Bx Height Control Lever

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Way2go
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Post by Way2go » Fri Feb 24, 2006 3:07 am

I think that you are getting caught up in "Smoke and Mirrors here".

The Citroen BX suspension concept is very simple and it is this simplicity that that borders on genius. To understand it just break it down into functional chunks.

1) Spheres are the ONLY thing that gives springing and shock absorption.
Compare this to conventional cars so that the diaphragm with the gas behind IS the spring and the Damping Hole IS the shock absorber. As the sphere deteriorates with age it loses gas pressure behind the diaphragm(good springing) until there is no springing (or suspension) at all and the car is jumping around as if the wheels were directlly bolted to the chassis. (When the sphere is full of LHM the suspension is hard not soft)

2) Struts are just hydraulic rams, LHM can't be compressed. When the sphere gas has gone the struts can't move exept under the control of the height correctors which has nothing to do with suspension in the conventional sense. The Anti-roll design effect of the struts cannot work either if the spheres are out of gas.

3) Height correctors are valves that control how long the struts are, they also do not give any springing. The lever inside the car sets the ride height. The lever attached to the Anti-roll bar allows fluid to transfer in or out of the struts and as there are 2 height correctors working in tandem this also gives the Anti-dive characteristic when braking.

4) Pumps,Accumulator Sphere, Reservoirs, octopus etc are for General housekeeping to maintain LHM delivery & leaky returns.

Now going back to where this thread started I would say that you can lower the ride height so long as you have operating travel on the struts without bottoming 1) the strut 2) the car (centre exhaust clamp?)
The acceptable lowering will be different for straight line motion to cornering activity because of the Anti-roll characteristic of fluid exchange between the struts on cornering.

However encountering roadhumps in a position below normal will result in "total beaching" and potential damage! :shock:
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Post by Vanny » Fri Feb 24, 2006 9:12 am

1) if thats the case then its the pressure of nitrogen that creates the spring right? But why are 16v spheres harsher? they have the same pressure but a different internal volume!

I was under the impression the gas escaped through the diaphragm, rather than lhm making its way into the cavity behind the diaphragm at least until the point of failure of the diaphragm?

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Post by jeremy » Fri Feb 24, 2006 9:51 am

Way - nice explanation - but - anti-roll? - the two struts at each end are directly connected - so compression on one means extension on the other - which assists roll rather than fights it. This is one reason why the sphere supply pipes are very small, citroen anti-roll bars are thick, and so much effort has been expended on Hydractive and Activa suspensions

This is of course where a comparison of the various fluid suspension systems gets interesting. BLMC Hydrolastic was linked front to rear (like the 2CV) and so fought roll. Currently something similar may be used on the Citroen rallycar and look how successful that has been.

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Post by Oscar » Fri Feb 24, 2006 10:24 am

This will provide most of the explanations we seem to be skirting around.

http://www.tramontana.co.hu/citroen/guide.html
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Post by Toddman » Fri Feb 24, 2006 11:36 am

Oscar - Not read your link yet.
Also never really studied a diagram of a sphere but this is how I see the suspension.
Spring - Provided by Nitrogen gas. More gas gives higher gas pressure and in effect longer spring hence softer ride. Less gads pressure gives shorter spring hence harder ride.
Altering the gas pressure is like shortening or lengthening the spring.
The hole provides damping no problems there.
The volume of the sphere merely governs the amount of suspension travel. i.e. if the LHM is merely an uncompressable link then a higher sphere volume allows more of the lHM link to move into thge sphere hence more travel.
This is a loose description and may well be wildly inaccurate but it is hoe I see things.
So when you raise the height you compress the nitrogen more making the ride harder and when you lower tha car you start to make the ride softer but you soon run out of travel hence the complications with bouncing etc which is why IF you do want to lower significantly you must use more damping to prevent running out of travel.

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Re: BX

Post by DavidRutherford » Fri Feb 24, 2006 12:03 pm

Toddman wrote:So when you raise the height you compress the nitrogen more making the ride harder and when you lower tha car you start to make the ride softer
No. You only affect the springing of the car if you get to the extremes of suspension travel, and start to encounter bump stops. Otherwise, the pressure of the nitrogen, and thus the pressure of the LHM is CONSTANT throughout the suspension travel (if the car is at rest of course)

The only thing that's not been accounted for here is the pressure of the nitrogen at rest, compared with the sphere volume.

The overall softness/harshness of the ride is not only governed by the sphere pressure, but by it's volume too, since the pressure is measured with the sphere "relaxed"

Thus (for instance) a 400cc sphere at 50 bar contains 20'000cc of nitrogen in total (at NTP)
wheras a 450cc sphere at 45 bar contains 20'250 cc of nitrogen in total (at NTP)

So these two sphere's will give fairly similar springing.

Another comparison.

Early BX 16/19 used a 400cc sphere on the front at 55bar. Later BX 16/19 used a 500cc sphere at 55bar.
The early models had a total nitrogen volume of 22'000cc. The later ones have a total volume of 27'500cc. Thus, despite the pressures being the same, the later larger sphere's gave a softer ride.

The GTI/16v models use the same 400cc/55bar sphere on the front, but with a smaller damper hole. Makes sense.

