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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

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Old 10-03-06, 12:37 PM   #51
humble_biker
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Quote:
Originally Posted by khuon
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.

Note - Writing equations with vBcodes sucks!
Damn, and all this time I've believed that...If it's just regular air I'm throwing it out and using a hand pump. Nobody is pulling one over on me.
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Old 10-03-06, 12:39 PM   #52
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Quote:
Originally Posted by Doid23
I'm sorry, I've got to be honest, his explanation was a lot more fun than yours. I'm sticking with the expanding molecule explanation.
ha! vindication!
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Old 10-03-06, 12:59 PM   #53
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Quote:
Originally Posted by khuon
Because Vcyl is fixed, in order to preserve the relationship, Tcyl must decrease. This is why the cylinder gets cold.
Doesn't n go down as well?

The real reason that T goes down is that the expansion is fast enough to be adiabatic (no heat transfer) and is also doing work on the surroundings (PV work). The first law of thermodynamics, Q-W=delta(U) where Q is the heat transfer (in this case equal to zero), W is the work done on the surroundings, and delta(U) is the change in internal energy implies that the internal energy of the gas must decrease. Hence the temperature decreases.
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Old 10-03-06, 01:20 PM   #54
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Quote:
Originally Posted by humble_biker
Incidentally CO2 is compressed air, meaning that when it exits the cartridge and enters the tube it expands to larger than the normal CO2 molecule(!). It usually takes the molecules around 24 hours to shrink to normal size. That is why you will have a flat or soft tire the next day after using CO2.
This may actually challenge R600DuraAce for most memorable post ever. I dunno how I missed it first time around.
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Old 10-03-06, 02:12 PM   #55
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I reached for my co2 inflator the other day only to find that the metal spike was gone. I screwed the cartridge into the head, put it on the valve and nothing happened and nothing happened. I unscrewed the cartridge and voila, no hole. Damn that was a long walk... ...to the bike store to buy a mini pump.
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Old 10-03-06, 02:13 PM   #56
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Second Winds are decent if you want a CO2... whatever your CO2 cartridge is, it can handle it, as well as work as a so-so pump (it does get hard on the hands though). I actually used one of these to inflate a tire on a pickup truck, using a couple large threaded CO2 cylinders.

Personally, take a Second Wind on your person, and a "real" mini pump that has a mount between a water bottle holder and the frame (so it doesn't take up much space) and has a Velcro strap so it doesn't launch itself on bad terrain.
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Old 10-03-06, 03:09 PM   #57
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adiabatic ...


...adiabatic ...

...a word that I never expected to ever see on this forum. Let alone spelled correctly.
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Old 10-03-06, 03:15 PM   #58
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I'm sticking with my CO2. If it fails me (it hasn't yet in lots of flats) then I've got my cell phone.

Bob
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Old 10-03-06, 05:18 PM   #59
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Originally Posted by hammond9705
I'm sticking with my CO2. If it fails me (it hasn't yet in lots of flats) then I've got my cell phone.

Bob
Me too. I haven't carried a pump in 8 years. CO2 has never failed me.

Al
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Old 10-03-06, 07:43 PM   #60
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Quote:
Originally Posted by mollusk
Doesn't n go down as well?
For simplicity, we assume a perfect transfer... the moles in the cylinder al wind up in the tube. Of course we know that in reality, this isn't exactly true.

Quote:
Originally Posted by mollusk
The real reason that T goes down is that the expansion is fast enough to be adiabatic (no heat transfer) and is also doing work on the surroundings (PV work). The first law of thermodynamics, Q-W=delta(U) where Q is the heat transfer (in this case equal to zero), W is the work done on the surroundings, and delta(U) is the change in internal energy implies that the internal energy of the gas must decrease. Hence the temperature decreases.
Yeah. I implied a closed thermal loop but didn't want to throw another equation into the mix.
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Old 10-04-06, 06:10 AM   #61
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REAL men use pumps
Nah, real men inflate their tubes by blowing with their mouths.
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Old 10-04-06, 09:46 AM   #62
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Nah, real men inflate their tubes by blowing with their mouths.
That's also how you engage the autopilot on modern airliners...

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Old 10-04-06, 10:00 AM   #63
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Quote:
Originally Posted by khuon
For simplicity, we assume a perfect transfer... the moles in the cylinder al wind up in the tube. Of course we know that in reality, this isn't exactly true.



