BX and GS spheres...
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Thread: BX and GS spheres...

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    Fellow Frogger! ajpolden's Avatar
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    Default BX and GS spheres...

    Hi All.
    Excuse me for starting yet another sphere thread, but they're all so terribly interesting and informing
    Rang Martin Bray today for a quote on second hand regassed spheres, offered me a good price, but said he had some difficulty finding front spheres, only one available and it was for a post 1987 car.
    The date issue as far as he and I know is not a problem - using spheres from post 1987 model year (fuel injected) vehicles would only make the car ride like the later models (I don't even think were was much difference except for the 16V).
    He discovered however that the genuine part numbers for the BX and GS front spheres are the same, implying they are interchangeable. Can someone here confirm this for me?

    Thanks and , Andrew.

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    1000+ Posts brenno's Avatar
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    Not sure if this helps any, but I bought some new spheres for the GS last week from Auto France and the box is marked as 'BX 122 Front sphere - 400cm3/55 BAR'

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    Moderator Alan S's Avatar
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    They're about as close as "**** it" is to swearing the only difference being a small variance in the bore hole, although I must admit every application list I see for spheres seems to offer different specs for various models.
    I've just regassed the spheres in the BX Estate and they were 1.5 front and 2.5 rears. Take a peak at the supposed specs.

    http://www.peter-wells.com/data/SphereDimension.html

    I have another table also, but that might just confuse the issue even more.
    FWIW, I understand the cylinders off the rear of the GS will fit the front of a BX slso (or something similar) so apparently overall, the specs of these two models aren't all that far apart. Accumulator is the same from the GS through to the Xantia also.


    Alan S
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    Fellow Frogger! ajpolden's Avatar
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    Hi fellas, thanks for the replies.
    I did a bit more research as time allowed and referred to this site: http://www.tramontana.co.hu/citroen/...s/suspens.html
    No good for GS info but was able to find the proper specs for my car. It seems from this and the table Alan posted that the spheres are a different size/volume. 500cc for this BX and 400cc for the GS and other BX models.
    What difference would this make? It seems most other BX models run on the smaller ones, so surely the suspension would only perform like these?
    I can't be sure though exactly what spheres are fitted to my car though, short of talking to my mechanic and he'd probably have to look it up anyway.
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    Moderator Alan S's Avatar
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    It's not all that difficult to find out.
    Your mechanic may not know how to find it if he's not a genuine Citroen mechanic as I found from personal experience when I had a "specialist" doing the service for me due to lack of time, that I had 7 spheres on a car and no two were the same and none were for my car and when asked, his response was "the only difference is between suspension spheres and accumulators" which was when I decided I'd better make the time to service it myself.
    On the outside of the sphere near the nipple on the top you should find a number stamped in (eg "55" for example) which is the pressure.
    The volume can be judged by the shape as well as the physical size. The 400 looks a bit like an egg cup with a lid on or a chalice with a dome on it. The 500 is slightly elongated; this seems to depend on brand.
    The bore hole is on the silver part on the base and the sphere needs to be screwed out to see this; depressurise the system and just screw it out. It's marked one digit either side of the bore hole like 1 * 5 or 1 * 25 or 0 * 9 and as a rule of thumb, the larger the hole the softer the ride however the rate at which it depresses is controlled by a damper which is a series of plates mounted within the sphere behind the bore hole and is why some soheres may appear to have similar dimensions but show different characteristics when being used. This is where those part numbers come in to help identify it's original application.
    The difference between a 400 and 500cc sphere is due to the length of travel of the spheres so by using a smaller sphere with a smaller bore size and gassed to a lesser pressure, it's equivalent to using heavy duty shocks attached to a car with cut down springs and stiffened sway bar in a conventional situation and is why hydraulic Cits handle and ride so well and are so easily modified to handle different circumstances; it can be done in minutes, literally. This is why we often hear of people complaining about bad handling in Cits. It's usually caused by someone fitting incorrect spheres whilst trying to get better ride. A typical case was a write off we bought that we knew had a handling problem; it supposedly felt unstable even at reasonably slowish speed (80 - 100 kph) so I swapped the front 500cc spheres onto my car, the 16V. It didn't seem all that bad until I braked hard at around 110 on dirt approaching a corner and all hell broke loose as I almost swapped ends.
    A thorough check when I got home revealed 500cc spheres with 2.9 boreholes pumped to 72bar, so what effectively happened was under brakes, the nose dipped so far it threw the geometry out, lifted a lot of weight off the rear wheels and began to fishtail. We never did find out what the spheres were off but it was suggested possibly a Mercedes or were off an imported car that had been fitted with 'comfort" spheres and that they had almsot no damping, so it was akin to driving a conventional car with stuffed shockies as a comparison.

