Why is the cartridge full of CO2?

are you missing pink here? helium is lighter than co2 because it is a smaller molecule. meaning you would need more helium than co2 to fill the same space. with the final product weighing the same.

though you are correct on the loosing pressure part as it is easier for a smaller molecule to escape.

As a chemistry teacher, I cannot let this inaccuracy go by. The size of the molecule has nothing to do with how much volume & pressure the gas occupies. It's the basic premise of Avogadro's Law. Helium tires would be lighter. The tire will have a volume of about .85 L with either gas. Assuming a pressure of 760 kPa and a temperature of 32°C, the mass of CO₂ inside the tire will be about 11.2 grams. Filling with helium would use only about 1 gram. It is true, though, that the helium would leak out much faster.

The nerd in my had to click on the link.  Apparently, I've been mixing up Avogadro's Law and Boyle's law for the last 15 years or so.  Doh!  I did know that PV=nRT

On a lighter note (ha, get it?) wouldn't helium gas also be flammable? 

you're thinking hydrogen. helium is non flammable.

Good call.  I always did hate Chemistry. 

Let's get the obligatory comedy response out the way.  It's CO₂ because jelly wouldn't work!!

Secondly, why has no-one marketed NO₂ for bikes??  Think of that final kick as you come into T2.

Reminds me of a joke:  How many engineers/chemists does it take to fix a bicycle tire?...

Seriously, you brainiaks amaze me.  Thanks.

Not such a good choice.  R-22 at 60 degrees/100 psi some would still be  a liquid, while at 80 degrees you're already over 140 psi.  Unless you're referring to r-12, which I think used to be lumped into DuPont's "Freon" class of refrigerants.  I don't have a chart handy for that one, but it's both really bad for the environment and banned anyway.  It was replaced mostly with r134a, which has an interesting strory:  The EPA says it's bad enough that it's illegal to vent into the atmosphere, but the FDA says it's ok for asthmatics to inhale as a propellant for their inhalers.  It's also the ingredient in cans of compressed air sold everywhere, yet I get a $10,000 fine if I let a little out of a refrigerator...

Hey Avagadro! whats your number?

Just reach me at 6.02 x10^23 anytime!

So no one has taken a swing at that softball?

The cartridge is full of CO2 because an empty cartridge wouldn't be much help when it comes to inflating a tire.

Normal air is also not used because when you compress it, water develops (there is water vapor in the air naturally).  Think of it like this:  If you have an air compressor in your garage at home, there is the bleeder valve at the bottom of the tank.  Why?  It's not there to let the air out of the tank when you are not using it.  It is there to bleed the water out of the tank so your cylinder doesn't rust from the inside - out.  Compressed air, aside from the volume limits, is not a good choice because of this.

Just my two cents......

Thank you for correcting this!

I remember when I first encountered Avogadro's Law in high school chemistry class I thought that it was the most astounding thing that I had ever heard.

mgalanter - 2013-03-26 4:53 AM I looking to custom fill my cartrages with propane. It has all the physical properties i need and as  bonus I'll really be able to burn rubber!  

actually there are products that fill your bike tires with propane, to my surprise.

If you check out Big Air canisters, they are loaded with propane, as it states on the label !!

also it appears cyclists have used Nitrous Oxide to fill bike tires...but people might be laughing more in the race.