Bicycle Mechanics - Does Co2 seep?

Just started using these Co2 cartridges this year as I got tired of cranking the mini to pump up 120 lbs. However, it seems that after I've inflated a tire with C02, it goes substantially flat over 2 to 3 days. I'm used to topping off my tires each time I ride, but this pressure loss is substantially more. I haven't meticuously tracked pressure loss each time I've used a co2, it's more just an observation.

For example, the last time I rode, I flatted. I changed the tire and inflated with a c02 cartridge. No problems, I rode for another 2 hours. A couple days later I find some time to ride, but the same tire is flat. So I change the tire and check the tube for the leak but there is no leak. I also check the inside of the tire, it's fine. I also check the wheel itself, nothing obvious and the liner looks ok. Then I remember that I inflated with CO2. So, I put the old tube back in and inflate with a floor pump and the tire is rock solid the next day. Hmmmm.

in a word, YES

I keep CO2 on me when i'm out riding since i don't want to carry a pump. I keep my tires full with a floor pump at home.

Yes, CO2 is apparently very permeable in rubber. It's meant to get you home, not to be used every time.

Why is CO2 more permable than air? It would be expected that the oxygen and nitrogen in air would diffuse at a faster rate than CO2.

Why is CO2 more permable than air? It would be expected that the oxygen and nitrogen in air would diffuse at a faster rate than CO2.
This question comes up from time to time here. Apparently CO2 is more soluble in rubber than O2 or N2 and thus diffuses faster than it's molecular weight would indicate.

This question comes up from time to time here. Apparently CO2 is more soluble in rubber than O2 or N2 and thus diffuses faster than it's molecular weight would indicate.I'll buy that. For whatever reason I know for sure that the couple of times I've had to use the CO2 that tire was lower than the other the next day.

Paintball taught me that buytl (that is black rubber) does allow co2 to be disolved in it. If you suddenly dump the air from around an o-ring that's been sealed for a while... well it blows up like a baloon to two or three times it's original diameter. Nitrogen does not do this. Air is mostly nitrogen ;-) Also keep in mind that CO2's vapor pressure is really wacky. Like if it were fairly cold, and you had 120psi in your tires you may well have liquid co2 sloshing around in there.

I'm surprised that someone hasn't packaged Nitrogen in a cartridge. The auto industry has started to move toward N in tires.

I'm surprised that someone hasn't packaged Nitrogen in a cartridge. The auto industry has started to move toward N in tires.

How about Helium? Think of the weight savings!

(I know, I know; smaller molecules...)

Yeah, I don't get that. My neighbor bought her tires at Costco and they made a big deal about using 100% N2 and put green valve caps on the tires to remind you of that fact.

What's the big deal? I'm perfectly happy with my 78% N2 air.

The Nitrogen molecule N2 is much larger than the CO2 molecule so it doesn't seep as much. AS a previous posted pointed out, air is 78-80% N2 so it should seep less quickly than just CO2. It was my understanding that those CO2 inflators were for short term emergency use only.

How about Helium? Think of the weight savings!

(I know, I know; smaller molecules...)


Yep. Helium is the smallest, leakest gas molecule known to man.

OK; so the thing to do if you've used a co2 on the road is deflate the tire and re-inflate it with regular old air at the next opportunity.

I guess I should have paid more attention in my chemistry class.

Yep. Helium is the smallest, leakest gas molecule known to man.

What about hydrogen?

OK; so the thing to do if you've used a co2 on the road is deflate the tire and re-inflate it with regular old air at the next opportunity.

I guess I should have paid more attention in my chemistry class.

Or just keep topping it up with regular air. Eventually, it will stop seeping as you come closer to a normal air mix.

I'm surprised that someone hasn't packaged Nitrogen in a cartridge. The auto industry has started to move toward N in tires.

Probably somebody who is a chemist or a physicist would the definitive answer to that question. As I unserstand it, the CO2 in your cartridges is actually stored as a liquid which is what allows it to expand so much when it's released.

I'm surprised that someone hasn't packaged Nitrogen in a cartridge. The auto industry has started to move toward N in tires.

