ArneeA

Air pressure in the air springs is adapted in accordance with load, NOT volume.

Supporting force of the air spring is in direct relationship to the internal pressure.

ArneeA

^This is true for all pneumatic suspensions in automotive applications. There's a good detailed document that I've read from Audi about this regarding the A6 air suspension.

That's why some systems have a pressure accumulator tank and pressure sensors. Pressure accumulators typically have a reference pressure of 14.7psi/1bar or thereabouts.

mcbit

If you change the weight of the car the pressure in the struts will change to maintain the selected suspension height and maintain the level of the vehicle. A certain air pressure will support a given load, no more no less.

Raising and lowering the vehicle is achieved by adding or removing air from the cylinder until a level set by the contol module is reached. This increases or decreases the effective volume of the cylinder. Greater cylinder volume = lower spring rate; Lower cylinder volume = Higher spring rate. If the weight of the vehicle does not change neither does the air pressure required to support that load.

ArneeA

Again, you're saying the volume is changed. It is not. Pressure is changed with right height changes.
And pressure in the airbags, not struts.

mcbit

So please explain to me how the ride height is changed....

and by the way this is what Porsche call them

Air suspension
with level control and height adjustment
– Fully load bearing air spring struts with integral shock absorbers
– Air supply system with pressure accumulator

techowner12

I have indeed seen a 2006 CS with wood + PDCC but it's definitely a rare one (here http://www.cars.com/go/search/detail...&&aff=national )

Seems everything was a combinational option - wood, suspension etc.

techowner12

Oh yeah my question was this - WITHOUT PDCC how should this thing hit bumps? In California, they spend money on social programs and not roads (Ohio.. North Carolina .. Texas .. good roads. Here? Youd be better off on gravel, Im not joking.)

I don't know if I need a strut or 2 or just to adjust my expectations. The caverns arent that deep and I expected this thing to roll over stuff better with 18" wheels and such. It currently hits them and is next to jarring.

mcbit

No PDCC until 2008 I believe.

ArneeA

I thought I already mentioned it. The pressure is changed.

Porsche, VW, Audi, BMW, MB, LR, Lincoln, etc... it doesn't matter. They're all the same concept. Application and execution may be different.

Pressure inside the air spring bladder is changed. Supporting force on an air spring is directly proportional to spring rate; it is not constant compared to a (non-progressive) coil spring. Supporting force is a function of internal pressure and effective surface of the air spring bladder.

F = pi x Aw

Aw is a function of the air bladder diameter, dw.

Aw (or dw) is constant in this case since you're not changing the dimensions of the air spring bladder (hence you're not changing the volume). You're changing the internal pressure, pi.

The pressure accumulator is only a "tank," to store pressure, to minimize load on the compressor when instantaneous pressure is needed (or during start-up, after wake-up, etc).

The valve units are there to transfer air to and from the air bellows during the necessary changes to maintain level control.

The compressor is there to compress air pressure. Volume within the system is constant.

Compressor's volume is the same, pressure accumulator's volume is constant. Air spring bladder is constant. Volume in the air lines is constant. Volume in the valve block is constant.

The suspension-ECU is there to of course be the watchdog for all the mechanical modules (hall sensor for ride height, valve block, pressure accumulator, compressor).

Since I've had problems with my 4.8is and ex-S500 air suspension, I have been studying how these work. They are all basically the same concept (and why shouldn't they? Who likes re-inventing the wheel?). They all use the same compressor manufacturer too, WABCO.

Let me ask you a question, when you use an air compressor to increase or decrease your tire pressure, are you changing the pressure or the volume?

ArneeA

BTW, I have a good read for the Audi A6's air suspension design but it also gets into light physics. I can send it to you, if you'r interested. The BMW documents that I have are pretty much useless and are designed for techs only.

GTS Jay

Correct - 2008 was the first year PDCC was offered. That one in the ad does not have it. You can easily tell which cars have PDCC and which don't. The ones with PDCC have silver controls on the console where as the rest have black controls (except for the big rotating switches themselves which are always silver). It also says "PDCC" in the lower right hand corner of the switch pod, facing the back seats. I know this is true for at least the 2003 - 2010 Cayenne models.

mcbit

Thanks for the offer but I'm an Engineer by profession and my physics is kinda OK.

The A6 system is a very simple one which only involves self levelling by means of two air springs at the rear of the vehicle, however the principle of operation is the same. I'll use some ballpark numbers just to try and simplify the explanation so don't take any of them as gospel.

The A6 unloaded weighs 2000 kgs with a front/ rear split 60/40 which means that the air springs at the rear are supporting 800kg. In order to support this load the 'U' bellow 'bags' which form the air springs have 8bar air pressure in them and the car is level. We now put 2 passengers (140kg) and 60kg luggage in the trunk which means that the air springs are now trying to support 1000kg. If we dramatically slowed down the operation of the self levelling system this is what would ensue, The 8 bar currently in the airsprings can no longer support the new 1000kg load so the back of the car would drop compressing (reducing the volume) of the air springs until an internal pressure of 10bar, sufficient to support the new load, is achieved. The ECU is unaware of the pressure change in the springs but is aware that the level sensors are indicating a deviation from the reference level. The ECU sends a signal to the solenoid valve on the appropriate spring, the solenoid valve on that spring opens and allows air to pass into spring which extends, due to the increased volume of air, until the reference level is reached and the solenoid valve closes. The increase in pressure is actually a load dependent passive process and levelling is achieved by addition of air at the required pressure to apparently maintain a constant volume. As the load/ pressure increases so does the spring rating.

The Cayenne air suspension has 4 air struts and, in addition to the self levelling mode, has 6 different height settings, it is in achieving these height settings that the obvious changes in volume occur. Height changes are achieved by lengthening or shortening the strut, which involves the 'u' bellows rolling out/ extending (increasing in volume) or rolling in/ shortening (decreasing in volume). When in special terrain mode the spring is at its greatest volume and lowest initial spring rate. The initial spring rate increases with each lower setting and reducing volume of the air spring.

This is actually one the reasons why air springs are ideally suited for off road/ on road vehicles. Increased ground clearance and lower spring rate off-road and aerodynamic efficiency and higher spring rate for improved handling on road.

GR997S

No way. I own an '09 Cayenne GTS and a 997s. The 911 is in a totally different league. Just because the Cayenne is an unbelievable handling SUV...it is still an SUV....GR

ArneeA

I'm an engineer by profession as well. I'm not talking about the A6's rear self leveling only (1-axle). I am talking about the full 2-axle pneumatic suspensions.

Pneumatic suspensions are all the same, RV, Porsche, VW, BMW, Audi, etc. etc. etc... even custom hot rods. Some use pressure-feedback for ride height and some use hall sensors for ride height.

Not taking anything gospel, as an engineer, I have to understand it myself. Volume doesn't change, pressure will. Agree to disagree if you don't believe it.

mcbit

I don't think that there's any engineer out there who could give a satisfactory explanation as to how you extend the length of an air or hydraulic cylinder (air bag) by more than 4" without increasing the volume.