Simple Air Ride question that I have not seen the answer to

[Mean 69]

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

[Steve Chryssos]

Time to just fire up the air compressor, order the pizza and get some answers. We bought a nice AZ car so the swap should go quick.

As it was explained to me, Air Ride has determined correct performance bag pressure at nominal ride height. That sounded like ad-speak so I asked for a translation: At "pro-touring" driving ride height, them bags are pretty darn well full. From there, improved ride quality comes at a lower "installed height" resulting from reduced pressure. And there isn't much room for increased pressure.

Air springs are available from a multitude of sources. So when you think about it, the correct ride height/bag pressure relationship is exactly what we are paying AirRide for--Their expertise. I sure hope they have it figured out. We'll know soon enough.

[TravisB]

I have done exactly what mark was talking about on my car.....it has adjustable mounts for the airspring(shockwave) both fronnt and rear that will allow the low ride hieght and a firm ride hieght if needed. Of course my car is not anywhere near done yet so it is just an idea of mine. we'll see i guess.

nice post mark

[wiedemab]

Here's an idea. My thought for the guys who want to retain or even lower their ride height and have an increase in spring rate is to design an airbag inside and airbag. The larger outer bag could be used to support the vehicle to a given height with lower pressure (more area would allow it to exert more force for a given pressure). For a more firm ride (stiffer spring rate) you would take pressure out of the outer (larger) bag and add to the inner bag. The inner bag would require more pressure to reach the same ride height. The inner bag would also have less rubber (bag) surface area which would also contribute less compliance in the bag and a higher spring rate.

OK - that's my idea. Who's got some money? Lets build some!

I'm probably missing something here - my good ole' engineering skills are a bit rusty.

OK - My brain is fried anyway - it's Friday - I'm outta here.

Later,

Brandon

[ProdigyCustoms]

Mark is so smart it makes me dizzy!

[camcojb]

Originally Posted by ProdigyCustoms

Mark is so smart it makes me dizzy!

True dat. I want to know why three of the Air Ride guys read this thread but none replied. Looking for a magic solution here guys.

Jody

[darren@ridetech]

You guys are correct; you can not theoretically change the air pressure in the air spring without changing the height of the vehicle. But small adjustments (5-10 psi) will only change the height a small amount, but will affect the ride and performace. And yes one of the keys to obtains a performance air suspension is using adjustable shocks. And we do offer double adjustable shocks as well, we may need to point that out a little more on the web site.

The new LevelPro utilizes height sensors mounted between the frame and the suspension to more accurately raise the vehicle to ride height. The RidePro E2 system only uses air pressure sensors which is fairly accurate, but varying loads and suspension bind leaves room for improvement. This is where the LevelPro system accels.

HRBS,
Neither of these system will level the vehicle out while cornering. As cool as that sounds, jacking around with the spring rates mid corner would most likely not be a good thing. There are way too many variables to make that happen effectively.

[darren@ridetech]

No, I was too busy answering the dang phone....

[camcojb]

Originally Posted by darren@ridetech

You guys are correct; you can not theoretically change the air pressure in the air spring without changing the height of the vehicle. But small adjustments (5-10 psi) will only change the height a small amount, but will affect the ride and performace. And yes one of the keys to obtains a performance air suspension is using adjustable shocks. And we do offer double adjustable shocks as well, we may need to point that out a little more on the web site.

The new LevelPro utilizes height sensors mounted between the frame and the suspension to more accurately raise the vehicle to ride height. The RidePro E2 system only uses air pressure sensors which is fairly accurate, but varying loads and suspension bind leaves room for improvement. This is where the LevelPro system accels.

HRBS,
Neither of these system will level the vehicle out while cornering. As cool as that sounds, jacking around with the spring rates mid corner would most likely not be a good thing. There are way too many variables to make that happen effectively.

so for an honest opinion. My Chevelle is fairly stiff in my opinion (though others may disagree!). It has a full Hotchkis a-arm/spring/shock combo which was sorted out by a GM suspension engineer. It is not set up for the best possible handling but a good compromise between ride and handling.

