Nice squidgy suspension spheres...............

[BX Bandit]

I have 'somewhere' on this very site listed the specified cc/bar combinations on the IFHS comfort spheres. I can't remember the pressures but the cc was definiteley a 400cc sphere....which is odd as the 500cc are softer still.

AEP in Glasgow will order them and you may have to wait a few months to get them, but the comfort spec didn't impress me. Laws of Physics and ale consumption dictate that the 500cc spheres will be more forgiving than the 400cc.

I'll try not to be boring and succinct, but there are 4 main (only???) factors that influence ride sphere wise

1) Cubic capacity - the bigger the softer, equivalent to 'spring rate'
2) Sphere pressure (unloaded nitrogen gas pressure) - the higher the softer
3) central bore diameter - the larger the better it absorbs 'slow' road undulations, equivalent to damper
4) 'disc dampers' - these open up to accomodate the damping effect not absorbed by the central bore

Both 1 & 2 are easily gauged in the various sphere specs. 3 doesn't seem to be readily comparable between the differing sphere specs.

If you get your spheres re-charged, then (as mentioned by a member above (aerodynamica?)) then raising the pressure will give you a softer ride and the 500cc will always out-soft a 400cc.

The central bore diameter can be drilled out but be careful. I have tried 400cc 1.8 central bore on a turbo diesel. 500cc is better, but I think a 1.85 CB is too large as it bottomed out on speed bumps and also rolled too much on undulating roads. A 500cc 1.8 may be better as the extra capacity would absorb and prevent the rolling effect. Another way to put it is that a lighter car would have lessened the rolling effect (as would a bigger anti-roll bar).

Sadly, it's all about compromise. Maybe, as your cause is related to preventing back ache, then a 500cc 1.8 bore will be best, so long as you take it slow over speed bumps and don't mind body roll over the undulations.

I have tried drilling a central bore to 2mm, but twas no good. No damping at all.

[Linegeist]

Wow! Superb info! Eye thenkyew guv'nor! I did look for that post you mentioned - I recall seeing it yonks ago .... but no joy.

The figures you've given ARE in mm, right? I'd hate to drill a pair of good spheres, and then find your measurements were in Transylvanian thrups or similar .......

[BX Bandit]

Yes mm! But, say for example you increase from 1.8mm to 1.9mm diameter, the increase in diameter is 5.56%. But, the actual effect of damping is related to area! So the same increase in diameter results in almost 15.5% increase in area!

[Aerodynamica]

a good few years ago i fitted front GS spheres to the rear, and if you just tickle the go go juice pedle the rear end would hit the bump stops,

Hmm, that's not a good condition.

1) Cubic capacity - the bigger the softer, equivalent to 'spring rate'
2) Sphere pressure (unloaded nitrogen gas pressure) - the higher the softer
3) central bore diameter - the larger the better it absorbs 'slow' road undulations, equivalent to damper
4) 'disc dampers' - these open up to accomodate the damping effect not absorbed by the central bore

1. The larger the capacity the shallower the rate of change of the spring over the stroke of the wheel and hence, the sensation that it's softer but the actual spring rate 'K' is the same for the same pressure - it's only the rate of change that 's different for the larger volume.
2. True that higher pressure (than standard) is softer BUT within limits. If the BX has 55 bar spheres 'on the shelf' the 55bar pressure will double to 110bar when the gas volume has been halved i.e. under the LHM pressure holding up the car. Assuming it runs at about half the original volume it gives an equal amount of sphere diaphragm movement in either direction and so can be considered to be about 'what you'd want'. But! with a higher pressure say, 60 bar, when the 110bar of LHM to hold the car at normal height was present, the amount of reduction in gas volume to reach this same 110bar in the gas pressure would be less than before on 55bar sphere gas pressure and so you'd have more gas volume to play with at normal height and no extra weight in the car. It means the bubble of gas is 'less' compressed than on only 55bar and has more 'distance' to move upwards (wheel) to move the diaphragm in the sphere. This equates to 'softer'springing but note the increase in pressure: only 5bar in this example. If you increase it further you risk the problem of speccing a gas pressure that's too high for the pressure holding the car up - I've said 110bar up^^ there but that assumes that the gas volume in the sphere is reduces to a 50/50 gas/LHM volume - this probably isn't the case because the engineers would have included the factors of additional mass added to the car and the additional loads that any one suspension will encounter such as a maximum spike for a given amount of moles of gas in the sphere. So they'd spec. the pressure higher than needed for this (i.e. 55 bar will be on the higher end of the spec for the pressures expected in the LHM acting on this gas)

