Spring rate conversion? Bruce??

[HYBRIDT]

I'm interested in what the Lb/in rate on springs is related to? If I wanted to switch to an air suspension system how could I calculate the difference in psi in the bag to the spring rate and compressed height of say a Moog spring. Going off of this link I can see that I'm looking for a compressed height of 11" and a spring rate of 420lb/in (1530 lbs spring force at compressed height). Most air bag systems run in the 145psi max range (usually 120psi). Could I then take the internal area (volume) of the bag and determine what psi I would need to get the same "spring rate" so not to have too soft or too firm a ride at "ride height" (11")?

Adam

 

[JP87GN]

You lost me kid! ,This is a question for Bruce!

 

[Greasemonkey]

Using the volume of the bag you would work with cubic inches, which doesn't really work with pounds/square inch. I'd say you really need the area of the top of the bag, but I'm not sure if the whole top of the bag is in contact with the frame or not. PSI * square inches = force, so you should be able to work from that. However, that doesn't really get you to a rate. Have you tried contacting one of the companies that sells the air bags?

 

[bruce]

I'm interested in what the Lb/in rate on springs is related to? If I wanted to switch to an air suspension system how could I calculate the difference in psi in the bag to the spring rate and compressed height of say a Moog spring. Going off of this link I can see that I'm looking for a compressed height of 11" and a spring rate of 420lb/in (1530 lbs spring force at compressed height). Most air bag systems run in the 145psi max range (usually 120psi). Could I then take the internal area (volume) of the bag and determine what psi I would need to get the same "spring rate" so not to have too soft or too firm a ride at "ride height" (11")?

Adam

This Bruce, has no idea of where to even begin.

 

[jgme]

I guess, some old chemistry can be dug up here. You're trying to shoot for an equivalent stiffness which is ΔF/Δx. Use x for the height of the spring. Start with the ideal gas law, PV=mRT. Pressure is force over area. The volume, assuming a cylinder, is cross sectional area times the height. Some substitutions, algebraic shuffling, and one differentiation results in

ΔF/Δx = PA/x

where
A is the cross sectional area
x is the height
P is pressure

Just make sure the units match. Also, it's assumed that the air bag wall has negligible stiffness.
(hopefully, I didn't miss a variable in the shuffling)

 

[HYBRIDT]

Word.


Adam

 

[Racer X]

<thinking out loud>

So let's assume you have the air bag filled to, say, 75% of it's full capacity...Once you load the rear end and the air bags get to the point where they have no more *give* to them, your 'spring rate' would then be infinity? Kinda the same as coil bind.

I would say that as you compress the air bag you would have something that behaves more like a variable spring rate rather than a linear rate.