Building a couple of rear bumpers that will hold air, made out of 3x6 tubing with 3/16 wall, 1/4" thick ends blah blah blah... anyway it will hold air at 125psi and I was wondering if anyone knows how to figure the capacity of it?

Assuming you would like to know in gallons: 1 gallon = 231 cubic inches.

You did not incluse the length of the bumper so you will need to figure it out the capacity for yourself. :D

Multiply your inner width, height, and length. Divide this number by 231 and the resulting number will be the number of gallons.

If I remember right you take one gallon milk juggs stand the bumper on in and begin filling.

Just kidding the fomula is radius squared x 3.14 x the length will give you your cubic inches.

Originally posted by pontiota
If I remember right you take one gallon milk juggs stand the bumper on in and begin filling.

Just kidding the fomula is radius squared x 3.14 x the length will give you your cubic inches.

Is that not the way to calculate the volume of a cylinder?

How do you get from there to the volume of a rectangular box? ;)

AIr volume has something to do with the pressure. Yes you can figure the liquid volume by figuring the space but it's not going to give you air volume. As an example. I have a 30 gallon IR 5 hp compressor. That tank is not much bigger that two 5 gallon pails. No way in hell 30 gallons of watter would fit in it. Might get 12 in it at most.
My sliders that are made out of 4x4 3/16 wall and are 66 inches long hold right at 4 galons of water. I have as yet to come across how to figure the compressed air volume.

lenght * diameter * pi= inches cubed

Dawn Geek Shield.... :eek:

Air is a compressible gas compressible - so knowing the volume is somewhat useless. Just for kicks....

V = L * W * H = 2.5*5.5*L for your bumper with 1/4" wall tubing.

So Volume = 13.75*L cubic Inches = 7.44*L gallons

I'm assuming that you really want to know how well this will do at inflating a tire. Looking at a Flintstone tire (true cylinder) I'll guess some dimensions of 15" ID, 33" OD and 8" wide.

The volume for this 'tire' is pi/4*(OD^2-ID^2)*w=5429 in^3

Using a bunch of assumptions about ideal gas, adiabatic process, etc. (and assuming you don't loose any air in connecting & disconnecting) the tire pressure you could get is

You must use absolute pressures rather than gage, Pabs = Pgage+14.7

p1*v1=p2*v2 Thus (125+14.7)*13.75*L=(P2+14.7)*(5429), most jeep bumpers are 54 - 60" long...using 57"

P2 = 5.5 PSI = In other words, don't humor yourself that you can do much good with the air stored in the bumper alone. It would help ease the burden on an air compressor slightly.

Put down Geek shield :smokin:

Originally posted by ross-sparky-h
lenght * diameter * pi= inches cubed

How do you get three inch factors outta that.....?

You mean pi * radius squared.


Figure your tank volume and tire volume.

When you connect a hose between the two and they equalize (you stop getting
anymore air in the tire) you end up with same pressure in both.

Equation:

ptank * vtank + ptire * vtire = pfinal * total volume

Say the tank is 5 gallons at 100 psi.

the tire is 10 gallons at 5 psi.

We have:

5 gallons at 100 psi = 500 some units

10 gallons at 5 psi = 50 some units

Total 550 some units.

Now divide the 550 by the total volume of 15 gallons (tank plus tire) and we see
we equalize at about 37 psi.

You can easily figure what a static tank can do for you. If you want to air 4 tires....
multiply tire volume by 4.

Geeky......

Jay

You will get about .85 gallons per running foot for a bumper of that dimension, or 4.2 gallons for a 5 foot bumper.

I love (not) some of the formulas here. I hope some of you don't build bridges, buildings or gawd help us rockets :flipoff2:

Whaaaaaatttt...me spew formulas :confused:

Ok....and you quote gallons per foot :D :D :D

Jay

Thanks for all the replies..all the math is giving me a flashback to highschool math classes that I didn't do so well in:rolleyes:
Here's a pic of the bumper with rough outside measurements, it's all 3x6 3/16" wall tubing. The overall length is 65.5", the center section is 44" wide before it tapers up to the ends which are 3.5" tall.
http://tellico.off-road.com/wwwthreads_uploads/792489-rear%20hitchm.jpg

Now if I try and figure this out on my own I'm liable to go into a fit of seizures and hurt myself. Just would like a close estimate at 125psi, the amount of air it would hold. Thanks again.

nope not gona do it

was tempted to enter a mathmatical guess but after I figured out the
volume I couldnt remember how much
air weighs at 125 lb of preshure.


:beer: :beer:

I can tell you it probably won't be sufficient to air up one tire.
Jay

Was thinking of using it just as a supplement to my compressor, don't have much room left for another tank is why.

Should work well for that application....
Jay

I think its a tad over 4 gallons +/- a quart or two.

If hadn't trimmed the corners it would have been about 4.6, and the two corners are about .5 gallon (for both).

Did you paint the inside? if not its gonna rust you might try to get some good liquid rust inhibiting paint on the inside and

Do you have a drain planned?

I have a 4x3x1/4 bumper that is about 58" long and it works very well with my sanden rotary a/c compressor. I would not use it with a 12v compressor (like Quickair). They don't put out enough cfm's to fill the tank very fast and it'll take a lot longer to air up (versus no tank).

I cleaned it out best I could before welding the end caps on, then completed the bumper with the rest of the fittings. There is a ball valve on the bottom back for a drain and the other fittings are located on the back too. I put in a handful of small nuts and shook the bumper around as much as I could (it's not exactly light) to clean it up a bit, then emptied it out. I then used a product used for lining gas tanks, very easy to use and makes a nice rustproof coating inside.

I get 3.8 gallons inside your bumper using external measurements and subtracting material thicknesses.
.............I must be bored................
Not sure of a formula for compressing air....but.........if you take it 280ft below sea level, then fill it up with air, it would have 125psi. when you get it back to sea level.:smokin: