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Damping improvement in a 6.9

Started by WGB, 31 October 2007, 07:08 AM

WGB

Having replaced the two rear spheres in my car there has been a dramatic improvement in rear damping which now shows up the state of the fronts.

I have replaced several sets of shockies in M-B's over the years as they start to go soft at about 80,000km and leak about 100,000km. I usually buy Bilsteins from the local Bilstein agent which usually run at about 1/2 the price of genuine originals and usually have a softer ride than OEM's but tighter damping.

There is a lot of talk about "Flat" spheres but little talk about poor damping which must occur long before they actually go flat. Certainly none of my spheres are or were flat but there is definitely a big difference in the quality of the damping with new spheres.

Bill

CraigS

One "test" I use when evaluating any future purchase is to drive over a speed hump at about 40 km/h. If I feel any more than the slightest thump, I look further at the suspension. Obviously I do this when the existing owner is not present.
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WGB

Quote from: CraigS on 31 October 2007, 08:36 AM
One "test" I use when evaluating any future purchase is to drive over a speed hump at about 40 km/h. If I feel any more than the slightest thump, I look further at the suspension. Obviously I do this when the existing owner is not present.

That I think is my point - that test will show up a "Flat" cell but it is the level of wallow that I would be interested in as well because even if a sphere is not flat the underdamped nature of the suspension - in my case the uneven underdamped pair of rears - begins long before alll Nitrogen is gone from the sphere.

It is rare in suspension terms that the suspension and the damping are included in the same unit - I am referring to the damping quality and the thump refers more to the suspension quality.

Bill

koan


Normal dampers are a piston in a cylinder filled with fluid, the pistol has valves to control fluid flow. They are no longer usable when the piston leaks too much fluid past its sides or the valves can't control the fluid flow.

6.9 struts have no valves to control fluid flow, the only valve in a strut is to return leak back oil to the return lines. The critical part is the spheres.

I've cut open a sphere, after carefully drilling a small hole in the rear section to relieve any pressure, there was none in this case. Two chambers separated by a rubber diaphragm, front chamber is the fluid section, the rear section is filled with heavy foam rubber and gas at high pressure.

There's not much to go wrong, rupture of the diaphragm allowing fluid into the gas chamber and the gas out, or a slow leak doing the same thing over a longer period. That's what appears to have happened to my opened sphere, hence the lack of pressure.

Could be that the foam rubber is different in the three types of spheres (red, green and blue dots) or  maybe it's just the gas pressure is different in the three types.

What I'm leading to is that there is only one failure mode, oil in the gas section and it shows up in different ways.

The harsh feel comes when front spheres are really close to the end of their life, or when less heavily loaded rears are starting to go on their way out.

In conventionally sprung cars the resonant frequency of the front and rear suspensions are designed to be different to prevent oscillations building up. In 6.9s maybe as the spheres change their characteristics they pass through a stage where the resonant frequency is the same front and rear which results in the wallowing motion., like a boat head on to the swell. 

What I assume provides the damping is resistance to fluid moving through the plumbing or possibly an orifice somewhere and I can't see that changing.

Any comments?

koan.
Boogity, Boogity, Boogity, Amen!

WGB

Well something sure controls the damping and it is quite different in new vs old spheres.

When you look into the wider orifice of the sphere where the flexible line connects there appears to be a white plastic object visible just past where the thread ends. ?is this a valve or is this a reduced orifice to control the fluid flow.

Bill

koan

Quote from: WGB on 01 November 2007, 08:41 AM

Well something sure controls the damping and it is quite different in new vs old spheres.


Not disputing your observations Bill. I agree, I mentioned in a thread awhile ago that my car seemed less stable that it did but is it the damping that's changed? Could it be the front/rear spring rate thing. Tell me when you feel the less damped condition.

Quote

When you look into the wider orifice of the sphere where the flexible line connects there appears to be a white plastic object visible just past where the thread ends. ?is this a valve or is this a reduced orifice to control the fluid flow.


I don't remember looking into the hole of a new one so I don't know. The old front one I butchered apart just had a hole, big enough to push a screw driver with about 3 or 4 mm shank through.

koan

Boogity, Boogity, Boogity, Amen!

