Hydraflush And "hydronet 2000", ...and the happy ending of an Odyssey!

You r right, you can't alter the flap valves on newer Cits, I think this was possible on cars with the dismantable spheres -DS, SM- as you could take the damper totally apart into its components.

Sphere damping operation: very simple and ingenious, it's a 2-stage operation, centre orifice allows some oil to free-flow, then, if the rate is higher, the flaps are obliged to open up and reveal the other 4-or more holes -so flap thickness and diameter also plays a damping control role-. Also, flaps can be assymetrically tuned to give different compression and extension characteristics -but I have not been able to find further info on what is the setup precisely for the various models-. See full description in photos below -from the doc that can be found in my Flickr page-:






cheers
George

Oh, there's a sphere driller amongst us! I was once tempted to try this but I didn't have a small enough drill bit. Trouble is, how do you prevent all the swarf from drilling finding its way around the LHM? Also how do you know just how much larger to make the hole?

As far as the operation of the damper is concerned: you're right, it's a combination of the 2 way leaf valve (disks) and the bleed hole in the middle.

I'm not sure that the hole does anything on its own - it works with the damper valves, a lot of us DIYers seem to think that the central hole IS the damper, but if the LHM had to squeeze through that alone with a necessary flow rate that's high enough in both directions for the kind of speeds that a wheel moves up and down would be like having no suspension movement at all! You could probably push the car down by hand by about an inch over like 30 seconds - it would be mega slow . I also think the hole allows the pressure on either side of the damper to equalise quickly. It may also serve to dissipate the heat build up from the friction of the LHM pushing the disks frequently.


With an old BX, I bent up the disks by < 0.5mm to give extra unrestricted flow. Even this was too much and I ended up with a ridiculous ride. The even cruder method of returning some restriction was to bash a brad nail into the bleed hole to eliminate it which worked for a while but soon dislodged and went missing in the hydraulic citcuit (hey, I was only 19 at the time) it did turn up eventually as the cause of a failed front strut...

But funnily enough, in later years I discovered that the old style DS type spheres had no bleed hole in the centre but that the inner damping disks were slightly 'unseated' by a little step so as to leave a gap around the edge for the same purpose.

You might have noticed sometimes there are different amounts of disks. That says to me that the different damping hole choices are paired with different specs of disk stiffness. And it can differ between the inner damping disks and the outer (rebound) damping disks.

Of course different sphere pressures paired with these various formats of dampers add a whole new factor in performance. Then there's the consideration of how thick the hydraulic strut is (a larger cross sectional area will have stiffer damping and a sharper rate of change of the spring over a given stroke for the same sphere)

I think it's reasonable to modify the pressure of a sphere to change the spring rate but to leave the damper as it is because the damper is more likely chosen for the weight, performance and anti roll bars of the car they're fitted to. I only say this because I increased the gas pressure on my GSA while the dampers were unaltered. The suspension was amazing after that! (the G has spheres with the same dampers as the BX14/16)

I also tried the Xantia fronts on the CX just to see what would happen. I'd say the stiffer damping was completely acceptable on the CX but the gas pressure, 55Bar, was too low for the weight, The CX is specced with 75bar and it really seems to need it, it had limited travel and the spring rate felt rubbery. The CX spheres in the Xantia however! that was interesting- probably the best suspension I've experienced!

So the sphere, when the cars weight is on it and the car is stationary, has about 50/50 LHM to gas ratio, it means that the initial pressure if the gas is about double. And when this is the case, the LHM pressure and the gas pressure are the same. So a car with a higher set sphere pressure will have a larger volume of gas left after the LHM raising the car has compressed it - giving a soft ride. Likewise a lower pressure sphere will take more reduction in volume to reach a higher pressure equal to the LHM, it needs to be more squashed, hence there's less of it to squash further as a spring, giving a stiffer spring.

AFAIK the damping operates totally independently of this pressure relationship and indeed the damper is only involved with LHM pressure (which is constant over all the damper's surfaces in static conditions) and fluid flow. It's what fluid engineers call a 'Control Volume' it means you can take the damper on its own and consider the input from one side or the other as being 'given' and then any information you get relating to the damper's performance can be taken as relating to it alone. Where it would have to be considered in great depth is where you've got a very stiff sphere or very soft one where the undamped frequency going through the control volume is very high or very low.

It's funny, because using a gas as a spring actually has a mild damping effect on its own, not enough to use but enough to be used with the damper. The higher the gas pressure the greater this damping effect. It's quite likely that this has been brought into the equation for designing the dampers.

The actual physics of the flow frictional forces that the damper deals with are pretty complex and I think you need to take the flow rate multiplied by the cross sectional area of the holes through the damper valve in one direction and take away the force required to lift the disks by a certain amount for this given flow rate, (who knows how much though)the value for the unrestricted bleed hole would be a constant. This also assumes the flow of fluid is not turbulent.

