Air bags 101
There are 2 different styles of airbags that are most commonly used:
Double convoluted bag
The double-convoluted bag has a high spring rate and has a fast rising rate (as the bag colapses, the spring rate gets stiffer). They are typically used in higher load applications like front suspensions, or mounted on a link bar under leverage to get more travel and still have an acceptable ride. If a double convoluted bag is mounted in a 1:1 position, for instance directly over the axle, your ride quality will suffer.
Sleeve style bag
Because of the way the sleeve style bag works, it has a lower spring rate and does not have a very fast rising rate. They are most commonly used directly over the axle, but do not have as high of a weight carrying capacity. The sleeve bags that you do see used in high load situations are quite large. The odd thing that I have found from using sleeve bags is that they work much better if the vehicle is heavier. For instance a minitruck would not ride as nice on sleeve bags as it would using a double convoluted bag mounted on the link bar. I have my theories, but they will have to be saved for another post.
Before I go any further, let me explain how spring rate works. A spring with a 100lbs rate takes 100lbs to compress the spring one inch, then another 100 lbs to collapse the spring another inch and so on… So a 500lbs rock would collapse a spring with a 100lbs rate 5 inches, but that same 500lbs would collapse a 250lbs spring only 2 inces.
Killing brilliant ideas
Okay, there is a misconception about mounting oversized double convoluted bags directly over the axle in order to cheat and get a nice ride. Well, there really isn’t any reason why you couldn’t run a large bag on your truck, but a nice ride is not always what you will get. The theory has merit, but the problems are worse than they appear. A giant bag will take very little pressure to lift your vehicle off the ground and the first inch or so will feel acceptable, but only in a small operation range. You can’t raise the suspension any higher and expect a tolerable ride, in fact the higher you raise the suspension the worse it will ride. This isn’t too big of an issue for around town where bumps are hit at slow enough speeds, but at freeway speeds the problem becomes quite noticeable. Until you go over a bump on the freeway at 60mph in a poorly set up vehicle, you won’t understand.