Spheres and comfort

With all those various spheres and damper diameter holes, for any given Citroën model or even accross models, it is difficult to see at first how and why all these physical factors influence the suspension comfort.

What we all know from our own experience is that lower pressure in the spheres give a harder suspension—actually, this is happening all the time as our spheres age. To understand why this is so, we have to note that the pressure stamped on the sphere has nothing to do with the pressure inside when the suspension is functioning. For any given static condition of load and vehicle position, the pressure inside the sphere is constant regardless of what kind of sphere is fitted as long as its initial pressure is below the operating pressure. This second only depends on the force pressing down the given corner of the car, nothing else—more precisely, it is equal to the force pressing up on the suspension strut times the area of the piston in the strut.

The initial pressure is the pressure stamped on the sphere when new (and diminishing as the sphere ages and nitrogen escapes through the membrane). This is the pressure of nitrogen gas with no LHM present on the other side of the membrane. A lower initial pressure means there is less gas in the sphere, which means that once the (higher) operating pressure is applied, the gas will compress to a smaller volume. It is this smaller volume that makes the suspension stiffer—an added fixed amout of LHM inside the sphere reduces a smaller volume by a larger relative percentage than it would a larger volume, hence the pressure increases faster than for a sphere with a higher initial pressure.

An example: let's suppose we have a 300 ccm sphere and the operating pressure is 100 bar. One centimeter of movement in the suspension results in 5 ccm of LHM being pushed into the sphere. For a 300 ccm sphere with an initial pressure of 30 bar, when the suspension rises to its normal position—in other words, when the operating pressure has been achieved—the fluid has compressed the gas from 30 bar to exactly the operating pressure. The gas opposes the liquid and the suspension is in equilibrium, not moving. In order for this to happen, the initial 30 bar to become 100 bar, the gas needs to be compressed to a volume 100/30 times smaller than the initial size of the spehere: to 90 ccm.

Let's add a load now that pushes the suspension down 1 cm. As this movement equals to 5 ccm of LHM displacement, this additional amount of LHM enters the sphere, compressing the gas even further to a volume of 85 ccm. The pressure will increase by the same ratio, 90/85=5.88% over the nominal 100 bar. Consequently, the additional load needed to push the suspension down by that 1 cm is 5.88% over nominal.

Now let's consider the same load but with a sphere with an initial pressure of 50 bar only. At operating pressure, the gas will be compressed into a volume 100/50 times smaller than initial—150 ccm this time. Reducing this by 5 ccm on account of a load that pushes the suspension down 1 cm will now seem smaller in comparison, because instead of reducing from 90 to 85 ccm we are reducing from 150 to 145 ccm, with a pressure increase of 3.45%. The same suspension movement with less added load results in a softer equivalent spring in the case of the higher initial pressure sphere.

Two parts in the system contribute to the damping effect of the suspension, a fixed part: the bore in the damper assembly and a variable part, the damping elements around this center bore. The damping elements are also holes, covered by a spring washer. The reason for this double setup is to provide different responses in different situations. For a sharp increase in pressure difference between the sphere and the suspension—characteristic of abrupt jolts caused by road irregularities—the spring cover opens up and lets the hydraulic 'spring' absorb the energy and dissipate it slowly. The center hole provides the damping—drilling it to a larger bore will decrease the damping effect.