Short answer? I don't totally know! but we all get the feeling of what goes: higher pressure than 'standard' for any given car gives a softer ride, higher volume of sphere gives shallower rate of change of spring, small damper hole (e.g 1mm or less) gives stiffer damping and certainly feels like stiffer springing but it would be the same spring, just overdamping.

That's a super diagram George! where did you find it? It looks like some sort of original citroen publication. That is the hydraulic circuit of a GS they've shown there.


Is there any more to this?


Graeme

Yes that's right. There is a sphere driller amongst 'US'.

I drilled the sphere's upside down at a very slow speed with a strong magnet on the drill chuck. I think I upped the hole size to 1mm.

As above

>> I drilled out the orifice in an old pair of Xantia spheres as an experiment

Which means none scientific suck it and see kind of thing. It was surprisingly softer pretty much like comfort spheres I had on my XM for about 4 days...

Steve.

?

Hi Graham

yes you can download the whole doc from my Flickr page shown down at my signature.

Something is mentioned in there about the relation of load and damping rate, from memory it said the higher the pressure in sphere, the lower the effective damping rate, but to an not-perceptible degree. This has striked me, as I had always noticed my XM seems much softer whenever I carry 4 people in it -and others have remarked that also-. So it's perceptible enough, if it's the said effect.

It's nice when you have various cars and play around with their spheres, isn't it? I've never done it yet...

Please if you have any other interesting documents share them with us!
cheers
G

'US' when you only just joined and stating things in such as way as to make it sound stupid drilling spheres.

Nice work offending me in your first few posts.

Steve.

That's not offensive^ it's a question. Any you gave an answer of 'strong magnet' that's a good solution, well answered, I just wanted to know as I've avoided it in the past.


Don't see that someone has to be on any forum for ages to earn any rights to cause offence or be one of 'us'. Besides, 'us' as I used it refers to the fact that we have a forum in common - 'the forum used by 'us'' that's all and not that I'm muscling in on your precious sacred space.


Anyway sorry for the upset.

Now children. Its against club rules to get upset or upset anyone else

Re the softer ride with increased load. This applies to all cars as it changes the sprung to unsprung weight ratio. If you have a light car with heavy suspension components e.g. Landrover the ride is hard because the 'tail is wagging the dog'. As you increase the weight of the 'dog' the suspension movement has less effect as inertia tends to to keep the car body where it is. The more weight the better ride - handling is a different storyof course.

Peter.N.

I'm hoping you're joking

Banter - yes please!.

Offensive comments - no thanks.

I can see how your 'sphere-driller' comment was misconstrued. Any sentence that ends 'amongst us' would seem to denote the 'sphere-driller' as a separate and distinct person, hence not part of the 'us'.

I think we need to get to know you a bit better to understand your humour.

Steve, as many times as I re-read aerodynamica's comments on drilling I cannot perceive any attempt to present it as stupid drilling spheres, to the slightest? My impression is he was just descriptive, no? Am I missing something So just a misunderstanding probably? Happens often on the net where you don't face the other person and the tone of speech is not there!

C'mmon, friends again?!

cheers
George

OK amigos, apologies all round. I re read it again and I see it could've seemed not nice. I can assure you it was not intended that way!


So Steve, again, very sorry for my silliness.


EDIT: George, nah, I can see how ic could come across now

This post has been edited by Aerodynamica on April 03, 2008 05:25 pm

Noooooo!

I cannot believe it ! Peter, your address please, I'm coming there to give you private and exclusive (free!) tutoring over the basics of hydropneumatics! To-ni-te !!

Rule 1 of hydropneumatics: thanks to the height correction system, spring ratio automatically adjusts and is the same under any load! And this represents one of the "Top 5" benefits of that suspension against coils -and u r right about the coils behavior-.

Anyway, I'm sure u knew it anyway, just the time of the day...



(Loads of smilies to emphasize my humorous mood)


cheers
G

This post has been edited by jorgy9 on April 03, 2008 05:43 pm

That's true but the Citroen system should tend to 'firm up' as load increases but maybe it's not enough to deal with the added inertia.


Another thing that strikes me about the LHM is how different the ride is first thing in the morning to later in the day when the fluid is well heated. Also I recall how soft it became in the heat of summer once cruising in the borders. I think the viscosity of the LHM is quite affected by temperature. When I had a BX in a tyres place, it was a warm day, more so in the forecourt and the LHM and the car in general were roasting. You'd never have known it until you sat in only for the suspension to flop softly to the bump stops - right down- something it never did otherwise. I thought it was quite cool, like a DS or something.

Ooooh, that's the explanation I needed. Thanks Wirdy for this mini-lesson!

cheers
G