Sphere diaphragms - a hypothesis

[noz]

First of all, I'd like to point out that I'm anything but an expert on this but I have applied a little bit of logic. I have heard the flexing argument many times on this and other forums but I'm afraid I can't bring myself to believe it.
Consider the status of the sphere when the car has been parked for a few days. The fluid has all leaked back to the reservoir and the gas pressure has pushed the diaphragm flat against the lower half of the sphere wall. Now start the engine. The pump pushes fluid from the reservoir into the system. If you have flat(ish) spheres it takes a long time to rise because a lot of fluid has to be pumped. If you have 'new' spheres the car rises quite quickly. I proved this last weekend beyond any reasonable doubt. The car rose within 10 seconds of closing the PR valve, after I had replaced the two front spheres only. That includes the time to pressurise the accumulator. Before replacing the spheres it took a good 30 - 40 seconds from a flat accumulator to normal height.
During the time the pump is filling the system what is happening? To begin with, the system pressure is zero (on the fluid side). As the fluid is pumped into the pipework the fluid pressure rises. The balance of forces at the struts means that the pressure to overcome the car's weight has not yet been reached. At 10 bar N2 in the spheres the fluid will take the easier of the two paths (either into the strut , raising the body or into the sphere, compressing the gas.) At this stage the easier path is into the sphere. The fluid flows into the sphere. As it does so it increases the pressure on the fluid side of the diaphragm. For arguments sake lets consider the fluid pressure as 11 bar. The fluid will continue to flow into the sphere until the moving diaphragm compresses the gas in front of it until 11 bar is reached. When the fluid pressure and the gas pressure are equal the diaphragm stops moving (Newtons first Law). However at 11 bar the pressure is still not enough to raise the weight of the body so the height corrector keeps admitting fluid to the sphere/strut. Follow the same process as described above for increments of pressure increase. The process continues until the fluid pressure exerted on the strut is enough to overcome the weight of the car and the body starts to rise. As more fluid is pumped into the pipes the car continues to rise.
In relative terms it takes much more time from PR valve close to the first sign of movement than it does from that point to reaching normal height. That's because it takes the first time interval to pump the required volume of fluid into the system, thus compressing the gas until the resulting pressure is high enough to overcome the weight of the car. Once the body starts to rise it only takes relatively a little more fluid to raise the body to the normal position.
When the car has reached normal position the height corrector switches off and no more fluid is admitted to the sphere. At this point let's say it took 80 bar of pressure to overcome the weight of the car. The sphere measured 10 bar gas pressure at the start of this diatribe. Sufficient fluid passed into the sphere to move the diaphragm such that the space left for the gas became smaller and smaller. As such the gas pressure rose as the fluid entered the sphere. Once at the normal height, lets say, to achieve 80 bar gas pressure and therefore fluid pressure the position of the diaphragm needs to be approximately 80% of the distance between the threaded end and the domed end ie more fluid than gas.

Now the body is suspended, floating on a cushion of gas. Almost exactly like lying on your back on a water bed.

Now lean on a front wing. The fluid is incompressible, the height corrector is closed from both additional fluid and the return path to the reservoir. As you put weight on the corner of the car the body sinks under your weight. The fluid which was in the strut has no where to go except into the sphere. As it does so it moves the diaphragm a little more thus increasing the gas/fluid pressure. At the same time the body sinks, the height corrector sees this. The HC admits fluid from the pump into the space between the sphere and the strut. The pressure rises until the additional pressure overcomes your bodyweight and the car rises back to the normal position. To support the additional weight the pressure now needs to be, say, 90bar. The gas pressure in the sphere is now 90 bar because the gas has been compressed into an even smaller space, say 85% of it's travel distance.

Remove your bodyweight from the car. It initially rises because the pressure of the gas/fluid pressure is greater than that required to support the new, lighter weight. The HC sees the increase in height and bleeds fluid back to the reservoir. As it does so the fluid contained in the sphere/strut decreases in volume as does the pressure and the weight on the strut pushes the piston back into the bore. When back at the normal height position the HC closes and we are back to where we started.

