Hmmmm, I put 30psi in the bag. Load my camper which has a 950lbs tongue weight. I check the pressure and it is exactly the same, within 0.5psi. Hammerz71, who also has airbags, noted that he too has no increase in pressure. I remember a thread on here a while back and guys arguing that the pressure must go up. But you ask the guys that actually have them, and check them, and they don't increase. I agreed with your point, but there was some guy that stated a bunch of technical data supporting the fact that they don't increase in pressure. I know, hard to imagine but they don't increase.

 

Not saying that I don't believe you. Just trying to grasp why it is that they don't increase.

 

I guess the change is so minimal that it isn't measurable.

 

If you air up your tires with your truck on the ground, and then check the pressure when it's up in the air...WOuldn't you expect the air pressure to be the same? It has to do with air pressure from outside as compared to the inside, not the amount of load overhead. It's all about Physics.

Loading weight on practically empty bags squats them(rubber expands) and can damage the internal pistons and bottom them out. It's best to slightly over inflate for what you need(stay within Man specs for the bags) and then reduce the air to bring your truck level.

 

OK, the engineer in me needs to chime in. The law of gasses is as follows;
Combined and ideal gas laws

Main article: Ideal gas law
The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume, and temperature for a fixed mass of gas:
With the addition of Avogadro's law, the combined gas law develops into the ideal gas law:
Where the constant, now named R, is the gas constant with a value of 8.314472(15) J·K−1·mol−1
An equivalent formulation of this law is:
where
k is the Boltzmann constant (1.381×10−23 J·K−1 in SI units)N is the number of molecules. These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature.
This law has the following important consequences:

1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

In a nutshell, if the bags are unconstrained as in Hammer's case the volume doesn't change and the only way the pressure goes up is with temperature. Just like tires, after a while when they get hot, the air molecules also get hot and expand thus higher tire pressure. In our case, since the bags are inside the coils, the volume may be constrained a little more than the ones on leaf springs so we should see a small amount of pressure increase. Probably very small as the bags start to squeeze through he coils.