Damping improvement in a 6.9

[WGB]

 Sorry I have taken the family away for a few days down to Busselton and have patchy internet coverage.

On the morning I left I was finally granted access to the M-100 members area after having paid a 2 year membership nearly 6 weeks ago.

There is a 22 page pdf on the functioning of the hydro-pneumatic supension which has 1 page on what actually does the damping.

They call the ram and sphere a gas pressurised shock absorber and the actual damping is done by a plate attached to the piston which has four holes in it that are exposed when the piston is in rebound and control the flow of oil between the ram and the reservoir.

The sphere reservoir obviously is part of the interplay and works better with  higher gas pressure.

The white plastic part that I saw down the large hole is attached to the rubber diaphragm and is presumably only a rest stop to protect the diaphragm .

I feel happier now that I know what actually does the damping and it will be interesting to see if the reservoirs can be pressurised in the future.

Due to the large number of 126's and others with rear self levelling the rear spheres will be available for a long time but the three fronts may in due time have to be re-gassed or else new rears "pumped up"to the higher pressure.

Do we know what pressure the citroen sphere's are pumped up to?

I will be back at the Pistol Club over the next few Sundays so will watch out for the Citroen guy who also shoots there - he seems to know a lot about refilling them and will certainly know about the pressures. He says they also refill the Rolls-Royce spheres as well.

Bill

[Big_Richard]

Sorry I have taken the family away for a few days down to Busselton and have patchy internet coverage.

On the morning I left I was finally granted access to the M-100 members area after having paid a 2 year membership nearly 6 weeks ago.

There is a 22 page pdf on the functioning of the hydro-pneumatic supension which has 1 page on what actually does the damping.

They call the ram and sphere a gas pressurised shock absorber and the actual damping is done by a plate attached to the piston which has four holes in it that are exposed when the piston is in rebound and control the flow of oil between the ram and the reservoir.

The sphere reservoir obviously is part of the interplay and works better with  higher gas pressure.

The white plastic part that I saw down the large hole is attached to the rubber diaphragm and is presumably only a rest stop to protect the diaphragm .

I feel happier now that I know what actually does the damping and it will be interesting to see if the reservoirs can be pressurised in the future.

Due to the large number of 126's and others with rear self levelling the rear spheres will be available for a long time but the three fronts may in due time have to be re-gassed or else new rears "pumped up"to the higher pressure.

Do we know what pressure the citroen sphere's are pumped up to?

I will be back at the Pistol Club over the next few Sundays so will watch out for the Citroen guy who also shoots there - he seems to know a lot about refilling them and will certainly know about the pressures. He says they also refill the Rolls-Royce spheres as well.

Bill

Bill, old buddy old pal - Can you email me that 22 page pdf 

[koan]

They call the ram and sphere a gas pressurised shock absorber and the actual damping is done by a plate attached to the piston which has four holes in it that are exposed when the piston is in rebound and control the flow of oil between the ram and the reservoir.

What I use as a reference are the 20 pages devoted to the "Hydropneumatic Suspension" in the Mercedes-Benz publication "Model 450 SEL 6.9". In addition I have a copy of a Mercedes-Benz manual, about 150 pages "Hydropneumatic Suspension System Models 116.036, 126.033 & 126.037".

I believe the passage quoted above refers to the rebound stop being damped, not the strut/sphere combination being damped as a whole.

From the first mentioned reference above:

"The rebound stop in the gas pressure shocks is hydraulically dampened. During rebound, the collar of the piston rod enters the cylindrical portion of the guide ring. The oil between the collar of the piston rod and the cylindrical portion of the guide ring can escape only slowly and the collar of the piston rod can therefore also settle only slowly on the inner face of the guide ring. To permit easy retraction of gas pressure shock from the stop position into its normal position, a ring valve inside on cylinder releases four bores during deflection through which oil can flow."

Basically at almost full extension the strut has an arrangement to damp the last few mm of travel and a simple four hole and O-ring valve in the bottom of the strut to allow it to release easily.

"styria" dismantled a strut, his picture and description confirmed that the piston is solid and without valves.

koan

[WGB]

We are obviously reading from a similar document and I apologise about the the plate with four holes in it - this is exposed on compression and is to allow for easier compression and then a more controlled rebound when the plate with 4 holes is occluded. Your document suggests that it is only the last few millimetres mine would suggest it is through mostly the whole compression travel.

Definition - to me compression is the striking on a bump and rebound is the strut lengthening again afterwards having traveled over the bump.

