Sphere specs

[Mandrake]

C5 spheres are spec'd differently, eg
385/57/1.9-1.3
- two figures for the damping

What do the two figures relate to? Could it be fluid in and fluid out - i.e. bump and rebound?

Nope, because the hole that everyone seems to refer to as the damping hole/orifice is actually the "damper bypass hole", and there is only one of them on a sphere, and the hole is not directional in any way.

It's only a small part of the damper valve function - most of the damping is provided by the leaf valves. The damper unit is a cylinder shaped disc with many larger holes drilled in it - I don't remember exactly how many and it has probably changed with the different designs of spheres over time (the DS spheres that can be disassembled differ from the later types quite a bit) but there are approximately 6-8 holes of around 3mm in diameter drilled into the disc towards the outside that you can't see as they are hidden under the spring steel washers that act as leaf valves.

On "modern" spheres (GS/CX/BX/Xantia/XM/C5) there are multiple depressions in the disc under the edge of the spring steel washers to admit oil into these 3mm holes. The depressions are alternated on each side of the disk so that they are only present on one side of each hole. Thus for oil trying to flow in one direction (say into the sphere) half of the holes are blocked by the washers on the near side of the disc (which act as one way valves) and the other half are open, but only on the entrance side.

For oil to actually flow through there has to be enough force to bend the washer on the opposite side to cause it to lift up slightly - then the oil can flow through in that direction. Thus it forms a threshold valve - no oil at all flows below a certain threshold but once enough force is present the valve partially opens and allows oil to flow, but the restriction provided by the partially open hole provides hydraulic damping.

Because the flow restriction in each direction is controlled by a different leaf valve, you can adjust the damping in jounce and rebound directions independently by changing the two different spring steel washer stacks on the two sides of the disc. (Only the outer one is visible without destroying the sphere, but the other one is there, inside the sphere neck)

Shock absorbers in conventional suspensions especially sportier cars typically have different jounce/rebound rates with rebound being stiffer but "normal" Citroen spheres (spheres on non hydractive models and the centre sphere valves in the hydractive block on hydractive models) are actually 50/50 with equal jounce/rebound damping, which gives superior ride.

The Hydractive corner spheres have unequal damping though with more rebound damping than jounce (about 30/70) and the reason for this I believe is that when the car rolls the increased rebound damping means the side of the car on the outside of the corner compresses more than the inside rises - thus during cornering it tends to keep the average height of the car lower and prevent "lift off" on the inside corner that can cause instability in hard cornering. So a hydractive model has near equal damping in soft mode (at least for small movements) for optimal ride and unequal damping in hard mode for improved cornering and stability.

In addition to these large holes that are restricted by leaf valves, there is also a "bypass hole" which is literally a very small hole drilled through the middle of the valve. This hole works equally in both directions and is there largely as an "equalisation" port, which allows the suspension to slowly return to equilibrium after a step in the road has passed. Whilst the diameter of this hole is very important, the stiffness of the leaf valve washers is actually more important to damping, so the damper hole size, pressure, and CC volume of the sphere does not fully characterise the sphere. (EG drilling out the hole size of a hydractive sphere of the same pressure to the hole size of a non hydractive sphere does not make the same sphere - the leaf valves are also much stiffer!)

Not convinced, Marc.
Other C5 mushroom spheres' damping figures are 1.75/1.3, 0.7/0.48, 0.9/0.48 1.4/0.94 . . .
Don't see why Citroen would suddenly have gone all approximate with respect to these apertures.
As tolerances, the variance is huge - eg 0.9/0.48 - that's getting on for twice the diameter, ie four times the damping area.

C.

I agree - there's no way its a tolerance for a single hole, the damping of the bypass hole is related to its area, so 4x is a massive change. The two numbers have some other significance, although I have no idea what, when the table says it is damper hole diameter and there is only one damper bypass hole...

