PeachParts Mercedes-Benz Forum - self leveling suspension question

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- - self leveling suspension question (http://www.peachparts.com/shopforum/mercedes-benz-performance-paddock/1083-self-leveling-suspension-question.html)

I am currently working on upgrading my 2.3-16 suspension setup. So far I have recieved the following parts.
1. Eibach Pro-Kit
2. Koni adjustable front struts
3. self leveling suspension plug
4. #4 pads to raise the car back up

Koni sent me the wrong rear shocks, hence I am waiting to recieve rear adjustable konis.

The reason I decided to upgrade my suspension was because th quality of the ride was too soft and floaty. I thought I should eliminate the rear self leveling suspension because the front shocks should match the rear and I doubt Koni developed their fronts to correspond to the OEM self leveling suspension. That was then.

Currently I feel that I might have dismissed the self leveling suspension too soon and did not investigate it enough to even know how it works. Hence I turn to you folks to educate me. I would like to mention that I only went with the Eibach springs for the increased compression, not ride height, as the Konis one way only adjust in Rebound, leaving the soft OEM springs with the task of handling the compression.

I read through the shop manual and was only able to speculate on how the self leveling suspension works based on the diagrams MB provided. They offer no explaination of how it handles load and how it responds to different condition, from slow response (ie high speed turns) to fast response (ie hard sudden bumps). I wonder if one is able to dial in different characteristics using the adjustment rods on the self leveling setup, or are these intended for ride height? Do different oils provide different responses, like motorcycle forks, or does one have no choice in the oil used other thanthe only one MB specifies.

I guess overall I am looking for someone to educate me of the inner workings of the self leveling suspension, as well as offer advice on whether I should eliminate it and use adjustable Koni's all around or not.

On a side not I am also replacing the strut mounts and bump stops to be on the safe side.

cheers

W.

------------------
Wael El-Dasher
http://www.efini.net

If you are racing the car then the self-leveling system should go, but for street use it should be able to perform fine. As you surmised the system is for maintaining ride height & not racing. By removing the system the car will weigh less & pick up a bit of HP (pump).

The purpose of load leveling on a this MB performance suspention system was to allow the vehicle to perform under various loading situations; after all this is a four passenger sedan with a trunk.

The system is not designed to handle sway. Stability was increased with reduced height and stiffer springs. Because it was lower, load affected ride height became more important.

The system is simple and defines the car. It also is trouble free (with the exception of the probable need for new accumulators every 60-100k). Also the system acts like a car with no shocks if it isn't charged.

This same basic system has been used on all station wagons and top S cars. MB Hydraulic fluid is the only recommended fluid.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by Webmaster (edited 03-14-2000).]

Hi steve,

Talkin' with a couple of fellow MBshop members, we are all in agreement that YOU ARE ONE SHARP COOKIE! I think we will all learn alot from you in the future.

As for my question-I have a 1984 500SEL Euro with the hydropneumatic suspension on all four corners (unlike the US version). Is there a way to tell via the ride, if my accumulators are goin' or shot? I don't think they have ever been replaced. I am the 4th owner of this vehicle. I'm considering replacing them but I don't have the funds to just frivolously throw away, if in fact they are fine. Also how often should the hydraulic fluid and filter be replaced?

Thanks for sharing your valued wisdom and experience.

------------------
FrankieZ
84 500sel EURO 87K

W,

I suggest you try the car out for a bit with the rear suspension untouched...you may be surprised that the rear will step up to the task. In a road car, do not dismiss the value of maintaining the car's attitude (in this instance I mean the fact that the car's rear doesn't dip under load) not only for suspension reasons but also your headlights pointed safely where they should be. Also consider that the 500E/E500 came with a self-leveling rear that I think works quite well...even throws a couple thousand PSI to the rear struts under hard acceleration...the car doesn't squat like you'd expect.

Try it out; you've nothing to lose.

------------------
Best regards, Michael
'92 500E
'88 300TE

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by M.B.DOC:
If you are racing the car then the self-leveling system should go, but for street use it should be able to perform fine. As you surmised the system is for maintaining ride height & not racing. By removing the system the car will weigh less & pick up a bit of HP (pump). <HR></BLOCKQUOTE>

Just wanted to say thanks for all those that responded. I appreciate the input . I posted my question to the Mercedes Realtime mailing list as well and thought I should share this very enlightening response from David Bruckmann:
----------

(Note that my frame of reference is the self-levelling system from the W123
T-series, although I believe that the 201 uses essentially the same wacko
scheme.)