This is why when I fitted CX front spheres to my BX 16, the 75 bar of rest pressure in the CX spheres was too high and although the sphere's damper hole on the CX spheres is larger at 1,9mm and that pushing down on the BX front end resulted in a satisfyingly soft downward movement, upon rebound, suspension would 'jam' at about 1" above the normal height (you know, as the suspension rebounds, it will overshoot a bit an then settle) it would not overshoot but would jam. It is because the 75bar of pressure must be something equal to the LHM pressure holding up the car and hence, the sphere is still almost 'all gas' when the car is sitting at rest on its suspension. You need to have a sphere pressure that is much less than the LHM pressure holding up a given axle on a given Citroen model - they are all different: a CX or DS will have a much higher LHM pressure than the BX due to the simple fact that the hydraulic piston is more than 1/2 way along the suspension arm and so must exert a greater force on the arm to give the same force AT the wheel. The area of the piston is the other variable - the BX will have a much smaller cross sectional area than (e.g) a CX to even come close to a similar LHM pressure for the same axle weight (though , of course the CX is heavier) so will have a higher LHM pressure anyway and of course, need higher pressure gas in the spheres.

3. I agree here, that the central bore size is a factor but I'd say it's the opposite to what you state: that it is actually the larger the better it allows the fast undulations - the slow, or longer range of energy dissipation is taken by the surrounding damping disk (leaf valves) - the the centre bore, among other things, takes the small, lower energy damping movements.

4.I agree here completely. However, the centre hole also governs how early on in the transfer of energy when this is taken by the movement of the disks. A small hole will see the disks bending earlier and requires more energy to do this and hence, firmer damping (more of the energy is absorbed by the resistance initially)

Anyway, without an mad lecture on fluid dynamics, gas laws, heat transfer and energy equations the best understanding is to consider what you already have on the car, how well/not well it serves and what SMALL changes you can make to these components to affect the overall absorption.

Finally! bear in mind that if you change the absorption characteristics of the suspension in one area E.g comfort at a given speed, you affect other areas of that suspension's characteristics such as at higher velocity and\ or when you're carrying much higher loads - it might be very underdamped due to the increase in potential-Kinetic energy.

Otherwise, enjoy experimenting if you've got the cash to acquire many different sphere volume/pressure and damper sizes!

[Linegeist]

I used to know a Colonel at Military Engineering School, who lectured just like that!

Thanks for the superb info gents. It's looks like I've uncovered something a little more complex than a simple sphere mod ....... The law of Murphy in operation methinks. I obviously need to go away (after digesting Graeme's learned dissertation here), stop being a lazy bugger, and work out what I need to do properly.

Thanks blokes. Top stuff!

[Aerodynamica]

Sorry, wasn't meant to be a lecture - it's just difficult to cover the essentials without a huge description

Anyway, if your car is a turbo diesel then it'll have the firmer damping anyway - I'd tend to think spalling on a pair of 500cc BX 16/19 fronts wil make quite a change.

[Linegeist]

Sorry, wasn't meant to be a lecture ..........

No criticism was intended I assure you Graeme - it was more of a tongue-in-cheek (am I allowed to say that any more? ) compliment in fact. A fascinating insight into the subject my someone who obviously knows their stuff. I just need to dust off my abacus, that's all.

[Ian_Fearn]

Anyway, if your car is a turbo diesel then it'll have the firmer damping anyway - I'd tend to think spalling on a pair of 500cc BX 16/19 fronts wil make quite a change.