    Hope that explains it as best I can in an understandable manner.


    Alan S
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    Too many posts! JohnW's Avatar
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    Quote Originally Posted by Alan S
    It's not all that difficult to find out.

    Hope that explains it as best I can in an understandable manner.


    Alan S
    Pretty good summary Alan! It's all a bit counter-intuitive, but when you get your mind around the subject it all becomes logical.

    Cheers

    JohnW
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    JohnW

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    Fellow Frogger! ajpolden's Avatar
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    I had a look today and oddly enough it seems that 500cc spheres are fitted to the back and 400cc ones to the front. The previous owner was a somewhat of a DIYer, and from some of the things i've seen, probably not a very good one. Anyway, I've had Mr Bray send a set sent over, GS 400cc front spheres, it seems second hand 500cc ones for this model are hard to come by, and CX spheres could cause lots of hassles with smaller bore holes, for the larger car.
    I'll report back when I've fitted them...
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    Hello again

    Well today I finally had a chance to fit the spheres that arrived last week. No problems, front ones came off with minimal effort, and the rear ones with a bit of fussing about - it seems the washers weren't sitting properly and this might explain the LHM seepage i've seen under there. It turns out the rear spheres were 500cc ones and must have been a fair way down, as they held a lot of LHM.
    Put all 4 suspension spheres on, new seals, ensuring that the seals slotted into the proper place, tightened up the regulator screw and started up the engine. Heard a bit of a creak, and the car began to rise...

    THUMP! Sudden thumping sound, and I thought one of the rigid lines had popped off or something, then I saw what looked like steam emanating from the front left corner of the bonnet, directly over the fluid reservoir. Opened the bonnet, note the the filler cap had blown off. Took a peek inside and the fluid was frothy! Had it boiled? no, there no reason it should get hot and it wasn't even warm anyway. No piping had blown off, and there was certainly no fluid on the ground below. I check the reservoir breather tube, and it wasn't blocked. I promptly drove it up the driveway to turn it around to get the front wheels on the ramps, and noticed the front certainly didn't feel right at all. I then found that the front left corner was ROCK HARD!
    It seems I have experienced a blown sphere . I can understand why this happens, but how often? This obviously meant that I had the nitrogen from inside the sphere flow through the circuit back to the reservoir, could i have damaged something, and is there something other than citrobics i'll need to do bleed any remaining gas out?

    Next question, I can't figure out an easy way to get at the accumulator sphere to remove it - it's just too cramped. I'd be interested to know which combinations of parts some people have removed to get at it

    Thanks and ,
    Andrew.
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    1000+ Posts Ken W's Avatar
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    Sounds like you lost a full charge of nitrogen from that front sphere quite quickly - you should expereince it when you are driving along at 100 kmph!

    I find the easiest way to get a BX accumulator off is to use a strap wrench that I have which takes a 1/2 inch drive to tighten it up and to act as the lever arm. After you have put the car up on ramps, lowered the suspension and opened the bleed screw on the regulator, you need to take off the clamp which holds the aircon hoses if you have them, and move the hydraulic pipes outa the way, slip the strap over the sphere (it just fits) and loosen it from below. Once unscrewed, it will then just drop out between the sump, gearbox and radiator. The fun part is when refitting it and getting the correct angle for it to screw back onto the regulator without upsetting the neatly placed seal!!!

    I think early is my BX days, I used some old GS rear shperes temporarily on the BX while I was having the ones it came with regassed. They appeared to have much less damping as when the car hit a bump, the back oscillated up and down with some amplitude for quite a few cycles after. I remember having a very uppsy and downsy trip to Noosa and back. So I didn't try that one again.

    When you buy new front spheres (and you will have to if you want to get some life out of them), I would either go for the BX 500cc spheres (you get a bit more time between recharges) or I have been using CX front spheres gassed at 55 bar or less with some success for quite a few years now. Makes the ride quite a bit softer but you can bottom out the front suspension on big bumps taken at speed quite easily.

    Cheers

    Ken W

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    Hi Andrew,

    sorry to hear about your recent sphere dramas. As was mentioned in another thread recently, a GS owner I know had a similar problem with a second hand sphere from Martin and he was quite happy to replace it free of charge, so give him a call about exchange or refund.

    I have some GS spheres that may be serviceable out in the shed (more rears than fronts I think, but I can check). If you are down Hobart way I'd be happy to lend them to you for a few weeks to get you out of a spot if they're helpful.

    Regards the nitrogen gas, it shouldn't damage any system components because even a fully charged accumulator at 62 bar runs a much lower pressure than the 170 odd bar regulator cut out pressure. So just open and shut the regulator bleed screw several times with the engine running to bleed any remaining gas back to reservoir. If the brakes lag or feel at all odd you might need to bleed them also (I would do them anyway as I don't like to take any chances with brakes- LHM isn't that expensive and you don't lose much during a bleed).
    Cheers,

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    Thanks fellas

    Had another go at the accumulator this morning - undid the bar under the regulator to allow a bit more swinging room for my already strained strap wrench, moved the air con hoses aside, and still couldn't budge it - I was even moving the engine, I was pulling that hard! I may have to take the regulator off to remove the sphere. Anyway, I've put the old spheres back on the front, and they don't feel too bad I suppose, but better to get them done as soon as I can.
    I have one remaining question though. I noticed the rear of the car is amazingly soft now - perhaps too soft? Backing out the drive the rear rose more noticeably than before, and upon acceleration, also drops quite noticeably as the weight shifts back. How normal is this?

    Regarding replacement of the spheres, I'll ring and talk to him on monday, and see what he suggests, I may yet just get a couple of newies from GSF.

    regards,
    Andrew
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    Previous:
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    Hi Andrew
    You probably need to take a chisel to the accumulator sphere to crack it loose. That is the easiest way to shift them - if they've been overtightened you'll break lots of filter wrenches trying to get it off. Best way is use a small cold chisel and give it a few whacks on the heavy welded seam close to where it screws into the regulator body -make sure you chisel it at an angle in the direction to loosen it. Just a few hits should do it - you'll see it start to move, then use your strap wrench to undo it. Make sure the car is on ramps and the pressure release is undone 1/2 turn or so. Had to do a few at the CCC tech day yesterday like that - including the accumulator on my 16V. When you put it back on, lube the seal with LHM and only tighten at most 1//8 turn after contacting the seal....if you want to get it off easily next time!

    Also your rear might have gas in the lines especially if you blew a diaphragm, so would be a good idea to bleed both sides back to the reservoir via a long length of clear tubing. Regassed spheres will make it softer and a bit bouncier, but it shouldn't jump about too much when braking or accelerating - most likely have air in the rear suspension lines.
    Cheers,
    Roscoe
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    As someone who has done a bit of tuning of spheres (changing the gas pressure and damper valve settings) I'd comment that the damper valve characteristics are VERY fussy, and seemingly small differences in the size of the hole makes for BIG differences in the feel and handling (and safety) of the car.