The sphere that always looks deceptive is the CX Safari rear sphere. it's a 700cc/35bar. On the face of it, 35Bar looks like a hard ride, but the total nitrogen volume is 24'500cc, hence it's fairly soft.

Since the springing is based on the total nitrogen volume, any loss of nitrogen gas makes the springing harder, until it's all gone, and you have no springing.
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Post by Way2go » Fri Feb 24, 2006 12:25 pm

Vanny wrote:1) if thats the case then its the pressure of nitrogen that creates the spring right? But why are 16v spheres harsher? they have the same pressure but a different internal volume!
Same pressure but different diameter gives a different pressure per unit area on the diaphragm
Vanny wrote: I was under the impression the gas escaped through the diaphragm, rather than lhm making its way into the cavity behind the diaphragm at least until the point of failure of the diaphragm?
yes, but as the gas pressure diminishes more LHM is forced into the sphere to the point when the diaphragm will rupture under the excess stretching. In other words the gas will take up a smaller & smaller space under LHM pressure as its original pressure falls. This is why regassing was only possible in the past while your springing still worked (diaphragm still intact).
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Post by DavidRutherford » Fri Feb 24, 2006 1:44 pm

Way2go wrote:Same pressure but different diameter gives a different pressure per unit area on the diaphragm
The different diameters of the sphere's is not relevant. The pressure of the Nitrogen is the same as the pressure of the LHM. If this weren't true, then the system would move until it was back in equilibrium.

The sphere could be 500cc or 50'000 cc for all the difference it makes once the system is pressurised and the car is stable.

The only difference having a larger sphere makes is that the "relaxed" pressure is lower (see safari spheres above) and that the system takes longer to come up to pressure from relaxed, as it has to pump more LHM into the sphere before the nitrogen pressure (=LHM pressure) is sufficient to raise the car.
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Post by Toddman » Fri Feb 24, 2006 1:46 pm

So why does the suspension get firmer when you raise the height with the lever from say normal to intermediate height?

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Re: BX

Post by DavidRutherford » Fri Feb 24, 2006 1:52 pm

Toddman wrote:So why does the suspension get firmer when you raise the height with the lever from say normal to intermediate height?
Could be that you are encountering the top bump stops. From intermediate to maximum isn't very far at all on a BX.

Also, the front suspension wishbone inner rubber bushes are quite a bit deformed at intermediate, and will add a fair degree of resistance (not applicable on MK1 models due to them having inner bearings rather than bushes)

And... at intermediate height, the front wishbone is further away from level than at normal, meaning that some of the suspension loads are transmitted along the wishbones, straight to the front subframe. (imagine if the suspension could go even higher so that the wishbones were almost vertical. The wishbone would take most of the load, and the suspension would be very stiff indeed)

but most likely that at intermediate height, you're using a bit of the suspension cylinder that doesn't normally see any wear, and is probably many many years old, hence there is a fair amount of friction that you don't see at normal height.

Certainly, my MK1 rides pretty much the same at intermediate height as it does at normal height. And my CX (which has a much better front suspension arrangement than a BX) rides exactly the same at anything from just above minimum height to just below maximum.
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Post by Way2go » Fri Feb 24, 2006 2:59 pm

DavidRutherford wrote:
Way2go wrote:Same pressure but different diameter gives a different pressure per unit area on the diaphragm
The different diameters of the sphere's is not relevant. The pressure of the Nitrogen is the same as the pressure of the LHM. If this weren't true, then the system would move until it was back in equilibrium.

The sphere could be 500cc or 50'000 cc for all the difference it makes once the system is pressurised and the car is stable.

The only difference having a larger sphere makes is that the "relaxed" pressure is lower (see safari spheres above) and that the system takes longer to come up to pressure from relaxed, as it has to pump more LHM into the sphere before the nitrogen pressure (=LHM pressure) is sufficient to raise the car.
:?: :?: So, what happens when there is no Nitrogen pressure? Are you saying the car doesn't rise?
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Post by adamskibx » Fri Feb 24, 2006 5:30 pm

If there is no nitrogen pressure whatsoever, id imagine that there is no nitrogen left so its purely the pressure created in the rubber diaphram due to it pressing on the sphere wall that allows the LHM to be pressurised.

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Post by Stewart (oily!) » Fri Feb 24, 2006 5:43 pm

I think the cars get stiffer when in the high driving setting (not the top one) due to the rear arms and wishbones being at a steeper angle, this places the wheel centres closer to the pivot centres, the cars weight is therefore producing force that is trying to compress the arm/wishbone in addition to moving them through an arc, now Im off for a lie down, physics makes my head hurt :)
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Post by DavidRutherford » Fri Feb 24, 2006 5:46 pm

Way2go wrote:So, what happens when there is no Nitrogen pressure? Are you saying the car doesn't rise?
Not at all.

If there is no nitrogen pressure then the car will still rise and fall as normal, but there will be no springyness in the system. The suspension will be rock 'ard, the only "give" in the suspension now being the rubber mountings, and the tyres.

Exactly as your point 2) above.
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Post by ernst stavro blofeld » Fri Feb 24, 2006 6:57 pm

If you take the pump belt off, it won't rise. Perhaps rising is something to do with the pump?
I cannot see how the suspension becomes stiffer if the spring is supporting the same weight. The only thing that has changed is the amount of fluid in the strut, which is to all intents and purposes a solid spacer.