I ran over some roots yesterday and got an instant rear pinch flat. Fixed that, rode 10 feet and had to deal with a less-than-instant front pinch flat. Fortunately it was a nice, warm, sunny Fall day and I was prepared.

While pumping up -- combination of CO2 and air -- I started pondering things I had read regarding the alleged tendency of CO2-filled tires to lose pressure more quickly than air-filled tires.

Is there any actual knowledge regarding this? My simple (ignorant?) thought is that a CO2 molecule is physically larger than O2 or nitrogen and therefore should pass through rubber with more difficulty. As is apparent, I'm no chemist and I expect there's more to it.

Other than cost and convenience, is there any known difference between bicycle tires filled with air (N+O2+CO2+H2O+etc) or pure CO2 (or pure nitrogen or helium for that matter)?

asmac, I don't recall any meaningful difference in the rate of tire pressure loss after repairing a puncture and using CO2 to inflate, at least when comparing the un-repaired tire's pressure to the repaired tire's pressure when the tires were next topped off.

Nitrogen is the largest molecule in the air we breathe and it also has lower water content than the air we breathe. This is good for tires that live through wide temperature changes, like an airliner or a racing motorcycle. The larger molecule also reduces pressure loss over a greater time span than the air we breathe.

Brad

yes , at the University .. maybe Masters level courses. Engineering, Physics, Chemistry, Materials Science.

CO2 passes through rubber inner tubes much more readily than either N2 or O2, but the effect is due to chemical effects, not the size of the molecules. As stated by the OP, the CO2 molecules are largest of these three but still diffuse through more readily. CO2 consists of carbon with two double bonds to oxygen atoms. When this comes in contact with the carbon compounds of the rubber tube it can readily form a temporary bond to them - in effect sticking to the rubber molecule. That makes it more likely that it will eventually migrate through the rubber and escape to the outside air. OTOH, N2 molecules just bounce off the inner surface of the inner tube and are much less likely to find their way through.

Yes it did not make sense to me until I dug a bit into google and the way it was explained was that the CO2 is more or less "soluble" in the butyl that most inner tubes are made out of.

Molecule size wise it makes no sense, but looking at the interaction between the gas and the butyl it makes sense.

There is another product that comes in a "CO2" type container that is not supposed to leak out like CO2.......I tripped over it while looking up stuff about this, and I am not finding it easily right now :-).

Bill

1. You need to run higher pressure to prevent pinch flats.
2. As you top off the tire with air in the future, you will be replacing whatever molecules that leak out the fastest and will eventually reach "equilibrium" with "generic air".
3.IF it really bothers you, you can deflate the tire when you get home and refill it with "generic air".

Is this the one: https://stayfill.com/. Interesting but at $200 for 10 cartridges, maybe not!

While it's true that CO2 permeates through butyl faster than air, it's not really relevant in the real world of bicycling. The difference isn't that large, so there will be a faster pressure loss over time in hours or days, but not over minutes of an hour or two.

So if you fill your tire on the road, you can rest assured that there's plenty of time to get home. Next time you top off you'll be replacing the CO2 with air, so the rate of air loss on the new mix will be slower, and with each top off cycle the percentage of CO2 left will be reduced until there's virtually none left.

All in all, the talk about CO2 is totally academic except for those who like to debate immaterial issues.

Well the reason the question came up when I was digging into it was a friend that had some kind of CO2 inflation system that he used on dirt bikes, and dirt bike trailers, etc, and his mountian bike tires were BOTH losing stupendous amounts of pressure in just 3-4 days....and eventually it all boiled down to using CO2 instead of air.

His CO2 use was not typical of most bicyclists....but interesting nonetheless :-).

Off gassing is much easier on a recumbent!!!!!

True, but I would rather point the tailpipe to the rear as opposed to the front when off gassing...

I'm guessing you'd get some pretty strong opinions about your plan if you posted this in the Tandem Cycling forum .

Ah yes, except I am always the stoker, so its always my complaint to voice...But then maybe its this philosophy that always relegates me to the back seat???

Oh, well, if you're the stoker, Fire Away (so to speak) .

There was a cartoon in our paper a couple of years ago showing a couple on a tandem riding past a Mexican Taco stand. The wife (stoker) is saying; "NO we are NOT stopping there for lunch."

Now that you mention it, there were a few times my wife vetoed my lunch suggestions when she was stoking .

I like to stand for that kind of action, impossible on a recumbent.

Standing and unweighting over bumps helps in avoiding pinch flats also.

Good job getting the thread back on-topic.