Following on from this:
Having squandered my money on a Brompton, I can't afford taxis, so the pump stays. As does the CO2 inflator that lurks at the bottom of my C-bag, because the Brompton pump is feeble.
If you give a Brompton a full canister of CO2, does the tyre pop off the rim? Assuming the tyre is a perfect torus, the volume of a Kojak is roughly 0.349*pi20.0142 = 0.66 litres. 16g of CO2 is (16/44)*24 = 8.7 litres at room temperature, which would give you 8.7/0.66 = 13.2 atm in your tyre, or 194psi. Allow for the volume of the canister itself, which you obviously can't evacuate when using, and you've probably still got 185psi. A Marathon Plus is quite a bit bigger so would be less likely to pop off. The gas cools substantially as it leaves the canister so the pressure will be less than this to start with, but will increase maybe 5-10% as it warms up to ambient temperature.
A road bike tyre is more like a litre, and a canister is about right for one of those; it gets it nicely hard (oo-er) at about 120psi.
Someone needs to try this in the interests of SCIENCE, using a proper inflator which screws firmly onto the valve.
I've tested it in the interests of not being stranded in the pissing rain, using a proper inflator that screws firmly onto the valve, with a twist valve for decent control of the flow. Unfortunately, I stopped when the tyre pressure felt about right, rather than giving it the whole cartridge.
Given the above calculations, a 12g canister would seem like the optimal approach. If I hadn't bought a job lot of 16g ones several yonks ago.
I've just sacrificed another 16g CO
2 cartridge in the name of
SCIENCE.
Here's the details:
- Brompton Shimano dynamo wheel with the double-wall 349 rim.
- Brompton 'green' (though this is the newer model with white labelling) 37-349 kevlar tyre, with "INFLATE TO 100PSI" written on it in large, friendly letters.
- Original Brompton-supplied tube with Schrader valve, never punctured.
- 16g threaded cartridge from https://www.co2cartridges.co.uk
- LifeLine CNC CO2 Inflator screwed firmly onto the valve.
- Pressure measured with a Topeak Smarthead Digital Gauge D2. This agrees with the gauges on my Road Moph and Cyclaire Plus, but not the wildly inaccurate one on my Aldi track pump, or the one on my Wilko 12V compressor that consistently under-reads by about 30PSI.
- Room temperature 18.8°C. It's been within a degree of this for about 18 hours. All materials have been in this room for days, so nominally at the same temperature.
- Ambient pressure 996hPa and climbing at about 1hPa/hour according to a BMP085 I2C pressure sensor in an adjacent room.
I started by removing the wheel from the bike and letting as much air out of the tube as I could by squeezing the tyre. This seems a reasonable approximation of a minimally inflated tube to aid correct fitting - in reality, I'd give a new tube a couple of puffs from the pump before fitting. Obviously the bead is known to be correctly seated at this point, where that's less certain when you've just installed the tyre.
Wearing eye protection, I attached the inflator firmly to the valve, and the installed a new, unpierced cartridge. No gas leaked in the process. I then opened the inflator's valve slowly until I heard the gas escaping into the tyre. I then took a couple of steps back and waited. After about 30 seconds, the hissing had stopped. Nothing went boom. I left it for about a minute, then closed the inflator's valve and unscrewed it from the tube - very little gas escaped when released, probably just that contained between the two valves.
I opened the inflator's valve and observed a puff of gas. If I'd been intelligent, I'd have released this into an empty inner tube and measured the volume by displacement.
The tyre was cold and had accumulated some frost on the side that was in contact with the ground, directly below the valve. I assume some liquid travelled into the tube and boiled there.
I then measured the pressure in the tyre with the tyre gauge, with minimal leakage. Its recorded maximum was 104PSI. This seems
lower than expected surprisingly accurate
[1], but obviously the gas is still very cold. I'll go back and measure the pressure when it's warmed up. Nothing's gone boom yet...
ETA: Having done the maths, I'm not expecting a boom. 16g seems about right.
In the meantime, has anyone had any issues using CO
2 inflators with small wheels?
[1] Tube: 0.349*pi2*0.01852 = 1.18 litres.
Cartridge: 0.060*pi*0.01252 = 0.029 litres (assuming a cylinder).
8.7/1.2 = 7.2atm = 106PSI
