Is an air bag a progressive rate spring?

[TundraBay]

I know it isn't exactly, but in effect, the more you compress air, the higher the pressure in the air bag, so in effect it is a progressive rate spring.

Or is my understanding incorrect?

[Corban_White]

Not really. To demonstrate, put a set amount of pressure in one of your tires with the truck jacked up in the air and then set the truck back down and check the pressure. it will likely be the same. Pushing on an air chamber will not affect the pressure unless the volume is decreased. Airbags are designed to maintain a fairly constant volume throughout the travel. Air shocks however are a progressive rate spring because the volume IS decreasing.

[tomhole]

I don't know. I do know that an inflated air bag acts like a spring with very low damping. As mentioned in the other thread, an air bag reacts like a large beach ball with a fat guy bouncing on it. It has no damping. An airbag may be desinged to maintain a constant volume throughout it's travel, but it obviously does not. If it did, it wouldn't bounce like a beach ball.

I am going to change my recommendation to folks that are towing heavy loads (hitch, pin or cargo). Airbags are good for leveling a truck, not good at controlling bounce. For heavy hitch weights, use the WDH to level the load. Add new shocks to control bounce. For heavy pin weights or cargo, add airbags to level the load, then add new shocks to control bounce.

I am going to take my own advice and install new shocks. The airbags will stay, but I won't need them anymore. Unless I get a 5er.

Tom

[SE2000]

If you look at a progressive rate spring, it is obvious. Instead of a smooth spiral coil, the coil has some close and some farther apart. you get the low weight spring working when the vehicle is empty and it is a softer ride. As you add weight, the heavier part of the coil comes into play and keeps you from bottoming out. On the tundra leafs, the bottom flat leaf does the same thing
On a shock, the only increased dampening is if the shock is designed to dampen at a higher rate if the piston is lower than normal.

[TundraBay]

All very interesting. The reason I ask is, if I put my trailer on my truck, the air bag pressure is 10 before putting the trailer on, and is 25 afterwards. It is also visibly compressed. Therefore my conclusion is that the increased pressure in the air bag must be pushing back more stiffly, which would imply progressively more resistance to compression, like you mention SE as the springs compress, more of the springs come into the picture.
And of course I'm aware that it's the shock absorber that does any damping or slowing down of the compressive and expansive action of springs, air bags, or whatever. In one of his posts, Dave of Jowett reports testing of the TRD shocks the Tundra comes with stock, and noted that there was a lot of damping on the return, which would explain why there's less bounce with a TRD equipped Tundra with airbags. As a wee tangent.

[Corban_White]

Originally Posted by tomhole

I don't know. I do know that an inflated air bag acts like a spring with very low damping....
Tom

No damping. As is the case with all springs (barring rust on leaf springs, coils hitting the links, etc. ) they supply no damping, hence the need for shocks. Also notice the bouncing utility trailers going down the road with no shocks

[SE2000]

you are confusing linear with progressive. air bags are like regular springs as they increase compressing on a linear basis. the progressive have an initial low spring rate and then switch to a higher rate

[Corban_White]

Originally Posted by SE2000

you are confusing linear with progressive. air bags are like regular springs as they increase compressing on a linear basis. the progressive have an initial low spring rate and then switch to a higher rate

Thank you.

[Tankerhank]

Boy I'm glad I used springs this airbag stuff was making me dizzy just reading about it.