Exlax

The discussion about hieght vs stiffness has been over simplified. If you look at a typical air shock it is progressive in nature. As it cycles through its travel it gets progressively stiffer.This is in contrast to a coilover spring which tend to be linear. An air shock with less air volume will ramp more rapidly and will feel harsher. A higher pressure higher volume air spring will ramp slower but feel firmer in its initial travel. At high speeds the lower volume may tend to damp the more rapid spikes in suspension travel. To determine the best setting for the highest performance we need to look at the pressure-volume-and damping charracteristics. Many things can be done in the damping department to improve shock performance. One improvement would be speed sensitive valving or position sensitive valving. These would allow tuning the shock to specific conditions or position within the shocks stroke. I have not seen a diagram of the current air shock set up. Does anybody have a cutaway drawing? This is definetly a problem a high end suspension tuner could resolve if the were so inclined. Lets hope that this will be the case.

HPH

You make a good point about the (unknown) true design of the allroad air springs. (The manual has lots of pictures of these things, but they're anonymous cylinders to be removed and installed, not taken apart and examined.)

It could be that they're designed to maintain about the same spring constant at all settings, and our perceptions are just psychological.

Thus, one possibility is that the height goes up due to more air in the bladders (not more pressure, just more air), and the spring constant is pretty constant. This is a physically consistent alternative explanation to my "taller is skinnier" one above. HPH

Dutch8griffin

still didn't answer the question on stiff and soft
handling =( but here's a question... if your in service mode, doesn't that "lock" the suspension into place? so in theory, if you put your suspension into level 4, then activate the "service mode", will that make the car stay in level 4 even if your traveling faster then 20mph? will it damage the suspension to leave in service mode to do this? just a question... im going to post this up top to get more views hopefully.

HPH

...the shocks are shown beside the air springs in the pictures, not inside them. (To be inside them, the springs would have to be cylindrical toruses (tori?)--tall donuts.)

GP's comment about the jack mode might work except it's supposed to automatically turn off over 3mph.

And I'll take this opportunity to go public with a comment I emailed to GP and someone else (on the theory that people who've read this far into the thread are *really* suspension geeks): The manual has a hint of how it might be possible to lower the car in all four positions. If you want to do this, get the manual, because I don't want to be responsible for passing instructions that may or may not work. It involves the VAG tool and a procedure in the manual for post-suspension recalibration.

You measure (with a tape) the distance between the center of each wheel and the apex of the fender well (the plastic 'cover', as Audi calls 'em) and use the VAG to enter this information into the car's computer. Then, after all four wheels are entered, you give it another instuction, at which point it adjusts itself to where it thinks it should be (uniformly 405mm in position 2). So if you faked it out by telling the car it's sitting higher than it really is, it should lower itself accordingly.

Of course, if the discussion in the current thread holds true, then you'd have a smooshy, low-riding allroad. Maybe a better approach would just be 14" wheels...
HPH

uktena

Does anyone, in this forum, know how these are designed internally? I think, if I were designing this system, I would have considered multi-staged chambers. Actually, it might work better if the air chambers raised and lowered a box section on which a entire conventional suspension system would ride. of course, this is mere speculation, as I am not an engineer; but I did stay at a Holiday Innn Express last month.

Dutch8griffin

I didn't stay in a holiday inn last night, and I still think the theory (no matter how many chambers) makes the suspension stiffer in higher levels...

I'm gonna do a test:
go to a empty parking lot.
go around in circle,stopping a few times to switch levels, till I vomit.
see and feel how much body role I get...
very unorthodox, but might work... brb

marlinduke

Since this thread started, I've been trying to find an in-depth schematic or technical paper on the allroad's pneumatic suspension. No go. So we're stuck speculating. Going with Ockham's razor, assuming you felt the suspension completely compress on a bump, Gary's suggestion is the simplest--you need a new damper (shock absorber).

I figure we can trust Audi and Conti-Teves that the air bladders are designed to operate within the full range of conceivable (and perhaps a few inconceivable) conditions. Until we get a Continental engineer in here to tell us, I think hph's second hypothesis that the air pressure in the bladders is relatively constant is probably correct. Since PV=nRT, it makes sense that the air compressor is feeding "n" into the spring, allowing P to stay constant while V increases. Of course the equation is more complicated because other forces are involved: the contractile force of the rubber bag, the sprung mass, among others. These added factors probably contribute to the impression that the ar feels "stiffer" on level 2, more energy might be displaced to the body at level 1 because there is lesser volume in the rubber bladder and since the rubber is less preloaded, it doesn't fully contain all the energy exerted by displaced unsprung mass when the ar hits a bump. Definitely the hardest thing to get a handle on with the ar is the fact that the springs serve two purposes: adjusting ride height along with isolation.

Still, I think this problem is ultimately a damping issue. It is fun to think about all the ar's engineering wizardry though.

Dutch8griffin

wow you musta stayed at a holiday inn last night.... =)

uktena

It would be handy to measure the lateral forces.

BTW, it will work, go round enough and you will vomit.

BSteely

Having gone all the way through this thread, I just grabbed a kitchen timer and went outside and tried a little empirical test. I set the ar to suspension setting 1 (lowest). Set the kitchen timer for one minute. Started the timer and at the same time started bouncing the front fender over one of the front wheels with all my weight at the natural frequency of the suspension. I got appx. 82 bounces in one minute. Raised the ar to suspension setting 4 and repeated the test. Result was about 93 cycles in 60 seconds, so indeed the effective spring rate is higher at the higher setting. Would have tried settings 2 and 3 but my arms were getting tired :-)