Classic Audi Technical Mech/Tech Suspension & Brakes » Self levelling suspension - a try of explanation

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Old 25-10-11, 12:44 PM   #1
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Default Self levelling suspension - a try of explanation

In case of trouble with the levelling suspension, my customers always asking the same questions about the coherences of hard hopping rear and leaking dampers. Most of them are thinking that the reason for the loss of damping is too much pressure inner the accumulator spheres. Its not so easy to explain and so that I've prepared a little document including some pictures, how the suspension especially the spheres is working.
Unfortunately this document is in German language. I‘d like to think, that the most people (even Audi garagists) don‘t know much about the self levelling suspension in Audi cars. Due I‘ll try a bit translation to explain this suspension here too.
I don't want to bore you with pressure rates, temperatures or spring ratios, I'll try a coherent explanation how the system works- and please pardon my English - this seems hard stuff for me.

First let‘s take an eye on the main components:

-Hydraulic pump (supplying the system with oil(pressure))
-Hydraulic oil (not compressible!)
-Height regulation valve (to direct the mass of oil flow to adjust the vehicles rear height)
-Dampers-( yes in deed - for damping and even for level adjustment)
-Accumulator spheres (contain a high pressure nitrogen reservoir carrying the damper pistons enable the dampers to plunge - that‘s very important and most people don‘t know).

I guess, I don't need to talk about the pump.

The regulation valve is even quite simple to explain. It is connected via a push rod with one rear trailing arm (wishbone, A-arm). So it recognizes the rear plunging at increasing load. Due, it regulates the mass of oil flow to the spheres and dampers. The valve is a rotary piston type an it has a free travel of 10 mm +- to prevent over-reaction of the suspension. The return of oil flow proceeds even through the regulation valve. At least, the regulation valve has influence on the breaking force regulator.

The dampers work a bit different to conventional ones. They are chargeable and not a closed system. Their piston rod is hollow and connected to the sphere via a high pressure oil hose. They naturally have to damp but even to transfer the load weight to the spheres.
Different to Citroen hydro-pneumatic, Audi self levelling suspension has additional coil springs. These springs and the dampers are carrying the load of the vehicle - a bit more later.

The first picture shows an Audi V8, 220v, S6 (and others) accumulator sphere.
It looks a bit different to the ones we‘ve seen there:, but that doesn‘t matter. They even work in the same way, only the design differs.

The next image is a cutaway view. It shows the inner components. The most important one is the diaphragm (marked white). The diaphragm separates the Nitrogen area from the oil area.
Btw: This sphere had a blown diaphragm and so it wasn't suitable any more

The next one shows an unloaded car. The most of the sphere capacity is filled by the Nitrogen (blue), that suppresses the oil (green) via the diaphragm.
The lighter the load is, the more is carried by the coil spring. From about half payload on, the spring and the sphere split carrying the vehicles weight. The heavier the weight is, the more the sphere is carrying the load.

This last image shows a maximum loaded car and it needs a bit more explanation.
Remember, that oil is not compressible! You can imagine it as an iron rod, useful to transfer power and motion, for lubricating and cooling.
The mass of oil inside the spheres is variable, directed by the regulation valve.
The mass of Nitrogen isn't variable - it is fixed by the charged inflation pressure. But the Nitrogen is compressible! And that's how the levelling suspension works:

As the car is loaded, the damper will plunge. The oil is ousted by the damper piston into the sphere and now the oil compresses the Nitrogen via the diaphragm. The regulation valve recognizes the plunge and increases the oil flow into the spheres until the pressure within both areas of the sphere is balanced again. Due increasing pressure, more oil is getting into the dampers and so they extend until the height level is adjusted again.
As you see, the way, the diaphragm is moving up and down, is the way the damper can plunge and extend (remember that oil is incompressible).
Please mind, that the diaphragm will never touch the upper wall of its Nitrogen area, as long as the charged Nitrogen pressure is within the tolerances.
But if Nitrogen pressure is insufficient, the oil will oust the diaphragm earlier and even further. This entails, that the diaphragm sooner or later will touch the upper surface of the sphere. And that causes, that the damper has no more space for any plunge motion. The rear starts hopping. And so, returning to the opinion, that too much pressure is the cause for hopping: The opposite is true, less Nitrogen pressure is the cause.

Why? As times passed, Nitrogen diffuses through the diaphragm and emulsifies with the oil. Sooner or later it disappears over the reservoir. The hotter the setting around (and inside the) spheres, the more Nitrogen diffuses into the oil. That's the main reason why self levelling suspension wears.
(Btw: The lifetime of this suspension is stated with 80000 - 100000Km)
But that's not all. Diffusion caused Nitrogen pressure decreasing is the matter for further and different problems.

Once the nitrogen pressure has decreased too much, oil pressure peaks will not be damped by the Nitrogen buffer. The pressure peaks (up to 200 bar / 2900 PSI ) extremely charge the damper and regulation valve seals. First the dampers get leaking (this is unavoidable), then the regulation valve will be destroyed.

The problem is, that the process of pressure insufficiency rises successively. Normally the driver doesn't recognize the slowly decreasing damper action. But once a hard rear is recognized it's time for quick reaction. Because as long as the dampers not leaking, they can be used further. But at any signs of worn sphere, they should be checked immediately, repaired or replaced .

Most important to know is the remaining pressure of the spheres. Actually all Audi garages should be able to check the spheres (the procedure is described in the repair manuals) but sadly the most aren't. My experiences are this way, that they have either no skills or no tools or both.

Trouble shooting, symptom and possible cause:

Hard (hopping) rear = Decreased Nitrogen pressure
High standing (not lowering and not plunging) rear = Decreased Nitrogen pressure
High standing (not lowering but plunging) rear = Regulation valve
Leaking dampers = Damper seals (mostly caused by decreased Nitrogen pressure)
Leaking dampers = Caused by corrosion
Leaking valve = Regulation valve (mostly caused by decreased Nitrogen pressure)
Self (excessive) lowering over night (one or both sides) = Leaking dampers

As you see, the most cause of trouble are worn accumulator reservoirs.

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Old 25-10-11, 08:07 PM   #2
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