I was just having this same conversation with a group of folks yesterday. It really comes down to simple physics when you talk about the ride height, relative to the spring "rate." I'll agree, I have not seen specific information on the relative "spring" rate of a air spring as a function of air pressure, I'd really, really like to see the data and I know it is out there, but yet to see enough for my personal liking.
Regardless, it's pretty simple. The car has a fixed sprung weight, meaning everything that isn't connected to the suspension (sort of, but for this discussion, let's keep it simple). Dealing with the rear of the car is easiest, because almost always, the installation ratio (i.e. motion ratio) is usually not a big variable (shocks/springs usually mounted near vertical, directly to the rear housing, and directly to the frame), makes the math really easy. For some simple assumptions, etc, let's take a 3000 lb car, with a 50/50% weight distribution, to make the math easy as possible (I'm lazy), for 1500 lbs on the rear wheels.
A typical firm street style rear spring rate is 200 lbs/in, and there are two of them on the rear axle, so the combined rate is 400 lb/in (again, for simplification, ignore the influence on the roll rate). For this setup, the rear springs will compress (1500 lb)/(400 lb/in) = 3.75" until they settle and find their happy place. All of the spring manufacturers figure this into the design/free height of the spring to arrive at a specific ride height for the given frame mounts, etc. For a air spring type car, you need to fix the upper and lower mounting points for the spring/shock mounting to arrive at the ride height for a specific "firmness," or whatever you want to call it, but regardless it's fixed and non-adjustable for the sake of this discussion.
A stock style rear spring rate, which would generally be considered a "comfort" type setting, is typically around 150 lb/in (each). With the fixed upper and lower shock/spring mounts in the above example, this would result in a spring compression of (1500 lb)/(300 lb/in) = 5.00". This means that reducing the spring rate from 200 lbs, to 150 lbs, LOWERS the car by 1.25" relative to the "nominal."
Depending upon the balance of the front and rear suspension systems (i.e. if the car is neutral, oversteers, or understeers), one way to trim the car is to play with spring rates. In almost all cases, because our cars aren't 50/50% weight distribution, AND because there is a physical limit to how much tire you can put on the car, the basic behavior is understeer. Independent of ALL other possibilities of tuning around this, and again being specific to the rear spring rate case above, one of the easiest ways to tune against understeer is to raise the rear roll resistance, and for the sake of this argument, let's do it with the rear spring rate (again, keeping it simple). Very fast muscle cars that are run on the track can, and typically run 250 - 300 lb rear spring rates. Not to freak out the math, let's assume a "softer" rear track setup of 250 lb/in (each side). Now the springs compress (1500 lb)/(500 lb/in) = 3.00". This raises the car 0.75" relative to the street performance trim, and a full 2.00" relative to the cushy cruising trim.
The math doesn't lie, and the fact that the springing medium for the bags is air rather than a coil spring doesn't matter either. Increasing the spring rate to firm up/improve the handling of a car all things being equal, raises the car, period. All of the same logic holds true for the front of the car as well, the math is a little trickier, but the story is all the same. Like a lot of folks, I have been following the AirRide folks in particular, they are doing an amazing amount of work and have no issues putting their cars to the test, which I think is terrific, I wish we had enough money to do the same (soon..). And further, because there is so much passion with regard to the air setups (folks are either 100% with them, or usually pretty strong against them), I want to make it clear that I am not attempting to detract ANYTHING from their specific results, it is all great stuff, and gives the hobbyists in the sport something else to choose from: everyone wins. Their setups are real, and no one can contest that.
But.... As it applies to your specific question, and in a somewhat critical case, here's my take on it. When I drive around the streets, I don't want my car to beat me up, meaning plush ride. I also don't want to hit curbs, speedbumps, etc, so the combination of a slightly higher ride height, and a slightly softer "spring" is what I'd be looking for. On the track, I want as low a ride height as possible (the lower the center of gravity, the better the handling, period), and I want to firm things up, sometimes considerably (race tracks don't have speed bumps, potholes, etc...). As the above math shows, this is opposite to what nature says happens when using spring rate as the variable.
THE solution for the air spring case would be to include different mounting location options for the spring/shock mounts for folks that are more 'core, or to incoporate a typical coil-over type adjustment setup on the shocks (the air stuff I have seen has the air spring physically fixed to the shock with no way to adjust it's height, through necessity). This isn't any harder than adjusting the ride height for a coil-over type setup, with those, you just turn the spring perch ring, it's as simple as it gets. It'd be harder to pull of on the front of the car than the rear due to packaging, but it can still be done. The other alterntaive would be to run a stiffer set of bags/shocks for the track than the street, and keep the same mounting locations. Slightly more expensive.
All that said, most folks are not going to run the crap out of their car on the race track, so trying to drain out the last bit of performance isn't a big deal. Heck, look at Boris' comments on the Air Ride setup, he stated that if he had a bit of a push, he'd just raise the rear of the car a bit to help balance it out (consistent with the math/logic above). For a street car that is driven occaisionally on the track, that's probably just fine. For a race car? Well, I don't think you could convince Boris' crew chief that raising the rear of the No Fear stock car to tune against a "push" would go over very well.
Just thought of something to help segment the types of folks that wish to run track events. Do you have a HANS device? Do you have a full roll cage, or at least a four point roll bar with a five point racing harness? Do you have at least a fire extinguisher on board, or better, a dedicated fire system? Do you run R Compound tires, or better? Do you change your brake pads when you go to the track to a hotter compound? If you answer "no" to any of the above, the suspension specifics are less important. (and by the way, in my opinion, one has no business on a race track getting after things if ALL of the above aren't met, with the possible exception of the R compound tires).
Peace,
Mark
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Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
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) and then being able to step up for a track day. But doesn't any air pressure change also give you a ride height change? So if you get the car setup for the track at a particular pressure wouldn't the height and alignment be off when you adjusted it for the street setting?
I hope you're correct, it would be a huge selling feature to the G-machine crowd.
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