Don't want to go too off topic here, but looking into this Sphere issue - I was interested to find a couple of links to pictures of inside the Saucer spheres - in this case comparing an IFHS Sphere with a Citroen Original Sphere.
Inside an IFHS Sphere

Inside a Citroen Original
[/URL]

The point being that according to the C6 guys the IFHS, MONROE and SUPPLEX saucer spheres are the same inside as the old green LHM spheres in terms of membranes and don't last as long as they still suffer with the nitrogen slowly escaping.

The Citroen genuine spheres have the newer membranes in which seal better, reducing nitrogen seepage and allows Citroen to guarantee them for 5 years / 200k Km.

Guess I know which ones I'll be replacing mine with!

The Citroen sphere has a 3 layer membrane, the so called long life types which were first introduced on the front suspension of some Xantia's - the middle layer is impermeable to gas while the layers to either side of that provide mechanical strength. In a conventional sphere there is only one layer that provides both gas impermeability and strength - but the material is far more porous so it leaks much faster. As far as I know none of the after market sphere suppliers supply the 3 layer long life spheres, that includes Xantia's too.

And back to the topic - no extra damping holes inside the sphere as far as I can see.

You wouldn't see any damping holes in either of those two pictures - the damper valve is inside the neck of the sphere, not the body, and you're looking at the damper valve side on. The damper is contained entirely in the neck of the sphere and is only about 10mm thick.

Is it possible the dampers are tapered holes?

A tapered hole would not be directional damping, any more than connecting two different diameter hoses together and feeding it from one end or the other is... directionality is provided by the leaf valves as described above, although many spheres are tuned with equal non-directional damping.

3 and 3+ ? Think not; the table above relates only to Hydractive 3. In addition, a single sphere with a single part number carries the two diameter figures.

Is it possible the dampers are tapered holes?

Maybe, but I can't see that affecting fluid flow in each direction by much - limiting factor is the smallest diameter.

3 and 3+ ? Think not; the table above relates only to Hydractive 3. In addition, a single sphere with a single part number carries the two diameter figures.

Is it possible the dampers are tapered holes?

Maybe, but I can't see that affecting fluid flow in each direction by much - limiting factor is the smallest diameter.

Bearing in mind that these mushroom spheres have been around for years now, I'm amazed their workings and specs are not better understood.

C5 spheres really aren't any different to the 3 layer (dimpled) Xantia spheres - they work exactly the same way and the damper valves work exactly the same. The only real difference is the change in aspect ratio to a wider flatter shape. What is it you're not sure about in how they work ?

The reason for the change in shape was that it was found that the materials forming the low leakage 3 layer diaphragms do not like to flex too far when cold, otherwise they can prematurely fracture and fail - this is why they were only ever trialled on the 3 front suspension spheres on a Xantia, in the warm engine bay, never on the rear spheres that are subject to cold. It's also why they were never used in cold climates, only warm climates.

For the C5 the shape of the sphere was flattened so that the diaphgram could displace as much oil, but without bending back on itself so tightly, thus the problem of cracking/failure of the diaphragms in cold conditions was solved.

[white exec]

Simon,

First, thanks for that extremely detailed reply.

The reason for asking about the two figures for damping hole diameter on these (C5-type) spheres is simply that, in the above table, this is the very first time I have seen two figures listed. SFAIK, this hasn't appeared before (although I do not have access to the full C5 or Xantia Citroen Repair Manuals). It's also interesting that in the later 2005 C5-C8 "Private Car" data pages, in the spheres spec section, there is no listing of damper diameters at all.

The news that the small central drilling is merely 'an equalisation device' is a surprise. My reading of previous documents has always been the central drilling was responsible for setting the (bi-directional) damping characteristic of the sphere under normal (non-violent) driving conditions, and only when a significant upward impact occurs does the spring-steel washer valve open to prevent system damage. For a mere 'equalisation device', Citroen seem to have gone to considerable lengths in using so many different drilling diameters.