Wael El-Dasher wrote:
>>>I see the reference to the relief valve in the shop manual, which is set to
>>>open at 143+/- 10 bar. However it seems the pressure in the feed line is
>>>directly related to the exhaust camshaft as the hydrolic pump is driven by
>>>the exhaust camshaft. So am I to deduce from this that the rate and pressure
>>>vary with engine rpm?

No. The excess pressure generated at higher RPM is bled off at the pressure
regulator. The fluid is simply routed back to the reservoir.

>>>Although I see no reference to nitrogen spheres in MB's shop manual, I am
>>>equating it to the leveling valve, or should it be the pressure reservoir?

The nitrogen-filled spheres are the "bombs" that are located near each
strut. MB calls them "pressure reservoirs". It may help to think of the
spheres as coil springs (which are really also "pressure reservoirs" in
their own way), and the hydraulic fluid as a steel bar or other
incompressible member. Fluid doesn't compress or expand (much), nitrogen
gas does.

>>>Could the rpm relation between the hydrolyic pump/leveling valve cause the
>>>pressure reservoir to over pressurize the rear shocks leading to less
>>>travel? (ie such as at slow speed, low gear but high rpm <2nd gear 5000rpm
>>>steadystate for eg.> ).

There's no such relationship between RPM and system pressure, except when
there is an internal leak and the hydraulic pump can't keep up with
pressure loss (in a worn-out controller, for example). The pump should put
out at least the nominal system pressure at idle, although I believe it
reaches full output at around 2000 rpm. So it will take longer for the
system to compensate for a heavy load added at idle than when the engine is
revving higher.

>>>So what you are saying is the rear spring's compression fights the
>>>overpressurizing of the spheres (pressure reservoir).

You'll have to ask the MB engineers why they felt it necessary to leave a
spring there when they could have just used system pressure to support the
car. As I mentioned before, I rather suspect they simply didn't want to get
nasty phone calls from Citroen's patent lawyers.

Let me explain a little more about the principles at work as I understand
them. Firstly, you have a steel sphere (well, more oblong than spherical,
but let's call it a sphere for now) with a rubber diaphragm bisecting the
two halves of the sphere. When there is no fluid pressure in the system
(i.e. sphere is in a box on the parts counter at the MB dealer while the
customer is being given smelling salts after hearing the cost), the
nitrogen gas fills all available space in the sphere, forcing the diaphragm
against the walls of the sphere. In operation, fluid is introduced under
pressure into one side of the sphere. It presses against the rubber
diaphragm, thus compressing the nitrogen gas (on the other side) and taking
up as much space as it needs until reaching equilibrium with the pressure
of the nitrogen gas, let's say roughly in the middle of the space. Since
the nitrogen is still gaseous, it can continue to act as a perfectly
progressive spring.

So let's say you hit a bump with normally-inflated spheres. The strut
piston forces fluid through the line into the sphere. Fluid doesn't
compress but gas does, so the nitrogen takes up the additional load. As the
gas is compressed, it's pressure progressively increases until the gas
pressure balances the load. The diaphragm stops, the strut travel is
arrested, and the wheel is stabilised. To prevent overcompression/bounce
etc., a flow restrictor retards the movement of fluid between the strut and
the sphere, thus achieving an effect similar to a conventional shock
absorber. Small, low-amplitude bumps are passed through to the spheres
because the fluid is flowing relatively slowly through the flow restrictor.
Larger bumps invoke larger and more sudden flow, which is restricted more
noticeably. This is where different fluid viscosities will make a
difference. Note that it is two-way damping: you hit a bump, the wheel
moves up and fluid is forced from the strut through the restriction into
the sphere. You move off the bump, the wheel returns downwards, and the
spring effect of the sphere is damped by restricting hydraulic fluid flow
to the strut.

So, if there's too MUCH nitrogen, the diaphragm will remain biased towards
the fluid side in terms of its "home" location, since the gas will be
compressed to equilibrium with the fluid earlier than expected. If there's
too LITTLE gas pressure, the fluid pressure will push the diaphragm further
into the space normally occupied by the gas, again until equilibrium
pressure is reached. If the diaphragm has ruptured or all the gas is gone
(it will leak through the rubber over time), there will be no spring effect
at all, since fluids don't compress (much).

