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Old 08-31-2013, 04:34 PM   #1
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Axle weight based on air bag psi

I have seen these widgets that will convert air bag psi to axle weight. I am thinking these things require a couple of data points be entered and the widget does a best fit curve to display axle weight. Any idea how much axle weight I have with 38 lbs in the rear bags?
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Rich
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Old 08-31-2013, 04:48 PM   #2
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That would be like guessing how much weight is on a tire judged by no more information that the tire psi?
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Old 08-31-2013, 04:58 PM   #3
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P.S.I is pounds per square inch. If you know exactly how many square inches of area are available for the air to push on, then you can compute how much weight is being lifted per air bag.

Find the maximum circumference of the air bag and compute the area of the circle in square inches. Multiply that number times the P.S.I required to inflate the air bag and you will know the weight on that bag.

It's possible the manufacturer of your air bag might produce a graph you could use to keep track of your unit weight.

I may not have explained this well but the theory is close.
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Old 08-31-2013, 10:45 PM   #4
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This is one of the widgets I was referring to.
Digital axle load indicator - YouTube

For a simple spring, F= Kx, X is constant since the leveling valve is adjusting pressure. The units on K are force per distance. Distance is cancelled by x, leaving just force .... indicating axle weight can be determined from bag pressure.

The simplest approximation would be knowing the axle weights and associated bag pressures for two conditions of loading, say empty and with a load. With that data, a first order linear equation for Axle Weight = A(pressure) + B can be derived. With more data, an Nth order polynomial can be derived for a more accurate axle weight indication.

In the absence of axle weight data, how many PSI are you seeing loaded....have to indicate singled or not.

thanks
Rich
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Old 09-01-2013, 04:08 PM   #5
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Rich, airbags are remarkably linear. Most are rated at 100 psi max pressure, meaning that at that pressure they will be carrying the maximum design load. So if you know what that max design load was at 38 psi they will be carrying 38% of that load. Most are manufactured by Goodyear or Firestone and these companies do publish the specs for all their models, an example:



Goodyear has particularly "rich" technical library available for design engineers, hence the reason I use these in my hitches.
I see this "gadget" as pretty simple product, read the pressures through transducers and write a simple software routine for a microprocessor to calculate weight vs. the pressure and drive an LCD display.
If you look at the spec you will also notice that at the max pressure the air bag will be at the maximum height, your suspension will be limited in that regard by the leveling valve. Typically the leveling valve will keep the suspension at something less than the maximum height and pressure. If you look at the chart above you will see the correlation between the air pressure, height and the weight at that pressure and height.

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Old 09-01-2013, 04:20 PM   #6
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Here are the search results for "digital axle load guage"

digital axle load gauge - Bing

Interesting.
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Old 09-02-2013, 09:57 PM   #7
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Hi Henry - good info from the OEMs on the bag designs. I see what you mean, around the center of the design height, displacement is linear for a delta weight ...... I assume this is for a constant pressure, although they don't say.

So if I am at about 38% of the design max for a 40K rated rear on the truck, that puts me at about 15.2K on the rear and from what I have seen of others loaded with the hitch as far back as mine, about 10.5K on the front. Does that seem reasonable for the truck, 2K of fuel, pin weight, with a 3K deck, 3K Mini Cooper, and 1K for the generator and platform? I will run it across a Cat Scale to see how close this approximation is when an opportunity presents itself.
Hope the excitement of your move is settling down. The best thing is you will not have to do another winter in the NE and FL has no state income tax.

Thanks again
Rich
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Old 09-03-2013, 08:31 AM   #8
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Yes, for constant pressure.
Your numbers seem to be in the ballpark, although I don't remember if you were singled or tandem. On tandem you will have four airbags so each will carry 1/4 of the weight, but they will be linear. The hitch "back there" will typically offload the fronts by about 1,000 pounds to around 10.5K, it does on mine. The heaviest I've been was around 24K on the truck, that was pulling (6.5K pin weight) and carrying two ET's in the drom to the Rally.
The cheapest way to find out is to go on the CAT scales in various load configurations ($9 bucks a pop) I don't miss those "opportunities". There's got to be a truck stop that has one around where you live.
It's hot in August where I moved to, no surprise there but the pool and air conditioning helps a lot. I am looking forward to the "other 10 months of the year" that I will truly enjoy. Building ETs here already, in the open under canopy, life is good. Also noticed that they were expecting a night temperature of 42 degrees in the place I just left, they can have it!!!!!

hjs
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Old 09-03-2013, 08:44 AM   #9
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Yes, for constant pressure the height will change with the load, for constant height the pressure will change with the load (with the help of the leveling valve).
Your numbers seem to be in the ballpark, although I don't remember if you were singled or tandem. On tandem you will have four airbags so each will carry 1/4 of the weight, but they will be linear. The hitch "back there" will typically offload the fronts by about 1,000 pounds to around 10.5K, it does on mine. The heaviest I've been was around 24K on the truck, that was pulling (6.5K pin weight) and carrying two ET's in the drom to the Rally.
The cheapest way to find out is to go on the CAT scales in various load configurations ($9 bucks a pop) I don't miss those "opportunities". There's got to be a truck stop that has one around where you live.
It's hot in August where I moved to, no surprise there but the pool and air conditioning helps a lot. I am looking forward to the "other 10 months of the year" that I will truly enjoy. Building ETs here already, in the open under canopy, life is good. Also noticed that they were expecting a night temperature of 42 degrees in the place I just left, they can have it!!!!!

hjs
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