I am not dismissing what you have written - I have far too much respect for your extensive expertise on all this. I am just surprised never to have read about this anywhere before. As such, we must be looking at a huge misunderstanding of the workings of current Citroen spheres by most writers and owners.

My stock of Citroen technical literature only goes back so far. Do you have any Citroen descriptive references to sphere damping working?

Chris

[Mandrake]

The reason for asking about the two figures for damping hole diameter on these (C5-type) spheres is simply that, in the above table, this is the very first time I have seen two figures listed.

I've never seen the two diameter figures before either, and can't think of what it could mean. It might have been a measure of the stiffness of the leaf valves (but with who knows what measurement unit) except for the fact that it clearly says diameter in the table. But if there is only one hole what are the two figures for....

The news that the small central drilling is merely 'an equalisation device' is a surprise. My reading of previous documents has always been the central drilling was responsible for setting the (bi-directional) damping characteristic of the sphere under normal (non-violent) driving conditions, and only when a significant upward impact occurs does the spring-steel washer valve open to prevent system damage. For a mere 'equalisation device', Citroen seem to have gone to considerable lengths in using so many different drilling diameters.

Well you are kind of right - the bypass hole is part of the damper tuning and is pretty critical to the overall ride. But it mostly affects the recovery and return to steady state, for example after riding over a step (not bump) in the road, not the damping of an initial impact, whether that impact be "jounce" (compression) or "rebound". (extension)

The cross sectional area of the bypass hole is very small compared to the holes under the leaf valves. If we assume there are 4x 3mm holes for each flow direction under the leaf valves, that's 28.3mm^2, compare that to a 1.1mm bypass hole with 0.95mm^2 area. So nearly a 30:1 ratio of cross sectional area.

So for anything other than the slowest most gentle suspension movement most of the oil must go through the leaf valves as the bypass hole is seen as a severe restriction by the rapid movement. So for these faster movements its the adjustment of the leaf valve stiffness that controls how the initial impact is damped, and whether the bump is hit "hard" (as in Hydractive hard mode) or more softly. If the leaf valve is stiffer you'll get a harsher ride due to small movements not being enough to cause oil flow and thus not absorbed.

What I was trying to draw attention to is the misconception that the bypass hole is all that matters and that its diameter is enough to define the damping characteristic of the sphere, when its only one part of the damping and not even the biggest part. If the leaf valve is too soft then no adjustment of the bypass diameter will give sufficient damping as the leaf valves will open too easily.

Likewise if the leaf valves are too stiff you won't get a good ride no matter what size the bypass hole is - if you make the hole bigger trying to improve the ride in spite of a stiff leaf valve you'll just loose all your damping before you get a good ride. There's a delicate balance between the leaf valve adjustment and the bypass hole diameter to get optimal ride and handling.

I've played with the tuning of spheres in the past including adjusting the size of the bypass hole, and found that the bypass hole largely affects the recovery and ride quality. Too large and the ride becomes floaty and starts oscillating, too small and the ride becomes firm with a slow recovery time. (Over damped)

The way Citroen spheres seem to be tuned is for the leaf valve threshold to be set such that movements from bumps (which tend to be rapid movements) are too fast for the bypass hole to pass, so they force their way through the leaf valves, however the reaction (rebound) from the suspension which is a slower less forceful movement is not fast enough to open the leaf valve in the opposite direction, so the recovery can only play out slowly through the bypass valve.

Lets consider two scenarios. One is you have a road that is otherwise flat but has a speed hump on it. When you hit that bump the velocity of suspension movement from the impact is far too fast to be able to pass through the bypass valve so it forces the leaf valve on one side to open to allow the suspension to move. The effort/resistance to open this valve and force some oil through its partially open restriction provides damping. After you pass the bump the suspension will try to move equally rapidly back in the other direction - also too fast for the bypass hole so the opposite leaf valve will be forced open for the oil to flow back out. This also provides damping of the movement in the other direction.

So in this situation very little oil has flowed through the bypass hole and most of the damping has been provided by the leaf valves.