Yeah. I implied a closed thermal loop but didn't want to throw another equation into the mix.
you guys are some brainiac mofos. blows me away when people start tossing out stuff like that. i thought high school algebra was hard. i've always been a severly right brained/visual learner (left handed/dyslexic/artist) and found math really hard. i'm curious, would you guys say you're left brained type/ analytical learner types or what?
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Old 10-05-06, 09:15 AM   #64
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Quote:
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you guys are some brainiac mofos. blows me away when people start tossing out stuff like that. i thought high school algebra was hard. i've always been a severly right brained/visual learner (left handed/dyslexic/artist) and found math really hard. i'm curious, would you guys say you're left brained type/ analytical learner types or what?
I hope that from my answer it is obvious that I am pure right brain. With a tendency toward analytical thought. I used that answer since the inception of CO2 cartridges and for about 10 years no one had ever questioned my answer, that is until you medaling kids came along.
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Old 10-05-06, 12:19 PM   #65
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Quote:
Originally Posted by Surferbruce
i'm pretty much over co2. costs too much, fails too often, and with 2 cartridges weighs more than the mini morph. ?
I have not had a problem with my Co2 system, it works like a charm.

I will have to tell you though..... I hate flats, so I have been riding SP Nimbus Armadillos. Jeez, I haven't flatted in so long, I think I may have forgotten how to fix a flat.

lw
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Old 10-05-06, 12:23 PM   #66
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Quote:
Originally Posted by Waxbytes
CO2 has molecules that are small enough to escape between the tube's butyl rubber molecules. So a tire inflated with CO2 goes flat quicker than one inflated with air.

It's not the size of the molecules, it's their viscosity. We covered this in an earlier thread. CO2 is more slippery and slithers through the rubber.
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Old 10-05-06, 12:28 PM   #67
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Quote:
Originally Posted by Phantoj

I've always been a pump user, but I'm thinking about switching over to CO2, not so much for the weight, but because it's quite a bit smaller. Where's a good place to put a pump on a modern road bike?
Mine sits in a bracket held onto the bottom tube by the bottle cage screws.
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Old 10-05-06, 03:28 PM   #68
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And 7 speed derailleurs, VCR's and film cameras (none of that newfangled digital technology for them). We shall boldly march into the 1980's at your side....
It just dawned on me that I use all of those things.
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Old 10-05-06, 03:35 PM   #69
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It just dawned on me that I use all of those things.

That may seem retro to some, but not nearly as mind blowing as some of the topics on this forum. The concerns shared here do make me wonder what some of the weenies on BF would do about REAL issues?

'Is the cold bad for my bicycle'? Come on boys. And don't bogart that joint. Let ME have some of what you're smoking when you post this stuff!
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Old 10-05-06, 04:44 PM   #70
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Soooo, what mini-pump works w/o pumping a bizillion times? I use CO2 because my m/p takes for-ever to get past 60psi...
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Old 10-05-06, 05:11 PM   #71
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I used to carry a mini-pump, but I just hate having anything tied to the frame. Plus, having to pump for 5 minutes with a mini-pump sucks. As long as it works properly, you cannot beat the convenience of CO2.

A couple of CO2 cartridges and a cell phone has worked for me for thousands of miles. Of course, I have probably cursed myself to three flats on my ride home today
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Old 10-05-06, 06:07 PM   #72
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Quote:
Originally Posted by hammond9705
I'm sticking with my CO2. If it fails me (it hasn't yet in lots of flats) then I've got my cell phone.

Bob
Amazing.
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Old 10-05-06, 07:42 PM   #73
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Quote:
Originally Posted by blue_nose
I used to carry a mini-pump, but I just hate having anything tied to the frame. Plus, having to pump for 5 minutes with a mini-pump sucks. As long as it works properly, you cannot beat the convenience of CO2.

A couple of CO2 cartridges and a cell phone has worked for me for thousands of miles. Of course, I have probably cursed myself to three flats on my ride home today
Here in AZ, goat heads actually move toward bicycles even when the bike is standing still. Goat heads have actually broken into my garage and attacked my tires overnight. Although not common, I have had 2 3-flat rides in 4 years. I have a Topeak attached to the bottle cage bracket. I also carry a little CO2 air chuck and 2 16 gram canisters. There is also a spare "good" tube in my seat bag. The topeak is brand new because when I last tried using it, the pumping plunger came right out.

Flats are part of the riding experience. 3 on one ride is really irritating though.

And, it is some kind of rule: Forgeting to take the cell phone actually enrages goat heads and causes flats.
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Old 10-05-06, 07:53 PM   #74
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I have a CO2 pump with a manual backup. Unfortunately, the manual portion failed. Got a high pressure mini topeak with guage on order.
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Old 10-05-06, 09:27 PM   #75
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OK, now I get it. Some weenies here use their CO2 cartridges like whippets.

That would explain much of this thread.
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