dlester

I have the CO2 for the road bike, but I use a Road Morph on the commuter and a Mountain Morph on the mountain bike. There are always scenarios where I may have wished I had brought a pump on a long road ride, but I haven't encountered one yet and if I ever did there is always the cell phone call to the wife.

Praxis

That's a pretty good looking pump; thanks for sharing. A bit smaller and lighter than the road morph G. Looks more solid (metal footpeg for example), but looks can be deceiving of course. The foot pedal *does* look longer than that of the Road Morph, which would be nice as the latter's is a bit short.

EDIT: They even have a model with a gauge, apparently.

echotraveler

both for me....110 psi on a kmart pump aint fun. how many tubes do you carry should equal your co2 cartridge count.

day1si

A trick so that you don't lose air between the C02 and the presta valve is a presta (schrader) valve adapter. Screw the presta valve adapter on to your valve, then attach your C02 inflator and you don't lose any.

Also you can buy C02 inflatrs that also double as pumps, I have an Innovations one that works great.

EKW in DC

Don't trust it to last for long. I had the same (or similar) from Target, and it lasted for two inflations before it completely fell apart.

Now I carry a MiniMorph on frame and have a nice Planet Bike floor pump for home use.

khutch

Whereas I buy CO2 cartridges in bulk anyway for airguns so why not use them on the bike too? Either option works well and either can leave you stranded when it breaks. Choosing between them on the basis of secondary reasons is as good a plan as any.

Ken

Polar Foil

Ditto. I'm on my third Schwinn pump from Target and it will be the last. It won't go over 90psi, it's a pain to use, and it sometimes seems like it's letting out more air than it's putting in. I know Schwinn does make some decent stuff but apparently none of it is sold at Target.

pharasz

I love using C02 but I've also suffered tarwheel's experience... The only time ever used a hand pump was years ago when I was stranded without C02 and a guy stopped who had a hand pump. I recall it was very difficult to use and I was wishing I had C02. But it worked, and the lesson is the hand pump is slow and clumsy but very forgiving. The C02 gives you one chance to get it right and if you screw it up you're walking.

Now I carry two cartridges, but that only improves my chances, it's still not as forgiving as the hand pump. I'll never use a hand pump, but I'm thankful that some people do!

itsthewoo

You're really going to debate with me using high school / early college level thermodynamics?

Somebody else already explained this to you. The CO2 cartridge contains mostly liquid CO2 at ambient temperature. It only stays in liquid form due to the high pressure of the cylinder. When you're filling up the tire, the liquid immediately turns to a gas, and undergoes free expansion into your tire. The gas has to do very little (approximately no) work to expand, and thus the temperature drops drastically since intermolecular collisions decrease at the higher volume. Since this is not a closed system, it will attempt to reach thermal equilibrium with the ambient temperature, thereby causing the pressure to increase since molecular speeds increase with temperature.

I never said "dangerously expand." I said they expand more than you expect.

Though it is possible for a molecule with more atoms to be "smaller" than one of fewer atoms (due to nucleus size and intramolecular electromagnetic interactions), O2 is indeed smaller than CO2. However, as previously stated, N2 is the prevalent gas in air, but that is also smaller. I cannot tell you off the top of my head why CO2-filled tires will deflate more quickly.

John Coloccia

Speaking a little loosely, the size of a molecule determines how quickly it diffuses through rubber, but there are other factors having to do with the structure of the molecule that determines how soluble it is in the rubber. In order to leak through, the gas must dissolve into the rubber, move through the rubber, and then come out on the other side. Nitrogen really HATES to dissolve in rubber. Oxygen is a little more friendly. CO2 is an order of magnitude more soluble that N2 or O2. Ideally, you'd fill your tires with nitrogen.

In fact, have you noticed that when you fill a new tire, it will quickly go flat? Then you fill it again, and it "holds" a little better? You do this several times until it really stabilizes. The O2 and CO2 leaking leaking out, leaving you with nitrogen. Next time you fill up, you have a little less oxygen and CO2. A little more leaks out, leaving you with an even higher concentration of nitrogen. After you do this a few times, you have a lot of nitrogen in there and your leak rate goes down. And then you get a flat and start all over again.

Hope that helps.

khutch

Actually I do not find this to be true of bicycle tires. It is true to an extent with automotive tires which are tubeless. Automotive tires are made of thick rubber and to the extent that nitrogen does not like to move through rubber they offer significant resistance to nitrogen diffusion. The thickness of the rubber in bicycle tires is irrelevant for most of us since our tires have tubes. The tubes are thin rubber and once a gas molecule has diffused through the tube wall the rim with all its spoke holes bypasses any resistance to further diffusion that the tire might have offered. My bike tires need to be refilled every week and perhaps twice a week if I were really obsessed about keeping the pressure right at whatever I consider to be optimum. They don't go flat, they just loose a few psi every week and I don't see that process slowing down, much less stopping, no matter how many times I top them off. This has been true of every bicycle tire I have ever owned, going back 50 years or so.