Scenario 2 is a road which is initially flat then suddenly rises a couple of inches to a different level which is also flat. The initial impact is much the same as the discrete bump - too rapid for the bypass hole to play any role with the leaf valve opening for the duration of the climb over the step in the road. Once that climb is finished the leaf valve will close again.

Now the suspension is not in equilibrium as it is compressed 2 inches below normal height so it somehow has to return back to normal height. The reaction force if the springing is soft is quite weak compared to the initial impact force, and is too weak to open the opposite leaf valve so the bypass hole provides a way for equilibrium to be restored - without it the suspension would remain stuck at the wrong height!

The hole allows a gradual flow for the suspension to return to equilibrium. If the hole is really small that can be very slow - anyone who has tried bounce testing a Hydractive car in hard mode will know what I mean - its very hard to push it down at all, but when you do it returns really slowly, and can take as much as a second to recover.

The movements during a manual bounce test are generally too slow to open the leaf valves, so you are only really testing the bypass hole and gas pressure of the sphere when you do this. This is why you can hardly move the suspension on hydractive hard mode by hand (very small bypass hole) yet it can still move a lot to absorb large bumps when the leaf valves open.

One interesting point here is that as spheres lose pressure the springing rate gets stiffer, stiffer springs store more energy for the same compression, so as the spheres deteriorate at some point the reaction force of the suspension becomes strong enough to actually open the leaf valves - when you reach that point of sphere degradation you lose a lot of your damping control because now some of the reaction can return via the leaf valve instead of just the bypass hole...

[white exec]

Reassuring to know that you're unable to interpret those two damping hole diameter figures as well. There's only one hole, so lord knows what the other figure is.

Thanks for taking the trouble to put together that last post. It's very thorough. I have been scouting about for some official Citroen description of the sphere valve system, but can't find one - even looking back at early Hydractive I pages. Probably because the Citroen sphere is such an established (and pretty well unchanged) item, it hasn't merited Citroen description for decades. Nearest I got was p.23 in The Citroen Guide by Gabor. This accords with both my understanding and what you have written above.

Leaf valves, then, are obviously specifically designed to have certain damping characteristics, although we don't know just how many different ones may have been produced. I have a couple of (grey) XM rears (400/30/0.7) on the bench, both with ruptured membranes, and tomorrow I'll try to extract the leaf valve assembly and take a look at it. Interestingly, the valve assembly itself is marked with a couple of digits, stamped on either side of the central drilling - an E and a 2. Could just be a readable reference, to aid correct manufacturing assembly.

Given that - a specific spec for the leaf valve - it occurs to me that there could be an obvious problem in using a sphere intended for one vehicle/sphere position on/in another vehicle/position, even if the basic spec (eg 400/30/0.7) is correct. It might explain why Citroen parts lists different part numbers for what looks like the same sphere spec. (Can't cite an example immediately.)

Also, knowing that pattern (non-Citroen) spheres are already using single material membranes, I do wonder, even if they get the central drilling correct in size, whether their attention to detail extends to variations in leaf valve spec.

The only item that gave me pause in your last post was Scenario 2: the sharp up-only ramp, on to a flat surface. Agree with you completely re: first scenario, that the leaf valves will open both on the rapid compression and rapid extension, to accommodate the large volume of propelled fluid. On the second scenario (up-only), my reading is this: wheel rises sharply, so leaf valve opens to admit fluid to sphere - but then the gas pressure in the sphere is sharply increased, and immediately after the bump, on the level, this high gas pressure will want to expel the received fluid with just as much force as it received it (it is perfectly elastic), and will therefore open the leaf valve in the opposite direction.

I think the central drilling is only operational for slow/restricted movements of the suspension. All else opens the leaf valve. As you say, it's a tiny central hole, providing a high level of damping. Agree with you completely about firm-mode bounce tests - the ride gets pretty unpleasant if you're relying on just the centre Hydractive sphere for comfort, with "wood" on the corners, especially at the rear!

