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Old 06-16-2014, 03:55 PM   #1
Andrew@ORT
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Air Suspension Discussion Thread - Let's Get Nerdy


The age old question of looks versus handling. It's a question (and debate in most cases) that has plagued the aftermarket community for years. There are so many schools of thought on handling and performance versus looks and styling. Words such as stance and fitment are so overused that it makes most of us cringe. The majority of us love a well put together car and in that 'well put together car', there is usually some sort of suspension tweaking or altering that has been performed.

Before we get ahead of ourselves, we'd like to set something straight on our end. This thread is not meant to push products or sell anything. That is the antithesis of what we're after. We're trying to put to rest the rumors, misconceptions and false truths about air suspension. We also are trying to give a bit of 'insider information' about the progression and evolution that air suspension systems have made over the last decade.

While at one point, we would agree that air suspension was more about looks than anything else, that's hardly the case these days. The amount of research, design, technology, etc. that goes into (some) air suspensions is beyond that of your normal coilover suspension. And air suspensions handle as well as, if not better than coilovers. Yes, we said it.

Not long ago, we had the pleasure of spending time with AirLift Company. We took a full tour of their facility as well as spent time going over their 550 AWHP STI which is currently riding on their new Performance air ride system.



AirLift's 05 STI is sporting 9.5" wheels wrapped in 265/35/18 tires with zero stretching. The car was previously on RaceComp KW V3's.



RC KW V3's weighing in at 14.605lbs



AirLift Performance Front Struts weighing in at 11.865lbs

That's a total of 2.74lbs per front corner!



RC KW V3 rears weighing in at 14.450lbs



AirLift Performance Rear Struts weighing in at 12.455lbs

Total weight savings of 1.995lbs per rear corner.

Total combined weight savings: 9.47lbs



Front Camber plates [included with each strut]



Making excellent use of the rear camber plates!

The AirLift Performance Series Struts are built on Custom BC Racing Coilovers. Each set of Performance Series Struts for the Subaru GC/GD platform is both height adjustable via the fully threaded body but is also height adjustable via the air spring.

The struts are 30 way dampening adjustable via the adjustment knob on top of each of the struts (the rears have remote adjusters for convenience). Realistically, you could have your car Cadillac plush during the week and canyon carving stiff on the weekends. Another plus of the AirLift struts is that they are not assembled using normal coil spring rates. The struts are revalved specifically for the use of an air spring. This is because air springs do not function like regular metal coil springs. Things such as spring rate, deflection, etc. are much different and must be compensated for when building an air suspension.

Media / Videos

[ame="https://www.youtube.com/watch?v=_uppK8z0lDg"]Air Suspension vs. Coilovers - YouTube[/ame]

[ame="https://www.youtube.com/watch?v=9Fl7PuPmcoA"]Air Lift Performance Track testing - YouTube[/ame]




Let's bring some math into the mix:

Calculating the spring rate of an air bag is much harder than calculating the spring rate of a typical metal coil spring. Spring rate is defined as the amount weight (typically in lbs) needed to compress a spring one inch. Typically spring rates are calculated using the equation:

k = Gd^4/8nD^3

in this equation:

k = spring constant, pounds of load per inch of deflection
G = modulus of rigidity
n = number of active spring coils
D = wire diameter or coil diameter - measured in inches

Because we cannot plug in raw numerical values for air springs, it becomes increasingly harder to figure out the exact spring rate. Furthermore in the world of air springs, both pressure and volume need to be taken into consideration when trying to calculate spring rate - again, it's not as easy as it would be with a set of coil springs.

There are a two things that are certain more often than not and they are: larger bags will lift more weight but have a lower spring rate and smaller bags will not be able to support as much load but will have a higher spring rate. Spring rate with regard to air ride is definitely a function of bag pressure and volume. Another variable that has to be taken into consideration is the material used in bag construction. The thicker the material, the more weight the bag will support and the less pressure it will need to lift the vehicle. It's very tough to calculate the exact spring rate of air springs because there are (in a sense) too many unknowns. Without breaking out paper and an abacus, you could always set your car at maximum height and consistently add weight until the vehicle drops one inch and do the same at ride height. This will show you that spring rate will vary dependent upon pressure and not just volume. It's a pretty 'raw' method of figuring out the spring rate, but it should give you a general idea of the spring rate of your air springs.

We welcome any questions, comments, responses, etc. We realize that air suspension is not everybody's cup of tea and we're not here to turn atheists into believers. We're just trying to create a good discussion and help people further their knowledge outside of proverbial box.

