Gallons of air in a tire?

Airborne Lee · Mar 16, 2006

I kinda asked this in a previous thread but I don't have any answer yet so I am re-asking.

First, I have a great work compressor. Can't justify getting a second on the WJ.

Therefore, I am looking to take a storage tank with me just to refill tires.

How do I calculate how many gallons I need to do this? I know you can have the proper PSI in a tank but if the tank is too small you can't fill all the tires.

Anyone know how a site to help calculate the gallons of air used at a particular psi in your tires????

ArloGuthroJeep · Mar 16, 2006

A 15 gallon tank filled to ~100 PSI fills 30+ tires I believe...

Edit: I may have misread your post...the above is for a CO2 tank...unless you cool work compressor can make liquid oxygen:drool:

PlatinumZJ · Mar 16, 2006

A VERY rough calculation indicates that a 31x10.5R15 tire is about 9 gallons internal volume. Using the ideal gas formula PV=NRT and assuming NRT of the reservoir tank = NRT of the tire, P1 (pressure of tank) x V1 (volume of the tank) = P2 (tire pressure) x V2 tire volume. Then V1 = (V2 x P2)/P1 or V1 = (9gal x 35psi)/100psi = 3.15 gal per tire @100 psi so for 4 tires 12.6 gal and adding a fudge factor of 25% say 16 gal. this assumes that the pressure in the reservoir stays at 100 psi throughout, which will not be the case as you remoove gas from the tank. It also assumes that there ambient pressure in the tire (0 psi).

ArloGuthroJeep · Mar 16, 2006

Doug did you subtract the volume of the wheel from your calculations?

EMTimZJ · Mar 16, 2006

Airborne, it will be very difficult to carry a fixed amount of compressed AIR to fill your tires after they have been aired down. People use CO2 because it is stored as a liquid, liquid is more dense than gas, this is why you can carry so much more CO2 than AIR.

Most people with OBA are either CO2 setups are have compressors that have a great duty-cycle rating, like a York 210 for example; they are able to have a continuous supply of compressed air.

HTH

Also Doug, I have the same question as Ryan, does that figure JUST include the tire?

Airborne Lee · Mar 16, 2006

Everything you say about CO2 and OBA are 100% right, I totally agree. the problem is $ I can get a 125PSI 10 gallon tank wiith regulator, hose and chuck for $40 bucks. More importantly I can refill it for Zero dollars.
http://aircompressorsdirect.com/catalog/product_info.php?cPath=5_54_111&products_id=283

there is a formula on this web page that indicates to go from 15psi to 35psi I need 1.64 gallons. and from 0 to 35 I need 2.87 gallons.

http://www.aircompeq.com/formulas.html

10achik · Mar 16, 2006

If all your looking for is to fill tires, why not just get one of those little compressors, I have a CO2 setup, but I also have a little HV-40 that airs a 32" tire up in under 3 minutes, I think I paid around $40 for it on ebay. Takes up hardly any space (about the size of a couple bricks), fits nicely in the tire well, a 10 gallon tank will take up quite a bit of space and then you have to secure it down, not to mention the added weight. Just a thought.

Here's a link http://cgi.ebay.com/ebaymotors/Supe...ressor_W0QQcmdZViewItemQQcategoryZ42614QQitemZ8026478292QQrdZ1QQsspagenameZWDVW

PlatinumZJ · Mar 16, 2006

ArloGuthroJeep said:
Doug did you subtract the volume of the wheel from your calculations?

yes, a donut shape with a cross section of about 64 square inches (like i said its very rough). the volume of a torus (Donut) is 2 x r x r x PI x PI x R, where r is the radius of the tube (if the tire would have one) and R is the radius to the center of the tube and PI = 3.1415. Hey, Ooops i made a mistake earlier, if r = 4" and R = 11.5" then the tire's tube hold 15 gal, not 9 gal so it 5.25 gal of 100psi or 4.2 gal @125psi per tire.

now my head hurts.
Thanks

rm2001wj · Mar 16, 2006

Airborne Lee said:
...
there is a formula on this web page that indicates to go from 15psi to 35psi I need 1.64 gallons. and from 0 to 35 I need 2.87 gallons.

http://www.aircompeq.com/formulas.html

Sorry to say, but the formula that you used at that URL gives the answer in cu-ft, not gal. Also, the conversion of cu-ft to gal on that page is written incorrectly. It should read cu.ft. = gal X 7.46 (or equivalently, gal = cu.ft. X .134). More important is that fact that the formula calculates the volume of "free air" which is air at atmospheric pressure (14.7 psi absolute pressure which is the same a 0.0 psi gauge pressure). What you need is the volume of air at the tank pressure.