WGB

The rear end now feels tied down but the front end now has that floaty feeling.

Bill

koan

Quote from: WGB on 01 November 2007, 09:37 AM

The rear end now feels tied down but the front end now has that floaty feeling.


Sounds exactly like mine before new front spheres. I put it down to the spring rate thing. It's not just the front floating but the whole car pitching, as if it's floating on waves. Would be interesting to know what an external observer sees.

koan
Boogity, Boogity, Boogity, Amen!

s class

Quote from: styria on 01 November 2007, 12:50 PM
     2. Gradual deterioration-in my view commences after about 12-18 months, with the car becoming

That is depressing.  This is a concern about future availability of spheres - we are all going to need them with monotonous regularity into the future. 

My red car is at the far side of step 3.  Completely undriveable.  It passed the roadworthiness inspection like that.  Go figure. 


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

CraigS

The good news is that they fit the 126 as well, so I don't think there will be any shortage of them. It will likely only be a pricing issue. Would be interesting to look at the Citroen pricing and see whether the sphere's will do the same job, or as well (or better). Might look into it.
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s class

Craig I suppose in theory if the Citroen spheres have the same pressure charge, they will function as long as the plumbing can be made to connect. 


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

CraigS

Can you (or anyone else) recall what the pressures were/are in the 116/126 spheres ? I did see it here in a post - perhaps your lengthy suspension monologue. I will look through and see if I can find it. I know the 116 ones had a higher pressure in certain spheres, but not sure if all were changed. They certainly changed to 126 part numbers.
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CraigS

This is an interesting article on the recharging of Citroen spheres. http://www.mars.dti.ne.jp/~ynar/bxorg_archives/t03/t03e.html

From a quick look,  Citroen spheres sell for between 22 GBP and 30 GBP (A$50 - 75) - or about half the MB price. However, to recharge one they only charge about $10 each from what I can see. For a 6.9 however, you would need to buy the Valprex valve (7.40 GBP) and drill a hole in the sphere and tap it. Seems like a lot of work, but you only need to do it once. The trick to it seems to be that there must still be more than 20 bar remaining in the sphere in order to re-gass it. Not sure how you can measure this when it is on the car, other than general ride quality, or perhaps you could fit a gauge inline for each one.

The best alternative would seem to be to find a sphere with similar pressures to the 6.9 and modify the lines to fit. Would be an interesting exercise, and something I might try when I need to change any of mine.
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koan

Quote from: s class on 01 November 2007, 02:55 PM

That is depressing.  This is a concern about future availability of spheres - we are all going to need them with monotonous regularity into the future. 


A quote from the 116 manual is more depressing,

"The gas filling pressure decreases during operation, the time factor is evaluated higher than the mileage. Under normal conditions, the minimum pressure is attained after approx 2 to 3 years or 80,000 to 120,000 km. Under extreme climatic conditions or in countries with very high outside temperatures, the gas pressure may decrease faster under given operating conditions, e.g. repeated slow bumper to bumper driving."

I read that as meaning they degrade quicker in high temperatures and it's occurring while car is sitting there not being driven.

Quote from: CraigS on 01 November 2007, 03:48 PM

Can you (or anyone else) recall what the pressures were/are in the 116/126 spheres ?


Since I have the book open...

New

Front 72 + 2 bar, rear 60 + 2 bar, 126 57+2 bar

Minimum Value

Front 60 bar, rear 45 bar, 126 45 bar

Max difference between left and right 8 bar.

Quote from: CraigS on 01 November 2007, 04:38 PM

... you would need to buy the Valprex valve (7.40 GBP) and drill a hole in the sphere and tap it.


I don't know how the gas escapes from Citroen spheres but no amount of gas would have restored the 6.9 sphere I cut open.

koan
Boogity, Boogity, Boogity, Amen!

CraigS

Styria. How come your figures are different than Koan's ? The figures you quoted are the minimum figures mentioned by Koan. Is that what you meant, or are you reading from a different book, or W126 figures ?
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