I hope I can extract a leaf valve tomorrow, not least to measure the holes it reveals.

Chris

[Mandrake]

Thanks for taking the trouble to put together that last post. It's very thorough. I have been scouting about for some official Citroen description of the sphere valve system, but can't find one - even looking back at early Hydractive I pages. Probably because the Citroen sphere is such an established (and pretty well unchanged) item, it hasn't merited Citroen description for decades. Nearest I got was p.23 in The Citroen Guide by Gabor. This accords with both my understanding and what you have written above.

Spheres and Hydro-pneumatic are a special interest of mine. Many years ago when me and Dad both had GS's we had a shortage of GS spheres (which are identical in construction to modern Xantia spheres - and were/are very expensive in NZ) but had a home made re-gassing rig and a few DS spheres kicking about. DS sphere's have removable damper valves of a different design to modern spheres - they're functionally equivalent but constructed quite differently.

Instead of a cylinder with washers riveted to either side in the middle with a centre bypass hole down the middle of the rivet, the whole stack is simply bolted together with a bolt through the middle, and also bolted into the sphere neck. This means that the damper valve can be easily removed from the sphere and disassembled, washers removed and different numbers of washers fitted.

The bypass hole is near the edge of the disc instead of through the middle (not that it matters) and instead of the leaf valve holes being bored straight through with a depression in the middle cylinder/disc to allow oil in, the middle disc is flat on both sides and the through holes are all drilled on an angle so that one end of the hole is inside the diameter of the washer and the other end is outside the diameter of the washer - to give the same one way effect. Alternative holes are drilled on opposing angles.

The neat thing about those DS spheres is you can remove the damper valve in minutes and disassemble it, change the washer configuration, (thus changing the damping of the leaf valves) bolt it back together and put it back on the car. On a modern sphere the damper valve assembly is an interference fit inside the sphere neck - they probably fit them in the factory by cooling the damper valve then pushing it in. Getting it out without destroying the sphere and using a press could be challenging. If you want to have a look at one a bit more easily find an old Hydractive 2 control block from a Xantia or XM - they have two damper valves the same as spheres inside and they are very easy to remove with just one socket! (Just remove the two plugs where the large steel pipes normally go and they fall out... )

Anyway, we experimented with gas pressure and washer stack configuration of the DS spheres on the front of Dad's GS, so I learnt a lot from that. Even though DS sphere's are a lot larger CC than GS spheres (500cc vs 400cc I think) you can compensate for that by filling them to a lower gas pressure to get the same effective spring rate. If you fill them to the "normal" GS sphere pressure the ride is much too soft.

Likewise we experimented with the number and thickness of washers (multiple different thickness's were available/used and fitted much like stacking shims in a bearing to get the correct thickness) and initially we had too many on which made the ride rather firm indeed. I remember the first test drive with the DS sphere's the ride was actually very firm at the front but very stable feeling - not unlike Hydractive 2 in hard mode. We had to remove some washers to get softer damping to get a more normal GS like ride but with the right thickness of washers we were able to achieve that.

A few years later on a Series 1 CX we had fitted some new front spheres and found the damping at the front quite a bit too floaty. It turns out that Series 1 CX's have smaller suspension pistons at the front than Series 2 CX's, so the gas pressures and damper valves for S1 and S2 CX's are different to compensate for this. Unfortunately by this time (1998) Series 1 CX spheres were no longer available, at least in NZ and Citroen only supplied one size fits all for CX's! This time we experimented with the damper hole since the damper could not be disassembled like the DS spheres. I don't remember exactly what the original hole was but I think it was 1.65mm.

To change the hole size we simply soldered the hole closed then drilled it back out with precision drill bits. Initially we tried 1.2mm - this gave really good damping and nice feeling handling with no floatiness at all, but the ride was too firm and lost its CX charm. Eventually we settled on 1.4mm I think - which was enough to remove the floatiness and overshoot but still maintain a comfortable wafting ride. It was an almost perfect balance. Yep, just reducing 1.65mm to 1.4mm was all it took...

