I suggest you follow the link near the top of the thread in my first reply (warning long thread!Deanxm wrote:I have never understood this quest to seperate air from the returning oil, almost any Citroen ive worked on will ride much softer when air is introduced to the system as a result of work being done. Usualy a side affect that rights itself after a dozen miles or so as the system self bleeds. what is the theory that leads some to suspect air could make the ride harsh?
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) as its not something that is conceptually easy to explain in a few sentences.But at its most basic, you have a system where you have nitrogen kept in one mass behind a diaphragm in the sphere, and an incompressible link between the nitrogen and the piston - the oil. The oil must flow through the damper valve to reach the gas so the gas can't compress or extend without oil passing through the damper valve - hence you get good damping control as even the tiniest suspension movement must cause oil flow through the valve since there is nothing compressible between the piston and the damper valve. (In contrast to conventional suspension with rubber bush mounted shock absorbers where small suspension movements CAN occur without the shock absorber moving, meaning they are not well damped for small movements)
Air or nitrogen bubbles loose in the oil side of the system are on the wrong side of the damper valve which means the piston can move a small amount (compressing the rogue air bubbles) before the oil is forced through the damper valve. So as more and more air gets into the oil side of the system one change is a feeling of loss of damping and control for small movements - the car loses the normal ultra stable feeling that a good hydro Citroen has. IMHO this noticeable loss of damping control is the first sign of air in the oil, and noticeable before the suspension gets really crashy over potholes.
The second problem is that these rogue air bubbles tend to coalesce at a high point in the suspension over time. When they're still widely distributed in the piping as microscopic bubbles or "froth" I don't think they make the ride crashy, although they do still cause a loss of damping as the air contaminated oil between the piston and damper is not incompressible as it should be.
However over time especially on a Hydractive model with the large diameter long pipes the bubbles work their way up to the top in the front suspension in particular, near the top of the strut top. Once they coalesce into a larger discrete bubble the ride then becomes crashy and harsh because you get an effect like cavitation. The small air bubble in the piping has a much stiffer spring constant than the separate bubble of nitrogen in the sphere, so as pressure changes when negotiating a bump the small bubble compresses quickly and "bottoms" causing a cavitation like shock wave.
Another way to look at the problem is instead of the overall spring constant of the car being a smooth curve, the air bubble isolated on the wrong side of the damper valve causes a non-linear "kink" in the spring constant about the medium point, which adds harshness to the ride.
Although all Xantias can be susceptible to the problem in my experience Hydractive models suffer worse due to the diameter and layout of the large feed pipes to the strut, which give the air a good place to coalesce as well as making self bleeding much more difficult. (it takes a lot more outwards oil flow to bleed air past the height corrector on a Hydractive model as there is so much more oil volume between struts and height corrector)
If accumulation of new air bubbles exceeds the self purging ability of the suspension then over time the ride will deteriorate, which is what most of us experience to varying degrees. Citreorobics is a temporary fix as it helps flush any bubbles out but if there is a high intake of air bubbles the cure is only temporary.
The extra good ride that you sometimes get just after Citreorobics is how the car should actually ride all the time!
