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Negative G Forces
I'm building a cage for my holding tanks, and I want them to be very secure.  They're living on the inside of my cargo trailer because I have enough room for them and I want them to be freeze-proof since I'm building a 4-season unit.

My question is this: What do you suppose the maximum negative g-forces are that the trailer will experience during its travel?

This will affect the design of the cages I build for the holding tanks with regards to how much they need to be "strapped down" from going up vertically.  I'm already planning on some very robust forward/backward/left/right supports to handle all of those directions.

Let's say it's a 10-gallon tank (just to keep the math easy).  If the maximum negative g-force is 1.0, then I'd have to build in some kind of support that could withstand 80 pounds of upwards pressure (assuming the tank weight is negligible and I'm rounding water weight to 8 pounds per gallon).  If however the negative g-force is only 0.1, then I only have to build in 8 pounds of force resistance.

The reason I'm concerned about this is because I don't want my plumbing lines or seals breaking if the tanks move too far in an undesirable direction.

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The natural G forces on your body sitting on earth is 1.0. Do you plan on driving your rig like a fighter pilot fly's a fighter pulling massive G's all the time? I really think your over thinking it, just secure them as you see fit and call it good. Think of a roll over accident. I'd be more worried about the non-secure stuff causing more damage then anything else. Good Luck
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So stating that you want to control 'upwards pressure' means you are talking about the negative G you would only experience hauling ass off road, over a rise, or doing a 'Thelma & Louise' off a cliff. I doubt that you would ever have more negative G than positive. I doubt you would ever see anything close to one G positive on street tires even in a full on panic stop.
Since you say you're keeping them inside the trailer, I'd mount some tie down rings (bolt thru floor and frame) and just strap em down with 1" or 2" ratchet straps. For a more permanent mount, use perforated metal hardware strap or some 1" steel or aluminum flat bar 1/16" thick...bend and drill as needed.

There are free apps to measure G force with your phone if you really need to get the numbers and do the math.
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hmmm.... interesting question. Here are a few thoughts.

If you hit a bump in the  road, then your tanks can be moved in directions upwards or sideways with several G's. To stop the upwards force , and to return to ground contact again, is only the earths natural G force. Of 1G

If your tanks were empty (or solid) that would be the end of that.

But as they are filled with liquids, then that liquid is accelerated with the force of the bump, and could freely move and hit the top, sides or front of the inside of the tank, with whatever speed the water was accelerated to, and would thus push the tank in an upwards (or sideways) direction. Even as the tank-top/walls themselves are only being stopped, and/or pushed downwards by 1G.

So the straps would mainly need to be able to hold the tanks in place whenever the water slushes against the top and walls. 

How that is expressed in G's, and how much of the weight of the water might do those G's, at the same second, that is a little beyond my wishes for math exercises right at this moment. 

But if the straps can manage 5 to 10 times the total weight of the water, and the tanks are not too long in the length/breaking direction (so the space for acceleration, over the time it takes the individual drop to move from one end to the other end of the tank, is limited), then I think you will be okay.
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I use one of these on my tank. They use them to mount the propane tanks on forklifts. They show them used with a horizontal tank but you can use them with a vertical tank. Heavy duty, probably stronger than whatever you bolt them to.
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