reverse air ride

[dannyho]

It seems that inherent in the design of air ride suspensions, they are only made to give their designed ride when inflated, and used to lower the car when at a show or cruising. In order to get a stiff suspension at all it has to be inflated and therefore not at its minimum height. Is there anything out there that does the opposite, meaning lets the car sit at its lowest point, with great handling, yet let you raise the car to get over speed bumps and such, to me it seems like this would be a much more useful setup to a pro-touring crowd than the setups that were made for scraping the pavement with your frame rails. off the top of my head I can think of a few things to take care of this, but they all seem overly complicated and pretty much prone to failure due to that.
any ideas? I dont think using a spring and an air shock is a good solution, too much redundancy, but then again for my car I saw it was recommended to retain the torsion bars, so i guess you never know.

[TonyL]

It's all in how you look at it. If you want a "low" car with stiff air suspension, you *can* have that. you just target your desired height when you install it. want over a speed bump? Over inflate it. want it to lay frame, air it out.

This is the way the systems are being installed these days. Now, also factor in that you can go from a soft to firm ride without changing ride height. There's a sweet spot to work with where a few psi either way will change the hight little, but the feel a lot.

But to answer your question. Yes there is a system that gets firmer as it gets lower. It was developed for the Subaru wrx.

It's made by praxis systems (http://www.praxissystems.com/) No muscle car applications yet. (and their probably wont be.) I'd sort it out with the guys at Air Ride Technologies though.

[dannyho]

I've looked into they system, pretty sick setup, but It just makes me curious how they're able to increase spring rate and lower the car with that setup, they have cutaways of their struts, and I still can't figure it out. I emailed the company, but I have odds on them not answering my question. Do you or anyone know how this works?

[TonyL]

I've also hurt my brain trying to figure this one out. the nearest I could figure is the conical shape of the shock. as the bellows is lowered over it, it gets "tighter" increasing spring rate.

But thats only a guess.

[bret]

It seems that inherent in the design of air ride suspensions, they are only made to give their designed ride when inflated, and used to lower the car when at a show or cruising. In order to get a stiff suspension at all it has to be inflated and therefore not at its minimum height. Is there anything out there that does the opposite, meaning lets the car sit at its lowest point, with great handling, yet let you raise the car to get over speed bumps and such, to me it seems like this would be a much more useful setup to a pro-touring crowd than the setups that were made for scraping the pavement with your frame rails. off the top of my head I can think of a few things to take care of this, but they all seem overly complicated and pretty much prone to failure due to that.
any ideas? I dont think using a spring and an air shock is a good solution, too much redundancy, but then again for my car I saw it was recommended to retain the torsion bars, so i guess you never know.

The real problem here is that to get any ride quality or handling performance you must have some suspension travel. If you drive your car at its lowest possible height then, by definition, you will have no suspension travel because you will already be at a fully compressed position.
I have found that in the real world, you need at least 4.5" of ground clearance at highway ride height and 2.5" of compression travel [3" with a heavy car] at that ride height. This will allow you a civilized ride quality and enable you to clear most standard speed bumps without damage. After these 2 criteria are met, handling performance can be optimized with spring rate, shock valving, and sway bar tuning.
I consider the definition of ride quality to be the vehicles ability to isolate the driver from normal road undulations. This same suspension compliance allows the tire to maintain maximum contact with the road through various road irregularities, thereby increasing maxumum traction and handling performance.

[dannyho]

my problem with this is how this system claims to offer the best of both worlds (dont they all). but more specifically it claims it can raise ride height and offer a comfortable ride i.e. softer springs, and then lower ride height and produce a stiffer spring setting. this seems completely contradictory to how an air spring works, and I'd like anyone who can explain it to do just that. as a studying engineer I hate to be puzzled, espcially by something like this.

[Thump]

Speculation:

Imagine a vertical cylinder with the bottom closed and the top open to the atmosphere.
Drop in a heavy piston. The piston will drop only so far as the air beneath it compresses.
The PSI of the air in the cylinder now equals the piston weight divided by its cross sectional area.

If we instantaneously accelerate the cylinder vertically at 1g,
the piston will drop to 1/2 the remaining distance to the bottom of the cylinder
and the air pressure will be doubled.
If the cylinder comes to an abrupt stop, the piston will bounce for a while and then stabilize.

If we have a valve at the bottom of the cylinder we can add more air.

If we add enough air to raise the piston to double its distance from the bottom of the cylinder,
we know there are now twice as many air molecules inside.

The air PRESSURE has not changed, the VOLUME has doubled.

If we instantaneously accelerate the cylinder vertically at 1g,
the piston will again drop to 1/2 the remaining distance to the bottom of the cylinder,
which is twice the distance it traveled the first time.

The 1g bounce rate has been halved.

Paul

[dannyho]

I see what you're saying, but this place claims to increase the spring rate when its lowered, not have it remain the same. they also have a few cutaways of their air spring and it doesnt show anything like that. still puzzles me

[Thump]

More speculation:

(Taken from i-club):

"...in contrast to other air spring systems, THE AIR BLADDER ROLLS OVER THE CYLINDER unstead of being compressed between two flat surfaces or perches. Picture it as compressing a balloon between your two flat hands compared to pushing your fist into the balloon. The larger your fist, the more the balloon will resist the compression. If you forced a cone shaped object into the balloon, the resistance to compression would increase in a non-linear fashion..."

http://www.i-club.com/forums/printthread.php?t=22806

Imagine a 2" diameter piston with a height of 1".
Atop this piston's center there is a 1" diameter piston also 1" in height.
Substitute your fist with this stepped piston.
Push the piston assembly into the balloon until the 1" piston is buried
into the balloon 1/2" and record the force used to accomplish the task.
By increasing and decreasing the force on the piston assembly
you can alter the depth it penetrates the balloon.
Now, maintaining the recorded force, deflate the balloon until the 2" step
of the piston assembly penetrates the balloon.
Now there is 4 times the surface area to push the air.
The travel of the 2" piston needs only to go 1/4 of the distance
of the 1" piston to achieve the same resistance force.