For this explanation, the initial sphere pressure started at 10 bar and got up to 90 bar to support the car body and your body weight. The diaphragm moved between 80 and 85% (ie the sphere had a greater volume of fluid than gas.) of it's travel when adding and subtracting your bodyweight from the car.

I won't bore you with all the details again but consider the above process now with a 'new' sphere at an initial pressure of 70 bar (CX Front, 500cc). The same experiment would require the pressure at normal height to be the same as before at 80bar. Thus to achieve this increase in pressure from 70bar only a small amount of fluid needs to be added to the sphere. Therefore the respective movement now seen in the diaphragm position becomes, say, 15-20% ie more gas than fluid.

Sorry to ramble on but I'm coming to my conclusion.

When the car is parked and 'depressurised' the diaphragms are pressed against the lower surface of all the spheres by the respective gas pressure in each of the spheres (no anti-sink in this hypothesis). When the engine is started the fluid is pumped into the pipework and continues until the body reaches the normal height. The time this takes and the volume of fluid required both depend on all the spheres' initial pressures.
When the car is travelling along the road the diaphragms are continually in motion as each change in 'weight' (substitute bumps in the road for adding more bodyweight to the car) requires a respective change in pressure to match the disturbance inputs from the roadwheels. I don't know how much they move in practice but a guess of 15-20% would not be unreasonable in my opinion. This continuous position change will happen at the location in the sphere dictated by the initial pressure. But move it will, and continuously.

Therefore, the sphere doctor ;-) rejects the concept of diaphragm failure due to flexing. The diaphragms flex through most of their permissable range every time you use the car. The inflated balloon concept is flawed because the balloon is inflated to it's full extent every time you park the car for more than 6 hours. The size of the diaphragm must be such that when the system is fully depressurised and the diaphragm is pressed against the lower half of the sphere, it cannot be stretched at this point. It must be exactly the right size to sit there indefinitely stress-free. At all times in it's life the diaphragm never sees a difference in pressure across the diaphragm itself so it is never stressed. In theory you don't even need a diaphragm. The sphere would work without one as long as the gas was always trapped above the fluid. You only need the diaphragm to keep the gas in the sphere when it's manufactured and in the box. (OK, OK it may also slow down the diffusion of the gas into the fluid, but I'm making a point here)

I concede that if you let the pressure go too low then when you hit a particulary large bump the resultant compression of the gas may cause the diaphragm to hit the top of the sphere and if it has the built in spikes then the diaphragm will be punctured rendering it useless.

No offence intended to anyone. If any of my argument is in doubt, please feel free to humiliate me in public. I have the stocks ready.

cheers

norrie

[combwork]

Ok, it's an old post so maybe my reply is a bit irrelivent, but it seems to me that no matter how hard a bump the wheel goes over, the diaphram should never touch the metal on the gas side of the sphere. The reason for this is that as long as gas is present, it can be compressed but it will never go away. In theory, if it's compressed enough it would turn to a liquid, but it would still be there between the membrane and the wall of the sphere. Coments, anyone?????

Have fun,

Jim.

[noz]

Jim,

Some posts just take forever to carry on a conversation.

I agree with your hypothesis completely. I don't believe the diaphragm will ever touch the inside of the sphere when the gas is compressed. At least not when the initial gas pressure is correct. Clearly, as the sphere ages and the gas volume reduces then the height that the diaphragm reaches going over a bump will get higher and higher the less gas is present.

In the old Citroen spheres there was a piece of jaggy metal at the inside surface at the top of the sphere. After a period of time as the gas content reduces, one day you will go over a pothole and the diaphragm will reach the spikes. At this point the diaphragm will be punctured. This allows the gas behind the diaphragm to leak through to the fluid side especially when the car has been parked for a while and the fluid side of the system has pressurised to zero.

There are only two conclusions I can come to for deliberately designing the sphere with a self-destruct mechanism. Either

It was Citroen's way of forcing the premature sale of spare parts or

It was some cockamamy safety feature to force you to replace the spheres before they got to the completely empty stage and were therefore dangerous from a road-holding point of view.

An interesting topic methinks.

Cheers

noz