Bag over coils performance

[KyleAnderson]

I would like very much if we can devise a relatively straightforward solution to describe instantaneous spring rate on air bags in real-world application, rather than the purely theoretical approach I've outlined above. Obviously the dynamics of each individual suspension system will affect the real-world precision of any mathematical solution somewhat, but it seems like we ought to be able to come up with something here.

Same here... the air ride community could really use something like this. It would be helpful in cases like the OP where they're unsure if the performance of air can meet their needs. Even if the calculation is complicated (which I think it has to be), I could make a simple Excel worksheet. Do you see additional variables, errors, or refinements in the way I worked through it?

[Wiggins3377]

Bump for updates on this thread. I drive a 370z and debating a "bag over" setup with stance coils and universal bags. I'd love to figure out a happy medium of a great looking drop and maintain "some" performance.

[Will@BagRiders]

European Car recently published the results from their Tuner grand prix event.

AccuAir's B8 S4 with their Sport Kit (Universal bags over the factory dampers up front - separate bags and shocks in the rear) held it's own against all the other cars. The AccuAir car wasn't the fastest around the track, but it did really well. This says a lot for a properly setup air suspension kit.

The goal shouldn't be to maintain "some" performance. Why not maintain all of it? Of course if you're goal is to have the car aligned at a super low height (this was not AccuAir's goal for the Tuner Gran Prix) then you might have to sacrifice a little, but it is possible to essentially have the best of both worlds.

As for the comments above. I agree, it would be nice to know our 'spring rate' equivalents on the fly. I'm really not much help in that department.

[i2ice]

Great information in this thread. Just some clarification, what your saying Sam is that at a lower ride height the effective spring rate is higher? my example here is I have a 2002 lexus gs300. I am running UAS aero sport bags and bilstein shocks and accuair elevel for management.

At ride height I would say I have an addition 1.5-2 inches to full lift, this is not at the bag but I'm roughly estimating from the tire/fender. when I ride at cruise height, .5-1 inch lower, I feel that the suspension is softer? I bottom out more, or generally more movement in the suspension not only because of the lower ride height but less "spring" to counter act bumps. When I ride at my #3 setting, it feels stiffer? less movement in the suspension,"more spring"

As of lately I have noticed that my rear shocks are wearing out, getting loose,semi bouncy, squeaky noise. yet when I ride at a lower ride height there is less bounce after a bump in the road. In my head I'm imagining that as the shocks have gotten weaker the effective spring rate at ride height was "over sprung" causing bounciness. with less air in the bag, a lower "spring rate", the shocks are still able to damper the motion? any thoughts?

sorry didnt mean to thread jack op, didnt think it was worth making a new thread since its sort of on topic.

[Oh Damn, it's Sam]

Great information in this thread. Just some clarification, what your saying Sam is that at a lower ride height the effective spring rate is higher? my example here is I have a 2002 lexus gs300. I am running UAS aero sport bags and bilstein shocks and accuair elevel for management.

At ride height I would say I have an addition 1.5-2 inches to full lift, this is not at the bag but I'm roughly estimating from the tire/fender. when I ride at cruise height, .5-1 inch lower, I feel that the suspension is softer? I bottom out more, or generally more movement in the suspension not only because of the lower ride height but less "spring" to counter act bumps. When I ride at my #3 setting, it feels stiffer? less movement in the suspension,"more spring"

As of lately I have noticed that my rear shocks are wearing out, getting loose,semi bouncy, squeaky noise. yet when I ride at a lower ride height there is less bounce after a bump in the road. In my head I'm imagining that as the shocks have gotten weaker the effective spring rate at ride height was "over sprung" causing bounciness. with less air in the bag, a lower "spring rate", the shocks are still able to damper the motion? any thoughts?

sorry didnt mean to thread jack op, didnt think it was worth making a new thread since its sort of on topic.

Hey, sorry for the slow reply to your query - finals week and all that.

What I'm going to do at this point is just totally punt on your question, for one single reason: I don't think the effects of bag pressure are adequately attested to in my cheesy little model.

You see, theoretically anyway, bag pressure should be a constant no matter the system height if two conditions are met: 1. the system is at rest, as in the corner is not being compressed or in rebound. 2. the system is not at either its maximum or minimum height, where pressure does indeed become a factor in the system's performance.

The physics of this are relatively straightforward since it takes a constant force (whatever the corner weight of the car is) to keep the corner at any given height off the ground. Whether the car is sitting 1" off the ground or 2' off the ground, the pressure inside the bag should be the same, assuming the conditions outlined above.

Except I'm not 100% convinced this is the case in practice. The issue, as KyleAnderson above alluded to, is that the bag changes shape according to both pressure and compression, extending outward under both conditions, which both decreases the pressure inside the bag proportional to the corresponding increase in volume, but also increases the area of the bag that can provide upward force proportional to the increase in diameter of the bag. These are not factors that my simplified model takes into account at the moment, and I'm not convinced that these are as negligible of variables as I initially suspected.

What I would like to do at this point is to get my car back on air (coming soon, I swear to God Almighty) and take some precise measurements of pressure and bag dimension under a number of conditions similar to what Mr. Anderson has outlined above. From this, I should be able to perform an analysis that will isolate the pertinent variables. The result will still be an approximation obviously, since suspension geometry also comes into play here and I'll be damned if I'm going to create a mathematical model for all of that shit. Nevertheless, I think it will still be useful.

Stay tuned.

[KyleAnderson]

You see, theoretically anyway, bag pressure should be a constant no matter the system height if two conditions are met: 1. the system is at rest, as in the corner is not being compressed or in rebound. 2. the system is not at either its maximum or minimum height, where pressure does indeed become a factor in the system's performance.

The physics of this are relatively straightforward since it takes a constant force (whatever the corner weight of the car is) to keep the corner at any given height off the ground. Whether the car is sitting 1" off the ground or 2' off the ground, the pressure inside the bag should be the same, assuming the conditions outlined above.

Agreed- in a simplified model (but not theory), pressure should be the same at any height between min and max. The volume of air would only increase, but this also assumes that the available air space would increase the same amount to accommodate it. In our case, we're adding more air to a container whose volume remains the same before a load is added. When a load is added, the volume changes due to bag deflection.

The other very big, and incorrect assumption in a simplified model is that air is incompressible. Air will compress by a certain ratio. The more air you add, the more there is to compress. This is the major reason why pressure increases exponentially as ride height increases linearly. If you want to make it even more complicated, this ratio is affected by temperature, humidity, and content of the air, but to much lesser effects.

I find it easiest to think about this problem in two steps: before and after loading.

Before, we have a container of nearly constant volume (and shape). A gas takes the shape of its container, so adding air will only increase pressure, and therefore force exerted on the end cap.

After a force is applied, the pressure must increase by reducing the available volume until the force exerted equals the force applied. It's in this step that the effects of changing bag shape (diameter especially) and compressibility of air need to be factored in.

...but also increases the area of the bag that can provide upward force proportional to the increase in diameter of the bag.

This is the one point I disagree on. The force is exerted by end cap of the bag, which is unchanging.