Thread: CO2 Question
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Old 03-22-14 | 09:00 PM
  #3  
FBinNY
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From: New Rochelle, NY

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

Yes there is a formula: Pressure / Volume = Konstant.

You don't have to know too much or do complex math if you know the pressure a single cartridge fills a specific tire to. Think of the tire as a long cylinder, wrapped into a circle. If the wheel is the same size you don't care about the length, so the volume is proportional to the cross sectional area.

A= Pi x Rsquared.
You can forget Pi, that's common to both, so it's simply the ratio of the radius squared. Then (proportional) area of a 24mm tire = 144, a 37mm tire = 342, so the pressure in a 37mm tire will = 144/342ths of whatever a 24mm tire would be. Note this is a square relationship, so twice the width mean 1/4 the pressure, or put another way 4 times more CO2 would be needed to reach the same pressure.

Now, there is a workaround, and that's to partly fill the tire with a hand pump (which is good practice anyway in case there are seating issues). Then use the CO2 to top to pressure. For pracical purposes you can consider pressures to simply add. So if the CO2 is enough to fill a tire half way, you can pre-fill it half way, then the CO2 will provide the 2nd half.
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Last edited by FBinNY; 03-22-14 at 09:09 PM.
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