Well, don't be surprised. They do. BUT they don't contain much volume. Nitrogen at 1800psi runs by straight gas law, because it's stored as a gas. CO2 is stored as a liquid, and expands by a factor of 100 when going from liquid to gas, while maintaining pressure. That is given you can feed enough heat into it to maintain the vapor pressure. :-) If you let it get cold, all sorts of funny things happen. Dry ice formation, puddles of liquid co2, very low pressures.... Solid co2 can exist at atmospheric pressure at fairly sane temperatures, and boiling co2 takes a LOT of energy.

Just to be silly, if you don't empty your tires of air, and just top them up over time.... they will eventually drift towards having only large gas molucules in them instead of things like hydrogen, helium, oxygen... All of that will diffuse out. leaving you with heavy stuff. In old racing shocks they used to use freon filled gas bladders because the freon WOULD NOT leak out of the plastic bag.

What about hydrogen?
Well, it has to do with atomic physics. Helium has two electrons, filling the inner electron shell, which causes it to more stable & smaller. And which is why helium in inert. Hydrogen, with only one electron, does not have a full electron shell, which causes it to be larger and hydrogen to be a very reactive gas.

Well, it has to do with atomic physics. Helium has two electrons, filling the inner electron shell, which causes it to more stable & smaller. And which is why helium in inert. Hydrogen, with only one electron, does not have a full electron shell, which causes it to be larger and hydrogen to be a very reactive gas.

Chemistry is far from my strong suit, but I thought hydrogen formed naturally as H2 molecules because of that odd electron.

Are these molecules larger than the naturally stable He ones?

Don't you just love the tangents these threads take?

Just started using these Co2 cartridges this year as I got tired of cranking the mini to pump up 120 lbs. However, it seems that after I've inflated a tire with C02, it goes substantially flat over 2 to 3 days. I'm used to topping off my tires each time I ride, but this pressure loss is substantially more. I haven't meticuously tracked pressure loss each time I've used a co2, it's more just an observation.

For example, the last time I rode, I flatted. I changed the tire and inflated with a c02 cartridge. No problems, I rode for another 2 hours. A couple days later I find some time to ride, but the same tire is flat. So I change the tire and check the tube for the leak but there is no leak. I also check the inside of the tire, it's fine. I also check the wheel itself, nothing obvious and the liner looks ok. Then I remember that I inflated with CO2. So, I put the old tube back in and inflate with a floor pump and the tire is rock solid the next day. Hmmmm.

On a completely seperate topic: 120psi? How heavy are you? May want to consider dropping that down to around 100-110psi, you'll get a smoother ride.

I'm a svelt 220 & 6'5" (down from 245), so I need the extra pressure. I ride 25's @ 120 in the back and 105-110 in the front. Seems to ride ok to me. I was using 23's at 120 but that was a little narrow, so I moved up to 25's.

Chemistry is far from my strong suit, but I thought hydrogen formed naturally as H2 molecules because of that odd electron.

Are these molecules larger than the naturally stable He ones?

Don't you just love the tangents these threads take?

If I remember right (but chemistry was a LONG time ago), hydrogen is found as H[I]2{/I] molecules and helium as in a monoatomic form, thus giving smaller molecules.

Tangents are fun.:D

Use a floor pump for filling your tires and save the mini-pump for flats on the road. (Or trail.)

I'm surprised that someone hasn't packaged Nitrogen in a cartridge. The auto industry has started to move toward N in tires.

This is mostly because it makes money for people who want to sell you nitrogen, not because it provides any benefit to the typical consumer. The "advantages" of 100% N in tires are so minuscule even for cars that I find it difficult to believe that they'd do any good at the temperatures and speeds that bicycle tires operate at. Besides, air is already 70% nitrogen, which is good enough for me.

The Nitrogen molecule N2 is much larger than the CO2 molecule so it doesn't seep as much. AS a previous posted pointed out, air is 78-80% N2 so it should seep less quickly than just CO2. It was my understanding that those CO2 inflators were for short term emergency use only.

No, a molecule of CO2 in vapor is quite a bit larger than N2.

Others have made some good guesses as to why CO2 might leak faster; on the basis of molecular mass or volume, it shouldn't. Permeability seems a good guess, and is mentioned by a post on this very site:

http://www.bikeforums.net/archive/index.php/t-154847.html

As I understand it the benefit to N2 in car tires is that it's dry ( no h20) which can cause the steel wheels to rust from the inside. I don't think I quite believe it as I have had a set of steel wheels for 12+ years with no leaks!