Air-gun CO2 cartridges: inflating bike tyres for a tenth of the price

CO2 cartridges sold as air-gun consumables hold the same carbon dioxide as the "bicycle" ones and cost roughly ten times less. With an inflator for threadless cartridges and a spacer they work exactly like the branded ones.

The economics

An inflator for threadless cartridges (Deemount, for example) costs about 5,000 tenge — less than many decent hand pumps. The cartridges themselves are sold on marketplaces in bulk: twenty 12 g cartridges for roughly 3,000 tenge, which is 150 tenge apiece, one per wheel. Bicycle-branded cartridges cost 1,000–1,500 tenge each in a shop — almost ten times more for the same contents.

A twofold difference would change nothing. A tenfold one changes the whole scenario: a cartridge stops being an emergency reserve kept for race day.

Packs of 12 g cartridges are sold as an air-gun consumable

Why it is the same thing

An air-gun cartridge holds pure carbon dioxide, partly liquid and partly vapour above it — exactly like a bicycle one. Manufacturers state 99.99 % purity and roughly 57–58 bar at 20 °C: that pressure is set by temperature, not by which market the cartridge was sold into. There is no evidence that bicycle CO2 is any purer.

There is one difference between them: the thread. 12 g air-gun cartridges are almost always threadless, while 16 g "bicycle" ones usually have a thread and cost noticeably more.

The inflator and the spacer

A threadless-cartridge inflator after two years of use

An inflator for threadless cartridges is built around the 16 g size, and a 12 g cartridge is shorter — without a spacer the pin cannot reach it. Anything of a suitable size will serve as the spacer: a washer, the inner cup of a bearing. The only requirement is that it be thick enough for the cartridge to be pierced.

What it looks like in use: a short video of inflating a tyre from a cartridge.

Inflators come with a push button or with a screw valve. Both let you meter the gas: press and release the button and a portion goes in. The valve gives finer control — crack it open slightly and the tyre fills slowly, so you can stop where you want instead of shooting straight to 4–6 bar.

How much pressure one cartridge gives

12 g of CO2 is 0.273 mol, which is 6.56 litres of gas at 1 atmosphere and 20 °C. One detail matters: the tube in the tyre is already inflated, otherwise it cannot be seated — so there is already air inside at roughly atmospheric pressure, and it counts too. That adds about one atmosphere to the result.

These are approximate calculated values: gauge pressure at 20 °C for an ideal gas. A real gas gives 3–5 % less, cold weather less still. The table works as a guide, not a promise.

One 12 g cartridge is enough to take a 25 mm road tyre to a usable 6+ bar and a 30 mm one to about 4.5.

What to know about CO2

It escapes faster than air. CO2 permeates a tube considerably faster than nitrogen and oxygen, so the tyre starts going down over hours rather than weeks. That is normal, not a puncture: at home the tyre is deflated and re-inflated with an ordinary pump.

The cartridge and the head get very cold. The gas expands sharply and the metal frosts over — this is not the moment to hold the bare cartridge, as it can cause a cold burn.

Pressure rises as the tyre warms up. Measuring a few minutes later gives a truer figure; measuring immediately leaves the tyre under-inflated.

Be careful with tubeless. Makers of latex-based sealant warn that CO2 shortens its life and can make it clot — some blame acidification, others thermal shock. Some other makers state compatibility. Their own advice: keep the valve at 12 o'clock, let the gas in gradually, and swap the CO2 for air at home.

Storage and travel

• Cartridges should not be left in a car in the sun: pressure rises with temperature and small cartridges have no relief valve.
• They cannot be flown — not in hand luggage, not in the hold, and they are not shipped by air either. Flying to a race means buying them on arrival.
• A used cartridge is ordinary scrap metal, but it is worth checking that it is empty.

Verdict

The approach works: it saves money and space, lives in a saddle bag and needs no maintenance. Two things are worth knowing — what one cartridge gives on your tyre width, and that the tyre should be re-inflated with air at home.