Air Pressure Cells

[WGB]

Apparently the full hydraulic suspension on the 126 was more common in the US than anywhere else - presumably cost/benefit.

I must disagree with Styria that that the spheres are the dampers - by themselves. However this is only my opinion from reading the documentation available to me.

The damping action appears to be a result of the interplay with the movement of fluid between the strut and the sphere and the primary damping action is in the strut. There is a circular plate in the strut that opens on compression to expose 4 circular holes that then close on rebound to restrict the flow of fluid. This is presumably the point of difference between the Sport and standard struts.

There is obviously further damping obtained from the flow of fluid into and out of the sphere but apart from the size of the orifi and the physical qualities of the gas-filled there is no physical restriction to allow damping.

Obviously when the spheres are flat there is a profound effect on the damping but this is probably more to do with some physical flow change than any change in damping from the sphere.

There was an extensive discussion between Koan and myself at this link http://forum.w116.org/mechanicals/damping-improvement-in-a-6-9/15/ if you want to find the references for this opinion.

Bill

 

[s class]

I have received my return-authorisation from AutohausAZ. I will send the failed sphere off tomorrow.

 

[Styria]

Mercedes Benz published a small booklet titled "Technical Data Manual" at various times - usually at three to four yearly intervals. The 1981 edition consists of some 600 pages, in small print, and any information not contained therein is probably not worth knowing. Obviously, it makes no reference to rust issues, so we don't want to know about it, do we ?

One little snippet on Page 436 relates to the much discussed air pressure cells, and what is supposed to fit 'what and 'where' - we have guessed for some time now, but no longer. It's all there in black and white.

Before I get onto the nitty gritty, let me refer to two matters, namely

1)The Company refers to "HYDROPNEUMATC SUSPENSION" - don't know how it relates anywhere to air - it's all hydraulic, isn't it ?

2/My information is restricted to and includes the Ch. 126 500SEL model. The book does not cover the 560SEL model, and it appears quite clear that full hydraulics were only available on the 6.9 and SEL Ch 126 models.

As far as the air pressure cells are concerned (and they are referred to as dampers), the following Mercedes part numbers (in 1981) are applicable:

Central storage unit - master 6.9 116 320 16 15 red dot 75lbs
380 - 500 126 320 03 15 red dot 75lbs

Front axle 6.9 116 320 16 15 red dot 75lbs
380 - 500 126 320 01 15 green 57lbs

Rear Axle 6.9 116 320 14 15 blue dot 60lbs
380 - 500 126 320 01 15 green 57lbs

-----------------------------------------------------------------------

That information should enable us to stop guessing. As a last point, would anyone have an opinion as in regards to fitting front units to the rears, given that possibly quality may not be as good as then, and that furthermore, we are normally dealing with after market units. Regards Styria

 

[s class]

Thanks for giving us the facts styria. The reason the book makes no reference to the 560SEL is that the 560 engine was only introduced later, some time around 1986 or 1987 I think.

 

[WGB]

Thanks Styria - I have the 1979 version of that book but as you are aware the 126 wasn't out at that stage.

So now it looks like there are at least three and maybe 4 different spheres - red dot, blue dot, green dot and the one with the 123 part number that is used in the self levelling rear suspension.

Bill

 

[Styria]

I would think that steel sprung SLS 126's would use the normal SLS speres at the back.

The full hydro 126's may get away with green dot spheres in the front as opposed to the red dot items used by the 6.9, on account of their alloy engines being lighter than the massive 6.9 plant.

Well S-Class, it looks as though you're spot on as far as front pressure cells for the 560 are concerned, with only the red dot master cell being used on the Ch. 126 models.

Has anyone given consideration to fitting red dot pressure cells to the rears ? One would imagine for the ride to be a little harsher, but roadholding could well be improved - who knows. As we all know, deterioration takes place gradually with these dampers, and an elapsed period of perhaps twelve months may get the rears back to what they would have been like when the cars were new in the 'olden' days. Regards Styria

 

[s class]

Well I'd be interested to hear the outcome of fitting red dot spheres to the rear. My limited 6.9 driving experience suggests that the rear is already plenty hard enough (well compared to my 280SE anyway), so new red dot spheres may be overkill. But partly used red dot spheres should work I would imagine.

 

[John S]

Am I correct in assuming that the pressure rating of 57lbs and 75lbs refers to the nitrogen pressure in the spheres? If that is the case the 126 spheres would probably give a softer ride than the 116 spheres.

 

[s class]

THose pressures refer to the nitrogen charge in the sphere, when new. If you were to fit the lower pressure 57lb spheres to the front of a 6.9, the ride would be harder, as the nitrogen bladder would be more compressed.

 

[s class]

Actually, my last statement makes me rethink my earlier comments on fitting red dot spheres to the rear of a 6.9 - would that not actually make the ride softer, rather than harder? - I think so!

 

[Michel]

Actually, my last statement makes me rethink my earlier comments on fitting red dot spheres to the rear of a 6.9 - would that not actually make the ride softer, rather than harder? - I think so!

I am trying to keep up with this conversation....

Not easy....

 

[s class]

Well the reasoning is simple really - if your rear spheres are getting on in years and have lost some pressure, the ride gets really hard and crashy right? So conversely, if one raised the pressure of the spheres (ie fitting red dot items at the rear), the ride should get softer....

 

[Michel]

Well the reasoning is simple really - if your rear spheres are getting on in years and have lost some pressure, the ride gets really hard and crashy right? So conversely, if one raised the pressure of the spheres (ie fitting red dot items at the rear), the ride should get softer....

Hmmmmmm

I think it makes sense

 

[Styria]

I am trying to keep up with this conversation....

Not easy....

THose pressures refer to the nitrogen charge in the sphere, when new. If you were to fit the lower pressure 57lb spheres to the front of a 6.9, the ride would be harder, as the nitrogen bladder would be more compressed.

Hi guys, my apologies in advance if ticking the multi quote option results in God knows how many posts being repeated - I simply could not work out how to untick the option. All I wanted to do was reply to Michel to start off with. "We are talking about the anomalies created by an apparent standardization by Mercedes regarding the pressure cells to encompass various models" - and this may possibly have led to handling and road behaviour compromises".

S-Class - I do tend to have an opposite view to yours - I would have thought that pressure cells with a higher rating would tend to give a harsher ride. As a matter of interest, would anyone hazard a guess how the manufacturer of these cells achieves a higher specification as far as poundage is concerned - is it simply a matter of "pumping" more nitrogen into the cell and how would they have done it ? I wonder. My thoughts, Styria

 

[s class]

styria, I'm happy to accept you may be right about higher pressure spheres giving a harder ride - I guess someone will have to try it ultimately.

 

[John S]

I would not think that the spheres loose their nitrogen very slowly, once the nitrogen started to leak (I'm assuming) through a minor fault it would leak fairly quickly (say in days rather than months). The initial effect of a slow leak would be the hydraulic oil becoming saturated with nitrogen, so the shock absorbers would loose their effect, and a very soft bouncy ride would be noticed. Once the nitrogen had dissipated from the oil the ride would be very hard as the "progressive give" of the nitrogen filled sphere and the nitrogen saturated oil would be lost. As the springs and spheres work in conjunction to giver the required ride, do the 126 springs have the same rating as the 116 springs?