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Old 08-05-2021, 07:07 AM   #1
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Does the pressure in airbags change with load?

I posted a while back about setting up my Anderson weight distribution hitch after airbag install. Using their chart, my truck needed 17lbs of air in the bags to support my load.

So I aired the bags to exactly 17psi (which of course raised the rear of the truck), measured the new hitch height, and re-set up my weight distribution hitch to be correctly set for the new height. I did this 1 week ago.

Now I'm hitched up preparing for an out of state trip, and checking all my air pressures in tires. I check the airbags and it is at 23psi...

So does the additional weight of the camper cause the bags pressure to read higher?
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Old 08-05-2021, 08:05 AM   #2
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Yes. The more weight you apply to it the higher the pressure will be. Be more concerned with ride height. Just air up to the ride height you want with the load applied.
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Old 08-05-2021, 10:03 AM   #3
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Absolutely. Loading airbags causes compression and changes the volume which increases the pressure (I.e putting same amount of air in a smaller balloon.)

Most often, the stated PSI ratings/specs for air bags, air ride pin boxes and hitches are to be measured under load.

As said above, target the correct ride height as the goal. However, it’s also important to stay within the min-max range for the bag to prevent damage to the bag or the equipment.

Example. I recently installed an air ride pin box and in experimenting with pressure and ride height to get the smoothest ride (on some terrible interstates) I noticed through paint chips and surface marks that at some point my psi was low enough that the hinge point on the pin box jaws had “bottomed out” a few times. (Think of an alligator with an air bag in his mouth…..pressure was low enough that his jaw closed so far it chipped his rear molars a little.). So at that point there is potential for damage.
Similarly, too much air can cause the bag to fail and create even more damage.

So my upcoming weekend project is now to raise the new pin box (again) to allow adequate air pressure and still maintain a level trailer, smooth ride and full benefit of the air ride.

Hope this helpful.
Safe travels,
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Old 08-05-2021, 04:10 PM   #4
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Thanks for the responses guys, that helps me wrap my head around the increase. I really thought I measured it incorrectly.
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Old 08-09-2021, 05:03 PM   #5
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Boyle's law - an experimental gas law that describes how the pressure of a gas tends to decrease as the volume of the container increases. Conversely, the pressure of a gas tends to increase as the volume of the container decreases. A modern statement of Boyle's law is: Mathematically, Boyle's law can be stated as: or where P is the pressure of the gas, V is the volume of the gas, and k is a constant. The equation states that the product of pressure and volume is a constant for a given mass of confined gas and this holds as long as the temperature is constant.
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Old 08-13-2021, 07:35 PM   #6
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Quote:
Originally Posted by Gsykora View Post
Boyle's law - an experimental gas law that describes how the pressure of a gas tends to decrease as the volume of the container increases. Conversely, the pressure of a gas tends to increase as the volume of the container decreases. A modern statement of Boyle's law is: Mathematically, Boyle's law can be stated as: or where P is the pressure of the gas, V is the volume of the gas, and k is a constant. The equation states that the product of pressure and volume is a constant for a given mass of confined gas and this holds as long as the temperature is constant.


Simplified: P1*V1 = P2*V2
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Old 08-13-2021, 07:47 PM   #7
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Need one more CRITICAL bit of information:


Temperature of the air bags when you set them at 17 PSI and when you later measured 23 PSI.


Ideal gas law from high school physics. PV=nRT
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Old 08-14-2021, 06:00 PM   #8
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I ended up having time to unhitch and take all measurements again before leaving for vacation. The truck/trailer combo towed better than it ever has, granted I've only had this camper for a year, so I've only towed a few thousand miles.

With the truck sitting, I inflated the bags to 17lbs of air. When the trailer is attached that goes up to 27lbs. The reason I wanted both readings was in case I needed to check it while the trailer was attached, or for some reason I forgot to set it until I was hitched up (I run around 5psi when unloaded for ride comfort). Granted the temperature of the airbags and the air inside will vary some, but I'd assume that to be less of a variable than tires hot and cold due to their being in contact with the road and the friction.
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Old 08-15-2021, 08:59 PM   #9
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Sounds like you’re getting it dialed in nicely. One thought/suggestion to think about is a small on board air compressor. This will allow you to conveniently adjust the air in the bags any time any where. Makes it easy to re-inflate after deflating to the 5lbs when not towing or tweak the PSI as needed if/when your trailer may be loaded a little differently.

I installed an Air Lift brand compressor with under dash gauge/switch and could inflate or deflate my bags to desired psi from the driver’s seat. About $250 as I recall, but very handy.

Happy Trails.
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