    Use a sphere with a damper bypass hole that is too big (or has the wrong washer parameters) and you WILL notice it when you get into an emergency situation like braking around a corner and you find that the cars handling is poor.

    Mixing and matching spheres from different models, and between front and back is usually a bad idea unless you really know what you're doing.

    Because of the different mechanical properties of the different cars, such as weight, hydraulic piston diameters, arm lengths and leverages etc, a sphere that might be optimal on one car can easily give a ride that is too soft and underdampled on another model. (Or too hard/harsh in the reverse)

    In extreme cases, damage to the spheres could be done - for example putting a very high gas pressure sphere such as a 75 bar front CX sphere onto the rear of a Xantia is likely to cause the diaphram of the sphere to keep hitting on the bottom of the sphere casing over every bump, which would rupture the diaphram in no time flat.

    Putting a rear Xantia sphere on the front of a CX would do the opposite - the diaphram would be getting hammered at the top of the sphere and would also rupture. It's important that the gas pressure is within a certain safe range which keeps the diaphram away from touching either end of the sphere over normal load variations and bumps.

    The gas pressure is stamped on the top, and the damper bypass hole is measurable by inserting a small precision drillbit into the hole, and that is 2 of the 3 important factors, but the stiffness of the damper valve itself (the spring steel washers) is not easy to evaluate other than driving the car, and is usually different between front and rear spheres, and between different models of car.

    (For example I notice that hydractive spheres have a different number and thickness of washers compared to the equivilent standard models, one has 2 washers the other has 4 although I don't remember which way around it is)

    When tweaking the ride/handling the main thing of interest is the damper bypass hole - assuming you start with the correct spheres for the car the gas pressure and washer valves will be correct for the car, so the bypass hole gives the most control.

    The bypass hole critically affects the rebound characteristic.

    A hole that is too big will cause a floaty ride that has an overshoot rebound characteristic during a bounce test - eg if you press it up and down vigorously and release, it will overshoot and possibly oscillate.

    A small amount of overshoot will give a better ride than critical damping, but handling will be worsened. Ride over small bumps will be good. Too much overshoot will cause the car to feel unsteady and also increase the cornering roll.

    On the other hand if that hole is too small then the ride loses the "magic carpet" characeristic, starts to become a bit fidgety and harsh over small bumps, but is still able to ride over larger bumps. Body roll is reduced a lot, and handling is improved.

    Taking the small hole to the extremes - such as the very small holes in hydractive spheres when in hard mode actually worsens the handling again, as the suspension is not able to absorb anything other than large bumps, so tyre contact starts to become a bit erratic.

    If you get the hole size just right, you can get a very nice compromise where the ride is good but not floaty, and the car has a feeling of rock solid stability. This happens at about the point where you have critical damping. (Q = 0.5 for those who know what that means )

    Ideally, the washer valves in a sphere should be calibrated (in terms of their strength before opening) so that the rebound of the suspension isn't quite strong enough to open them, allowing the rebound to flow only through the bypass hole - this gives the optimum balance between ride and damping, as even small bumps are able to open the washer valves and thus be absorbed easily, while the rebound through the bypass hole is controlled.

    If the washer valves are too weak and open with the rebound, it will be impossible to control the rebound regardless of the bypass hole size - thus you get a ride that feels a bit unstable and uncontrolled, and making the hole smaller in an attempt to control the rebound better just makes the ride harsher without fixing the rebound problem.

    On the other hand if the washer valves are too strong, a small hole will make the ride very harsh and fidgety, and yet a larger hole will make the rebound uncontrolled.

    So the balance between the washer valve strength and the bypass hole is pretty fussy to get optimum results, and this is the main reason why spheres from one model don't suit another model, even if you adjust the gas pressure and bypass hole size.