In your opinion, is it possible that your full system could give a noticeably better ride while retaining at least the same level of handling as I have now? I understand that's not really possible to answer without testing my car currently and then after installing your system. But looking for a rough idea. I mean, to make it handle as well as a full Hotchkis setup would your air pressure be high enough that the ride would not be much different than what I have now?

I'm skipping the ride height differences right now and wondering if I can get the same handling at a softer ride, or if in order to control body roll and handling the airride will likely need to be as stiff as my current Hotchkis setup. I understand yours gives me the ability to lower the pressure, but then we have less handling and a different ride height, alignment, etc.

Jody

[ridetech bret]

A few things to consider for the guys who may struggle with math and theory...
The change in load capacity [ride height] and spring rate [stiffness] can be manipulated by changes in the fabric of the airspring and the profile of the lower piston over which the airspring rolls down during compression. Since Firestone [or whoever the manufacturer may be] controls the parameters of the airspring itself, we'll dismiss that variable for now.
Piston profile can be customized a bit easier. Most straight sleeve type airsprings roll down over a staight wall piston. A straight sleeve airpsring rolling over a straight wall piston will create a fairly linear spring rate...at a constant air pressure. Remember thought, that as the airspring compresses the air volume decreases thereby raising the spring rate and making a slightly progressive sprin rate. For example, if you take a ShockWave 7000 and inflate the unit to 80psi you have a load capacity of 641lbs at its ride height of 14.75" As you compress the ShockWave by 1" the air pressure raises to approx. 90psi and the load capacity raises to 772lbs. Compress it 2" andthe air pressure raises to 105psi, the load to 915lbs. At 3" of compression the pressure is 124psi and the load is 1094.
The slope of the spring rate curve can be manipulated by the shape of the piston it rolls down over because that shape determines the effective volume of air inside the airspring. If we were to build a tapered piston so the large portion of the taper is at the bottom of the piston, the spring rate would be more progressive...the farther you compress the airspring the stiffer it would get. If we would build a "reverse taper" piston with the smaller portion of hte taper at the bottom, then the curve would be more flat or maybe even "regressive" [spring rate gets softer as it compresses" if the reverse taper is steep enough.
What does this really mean in the real world? Well, for those who want to spend enough time and energy to play with different piston profiles, you can tune until you have a "perfectly" handling car...or are completely bored with tuning.
For most it is easier to play with variables such as air pressure, adjustable shock valving, and sway bar combinations. Here is a good example: I went to the Run To The Hills Autocross in Pigeon Forge last fall with my 70 Buick GSX. Double adjustable ShockWaves, StrongArms, MuscleBar swaybars and a RidePro e2 compressor system. I made a run throught the course and found that the car wanted to bottom out going into the banked transition [yeah, there was a 25 degree banked turn at this autocross!]. The next run I increased the air pressure slightly along with a couple of clicks more compression valving in the front shocks. OK, that problem was solved but not the car wanted to push a little on the flat corners. So I increased the rear air pressure very slightly to stiffen it up so the rearend stepped out just a little to balance the car. The car got faster. In theory this combination should have slowed the car down because I "dumbed" the rear of the car down to match the inferior handling front end. I was forced to compromise the handling of the front to accommodate the unusual banked transition at that track. In the real world...the car was the fastest car there on non R compound tires because I was able to adapt to the real world track conditions more effeciently than anyone else that day.
There is a real need for published data on this subject. My problem is how to find the time to "publish" the research and experience that we've done over the past 15 years. It's not that I don't want to...I just need to "pay the bills" by getting everything else done in a days time as well. Insert excuse here.
I really think most people want "cut to the chase" information about any suspension product, including air. Thats what I'm trying to do here. I hope at some point that I can expand into more detail because obviously there are many people looking for that info as well!
We do have spring rate and load data on our ShockWaves. I will figure out how to get it on our website in the next few days [I'm kinda slow].
If I don't get a chance to lurk on the forums, my email is bret@ ridetech.com. I may not be able to fix your problem or even answer your question, but I can probalby share several dozen ways NOT to fix it!