my first impression was: Citroen has been doing this for 45 years!
Second impression: well, adjustable spring rates and shock rates- nice for the street/rally/autocross user!
Ok, so Citroen doesn't do exactly this...
Of course- you are probably better off spending some of that money on a grocery-getter and beater, and keeping the race car for the race track?
re: basic air bag suspension vs. Praxis system:
Does the 'normal' system change spring rates with ride height?
What about adjustable shock rates? I'm sure that adjustable rate struts are available, but they probably aren't cheap...
It is a clever add on. You would get better tunability for the track by going to adjustable shocks and conventional road springs, but for the occasional racer they are a clever solution. Most of the handling benefit is from reducing the CG height, of course. Doubling the spring rate is not necessarily a good idea traction wise, they've done that so you won't smash the body about due to the reduced suspension stroke. It will also help disguise the lack of antiroll bar stiffness.
Note that air springs in themselves have significant stiction issues which means they will not be as good as a conventional suspension.
Lowering a vehicle 50mm typically does not optimize its handling. While true that the CGH may by 50mm closer to the ground, in most cases the roll center will end up somewhere off the map. Most OEMs keep the lower control arm parallel to the ground. Now it is 50mm closer to the ground at the chassis pivot point, but the outer ball joint is still at the original location. Chances are that the RCH is now well below ground increasing tire scrub and requiring more aggressive anti roll bars to compensate.
A quick question:
Greg writes that air springs "have significant stiction issues" - that surprises me!
Is that in strut applications, where the spring surrounds the strut? I would expect the classic air bag to have about the same stiction as a coil spring.
I don't know where the stiction comes from, but if you compare an airbag Mercedes with a steel sprung Mercedes there is a 5-10% degradation in ride, subjectively.
Air springs don’t have stiction, but raising the operating pressure of hydraulic cylinder does. Increasing the side loading of a strut does too, and adding an air spring to a strut system may do that. I do not have any information on the Mercedes system, but if it is not a hydropneumatic system or use struts, I don’t see how the air suspension would increase stiction. Have you considered that you may just have sampling error?
I'm assuming this Praxis system includes the concept of pumping a liquid (antifreeze?) in and out of the bags. Am I correct? I've often thought this idea could be used for chassis tuning of oval track cars in this country (where allowed by sanctioning bodies). And, come to think of it, it could be used at the front end of those RWD, beam rear axle drag race cars to either provide a static preload or to provide different rates for a dynamic solution to the unequal rear tire loading problem during acceleration.
