effect. AKA terminal understeer.
So, the "Mk 2" version of the solid roll bar mount has the hole through which the ARB passes moved aft of the standard middle point to give zero toe-in. 
How did I figure out how far to move back the ARB hole?
I slotted the mounting holes (mounting to the chassis) in the aluminium billets so that I could hammer the whole billets aft. This was done on a wheel alignment machine and I could move the ARB mountings aft until I got zero rear wheel toe-in reading. (Normal the SEC has no adjustment for rear toe-in) 
Back at the workshop I accurately marked the centre line of the front mounting bolt while still leaving the rear bolt tight. (using a temporary long bolt & measuring with a vernier to the ARB) I now knew the distance from the front bolt to the ARB.
New mounting billets were made with the hole for the ARB drilled in the perfect position for this particular car. (I couldn't trust running the car on slotted holes in case it moved during a race).
Track testing to follow will confirm (hopefully) a more crisp turn-in character since the rear wheels won't be on their own mission.

Being a bit of a maverick, I get a kick out of getting a big Benz to handle where the general consensus was that it was not possible.
Perceptions have changed and people now are starting to take the racing SEC Benz a lot more seriously.
The problem with being one of the very few nutters to try and race a SEC is that there is no data base of information so you have to try and work it out for yourself.
It is a challenge but so rewarding when you get it right.


>From mucking about on this race car, what emerges applicable for the normal road car user is that these rubber ARB mountings do a real man's job and it would be a good idea to make sure your car's rear ARB bushes are in tip top condition. Worn & sloppy bushes will seriously adversely affect a SEC's handling.

They are really easy to replace and I'm sure can't cost much from the Merc dealership.
If your particular car happens to have "out of spec" rear alignment and your alignment specialist tells you there is no adjustment so "tough luck" - don't believe him!
This is what you can do:
You can slot the holes in the saddle brackets which hold the rubber half moon bushes. Get nice quality thickish washers which are snug to the mounting bolts and spot weld the washers to the saddle bracket once you have got the alignment within spec. Remove the brackets & then thoroughly weld & refit in the same orientation. Problem solved. 

I am so enthusiastic about this project and hopefully the development of this racing Benz is of some interest to a fellow readers and I'm not boring everyone to tears. 
If so, tell me to shut up and I won't be offended!

Regards

Alec D.
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