Use the numbers that PlatinumZJ calculated; he has a pretty good handle on the theory.

CNY · Mar 16, 2006

Yeah, there's no way that "15psi to 35psi I need 1.64 gallons" estimate is right.

I have a 2 gallon tank that starts at 105 psi. By the time it is eqaulized, my 31" tires haven't gone from 15 to 18 psi.

I don't have the gallon answer for you, except to say you probably cant hold a tank that large in a Jeep.

Try this, get a 30 gal garbage bag, and air your tires down INTO it. See if you can fill it. Heck, I'm going to go do that, I'll be right back....

CNY · Mar 16, 2006

OK, here's the non-scientific results....

13 gallon trash bag, 31x10.5X15 tire at 29psi

Filled the bag to firm, tire now at 17psi.

The 13 gallon label on the bag must be if you fill it wide open. Pinched off at the top (like I had it), then set it next to a 5 gallon pail, it only looks like it's taking up the volume of 7 or 8 gallons.

now's the time for somone to step in and do some math. Let's say I took out 10 gallons, and got to 15 psi.....

But my 10 gallons are at ambient pressure, now (not eqaulized pressure, like you can do with a tank). But remember, a tank at 30psi is effectively empty (because you can't get anything else out of it).

So if you had a 10 gallon tank, it seems reasonable that you could fill the tank to 60psi, and get it to equalize with my tire at 30 psi. Fill it to 90psi, do two tires.

20 gallon tank at 100 psi should do 4 tires.

CNY · Mar 16, 2006

Here's cheap onboard air for the brave and stupid.............

fill your spare tire up to 100psi. Use a hose to connect to the valve stems between your tires and the spare.

You'll only use 15psi out of the spare per tire.

Airborne Lee · Mar 16, 2006

At $200 dollars per BFG AT/KO's Mounted, Balanced with road hazard. I can buy a compressor and not pop my spare.

CYN, Now - squeze the bag and put the air back into the tire!!!!:wave:

OK, OK I give, I'll buy a compressor (cheep one anyway). :thumbsup:

Reswob · Mar 16, 2006

CNY said:
Here's cheap onboard air for the brave and stupid.............

fill your spare tire up to 100psi. Use a hose to connect to the valve stems between your tires and the spare.

You'll only use 15psi out of the spare per tire.

If the tires are only supposed to take around 38psi max how do you propose to fill one to 100 w/o blowing it up?:rofl:

CNY · Mar 17, 2006

CNY said:
Here's cheap onboard air for the brave and stupid.............

Reswob said:
If the tires are only supposed to take around 38psi max how do you propose to fill one to 100 w/o blowing it up?:rofl:

I'm saying it's a BAD idea. The 38psi is max under load on the road. The bursting strength will be MUCH higher, but I wouldn't try it.

michiganmaveric · Mar 17, 2006

I never should have read this thread as I now have one hell of a headache, although I did get a good laugh out of it also.

PlatinumZJ · Mar 17, 2006

I just got a headache. I still don't think I figured it right.

greasefingers · Mar 17, 2006

I have spent an hour so far doing pv=nRT calculations. Still not sure I got it right:brickwall

I do like this kind of stuff.

PlatinumZJ · Mar 17, 2006

greasefingers said:
I have spent an hour so far doing pv=nRT calculations. Still not sure I got it right:brickwall

I do like this kind of stuff.

Yea me too, until my brain hurts

Phil Gerard · Mar 17, 2006

So THIS is where the geeks hang out!!!:rofl: :rofl:
Hey, I went to college as a physics major on a full schlorship.....changed my major week 2!!!
Phil :wave:

Gallons of air in a tire?

Airborne Lee

ArloGuthroJeep

PlatinumZJ

ArloGuthroJeep

EMTimZJ

Airborne Lee

10achik

PlatinumZJ

rm2001wj

CNY

CNY

CNY

Airborne Lee

Reswob

CNY

michiganmaveric

PlatinumZJ

greasefingers

PlatinumZJ

Phil Gerard

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