The pressure of the fluid and the resultant equilibrium pressure of the gas
do not change in either case: what changes is the volume of nitrogen gas.
So if you have a lower initial nitrogen fill pressure resulting in, say,
half the normally acceptable gas volume once compressed to equilibrium, you
will have twice the spring rate (or thereabouts; the actual relationship is
likely different but that's the idea) und thus HALF THE TRAVEL before
reaching a given compression. So hitting a big bump in a car with low gas
pressure could cause wheel travel that results in very high pressures in
the sphere/strut, and this would happen in a time shorter than would be
required to vent the excess through the relief valve. The risk of damage is
not so much a blown line as it is blown strut/piston seals (larger bore,
moving part, etc).

When you add a load to the car trunk, the car becomes heavier. The weight
forces the strut piston further into the strut cylinder. This causes fluid
to move from the struts into the spheres as the nitrogen gas compresses to
balance the load, causing the car to sink. The level controller reads the
change in wheel height and introduces more fluid into the system, which
takes up the displacement of the now-compressed gas, causing the struts to
extend again. The level controller is designed to admit fluid slowly so as
not to react to every bump in the road. Relatedly, the controller reads
wheel position in the middle of the stabiliser bar so that cornering
doesn't induce a false reading.

>>>I can also see were such a rate rebound quicker, relegating the rear shocks
>>>to play a much smalled role. Please correct me if I am wrong.

As to the damping effect, it is important that the diaphragm rest in such a
way that it can move in both directions. This is why there's a nominal
pressure in the system even without additional load in the car. There must
be enough pressure in the system to overcome the effects of gravity AND the
steel spring when, for example, the car goes over a deep pothole or
suddenly becomes airborne (hey,

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by fz500sel:
Hi steve,

Talkin' with a couple of fellow MBshop members, we are all in agreement that YOU ARE ONE SHARP COOKIE! I think we will all learn alot from you in the future.

As for my question-I have a 1984 500SEL Euro with the hydropneumatic suspension on all four corners (unlike the US version). Is there a way to tell via the ride, if my accumulators are goin' or shot? I don't think they have ever been replaced. I am the 4th owner of this vehicle. I'm considering replacing them but I don't have the funds to just frivolously throw away, if in fact they are fine. Also how often should the hydraulic fluid and filter be replaced?

Thanks for sharing your valued wisdom and experience.

<HR></BLOCKQUOTE>

Frankie,

Thought this was interesting for your question, particularly the paragraph before last. This was a discussion on the Mercedes Realtime mailing list. I appologise for the cross posting but I do think it will help.
-----------

Ken Tyler wrote:
>The pressure reservoirs are what Mr. Citroen would call spheres. My 6.9
>has 5, one for each corner and one central reservoir. There are no
>springs. The central reservoir is there to supply large volumes of fluid
>when wheel travel demands it. As I recall, the pump can supply only about
>0.4 litres per min at 2500 engine rpm.

Yes, AFAIK the 6.9 was the only fully hydropneumatically suspended (i.e.
not air-suspended as in W100 limo or the W109) Mercedes-Benz sedan that
totally dispensed with springs, and so earns my utmost respect! They did
not cut corners on the 6.9's system. Unfortunately, the other models
(AFAIK) all have a hydropneumatic strut/spring paired with a coil spring.
The logic in this escapes me, and if anyone knows why they did this I'd
appreciate hearing about it. I can't see how it could be cheaper to
manufacture, less complicated to maintain, or more reliable in the long
run. Also, if the only thing holding you up is the hydropneumatic system,
any problems are immediately noticed and rectified. With the spring combo
it's more difficult for most owners to notice problems and thus flattened
spheres beget toasted struts etc.

My understanding re: the central accumulator is that it also provides
residual pressure when the engine is not running. This means that when a
load is added to the car, it will attempt to return to proper height even
when it is turned off. It also stores pressure so that it can attempt to
maintain the vehicle's height if there is seepage past the struts into the
return lines. In a Citroen, the central accumulator also provides pressure
for the power steering, clutch control, and moves the selector forks in the
semi-automatic transmission. There is a further, separate accumulator for
the brakes that is normally fed by the central pump or, in case of pump
failure, by suspension pressure.

I'd love to take a 6.9 for a spin to see if it feels anything like a DS.
Any Torontonians want to swap your 6.9 for my 2CV for a few hours? Oh,
wait... how about AFTER the gas prices come down again! :)

D.