How Do Airbags Work?

I've seen a couple posts of people recommending installing airbags on the TRD Pro and OffRoad to reduce squat. The question that I have is, how do airbags actually work. I understand that the bladder is filled with air, but how does it take the load off the truck suspension? Is the suspension basically bottomed out and the airbags lift the truck up? Do airbags increase the payload capacity?

 

You can adjust the air pressure in the bags. Increasing the payload makes the truck squat more. Increasing the air pressure inflates the bags more, bringing the truck back to level while maintaining the function of a suspension. Take all the weight out and leave the air pressure and the ride height will go up.

 

You springs are rated at some amount of force per inch of compression. Lets say 200 lbs/inch. So if your trucks rear axle has 2000 lbs of weight with nothing in the bed, your two 200 lbs/in(400 lbs/in total) springs will have to compress 5"(2000/400=5) to carry that load. Now say you throw another 1200 lbs of payload on the rear axle. The springs compress an additional 3"(1200/400=3). So now the trucks rear is sagging 3" lower than unloaded.

The air bags act like a second spring. It pushes up on the frame and down on the axle with some force determined by the pressure. As you increase the pressure, the air bag lifts the rear. This unloads the coil springs.

So say you have the 1200 lbs of payload compressing the truck an extra 3". You pump up the air bags and get the 3" back so you are at the same height as when its unloaded. In this situation, the coil springs are extended back to the unloaded state, and are pushing up with 2000 lbs of force and the air bags are adding another 1200 lbs to compensate for the payload.

You basically get an effective increase in spring rate. Now instead of it just being 200 lbs/in springs on each side of the truck, its 320 lbs/in.