If you'd rather speak to someone directly regarding these suspensions, feel free to contact us:

E: info@openroadtuning.com
P: 610.572.2898
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Old 06-16-2014, 04:48 PM   #2
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Do you know if there are any race car teams or owners running this kind of suspension?
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Old 06-16-2014, 05:07 PM   #3
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Do you know if there are any race car teams or owners running this kind of suspension?
Ken Block has a Bagged Focus ST that he daily drives ;]
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Old 06-16-2014, 05:36 PM   #4
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If this is anything like the brake thread, this should be good.
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Old 06-16-2014, 05:44 PM   #5
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You can throw up all the math you'd like but in layman's speak and air bag has essentially an infinite spring rate. You can optimize the damping for a specific air bag height/pressure but in the real world that is extremely difficult because of the "infinite rate" nature. If you can have dynamic ride height while maintaining constant bag pressure then, sure... its possible. This is why GM's magnetic ride control is so amazing - it can compensate on the fly.

Still not sold on air ride but please keep up the R&D. Hope it gets there soon!
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Old 06-16-2014, 05:52 PM   #6
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You can throw up all the math you'd like but in layman's speak and air bag has essentially an infinite spring rate.
"Infinite spring rate". I very much doubt that. Would you care to elaborate? Do you mean rising rate?
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Old 06-16-2014, 06:05 PM   #7
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"Infinite spring rate". I very much doubt that. Would you care to elaborate? Do you mean rising rate?
You're welcome to doubt all you'd like mate

essentially at 0 psi on a bag you're at 0 lbs/inch or effectively 0 spring rate. lets say a Firestone made airbag has a max psi of 400psi (for discussion purposes) and in youre avg car that results in a 1000lb/in rate. This means that the spring, or in our case bag, will compress 1" under 1000lbs of weight. You effectively have an infinite amount of settings between 0-400psi of bag pressure depending on your desired ride height for that day. Get it?
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Old 06-16-2014, 06:31 PM   #8
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"infinite amount of settings" is not the same as "infinite spring rate". Infinite spring rate would mean that the bag could handle 5x10^66 lb/inch spring.

I am suggesting your statement "but in the real world that is extremely difficult because of the "infinite rate" nature." should have read "but in the real world that is extremely difficult because of the "rising rate" nature".
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Old 06-16-2014, 06:35 PM   #9
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Old 06-16-2014, 07:21 PM   #10
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I think in this case the fact that there is a much larger range of rates for airlift suspension than a conventional coil setup means that critically damping is multitudes harder than with only a static spring rate.

Albeit that could be changed by something like the Bilstein DampTronic dampers idea, but then it would be a pretty penny.
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Old 06-16-2014, 08:00 PM   #11
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If you are going to show weights, you should include the weights of the rest of the system.

Air springs are progressive in spring rate. How progressive is based largely on volume and pressure of the airbag, but also airbag material and design. Instead of basically saying "it's complicated" though, how about you actually put up some relevant data of spring rate vs. travel at various base pressures. It will give people a much better idea of how this all works.

Air springs are HIGHLY developed in other markets (motocross bikes and mountain bikes for example). They have done a lot of stuff to try and reduce the progressive nature of airsprings, but ultimately, an air spring is going to be progressive in spring rate.
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Old 06-16-2014, 08:25 PM   #12
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If this is anything like the brake thread, this should be good.
We're going to play nicely, can you?
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Old 06-16-2014, 08:27 PM   #13
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If you are going to show weights, you should include the weights of the rest of the system.

Air springs are progressive in spring rate. How progressive is based largely on volume and pressure of the airbag, but also airbag material and design. Instead of basically saying "it's complicated" though, how about you actually put up some relevant data of spring rate vs. travel at various base pressures. It will give people a much better idea of how this all works.

Air springs are HIGHLY developed in other markets (motocross bikes and mountain bikes for example). They have done a lot of stuff to try and reduce the progressive nature of airsprings, but ultimately, an air spring is going to be progressive in spring rate.
We're working on some statistical data to show the correlation in height / psi to the spring rate. We're just scratching the surface here as this "scene" is very new to air suspension. We don't want to throw too much knowledge down in the first post.
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Old 06-16-2014, 08:41 PM   #14
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A bit off topic, but our RS7 has the standard air suspension, that shit is nuts. I've driven cars with air suspension both OEM and aftermarket and I've never been impressed until we drove that car. Defiently made me a believer that you can have a good handling car with air suspension.

Besides that, the technicality of air suspension is a subject i have yet to wander into.
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