Many years later I experimented with damping adjustments on my Hydractive 2 Xantia - in particular I found the ride in soft mode a wee bit too much on the "floaty" side for my liking, so I removed the damper valves from the Hydractive control blocks (which control the slow movement damping in soft mode) and reduced their bypass holes from 1.1mm to 0.9mm. (Using the same solder then drill technique) That doesn't sound like much but it was enough to take the soft mode from a bit floaty with a bit of overshoot to almost perfect critical damping where the car returns to a steady state after an impact as quickly as possible but without overshoot. I liked that balance a lot.

Leaf valves, then, are obviously specifically designed to have certain damping characteristics, although we don't know just how many different ones may have been produced. I have a couple of (grey) XM rears (400/30/0.7) on the bench, both with ruptured membranes, and tomorrow I'll try to extract the leaf valve assembly and take a look at it. Interestingly, the valve assembly itself is marked with a couple of digits, stamped on either side of the central drilling - an E and a 2. Could just be a readable reference, to aid correct manufacturing assembly.

Yes it could be a code number for the different leaf valves. What I do know for sure is that spheres off different models do have different leaf valves, for example you can't exchange CX and GS spheres, even if you adjusted the gas pressure and drilled the damper hole. Also front and rear spheres cannot be swapped, the leaf valves are different there too, due to the very different characteristics of the front and rear suspension and the load they carry. Also Hydractive 2 and non-Hydractive sphere's in the same locations also have very different leaf valves as well as the different bypass holes.

How many different variants of the leaf valves there are across all models, I don't know.

Given that - a specific spec for the leaf valve - it occurs to me that there could be an obvious problem in using a sphere intended for one vehicle/sphere position on/in another vehicle/position, even if the basic spec (eg 400/30/0.7) is correct. It might explain why Citroen parts lists different part numbers for what looks like the same sphere spec. (Can't cite an example immediately.)

Yes, very likely. But remember there are different part numbers for spheres with the same damping/pressure specs but different diaphragm materials too. Some spheres used Desmopan, some used Urepan and the multilayer spheres used a 3 layer sandwich of two different materials. (not sure what materials were used in the multilayer spheres)

Also, knowing that pattern (non-Citroen) spheres are already using single material membranes, I do wonder, even if they get the central drilling correct in size, whether their attention to detail extends to variations in leaf valve spec.

I've sometimes wondered this too. I've fitted both Citroen and after market spheres in the past and my gut feeling is that the after market spheres don't always get the damping quite right - they're not a long way out but the damping doesn't feel spot on to me like the original Citroen spheres usually do.

The only item that gave me pause in your last post was Scenario 2: the sharp up-only ramp, on to a flat surface. Agree with you completely re: first scenario, that the leaf valves will open both on the rapid compression and rapid extension, to accommodate the large volume of propelled fluid. On the second scenario (up-only), my reading is this: wheel rises sharply, so leaf valve opens to admit fluid to sphere - but then the gas pressure in the sphere is sharply increased, and immediately after the bump, on the level, this high gas pressure will want to expel the received fluid with just as much force as it received it (it is perfectly elastic), and will therefore open the leaf valve in the opposite direction.

I see where you're coming from but you're missing something. In the step scenario the initial impact is a high velocity impact (the wheel suddenly moves up, very quickly) because the road and therefore the wheel changes height suddenly but the car body has a great deal of inertia, so depending on the softness of the springing the car body doesn't move much at all.

The suspension is all but forced to move in response to this step in the road, unless it is very stiff. Whether the springing (gas pressure) is moderately soft or very soft makes little difference to how the damper valve behaves - the small bypass hole simply cannot pass the quantity of fluid required quickly enough to respond to this impact. For all intents and purposes the hole might as well not be there and this causes an instantaneous spike in pressure that is sufficient to open the leaf valve to relieve the pressure differential.

