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Technical > Tech

September 2004 Issue

Gimme Some Air!

by Jim Allen

Lower tire pressure is a great way to fine tune your rig for the terrain, but you should’nt air down if you can’t air up. In order to air up you need either a compressor or a CO2 tank. A compressor suitable for bigger tires is the more expensive buy-in, but you’ll always have air. A CO2 tank is cheaper initially, and performs better than many compressors, but you can run out of CO2 at a bad moment. Plus, you will have to get the tank filled regularly at $15-20/pop (give or take). The cost between the two systems generally evens out over time. We’ll talk about compressors here because their performance is so variable among available units. Compressors are commonly rated by airflow in cfm (cubic feet per minute) at a specific pressure. Because it’s harder for a compressor to pump against pressure, specs that show cfm at a usable working pressure are most valuable. A compressor might be rated for 2 cfm at 0 psi but only .4 cfm at 35 psi. A compressor that makes 2 cfm at 100 psi is significantly more powerful than one that makes 2 cfm at 30 psi.

Cfm is only part of the story. The other important factor is duty cycle. This describes how long you can run the compressor continuously before it reaches an overly high temperature. Some companies list run/rest times, i.e. 15 minutes on/20 minutes off. Some will list a duty cycle in percent. A 100 percent cycle means it can run continuously. A 25 percent duty cycle means it can run 15 minutes in every hour, 10 on/20 off, or however you divide the time up.

Your compressor choice will be based on tire size. A tire of a given size will hold a specific volume of air (in cubic feet) at atmospheric pressure. One atmosphere equals 14.7 psia (absolute pressure), which reads zero on a standard pressure gauge. Standard gauges read psig, or “gage” pressure, which is only the pressure above one atmosphere, or 14.7 psig. How much air in cubic feet does a tire hold? The shape of a tire is called a torus and there are some complicated formulas used to figure out the volume. Let’s avoid more brain twisting; I found a website calculator that will do it for you easily:

(http://grapevine.abe.msstate.edu/~fto/tools/vol/index.html).

The average 35x12.50 tire holds about 4.2 cubic feet of air at one atmosphere (14.7psig) and 12.6 cubic feet total at 30 psig (about 3 atmospheres). Since we are seldom filling up a flat tire, let’s figure going from 15 to 30 psig. That’s about one third of the total volume, or 4.2 cubic feet. If you have a 2 cfm @ 30 psig compressor, it’ll take you about two minutes to fill the tire, right? Maybe. See the Stems and Valves sidebar for more info.

How can you calculate fill up time with any compressor? Find the approximate volume of your tires in cubic feet and the amount you want to air up. Four 35x12.50s from 15-30 psi would be about 16.8 cfm. If you can pump in 2cfm, that means 8.4 minutes, plus hookup and walkaround time (16.8/2= 8.4). If you can get 5 cfm, that’s 3.36 minutes. Don’t forget to correlate cfm to pressure. The cfm you choose should be available at a useful pressure at or above your standard street pressure.

Finally, let’s talk about the subject of air tanks. Do you need one or not? If you have a high volume, high-pressure compressor, you don’t... unless you want to run air tools a lot. If you have a smaller compressor, it makes more sense to run a tank because that initial volume can really speed up the airing up process. A 2.5 gallon tank at 100 psi is holding about 2.27 cubic feet of air. That will get the first 35-inch tire halfway filled. After that, it’s the compressor that does the work.

So, bottom line, the basic idea in choosing a compressor is to either have enough cfm to air up all four tires before the duty cycle is exceeded, or have a duty cycle long enough to air up all four tires regardless of cfm. There are compressors available at both ends of the spectrum.

Buy the

For more information on the compressors shown in this article, see the manufacturers web sites:

• Oasis Manufacturing Products
• Sun Performance Products
• Viair Products

Approximate Tire Volumes at 30 PSI (In Cubic Feet)

31x10.50-15	7.80
33x12.50-15	11.46
35x12.50-15	12.51
37x12.50-16	15.84
38x15.50-15	22.74

Slowing the Flow: Stems and Valves

The valve stem can slow your airing up and especially your airing down. There is a formula for calculating airflow based on pressure and the orifice area of the stem. I measured a bunch of valve stems in my shop and calculated that the rubber stemmed types can theoretically flow about 5.46 cfm at 30 psi with the core out. The airflow is less as pressure drops; only 1.4 cfm at 15psi for example, but more at high pressure; about 14cfm at 100 psi. I had some metal stems too and they had a smaller I.D. so they will flow less with the core out but the same with the core in.

It’s harder to figure cfm with the core in, but I took one apart, measured the guts and figure it would have about 50 percent of the airflow of the empty stem; a max of 2.75 cfm @ 30 psi or 7.0cfm @ 100 psi. To do a real world test, I went out and aired down a 35x12.50 tire from 30 to 15 psi with the core in and out, and the real world results were in the same ballpark as the paper calculations.

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