Another mini video by Bollinger, this time going into more detail about the suspension.
This video answers the question we had of why is the sphere mounted on the side of the ram ? It turns out that there are actually two
spheres per wheel, and it is a double acting ram!
Previous videos did not show inside the ram nor did they show what turns out to be the main sphere for each wheel, which is mounted separately with a flexible hose.
All Citroen Hydropneumatic strut rams are single acting pistons. That means on the compression stroke the piston forces oil into the sphere through the damper, however on the rebound it relies on the gas pressure in the sphere to push the oil back out through the damper and follow up the piston. The only use of a double acting ram on the Citroen system is for the Activa anti-roll rams, which need to be double acting to allow the roll bar to be twisted in either direction.
On the Bollinger system they have a double acting ram for the strut rams for each wheel - the top end with the large piston area is connected via a flexible hose to a much larger sphere than we have not previously seen depicted, but which is shown in this video. In addition the smaller sphere mounted on the side of the ram is connected to the "back" side of the piston at the bottom - because there is reduced cross sectional area on the back of the piston, (like on a power steering rack) the sphere connected to this side has to be smaller for the same springing rate.
So why go to this extra trouble of two spheres per wheel and a double acting ram ? One reason is shown in the video - making the springing rate very linear over a very wide range of loads and suspension travel.
Normal hydropneumatic with a single acting piston is non-linear when considered over a wide range of oil displacement in the sphere - as you compress the gas further you reach a point where the gas volume is small relative to the oil displacement and it rapidly gets stiffer in a non-linear way.
Normally you don't notice this effect because in an unladen Citroen the entire suspension travel is only a small proportion of the total possible diaphragm displacement inside the sphere, so over this reduced operating range the suspension approximates a linear springing rate. However if the spheres are low on gas and you load the car up you'll notice it gets hard and suspension travel becomes limited very quickly.
In the two sphere double acting ram scenario used in the Bollinger when the suspension is compressed oil is pushed into the larger top sphere by the large piston, and simultaneously pulled out of the smaller bottom sphere by the gas in the bottom sphere following up the smaller back side piston. Total springing stiffness at any given point of travel is the sum of these two spheres which are operating in opposite configuration.
The beauty of this is it would make the springing rate very linear over a very wide suspension travel and load range.
When the gas in one sphere is being compressed and reducing in volume and is starting to become non linear, the expanding large volume of gas in the opposite sphere will provide the dominant compliance, and vica versa. I haven't studied it enough to work out whether it makes the springing rate perfectly linear over the entire range, or whether it just becomes a much better approximation than a single acting system, but looking at the graph they show in the video tends to suggest that it makes the springing rate almost perfectly linear. Bravo!
I can see a second major potential benefit of using a double acting ram for the suspension which they don't talk about at all, and that is avoiding cavitation on the piston under rapid large suspension movements, especially if you have stiff damping.
In one of the mega long harsh Hydractive 2 ride threads I remember discussing the possibility of cavitation at the piston being a source of ride harshness under extreme conditions. With a single acting piston you have no problem on the compression stroke as the oil between the piston and damper valve, and from damper valve to gas interface at the diaphragm will always remain compressed no matter how hard or sudden the movement is.
However there is a potentially significant problem with rapid rebound movements because when the piston moves away from the sphere the only thing pushing the oil out to follow up the piston is the spring rate of the gas pressure in the sphere. Furthermore it is trying to push it through the damper valve, which will restrict the flow of oil trying to follow up the piston. The stiffer the damper valve is the more it will restrict that flow.
If the damper valve is sufficiently stiff and the rebound movement is sufficiently large and abrupt, it's possible that the oil can't follow the piston and a void will form in the oil between the damper valve and the piston, and then momentarily later it will catch up and "crash" into the piston harshly, in essence a kind of cavitation as it would cause a large shockwave. This places an upper limit on how stiff you can tune the damping without risking cavitation, and I suspect Hydractive 2 comes close to that limit under certain conditions.
On the double acting ram design this can be avoided completely because no matter which way the ram moves one sphere always has oil going into it, and thus the damper valve for that sphere in that direction can restrict movement without risking cavitation. So you would use asymmetric damper valves in each sphere which provide most of their damping on the "in" direction. So your compression damping would be set primarily by the damper valve to the top sphere and the rebound damping by the damper valve for the bottom sphere. The damping for the outflow direction for each sphere would be kept to a minimal level to avoid cavitation occurring.
Clever design using two spheres and a double acting ram for each wheel - I don't think this has been done before anywhere else ? Certainly not by Citroen.