gmerry wrote ...

I think we have established that the spring stiffness (aka nitrogen volume/pressure)) remains the same for low, normal, intermediate high and extra high positions. For the totally low position, (depressurised) the spring stiffness will be lower because all the weight will be on the suspension stops.

So what is happening at the intermediate high position. It must be something to do with a restriction in the fluid flow. So either at the damper (AMVAR) or back at the electro-hydraulic valve block.

Any other possibilities?

ul9601 wrote ...



I thought about this as it didn't make sense to me.
The nitrogen volume is NOT constant by name of the sphere design. What is constant is mass of nitrogen as PV=nRT assuming it's an isothemal process (which is not an unreasonable assumption).
For a given pressure increase on the strut side of the sphere to increase the ride height, the same pressure increase is required within the nitrogen chamber of the sphere to maintain equilibrium. As PV = constant, the volume of the nitrogen must decrease. Now you have increase in nitrogen pressure, which means spring rate is higher and decrease in nitrogen volume, which means there is less compliance in the spring. In combination of the above, you get rock hard suspension at high setting.
Class dismissed.

ul9601 wrote ...

The basic principle of the load leveling and manual ride control adjustment in hydroupneumatic suspension is that increase in ride height must require increase in hydraulic fluid volume, which can only be achieved by increase in hydraulic fluid pressure. This is not an assumption as there is no other means to control the ride height. The ride height adjustment must be explained with the principle of hydropneumatic suspension (and electronic control of it, in the case of Hydractive Citroens).
Constant ride height is ensured by ride height adjuster regardless of the sprung weight. Manual ride height control overrides this by allowing additional hydraulic pressure to be introduced to the strut side of the sphere, simulating large sprung weight condition. As there is not an actual load to maintain the constant ride height (at normal setting), the suspension rises to mid-high or high, depending on the manual override setting.

arconell3 wrote ...

...must require increase in hydraulic fluid volume, which can only be achieved by increase in hydraulic fluid pressure. ...
This is where I think there is a problem in the equation. As for the increased volume, that is correct. As for the increased pressure, it is not. At least not in the static condition, i.e. once the force to increase the height has done its job and the car has reached the high(er) position. In other words, there is an increased pressure while the car is being lifted, delivered by the BHI (hydraulic pump unit). Once it reaches the higher position, the pump stops and the system returns to its static equilibrium. The static forces on the hydraulic system and therefore on the membrane of the suspension sphere return to the same values as before the car was lifted, simply because the forces working on the hydraulic system haven't changed between the static low and the static high position.

Again, I am referring to a static condition, or rather 2 static conditions, and a lifting action to get from one position to the next. It is only during the lifting action itself that the hydraulic force in the system increases.

Once that (higher) position is reached and we start driving the car, the strut contains more hydraulic fluid, but under the same pressure as when it was in the normal (lower) position.

Now, while driving in the higher position, the dynamics may have changed as compared to the normal position, not so much as a result of the higher liquid volume but more as a result of the change in geometry of the higher position as compared to the normal position. Another (additional) explanation may be that the fluid flow within the strut is different in the high position as compared to the low position, i.e. more restricted.

Robert

gmerry wrote ...

The suspended mass is the same at whatever ride height the car is.

gmerry wrote ...

The nitrogen pressure, for a STATIC situation, therefore must be the same.


The dynamic stiffness depends on the hydraulic throttling:-

TRY THIS TEST! In Lexia, Diagbox, perform the AMVAR suspension test. Its possible via the test to have "soft as a pillow" suspension rates, or rock hard. It completely depends on the suspension damper settings.

Best regards

gmerry wrote ...

PS, if the pressure was not the same at whatever height, this would imply a NET force which would accelerate the car skywards (until stopped by the dampers of course)

ul9601 wrote ...


............
When you increase the ride height, you are increasing its potential energy, which is achieved by work. The work is carried out by force, which is pressure x area. The area remains the same, so pressure must be higher. At higher ride height, therefore, the static pressure of the hydraulic fluid is higher. For equilibrium, nitrogen pressure must be the same as that of hydraulic fluid.
Higher nitrogen pressure means higher spring rate, stiff ride.