Now the theory you espouse about gas diffusion in tires is widely quoted by people who want to convince you to pay extra money to fill your tires with N2. And I believe it is true to an extent. But the notion that you eventually get pure N2 in your tire or keep your tire filled with pure N2 is nonsense if the claims that are made about O2 diffusion that are an essential part of this theory are correct. Gas diffuses according to the partial pressure of the components. Fill your tire with pure N2 and the partial pressure of O2 inside it is zero while the partial pressure of O2 outside the tire is about 3 psi. If all the N2 stays inside the tire then the pressure inside the tire will eventually increase by 3 psi as O2 diffuses into it. The much touted pressure stability of N2 filled tires must be due in part to the fact that O2 diffusing into the tire replaces the N2 diffusing out initially. After the internal O2 pressure is 3 psi the tire pressure can be only as constant as the N2 diffusion rate allows. And after a period of time equal to the rate at which you believe the O2 diffuses in/out the composition of the gas inside your tire is the same whether you filled it with pure N2 or ordinary compressed air, contrary to what so many believe. No matter how you fill your tires the internal O2 pressure will be 3 psi eventually. If you filled with compressed air to 120 psi the initial O2 pressure is 25 psi. If you find that your tire drops below 98 psi some time after a compressed air fill, it is not due to O2 diffusion because that is as low as O2 diffusion will take it.

Ken

pharasz

Wow. Great threads on gas diffusion and molecule sizes. I always thought C02 leaked out faster because the molecule was smaller. Now I know better. The diffusion explanation makes perfect sense. Ken, I think, brought it all to an excellent conclusion. I agree with Ken that my tires need about the same amount of air every week I fill them up. John, I have not ever noticed tires "stabilizing" and needing less air over time.

John Coloccia

Actually, I think it's mostly the co2 that causes any significant leaking. O2 and n2 solubility are nearly the same. For bikes, plain old air vs plain co2 is certainly different. The reason I normally see quoted for filling with n2 is to prevent tire oxidation. I'm not sure I believe that. Anyhow, forget I said the word tire and replace it with tube, which is obviously what I meant. I also never said that you end up with pure anything.

I'm not really into bike or car culture so I don't know all if the lore surrounding nitrogen filled tires. I'm just trying to answer why co2 tires go flat more quickly than air filled tires.

truman

I don't flat often on my commute, but I take CO2 because I have a train to catch. If I flat, CO2 buys me a bit more time.
I also carry a RoadMorph pump for when time isn't a big factor.

mrrab

I was watching re-runs of the Hawaii triathlon and this female cyclist got a flat and all she had was a CO2 pump. So, she attempts to inflate her new tube, but somehow accidentally punctures the CO2 cartridge and she didn't have a back up.

Lesson learned, hand pumps require additional effort, but there almost always guaranteed to work.

khutch

A lot of what I wrote was in reaction to all the nitrogen culture nonsense I have read over the years, not to you personally. Your post was just a convenient place to hang a reply on. I was not trying to put words in your mouth. I have only used my CO2 inflator once and that was to test it out. I don't recall how long the fill lasted so I have no real experience with the diffusion rate of CO2. The atmospheric concentration of CO2 is so small that the only time you would be concerned about the CO2 diffusion rate is when you inflate with pure CO2. Most of us do that only on the road, to fix a flat, so if we need to top off the tires more frequently during the following weeks because of the CO2 fill it would not be a major inconvenience and we might not even note the difference. As I recall it took a whole CO2 cartridge to fill a 700x32 tire and then only to a decently rideable pressure, not the pressure I would normally inflate to. If you have the time and are in a situation where you really want to get to your optimum pressure for the remainder of the ride then you would want a pump or you may (depending on the size of your tires) need more than one CO2 cartridge per tire. I think I normally carry four in my trunk bag, they are small and don't weigh that much.

You are right to suspect the tire oxidation theory. That comes mostly from the automotive world and the snake oil salesman's pitch there is that the O2 in your tire will rust your steel radial belts. As I say, if the fundamental principle on which nitrogen filling is based, the supposedly high rate of O2 diffusion, is true then no matter what you fill your tires with the O2 level inside the tire and inside the rubber of the tire will quickly become the same because of the assumed high O2 diffusion rate. Therefore there can be no difference in the rusting rate of the radial belts or the oxidation rate of the tire rubber.

Ken

josephjhaney

Chemistry or not it does dissipate though the tire. I can find and post the article, I found this because I inflated my tire to 110 using my co2, tested it when I got home, still at 110, when I went to leave the next morning, my tire was at 60. I thought I had a slow leak, so I pumped up, left for work, fully thinking I'd have to fill it again before pedaling home, and was shocked to find it fully inflated after my work day. THat's when I looked it up- Article below.

Oh, and on topic, I carry the Road Morph G as well.

So, here is the explanation- My wife is a Chemist by trade, and has verified this to be accurate.