The reaction that follows afterwards however is very different. There is no sudden movement involved to cause an instantaneous pressure spike. You simply have the gas compressed slightly more than normal so that there is a pressure imbalance, but the softer the suspension is, the smaller this pressure imbalance is for the same fluid displacement. (change in suspension height) On a very soft suspension like a Xantia, there isn't that much energy stored by the gas spring, so the restoring force is also weak and doesn't require much damping. This restoring force is too weak to open the leaf valve on the return so it bleeds slowly (over a good fraction of a second) back through the bypass hole.

Another thing to keep in mind is that some of the energy of the initial impact was already absorbed by the turbulence through the leaf valve, so there is less energy to be returned than there was put back in.

The way to test all this empirically (which I have done) is to make the bypass hole very small without changing anything else. Just enough so that the ride height can equalise (slowly) under static conditions, a bit like a hydractive sphere. Then observe that when you hit a step in the road the initial impact is absorbed well but the car returns very slowly to its original height - over a second or more.

If the return was through the leaf valve reducing the bypass to a tiny hole wouldn't have this dramatic effect. So my conclusion is that the vast majority of the rebound returns through the bypass hole.

I think the central drilling is only operational for slow/restricted movements of the suspension. All else opens the leaf valve.

Correct, but with very soft springing rates (Hydropneumatic springing rates are some 2x to 3x softer than typical sedan coil springs rates) the reaction/rebound from the compressed spring is a very slow movement. It is just a slow pressure equalisation in response to a pressure imbalance and not directly in response to an impact. The initial impact imparts energy over a short time, the release of that energy is slowed down and spread out over time by the damping.

And I think that's why the damping on a properly set up Citroen feels so good - it has a very stable unruffled feeling to it. Each impact is absorbed by a leaf valve (whether it's a bump or a dip) and the "return to centre" reaction plays out slowly and gently via the bypass hole.

This is not the case when you have stiff springing rates - now the energy of the rebound is much stronger and more similar to the energy of the initial impact, so a threshold damper valve can't distinguish between an initial impact and the rebound force from the spring, thus you end up with heavy ride destroying damping to keep the car from rebounding wildly. (The damping on a Hydropneumatic Citroen is actually very light and gentle compared to a typical coil spring setup, because with the soft springing you don't need really stiff damping to control the rebound because the rebound is so weak)

Hydractive 2 is even more complicated to analyse, because the centre sphere is connected in "parallel" to the strut spheres effectively. Now you have four different paths for oil to flow in response to a movement - the bypass holes in the strut spheres, the leaf valves in the strut spheres, the bypass holes in the centre damper valve or the leaf valves in the centre damper valve.

As the damper valve tuning of the strut spheres is very different to the damper valve tuning in the Hydractive block, this effectively gives 4 stage damping instead of conventional 2 stage damping.

In addition to this some of the oil that flows towards the centre unit flows into the sphere and some flows back out through the second damper valve towards the other strut! In addition, roll movements are damped by the centre damper valves even in soft mode.

In Hydractive 2 in soft mode small gentle movements tend to be absorbed by the centre sphere while large sudden movements are absorbed by the strut spheres. This is good because this means the rebound damping for small gentle movements is light, while the rebound damping for large sudden movements is stiff! The damping rate is dynamic depending on the nature of the initial impact even if the suspension stays in the "soft" mode. Think that one over for a minute...

There is more to Hydractive 2 than just a switch between soft and hard modes under computer control, the soft mode is a lot cleverer than it appears due to the complex way the centre sphere and sides are interlinked via additional damper valves.

[Stickyfinger]

Great thread....learning a LOT.....cheers guys

[GiveMeABreak]

@ Chris
I've just dug this up which shows a slightly better view of the internals of a slimline sphere - specifically the valve area - in case you didn't particularly want to start hacking one to bits in your vice!

Oh, and again, note the 'umlimited life' claim here again.