For info, I put BX 16/19 spheres on the front of my TZD a couple of years ago. I'd bought some standard TD spheres only a few months previously and found them too hard. The 16/19 spheres made a big difference. Well worth doing.

[BX Bandit]

3. I agree here, that the central bore size is a factor but I'd say it's the opposite to what you state: that it is actually the larger the better it allows the fast undulations - the slow, or longer range of energy dissipation is taken by the surrounding damping disk (leaf valves) - the the centre bore, among other things, takes the small, lower energy damping movements.

4.I agree here completely. However, the centre hole also governs how early on in the transfer of energy when this is taken by the movement of the disks. A small hole will see the disks bending earlier and requires more energy to do this and hence, firmer damping (more of the energy is absorbed by the resistance initially)

I'll take your word for it on all that Aero (as I am a bear of little brain!) but....

3. Hmm, interesting. I understood that the LHM will always pass through the centre bore (slow undulations) until at such a point, when the force exerted by the pressure of the LHM exceeds a pre-determined value and causes the damper discs to open (rapid undulations e.g. pot hole), so allowing an increased area for the pressure to dissipate to the Nitrogen via the membrane.

Somewhere on here there is a scanned image of the sphere specs showing the discs with a differing array of patterns on, as if to suggest the damping characteristics. Damned if I understand it though!

I have found the comfort sphere specs:

Our BX comfort spheres (BX VA COMF; front and BX HA COMF; back) are both 400 cm³; pressure front = 55 bar and pressure back = 40 bar and the central bore = 1,8 in front and 1,2 back.

The 400/55/1.8 are the same as a 1.4 and early 1.6/1.9 and are what I had on my TZD
The rears however are 0.1mm larger on the bore vs the usual 1.1

[Linegeist]

Strewth! This place is a veritabubble mine of information ........

Superb!!!!!

[BX Bandit]

This may be of interest to you Mr Linegeist and possibly to a few others.

Scroll down to find a little program that tells you what the eigen frequency is of your Citroen. You can manually input different sphere specs to see the results. e.g a 16v with 1.4 spheres etc. The turbo diesel rear sphere volume is incorrect though!

What it doesn't accommodate is the effect of the central bore diameter on the eigen frequency, even though it's clearly in the program, so it's either there for reference or the bloke who wrote it couldn't/didn't complete the program.

If you right click over the program and select view source you can see all the blurb that makes up the program. If someone on here is into programming and knows what effect the bore has on damping then it would be nice to see a modified version working somehow? Tis beyond me.

[Aerodynamica]

3. Hmm, interesting. I understood that the LHM will always pass through the centre bore (slow undulations) until at such a point, when the force exerted by the pressure of the LHM exceeds a pre-determined value and causes the damper discs to open (rapid undulations e.g. pot hole), so allowing an increased area for the pressure to dissipate to the Nitrogen via the membrane.

Yea, now I think of it more - you're still right. i actually think both the disks and the central hole are always working I think that the amount of the disks' influence it varied by the center hole size.

I only say this because the disks are quite easy to carefully lift up a bit with a little screwdriver and given the high pressure of the LHM I'd guess that every movement makes the disks move if even just a tiny bit.

Interesting link you provide too. I must admit that I had no direct experience of the comfort spheres and I notice they're still the same pressure as standard and that it's just the damper that's softened up a bit.

Anyway, best of luck Linegeist in getting it spot on.

[Tim Leech]

Im lost now, is there a place we can get them from off the shelf?

[DavidRutherford]

I have to say, over-size and slightly over-pressure spheres on the front, combined with front spheres on the rear will give the softest ride possible (where spheres are concerned).

You also need to be looking at spheres with the largest damper holes possible, which will improve the "bump" softness (IE pothole response)

[BX Bandit]

I've not tried fronts on the rear, any in particular?

The one thing I'd say about having had 400cc 1.85 bore spheres on the front is that over steep speed bumps (and going slowly) the car would bottom out coming off the bump. That suggests to me that 1.85 is just a smidgen too large.....?