[W126 Coupe] Rear suspension mods.
Divov
divov at tiscali.co.za
Mon Apr 30 08:18:32 EDT 2007
Dawid,
It took 3 years of trying before the association finally accepted a SEC to
race in Historic Car Racing.
I am there on "probation" this season as strictly, the cut-off date is 1977
and this date doesn't move up each year.
In other words, if you are post 1977, you are theoretically banished to the
Wilderness for time immemorial.
As they put it in writing to me "your continued participation will depend
largely on the attitude of your fellow competitors on having your cars
participating with them"
After 3 events I get the feed-back that they see me as just another
competitor and I race clean and the car is perfectly capable of maintaining
clean & predictable lines through corners and, much to everyone's surprise,
is fairly competitive. So far so good. What I fear most is them saying that
by their accepting my 1980, they are being pressured to accept other entries
which they consider unsuitable and now they can't justify letting me
continue participating if they have to refuse others....
I've now got hooked on racing Benzes so my plan B would be to get a 450SLC
and transfer my 560 motor etc into that shell if they boot the SEC out.
They saw my concours SEC and a lot of their accepting my car was due to its
pristine condition. My plan was really to race my other 500SEC (which is a
bit tatty) but has a 560 engine. This car I am slowly preparing to replace
my road car which I am currently racing but I want my participation to be
unconditionally accepted before I pull the 560 out of the wood-work. No
amount of effort can get it to look as good as the 500.
All the development work done on the 500 can be transferred into the 560
when the time is right.
The 560 has been stripped of everything possible to reduce weight and will
only ever be able to be race car. I intend to restore the 500 back to being
a 1st class road going example once I stop racing her. This said, I don't
really want to do much to the 500 motor as she only has about 5 or 6 races
left to do. So I'm stuck with stock power output and almost stock total
weight. I am looking forward to getting better final drive ratios into the
500 which must perk her performance up greatly. The standard ratio is
ridiculous and was clearly aimed at fuel economy during the fuel crisis of
the 80's.
The disappointment was that a (W116) 350SE diff won't fit - the ratio is
perfect... So, the search continues. I hope a 450SL will fit as the ratios
are almost as good.
Unfortunately I miss the next race on 9 June at Welkom (Phakisa circuit -
which would suit an SEC better than the tighter circuits) as I will be in
the UK.
On 25 June there is the re-opening of the Maputo circuit in Mozambique which
I doubt I can do as my car trailer is too small for an SEC (perhaps I can
borrow a trailer somewhere). I can't drive it 600 km to a race meeting. What
if I break something? That event will be such a gas. Lots of prawns & vino
verde!
Realistically, my next event will be Zwartkops (again..) on 28 July.
18 August is Wesbank raceway (old Gosforth Park) which should suit the SEC
having a very long main straight. I don't know the circuit so it will be a
first for me.
15 Sept is Kyalami which suits the SEC and I know the track.
13 Oct is Midvaal (Meyerton) which is a tight Mickey Mouse circuit and also
unknown to me.
3 Nov is Wesbank Raceway again.
15 Dec is East London - but that's too far & I'll give it a miss.
Regards
A.
----- Original Message -----
From: "Dawid Loubser" <dawidl at solms.co.za>
To: "Mercedes Coupes Mailing Lists" <mbcoupes at mbcoupes.com>
Sent: Monday, April 30, 2007 12:46 PM
Subject: Re: [W126 Coupe] Rear suspension mods.
> Alec, I follow your brave and enthusiastic journey with great interest,
> you are adding valuable knowledge to this group! I only have the guts to
> meddle with my car in very small ways, i.e. fixing and maintaining basic
> and cosmetic things - it's great to read about your experimentation.
>
> Attie and I have, on a few occasions, discussed how much fun it must be
> to
> convert a 560SEC into a special "lightweight" racing model - there is so
> much
> crap you can throw out of these cars if comfort is not your goal - and in
> a
> way what you're doing is very similar to this.
>
> I gather your 500SEC is not your daily driver, in is only for race
> purposes?
> What measures have you taken to reduce weight? I have a feeling you could
> easily
> get rid of 300kgs if you're determined enough.
>
> Next time before you go race, you should post here... If it's not too far
> from
> me, I'd love to come and watch.
>
> regards,
> Dawid
>
>
> On 27 Apr 2007, at 7:32 nm., 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.
>> The MB Coupes Website!
>> W126 SEC Mailing List
>> Postings remain property of MB Coupes, L.L.C.
>
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