Re-gassing spheres to a lower pressure?

[assembled]

Typical sphere has >30 Bar or >400 PSI (roughly) in it. If your system has only 150 PSI on it, you are not even compressing the sphere. Until you hit a speed-bump and pressure exceeds sphere pressure, your system has zero compliance - it is completely stiff. Assuming you want to have 50% compressed gas at 150 PSI equilibrium pressure, you would need a sphere with only 75 PSI in it. From there regular Citroen hydraulic behavior starts: reduce pressure to increase stiffness, increase pressure to increase softness.

To clarify, central hole serves for pressure equalization. increasing it beyond some point just defeats damping. Dynamic damping is done by leaf springs and holes around the central hole.

There is this calculator:
http://citroen.tramontana.co.hu/en/susp ... athematics
It does not do sanity check, so you can input nonsense and still get a result of some sort.

[Kyleb]

Typical sphere has >30 Bar or >400 PSI (roughly) in it. If your system has only 150 PSI on it, you are not even compressing the sphere. Until you hit a speed-bump and pressure exceeds sphere pressure, your system has zero compliance - it is completely stiff. Assuming you want to have 50% compressed gas at 150 PSI equilibrium pressure, you would need a sphere with only 75 PSI in it. From there regular Citroen hydraulic behavior starts: reduce pressure to increase stiffness, increase pressure to increase softness.

To clarify, central hole serves for pressure equalization. increasing it beyond some point just defeats damping. Dynamic damping is done by leaf springs and holes around the central hole.

There is this calculator:
http://citroen.tramontana.co.hu/en/susp ... athematics
It does not do sanity check, so you can input nonsense and still get a result of some sort.

[senmathal]

From what Citroen model have you take the spheres ? I think that most Citroen are lighter in the rear so if you test a rear sphere from a GS/GSA for example. The spheres is also in different volume, 400 and 500 cc and I think bigger volume will give softer suspension. Here is a list with different spheres and there specification. http://oto.to/schematy/citroen_kule.pdf

Of course if the car is to light to compress the sphere lower pressure can help and rear spheres has typical lower pressure.

/Mats

[senmathal]

Typical sphere has >30 Bar or >400 PSI (roughly) in it. If your system has only 150 PSI on it, you are not even compressing the sphere. Until you hit a speed-bump and pressure exceeds sphere pressure, your system has zero compliance - it is completely stiff. Assuming you want to have 50% compressed gas at 150 PSI equilibrium pressure, you would need a sphere with only 75 PSI in it. From there regular Citroen hydraulic behavior starts: reduce pressure to increase stiffness, increase pressure to increase softness.

I don´t think you can calculate it that way. It´s more parameters to take care of. It depends even of the ratio between the area of the diaphragm in the sphere and the area of the piston in the strut which move the oil in to the sphere. The smaller amount of oil running in to the sphere when the suspension move a given way the higher pressure on the diaphragm will be. So if the car is light you can compensate that with a smaller piston in the strut. This is fore example hove I as a weak person can lift a heavy car with a hydraulic jack.

/Mats

[assembled]

Sure it is not so simple. My point is that if there is 150 PSI in struts and >400 PSI in spheres, suspension is completely hard. 75/150 PSI is just a simple example. This suspension project needs to be either properly calculated or developed in some other way. We don't know strut dimensions, their relation to wheel travel, weight of the car, etc.

[Kyleb]

Cheers for all the replies! I'm going to make a screw in valve that goes on the accumulator and I'm gonna have a play with the pressures!

I know quite a few people running similar setups and obviously there's thousands of low riders in the USA that run this setup so it will work, it's just a case of playing with it.

[Kyleb]

Sure it is not so simple. My point is that if there is 150 PSI in struts and >400 PSI in spheres, suspension is completely hard. 75/150 PSI is just a simple example. This suspension project needs to be either properly calculated or developed in some other way. We don't know strut dimensions, their relation to wheel travel, weight of the car, etc.

I can supply all the specs, gimme a second and I'll find them!

[assembled]

Sure it is not so simple. My point is that if there is 150 PSI in struts and >400 PSI in spheres, suspension is completely hard. 75/150 PSI is just a simple example. This suspension project needs to be either properly calculated or developed in some other way. We don't know strut dimensions, their relation to wheel travel, weight of the car, etc.

I can supply all the specs, gimme a second and I'll find them!

Then it might become quite interesting

[RobC]

Correct me if I'm barking up the wrong tree, but are you saying you're planning on using accumulator spheres for the suspension? This is a very Bad Idea... The acc sphere is merely a pressure reserve and has no damping qualities whatsoever.

[Way2go]

Thread post does say he is running on accumulator spheres with no damper! If conventional shock absorbers are being used too then these would short circuit the spheres anyway so minimal ride comfort could be expected. Spheres with dampers should be used with the struts and an accumulator is needed in the system to supply the fast replenishment on demand that the pump could never keep up with instantaneously.

[xantia_v6]

Conventional shock absorbers would work as well as spheres with damper elements, perhaps better, as they can have more sophisticated damping calibration. All academic until the spring rates are fixed.

[Way2go]

Conventional shock absorbers would work as well as spheres with damper elements, perhaps better, as they can have more sophisticated damping calibration. All academic until the spring rates are fixed.

Yes, conventional shocks may be more sophisticated but when used with hydraulic spheres/struts they are being subjected to a movable start point (unlike a conventional spring which is in equilibrium at rest) which is not usually part of their design? So in this instance surely it would both impair the ride over a damper orifice and also the life of the shock absorber as a result?

[Rosieuk]

The question is what car are your spheres made for? If they're bx spheres I'd imagine their settings are different for those for Xantia, XM, C5, C6, not to mention the weights the system is used with.. A BX is around 950kg, a C5 estate is positively lardy by comparison.
Then you factor in the weight of your vehicle.. Which is?
The easiest way at that point would be.. Find the Citroën car that is closest in weight to yours and either pull the parts from it, or have an expert make it up with those settings.
Eg my Accent is, with everything full up, plus me 1200kg. I'd imagine that's roughly the same as a Xantia in weight. So I'd get Xantia rated spheres and have them charged according. But I really don't want complicated electronics so I'd pull the control (and any other requisite bits) from the bx so I get the closest spheres for my all up weight, but the simplest control system.. That way at the normal charge pressure you get something that is a happy medium instead of either something that has all the suspension movement of a BRM or something that best resembles the heap from Uncle Buck (ie rock solid or soggy as hell to the point of being dangerous).
Of course the problem will come if your application is either much lighter or much heavier than any of Citroëns applications..

[Rosieuk]

I just had a look. A stock beetle is about 800-850kg kerb weight, even a BX is 100kg plus above that at best. The closest I can think of would be the gsa/gs parts. Even those are 900+kg, so about the same as BX. It might be possible to accurately measure the port size in the spheres *accurately*, work out the difference between your kerb weight and that of the donor in percentage and then apply that to fill and remake the ports in a tighter size to work with the lower application weight.
I don't know if this would work but have you tried underdriving the pump by changing the ratio of the pulley to engine rpm? Assuming you're using the pump etc.

[Kyleb]

Let’s see how this goes!