    Just as some examples, on my Dad's CX when he bought some new front spheres a few years ago we were pretty unhappy with the damping, although the ride was soft there was lots of overshoot that was quite obvious while driving, let alone the oscillation when doing a bounce test, so we decided to do something about it, and experimented with the bypass hole.

    The standard size for those spheres was 1.9mm, and we tried a few sizes between 1.2 and 1.7mm. (By making a small interference fit insert) With 1.2mm there was no trace of overshoot or floatiness at all, and handling was excellent, however the ride was very un-Citroen like, definately too firm.

    Eventually we settled on 1.5mm as a good compromise between ride and handling. Just enough extra damping to prevent overshoot, but not enough to spoil the ride.

    We did a similar thing on his (standard) Xantia at the front, reducing the hole size from 1.5 to 1.2mm.

    As for what difference the volume of the sphere makes (400cc vs 450cc vs 500cc) you can use a larger volume sphere, provided that the gas pressure is lower by the same proportion.

    For example on the Xantia there are two versions of the front spheres available - 50 bars, 450cc and 55 bars 400cc, which are roughly equivilent.

    What matters is what volume the gas is compressed to at the operating pressure. The larger sphere has to have its gas compressed further to reach the same volume as the smaller sphere, which means more pressure rise, which means the starting pressure needs to be lower.

    What the larger sphere gives you is better linearity in springing rate, and a wider useful operating range between minimum and maximum load weight.

    However unless there are also changes made to the suspension arms or pistons that affect the leverage, the extra available operating range is at the MINIMUM weight end of the scale - assuming that the pressure of the sphere was lowered in proportion.

    Keeping the same pressure as the smaller sphere would give you more operating range at the maximum weight end of the scale instead, but you'd also be changing the springing constant in the process.

    Next on my list is modifying the damper valves in the rear hydractive control block of my Xantia for a little bit more damping in soft mode.

    Regards,
    Simon
    Last edited by Mandrake; 17th November 2005 at 11:31 PM.

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    Quote Originally Posted by ajpolden
    THUMP! Sudden thumping sound, and I thought one of the rigid lines had popped off or something, then I saw what looked like steam emanating from the front left corner of the bonnet, directly over the fluid reservoir. Opened the bonnet, note the the filler cap had blown off. Took a peek inside and the fluid was frothy! Had it boiled? no, there no reason it should get hot and it wasn't even warm anyway. No piping had blown off, and there was certainly no fluid on the ground below. I check the reservoir breather tube, and it wasn't blocked. I promptly drove it up the driveway to turn it around to get the front wheels on the ramps, and noticed the front certainly didn't feel right at all. I then found that the front left corner was ROCK HARD!
    It seems I have experienced a blown sphere . I can understand why this happens, but how often?
    Not very often, but it can certainly happen on regassed spheres... I've personally seen it happen on a regassed CX accumulator.... as it pressurised WHOOSH! a loud noise, a little squirt of oil, and an o-ring sticking half way out under the filler cap.

    It's caused by one of two things - either the o-ring seal under the filler cap is not good enough after the regassing (due to the wrong size seal, or a damaged/pinched seal, etc) causing the seal to blow out at a certain pressure, which causes all the gas to rush out the filler cap, or it can also (I think) be caused by catastrophic failure of the diaphram inside the sphere, which would cause enough of a shock wave inside the sphere to also blow the seal out the top, whether its good or not.

    Either way, once it happens, the sphere is a boat anchor, its not worth regassing.

    This obviously meant that I had the nitrogen from inside the sphere flow through the circuit back to the reservoir, could i have damaged something, and is there something other than citrobics i'll need to do bleed any remaining gas out?
    If it blew the seal out 90% of the nitrogen will have escaped the filler cap, and any that did find its way into the hydraulic system will not do any harm. Just put a new sphere on and do Citrobics to bleed any out...

    Regards,
    Simon

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    Simon

    Great information - thanks!
    Craig K
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