How Much CO2?

One of the A riders in the club posted on the Yahoo group that he was making the move to CO2 cartridges, after many years of resisting them. That started online discussions that took two separate tracks.

12g threaded

Another member asked, “How many pounds of pressure does a 12 gram cartridge impart to a 700×23 tire? I use 16g cans and still have trouble getting home with the optimum 92-94 lbs onboard.”

The Internet is a wonderful thing. It doesn’t take long before answers start rolling in. Multiple answers.

“I’ve never checked the actual pressure,” answered a fast rider, “but my 23’s are quite firm after using a 12 gm cartridge. I just did a bit of research and it appears that you get about 100 psi in a 23mm tire from a 12 gm cartridge and about 125 from a 16 gm.”

But that doesn’t allow for individual differences. The fellow who posted that probably doesn’t weight more than 130 pounds soaking wet. The original poster followed up:

“Am I the only one who rides 25 mm tires? What did your research say about the pressures in the larger road tire?”

Cripes. We haven’t answered the cartridge size question yet, and now we’re introducing tire size variations? It’s gonna get confusing.

“I ride 25s too,” chimed in another member, “and am somewhat heaver than Steve so you probably will not be surprised to hear that I only roll with the 16s on the road bike.  However, I also put some Gatorskins on the wheels so I don’t get flats very often any longer [although I just jinxed it and am sure to get a flat on my next ride] so the extra cheese spent on the threaded 16 gram cartridges isn’t a big deal.  If you use less than 6 of these a year, it’s not a major expense.” But he also had a practical suggestion.

“Go buy a 12 gram and a 16 gram cartridge.  Let all of the air out of your rear tire, fill it up with the 12 gram cartridge, and go for a ride.  Repeat with the 16 gram.  Which did you like better?”

I imagined pffffft! sounds coming from garages all over the neighborhood as cyclists took him up on that idea.

16g unthreaded

Then, of course, there’s that complication you get with CO2:

“Bear in mind that CO2 bleeds out of a tire much faster than pumped air,” wrote the 130-pounder, “so if you fix the flat in the early part of a three-hour ride, you will find a lower pressure when you get home. This is why, after fixing a flat on the road with CO2, I always deflate the tire and refill it with a pump at home. However, unless it gets down in the low 80 lbs range, you shouldn’t have any problems short of a pinch flat if you hit a big pothole or rock.”

Okay, this is all good information. But one member had a great idea: Go right to the source. The Genuine Innovations website has a link to an inflation chart.

“Unfortunately there isn’t a measure for the 25mm tire,” said the guy who suggested experimenting. “But there is one for 23 and 28. You should be right in the middle. It looks like a 12gm will get you 90psi in the 23mm tire and 62psi in the 28mm tire or about 80psi in the 25mm. If you want a higher pressure, go with the 16.”

What was the second discussion track? Cost, of course.

The original poster wrote: “If you buy cartridges from a bike shop they can be pretty expensive. I just checked online and it appears that you can get them reasonably cheap if you get a bunch of them. For example you can get a box of 50 16 gram non-threaded cartidges for $50 including shipping. And different prices for different amounts. but 50 for $50 seems about the best.”

Another replied, “Buy a box of 12 gram cartridges at Walmart in the BB/paintball gun section. 15 for $7.97.”

“+1 on that recommendation,” said a third.  “I buy mine in bulk from a discount sporting goods store.  The only thing is that these do not have the threads to screw into some of the smaller nozzles.  I had to use it this weekend and a single Crossmark CO2 cartridge inflated my tire in one shot.”

Well, that was settled pretty quickly. Looks like a bunch of my club pals are headed for Wal-Mart to get their CO2. But one of the previous commenters signed off with some wise advice:

“Even though I’ve had good luck with cartridges, I still have a frame pump attached to my bike just in case Mr. Murphy is drafting.”





  1. says

    Another variable…It also depends on the quality of the inflation tool. I’ve seen guys use CO2 cartridges and probably put a third of it in the atmosphere due to a lousy device.

