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Math wizards: air tank size for filling tires?

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I need some help with figuring out how large of an air tank I would need to fill up tires with. I'd like to mount an air tank to my truck to use on the trail, so that I can air my beadlocked tires down to 10 psi and then air them back up before hitting the pavement again.

Tire size: 41.5x13.50R17
Deflated pressure: 10 psi or maybe lower
Street pressure: 30 psi

I'm having trouble figuring out how air volume and psi relate to eachother. The math equations I've seen online are perplexing to me.

I've thought of comparing my idea to what PowerTank says will fill X sized tires from X psi to X psi, but those tanks use CO2, not regular air. I will be filling up my onboard air tank via my garage air compressor, or at a gas station.

Would, say, a 10 gallon air tank be enough for me?

Thanks in advance
 
There is a very thorough post on here relating to tire pressure and volume, but it's actual math, and I don't like that. I'm not aware of an actual calculator that you just input numbers and it spits out the result, I looked.

Just throwing this out there, CO2 is probably a far better idea. ~10 fills (powertanks chart 42x15R15) off of one 20lb tank. Didn't cost me but $100 to get into a CO2 bottle and regulator from someones kegerator project.

Unfortunately the 10lb size is nearly impossible to find from what I saw.
 
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Boyle's Law: P1V1=P2V2. It's oversimplified, but will be close.

Basically saying that the (1)s are the pressure and volume of air in the tire and the (2)s are the pressure and volume of air in the tank.

So far you know:
P1=30psi (I would account for inflating from 0psi rather than 10 for margin)
V1=? (You're gonna have to figure out the volume of your tires - and then multiply by 4 since there are four of them)
P2=? (there's a couple of ways to look at this. See below)
V2=10 gal

So you have 2 variables. You need to get down to one before you can solve. I would try to find a way to get the volume of air in your tires. That would be the easier one because....

You can fill up a hot air balloon with a 10 gal tank if the pressure is high enough in the 10 gal tank. Personally, I would assume the highest pressure you can reasonably get in it from your compressor OR the maximum safety rating of the tank - Whichever is less. So let's say you can store 100 psi in your tank. (10gal x 100psi)=1000psi-gal. But we're not done. You have to still have 30 psi in your tank when you're done with you're last tire or the air will want to flow back in into the tank. So (30psi x 10gal)=300psi-gal. So you subtract that from your 1000 and now you're left with 700psi-gal to play with.

Using your desired 30 psi in your tires...(700/30) = 23.3 gal. Then you have to divide that by 4 (4 tires, remember?) so now you're down to 5.83gal.

So theoretically, with a 10 gal tank @ 100psi, you can fill up 4 tires from 0 to 30 psi if the volume of each tire is 5.83 gallons or less. So you basically have to determine your tire airspace volume and keep playing with air pressures/tank sizes until you're happy.

You probably regret asking the question now.
 
Assume the inside dimension of the tire is a hollow cylinder 40" diameter or 20" radius x 12" tall that's 15000 cubic inches. Subtract the wheel at 9" radius x 12" tall deletes 3000 cubic inches so roughly 12000 cubic inches in your tire.

Not sure if you would use .0043 gallons per inch for liquid or .0037 gallons for dry so split it for easy math and .004 x 12000 = 48 gallons.

This is high because the inside isn't a hollow cylinder but it's close. Probably 40 gallons.
 
That's all good info. Thanks everyone.

If what "Stein" and "dyeager535" are saying is correct, CO2 might make better sense for me. I certainly don't have room for a 40 gallon air tank. And as mindful as I am about air pressure, I probably should be filling my tires with CO2 anyway, since I live in an area where daily temperature fluctuations of 50 degrees isn't uncommon.

More food for thought.
 
I may be putting a big "moron" sticker on my face right now but I thought the "C" on the CO2 air tanks was Compressed not Carbon....
 
lol. Nah. Carbon dioxide. But if you have a source for pure oxygen, you need to get it. That stuff cures a hangover like nothing else.
 
Most home air compressors have a known volume tank and a pressure gauge, so just do some experimenting. For example if the compressor has a 30 gallon tank and can go to 150 psi, then turn off the pump and see how many times you can fill up a tire from 10 to 30 psi with that 30 gallon tank. If you can fill up a tire 6 times on a 30 gallon tank then that means it would take about a 5 gallon tank per tire.

I need to pump up the 39.5's this weekend and might play around a little and see what it takes. However I'm going to say it would take a pretty big tank to fill up a set of tires that big from 10 to 30 psi.
 
6.2Blazer said:
...If you can fill up a tire 6 times on a 30 gallon tank then that means it would take about a 5 gallon tank per tire...
Click to expand...

Good idea.

But also what "rebelgregory" said also makes sense, in that I'll need at least 30 psi left in the tank before I finish filling up the last tire. Makes sense that the pressure in the air tank has to be greater than the desired tire pressure, otherwise no more air goes into the tire. That's a good theory to "experiment" with my garage compressor too.
 
something else to keep in mind too is if you are cutting it too close the last couple of tires are going to fill awfully slow.
 
Co2 all the way. Much more volume as they pump liquid co2 into the tank, which releases gas on top of it.

I was going to mount my scuba tank in mine, but it's not even worth the weight and hassle for a couple tire fills.

I should be able to fill my 33s like over 20 times from 10-30 psi with my co2 tank.
 
I have a suspicion you will find that a regular air tank big enough to reliably inflate a set of 41" tires from 10 to 30 psi is not going to be practical to carry around. Also as mentioned above once the air tank pressure starts dropping down the tire inflation is going to become painfully slow. However I am interested in hearing some results on this.
 
I can't tell you the math.

I can tell you without a compressor on board, it isn't going to work.

Martin
 
82355 said:
I can't tell you the math.

I can tell you without a compressor on board, it isn't going to work.

Martin
Click to expand...

Yes. I can't fill all 4 of my 42s from 8 psi to any more than maybe 15 with my compressor not plugged in. Its a 30 gallon tank. Could probably get 2 out of it to 25 psi but it would be a stretch

Another thing 30 psi on the street? The radials are Definately different than my bias. I only run 20 psi for street pressures. 30 psi on that big of a tire seems high
 
blazinzuk said:
Yes. I can't fill all 4 of my 42s from 8 psi to any more than maybe 15 with my compressor not plugged in. Its a 30 gallon tank. Could probably get 2 out of it to 25 psi but it would be a stretch

Another thing 30 psi on the street? The radials are Definately different than my bias. I only run 20 psi for street pressures. 30 psi on that big of a tire seems high
Click to expand...

Same here: 30 gallon compressor 37 x 12.50 x 17's, It definitely runs out of air before I even get 3 filled from 12 lbs. to 25. I tried it with the compressor unplugged when I was contemplating on board air of some type. I had two york compressors that I was thinking about hooking up and opted for a 15 gallon CO2 in the rig. The CO2 just reseated the bead on a tire while wheeling a couple weeks ago. It runs air tools and has way more pressure than any air tank/compressor would give. Besides I got it for half off after winning some tickets at the Blazer Bash raffle.
 
Like previously said, straight air tank won't cut it. You'll need a CO2 tank or a compressor. The Viair compressors are a good, compact alternative to running a full blown OBA system. Nicely bagged w/ battery clamps and coiled hose w/ chuck.

I have one of their older, smaller models and it takes about 3 minutes to take a 35" tire from 12 to 35 lbs. Some of their newer ones are bigger and should be considerably faster. I didn't like their chuck so I cut it off and clamped on a better one w/ a good inline pressure gauge.
 
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