[Mandrake]

Thanks for the picture - very handy You can see that the damper valve is right at the base of the sphere neck - there is quite an empty region above it. You can kind of make out the washer stack on either side of the damper too.

"Unlimited" is a bold claim indeed, but in a way they're right. Conventional spheres progressively lose gas at a steady rate and apart from ceasing to ride properly when they get too low in gas they eventually puncture and fail because of the loss of gas if you don't top them up in time.

The long life types do not leak gas - I have seen long life front spheres on Xantia's (which use the same 3 layer diaphragm principle and material) which measure exactly at their original 45 bars at over 10 years old and 100k miles and still work perfectly, no loss in performance or ride at all.

However they can eventually fail due to fatigue of the diaphragm - when that happens they lose their pressure very quickly indeed. So the multi layer membranes effectively change spheres from a "wear and tear" service item like a brake pad, which is expected to wear out at a certain rate and need replacing after x number of miles or years, to a part that might potentially fail at random within a 10-20 year life time, but probably won't.

From what I've seen you can expect these type of spheres to last at least 10 years without any loss in performance, and probably a lot more.

[GiveMeABreak]

I thought it might be useful (or not!) to see what's inside. Why is it that they expressly 'forbid' re-pressurising of the slimline spheres then? Does anyone know why they have specifically mentioned this type of sphere?

[Mandrake]

I believe the reason for the advice against re-pressurising the multilayer spheres is that because they don't leak pressure at all under normal conditions, a big loss in pressure that would warrant re-gassing is a sure sign that the diaphragm (or at least one of its 3 layers) is cracked/fractured and is about to fail. Thus re-gassing the sphere is almost certain to leave you will a sphere that will fail completely in a short time.

The few spheres of this type that I've heard of failing failed completely and very quickly after the first sign of pressure loss. (Just a few weeks)

While with a conventional sphere pressure loss is expected and if caught and re-gassed before it goes below about 50% can be safely re-gassed and have its life extended.

[GiveMeABreak]

That makes sense Simon - I just saw the huge red warning and the words 'forbidden' and it conjours up images of cranking open the top cylinder nut then 'bang' and it's strawberry jam on the wall behind you!

[white exec]

That's a really helpful cross-sectional diagram, Marc, showing the exact layout of parts for the compact sphere.


As promised, I did get to work on an XM-type sphere, to extract and examine the leaf-valve...


^ First, sphere neck sliced off.



^ Central damper drilling, seemingly marked 32 (not 2 and E, as I thought before), and marked with what appears to be a manually applied centrepunch. Very odd. Note the countersinking for the drilling, added to later spheres, to improve fluid flow.



^ The interior end of the valve, with peened-over central sleeve, which holds the valve parts together. The peening was simply drilled off.



^ This then allows the central sleeve, containing damper drilling, to be pushed out (it's a very snug fit), freeing up the washers on both sides.



^ Leaf washer positioning, with apologies for the mixed units. Interesting to note that the individual washers have extremely tiny amounts of flex.
>>> Now edited, to show correct 3.5mm central drilling, narrowing to countersunk 0.7mm damping aperture.





^^ Outer and inner views of the damper body. Large central drilling for the central sleeve is 7mm diameter, and all other holes are 4mm, parallel sided. Note the raised and machined surface for every alternate hole; on this surface seats the leaf washer. This provides two one-way and pressure-sensitive valves, one in and one out.





^ Side views showing raised seats, and clearance between the flex-washer and the lower set of holes.


This is a part I haven't dismembered before, and it has provided quite an insight into a component which is crucial to the Citroen ride.

Chris

[GiveMeABreak]

Great pictures Chris - a round of virtual applause is due. It's hard to imagine all those components inside the space of the neck!

[xantia_v6]

Very interesting. Can you tell anything about the different materials of the washers? hardness, springiness, flatness etc?

[white exec]

No, but I will take a closer look, and try to assess the flexibility.