The longer I hang around cyclists, the more I learn that there are two camps for everything.
26 versus 29 inch mountain bike wheels, compact versus triple cranksets, tube versus tube-less tires and bike pumps versus CO2 cartridges - can't we all just get along? Arguments regarding pneumatic devices aside, there is truly a difference when it comes to choosing a CO2 cartridge size for your mountain bike. The volume of cartridges is measured in grams and common sizes are 12, 16, 20 and 25 grams.
We do not see 12 gram cartridges used much anymore and they were most likely available because that is what BB guns used when we were growing up and trying to shoot each other's eyes out. 16 gram cartridges are slightly larger than the 12 and great for re-inflating a 700-23 road tire or 26x1.5 inch mountain bike tire to a respectable pressure. What they are not great for is refilling a large volume tire like a 26x2.5 or 29 inch tube, commonly referred to as a 2 Niner.
Some of us have to learn things the hard way and it was my turn this weekend. I went to go ride my Gary Fisher Superfly with 29 inch tires on the Bluff and found the rear to be Superflat. Not to worry, I had a spare tube (yes I still run tubes) and a whopping two, 16 gram CO2 cartridges. I thought I would slap the tube in, hit it with all 16 grams of CO2 and go tear-up the Bluff. That would leave me a spare just in case I got another flat during the ride.
Needless to say, the single 16 gram cartridge gave me just enough pressure to give some shape to that big 29" tire, but not enough to make me feel like a pinch-flat wasn't just over the rock. What to do; use the second cartridge and hope I did not re-puncture, or keep one in the bag and pray I did not get a pinch flat?
The obvious solution is to carry the right size cartridge and get on with life - which in this case is a 25 gram cartridge. Whoa, not so fast my CPA brain screams. Which is the best value? Is it less expensive to use 2 of the 16 grams or one 25 gram? Let's compare prices.
A 16g sells for about $3, a 20g sells for $6, and a 25g sells for about $12. In geek terms, this translates to a cost per gram (CPG) of 19 cents, 30 cents, and 48 cents respectively. Are you beginning to see a pattern? From a cost perspective, you are still ahead if you use 25 grams of 2 - 16 gram cartridges and waste the remaining 7 grams "playing" with the cat at home.
The choice is yours whether you want to bother with 2 CO2 cartriges each time you fill or go for the gusto with one big one. Either way, make sure you have enough CO2 for the job or plan on carrying a portable pump to top-off the tire to the desired pressure.
I got lucky on my ride on the Bluff last weekend; one cartridge and no pinch flat. Next time though, there will be more cartridges available when I take the big 29" wheels out for a spin.