I just asked this in the General Forum but realized that you guys are the experts.

I use 16g cartridges on my 700x23c tires and they work well. How will the cartridges do on 700x35c and 700x37c tires? They only have to get to 80 psi or a bit less; will 16g cartridges be sufficient?

Is there a chart or a formula you can refer me to?

Thank you.

Mike

CO2 Inflation Chart - Genuine Innovations

I was never able to inflate 700x42 and 700x45 to the proper pressure with just one cartridge. That's why I carry a pump with me.

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.

Sure the ratio of the radius squares will give you a ball park answer. Differences in tire shape and wall thickness would account for any departures from theory. On the other hand the empirical method is just too easy for you not to use it. Do the experiment and see what you come up with. And don't forget you can/should use lower pressure in the wider tire, so that will help. Quite likely it is two cartridges for the larger tire, but it won't cost more than a few dollars to test for yourself. BTW, cartridges are cheap when bought in bulk from a mail order source not related to hobbies or bikes.

If you keep an eye on them at Amazon you can often snag 'em for under $1.00 each.

One 16g will not get a 35mm tire up to 80psig, but it'll be enough to get you home if it's a commute bike or something.

Thass wut I'm talkin' about.