[W126 Coupe] Rear suspension mods.

[Dick Spellman]

Divov wrote:

> 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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Great write-up Alec.  Keep the information flowing.

Thanks,

Dick