  2. says

    I’ve never used a CO2 cartridge. Are they refillable? If you are ordering 50 at a time, makes me think no. My dive shop can redo the ones I use for scuba (emergency vest inflation).

  3. Tim says

    I don’t get the attraction to CO2 cartridges. Too much down side for very little convenience. A good frame pump doesn’t weigh much more than a few cartridges and the inflation tool.

  4. Ronan says

    Here Comes Physics; If this do not interrest you, please skip to the end where I discuss when I like to have the cartridge handy.

    Beware; sane units are going to be used.

    In the pressure and temperature ranges that tires see (0-10 bar, -20°C to +40°C), the CO2 behaves a lot like an ideal gas (ask wikipedia for what it is). The ideal gas law is (after a bit of thinking):
    m (mass in grammes)= M (molar mass of the CO2) * P (Pressure in Pascal)*V (volume in cubic meters) / ( R (gas constant) * T (temperature in Kelvin)

    First let’s solve the problem of the volume. This is were mistakes will happen.
    A tire looks a lot like a torus (especially a tubular, but this is still pretty close to the reality for a clincher). The volume is:
    pi²*(tire width²/4)*diameter of the tire (more or less the ETRTO diameter of the wheel).
    A 25mm will have a volume of: 0,96 *10^-3 cubic meter. That is a bit less than a liter.

    I am looking to get at least 6 bar in it (I used to ride on 25mm, and I liked this pressure. I weight 59kg). That is 600,000 Pascal.

    Let’s go for the worst case scenario, and pick an outside temperature of -10°C. That is 263 K.

    The gas constant is 8.314

    The molar mass of CO2 is 44.

    After crunshing the number, the answer is 11.59g. So a 12g cartridge will be close to perfect.

    I like cartridges. If I flat, I know I will be back on the road in less than two minutes using tubulars, and less than 4 minutes using clinchers.

    Now, when are cartridge especially useful?
    – When you flat during a group ride. I (used to; see way down) know I won’t be a hindrance to the group if I flat. During fast group rides (or races), this is especially cool.
    – When you flat and the weather is cold. Less time of the bike = less time to freeze.
    – It is even more important when the weather is cold and windy.
    – When the ride will include a fast descent. It is pretty hard (for me) to get a tire up to 6 or 7 bars, even with a high quality old school frame pump. Fast descents are the placse where it is the easiest to get a pinch flat, and where it is the most dangerous. With a cartridge I am confident in the pressure of my tires. But Beware! the CO2 will cool a lot while filling the tire, so the pressure in the tire will be low as long as it has not heated up to the outside temperature. Be aware that you will miss a couple of bars for the first few minutes after a repair! Beware of the Pinch!

    When do they suck?
    – When you don’t know how to use them. Try one at home. This is true for all the stuff you bring on a ride. It is very useful to discover that your inflator is bad (as Tommy F remarks) before you need it.
    – When you flat more times than you have cartridges. This has yet to happen to me. I carry two cartridges when I bother with them.
    – A pump is very reassuring when I ride alone in unknown places, like when I quit my job and moved 600km last year, or when I am going on a trip.

    When are they dead weight?
    I do not flat often. My last flat was two years ago (not counting MTB riding). I use good tires (durano’s on the commuter, Veloflex Carbon on the week-ends bike), make sure they are correctly inflated, check the rim tapes now and then and keep an eye on the wear. I no longer carry cartridges or spare tubulars during group rides or races (I keep a couple of cartridges when I commute, but no pump). Obviously, this depends on what kind of road you ride on. My roads are full of potholes but with very little crap that puncture tires.

    I weight my cartridges every few months to check if they are still filled; I think that this is useless, my four years old cartridge is still good. I am using one or two cartridges per year to check if the inflator works correctly.

    If you read everything I wrote, you should try randonneuring. Your stubborness is well suited to this activity.

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