I suppose I should say something insightful rather than simply reacting...

Okay. Let's start with the Ideal Gas Law which is:

PV=nRT

Where:
P = Pressure
V = Volume
n = number of moles
R = Universal gas constant
T = Temperature
We assume for now that this is a closed system which is not entirely true as we will see later. Thus we have:

Pcyl Vcyl / ncyl R Tcyl = Ptube Vtube / ntube R Ttube

Now n is a constant and is fixed (ncyl = ntube) as of course is R, thus our relationship becomes:

Pcyl Vcyl / Tcyl = Ptube Vtube / Ttube

Initially Pcyl > Ptube. As the gas from the cylinder flows into the tube, Ptube starts to go up and Pcyl starts to decrease. However, bear in mind that Vtube also increases. Because Vcyl is fixed, in order to preserve the relationship, Tcyl must decrease. This is why the cylinder gets cold.

The gas expands from a smaller volume in the cylinder to a larger volume in the tube. The ratio of this expansion combined with the initial pressure in the cylinder directly governs the final pressure inside the tube when everything has come to equilibrium. The molecules of the gas do not expand or shrink. The space between them does. All molecular sizes are governed by the composition of their atoms and their atomic bonds.

Now we come to why tyres filled with CO2 go flat faster than with air. CO2 molecules are more permeable and soluable in butyl rubber than other molecules in air. Thus when a tube is filled with air, the CO2 molecules will tend to permeate and leak through the rubber faster than the other components of other gases in the air. This leaves other gasses such as Nitrogen and Oxygen (amongst others) to linger around longer. When a tube is filled with just CO2, the rate of leakage is the same as that of the CO2 leaking out through the tube filled with air but since there's only CO2 to leak, the tube will go flat faster.

BTW, CO2 doesn't necessarily leak through rubber faster because of its size but because of how the molecules in rubber attract CO2 better than Oxygen or Nitrogen. As a result, the CO2 permeates the rubber which then swells and thus allows more molecules to escape.

khutch

Actually this is true only briefly at the beginning and then again during the final stage of a CO2 fill. There is nothing wrong with the gas law or your logic, but take a fresh CO2 cylinder and shake it. Does a gas make a sloshing sound? No, but liquid CO2 does! Most of the CO2 in a fresh cylinder is liquid. At room temperature liquid CO2 has a very high vapor pressure and when you first open the valve that pressure does bleed off according to the gas law at least briefly. But as the pressure drops even a little the liquid CO2 starts to boil off by pulling heat from the wall of the cylinder and during the boiling off stage which should be the bulk of the fill the pressure inside the cylinder stays pretty constant. Not perfectly constant, there are many complex things going on, but well above what your simple application of the gas law predicts. Once all the liquid has boiled off the cylinder pressure once again follows the gas law prediction.

Ken

josephjhaney

THe key point being-


CO2 molecules are more permeable and soluable in butyl rubber than other molecules in air

When you use co2, you are introducing co2, and since co2 is soluable in butyl, it leeches though. Now I can't say this is 100% accurate, but I can say the time I filled to pressure with co2 only from a flat, and then came back to 60lbs pressure, never happened with that same tube again when I used my good ol floor pump.

khutch

I neither affirm nor dispute this, I simply don't know. I was speaking to your use of the gas laws and pointing out a flaw in the way you used them to calculate the tire pressure. I do know that while any liquid CO2 remains in the cylinder the gas law alone does not predict the pressure curve with time. Furthermore the gas law is of minor use in predicting the loss of pressure due to diffusion. It accurately predicts the internal pressure of the tube once the liquid CO2 has boiled off and the system has come to thermal equilibrium. It also accurately predicts the internal pressure over time if by some means you know how much CO2 escapes over time. The gas law is of no direct use in predicting the diffusion rate of CO2, it knows nothing about that whatsoever. All it does is supply the internal pressure which is an important variable in the diffusion process but one that can be had by other means, such as a pressure gauge. Appeals to the gas law are an amusing side discussion but they can prove nothing about which gases diffuse faster than others.

Ken

chipcom 

So basically you are saying that, as we continue to pump more CO2 into the atmosphere, besides climate change we're gonna lose air in our tires faster too?

DallasSoxFan

I carry a CO2 cylinder and a presta to schrader converter. The first gets me to the gas station. The second makes it usable.

itsthewoo

Looking back this is a more correct explanation for the cold gas in the tire. Though I'm pretty sure the free expansion approximation does have an effect in the process, the latent heat absorbed during the flashing of liquid CO2 to gas probably has much more of a contribution. I'd do the calculations but I'm too lazy to be bothered with that .

prathmann

Not to worry - we'd all be dead long before there's enough CO2 in the air for this to become noticeable. Dry air is about 78% N2, 21% O2, 1% Ar, etc., and 0.04% CO2.

The climate change concern is that the CO2 level used to be 0.03%.

akohekohe

Seat Post Pump. Better than the Morph. CO2 is still faster though.