Quote from my document which is from a summary at the end of the paragraph on page 10 -  " The purpose of the gas pressure shock, in combination with the pressure reservoir, to act as a shock absorber is attained by the piston which is mounted on the piston rod together with the respective bores and valve plates." The English is not totally precise but the meaning is thre.

Although there are no active moving valves themselves it would appear from the description that the clearances engineered into the strut itself are what allows the rebound damping.

It would appear from which ever document we read that while the pressure reservoirs can affect the quality of the damping as they go flat due to their inbility to transfer fluid easily, the real damping occurs in the struts themselves. If a sphere is going flat and therfore has little resiliency presumably more of the fluid expelled from the strut on compresion is transferred to the unit on the other side or elswhere within the system and explains the weird "cork-screw" movement these cars can do when the spheres are past their use by date but before they go totally solid.

Bill

[Brian Crump]

He says they also refill the Rolls-Royce spheres as well.

Well that bit got my attention and I would certainly like to know more. SSII spheres are only $200 each though so the cost is not a drama (yet...)
Regards,
BC

[WGB]

He says they also refill the Rolls-Royce spheres as well.

Well that bit got my attention and I would certainly like to know more. SSII spheres are only $200 each though so the cost is not a drama (yet...)
Regards,
BC

I'll try to track the guy down in the next week or so and get a bit more detail.

Bill

[koan]

We are obviously reading from a similar document and I apologise about the the plate with four holes in it - this is exposed on compression and is to allow for easier compression and then a more controlled rebound when the plate with 4 holes is occluded. Your document suggests that it is only the last few millimetres mine would suggest it is through mostly the whole compression travel.

I suggest it's the last few millimeters, not the document. Reading the paragraph (quoted in my previous post) along with the cross section diagrams, particularly the one of valve at bottom of the cylinder is what leads me to that conclusion.

I agree with your description of compression and rebound.

"The purpose of the gas pressure shock, in combination with the pressure reservoir, to act as a shock absorber is attained by the piston which is mounted on the piston rod together with the respective bores and valve plates."

That section is on my page 8. I can't have read the passage closely enough as I see a new meaning in it now, especially the bit "with the respective bores and valve plates" which implies the pistons do have holes and valves, these valves being separate from the "rebound valve". The cross section of the piston does look as if it might have some valve arrangement in it.

If that's the case I'll have to have a bit of a rethink about how the struts work. I've always assumed the volume below the piston to free of fluid, maybe it isn't.

The english is a not precise, but what I've learned about confusing sections of M-B manuals is that when a job is finally completed, a read of the troublesome section again is spot on and could not be understood any other way - which hardly helps here!

It would appear from which ever document we read that while the pressure reservoirs can affect the quality of the damping as they go flat due to their inbility to transfer fluid easily, the real damping occurs in the struts themselves. If a sphere is going flat and therfore has little resiliency presumably more of the fluid expelled from the strut on compresion is transferred to the unit on the other side or elswhere within the system and explains the weird "cork-screw" movement these cars can do when the spheres are past their use by date but before they go totally solid.

Again I'm having a bit of rethink, spheres this time. I previously viewed them just like a spring, pressure them with fluid from the shock and the foam compresses, take the pressure off and they spring back, just like squeezing a bit of foam rubber. If they are filled with the sort of foam that has a memory, like that used on dashboards, poke a finger in it and it leaves a dint that stays there but disappears after 2 or 3 minutes. That duration would be far too long, the foam would have to absorb energy quickly but return it relatively slowly - now that would be damping. As the foam became tired after millions of cycles or contaminated with fluid it would lose its damping properties.

If cork-screwing is the asymmetric motion I feel on speed humps taken slowly I put that down to  left and right  bumping and rebounding differently, not any side to side fluid transfer.

koan

[koan]

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

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.

Here's a table of Citroen accumulator and suspension sphere pressures that maybe of interest.

http://www.kolumbus.fi/perhe_pitkanen/xw/spheres.htm

koan

[CraigS]

Thanks. Interesting that the Citroens have 8 spheres, on specific models.

[s class]

This thread has been very interesting to me as it reveals that the struts also have a role to play in damping.  My red 6.9 has 195 000km, and new spheres but original struts.  It feels fine to me.  I am now really looking forward to seeing how the blue car behaves with new spheres AND new struts.  Perhaps I will then discover what the cars are really supposed to be like.   

[CraigS]

Have you read this as well ?

http://www.starmotorsny.com/pdf/HydroSuspGuide.pdf