|First blown tire today...||Matno|
May 12, 2003 7:17 PM
|Not to mention my first flat in over 3 years! I guess my new tires aren't as tough as the old ones were. Serfas Seca RS. They look good, but I guess the "flat protection" doesn't hold up well to glass. (Then again, what does?) The hole isn't huge, so I superglued it (Krazy Glue actually) then added 3 layers of duct tape to the inside just to be safe. We'll see how well it holds up. I have a feeling I should carry a spare tire with me on the morning 35 miler tomorrow... I just don't like the idea of carrying something that big. I guess maybe I'll be using the Camelbak this time. Unfortunately, my spare tire doesn't match! (I'm such a slave to fashion!) Guess I'll have to buy another...
On another note, I used a CO2 pump for the first time. I was impressed with how fast they work! Two quick taps on the lever and the tire was at full pressure. I'm not sure how many psi it was, but it was comparable in stiffness to what I usually ride (about 100). This was in a 700x23c tire. What really surprised me was that after I got home and fixed things up, just out of curiosity I added the remaining CO2 to my new tube. It filled it up enough that it could be ridden home in a pinch! I was under the impression that a 12oz CO2 cartridge would only pump one tire up to 90 psi, but I filled mine up almost twice. Hmmm.
|quick note on CO2||lonefrontranger|
May 12, 2003 8:45 PM
|you may already know this, but CO2 is only a "temp" fill. It's good enough to get you home but won't last the typical week most roadies go before remembering to pump their tires back up. The stuff leaks slowly out the tube by osmosis (smaller molecule I think; science geeks anyone?), similar to what you'd see with a latex tube, so that in about 24 hours you'll have about 40-50 psi in that tire and wonder why it went "flat" again or if you have a slow leak.
Best bet is to let out all the CO2 as soon as you get home and pump it up with plain ol' air.
|2nd law thermodynamics: entropy, chem concentration & ficks law||telebiker|
May 12, 2003 9:57 PM
|The pure CO2 in the tube, surrounded by an atmoshpere of 78% Nitrogen and 21% Oxygen results in a thermochemical pressure to pull the CO2 out of the tube. (It also pulls O2 and N2 into the tube). The atmoshpere is so large that the tube's CO2 never changes the CO2 concentration in the atmosphere, but the tube is so small, the O2 and N2 concentrations increase dramatically, slowing the exchange of N2 and O2 into the tube, but the Co2 continues to leak out at the same speed. The result is more CO2 out than N2/O2 in equals a flat.
The pure CO2 in a bike tube is equivalent to the Helium leaking out of a helium balloon faster than an ordinary air filledd baloon(the pure helium balloon deflates faster).
We all breathe by a similar mechanism. CO2 dissolves readily in our blood but the concentration of CO2 is so low in the atmosphere, it is sucked out of our blood in our lungs without sucking out the Oxygen from our blood. In biology, this is Fick's Law:
the net diffusion rate of a gas across a membrane (our lungs or the tube) is proportional to the difference in partial pressure, proportional to the area of the membrane and inversely proportional to the thickness of the membrane.
Fick's law also explains why thin tubed latex or extra light butyl tubes leak faster (thinner membrane is a faster leak).
In summary, filling the tube with pure CO2 produces an additional thermochemical force (entropy or diffusion or fick's law, depending on your science degree) moving the CO2 out of the tube into the atmoshpere. When you pump your tubes with air, this additional thermochemical force is much smaller because its the same N2/O2 mix inside and outisde the tube / tire.
|Thank you Professor.Is there homework tonight? :-) nm||PEDDLEFOOT|
May 13, 2003 4:42 AM
|It all sounds good, but it's mostly wrong||Continental|
May 13, 2003 5:45 AM
|The primary reason for the faster pressure loss for tires filled with CO2 compared to tires filled with air is that the CO2 passes through the rubber faster than oxygen and nitrogen. Or, in other words, for rubber CO2 has a permability coefficient about 5 times higher than oxygen and about 10 times higher than nitrogen. So if you have two identical tires, one filled with air and one filled with CO2 to identical pressures (say 100 psig), the tire filled with CO2 start to loose pressure nearly 9 times faster than the tire filled with air because the CO2 passes through the rubber more readily. This faster pressure loss for the CO2 tire would occur even if the tires were place in a room with no air, and even if the room were filled with C02 at atmospheric pressure.
The difference in what telebiker terms "chemical pressure" is a secondary effect that also contributes to faster loss of CO2 than air, but it is a much smaller effect than the difference in permeability. It only makes about a 15% difference in the rate of pressure loss, compare to an 800% difference due to the permeability effect. Telebiker and other geeks, check the link below:
|Good science, wrong application. nm||Mel Erickson|
May 13, 2003 7:52 AM
|Contental doesn't understand ficks law is permeability||telebiker|
May 13, 2003 9:34 PM
|This might be a silly semantic arguement. Fick's law encompasses your understanding of "permeability". Maybe you"re a silly Brit, and that explains this silly arguement (Monty Python style, with Vikings , men goose stepping and women chanting the benefits of pork luncheon meat).
Diffusion is the mechanism by which components of a mixture are
transported by means of random molecular (Brownian)
Permeation is the ability of a diffusant to pass through a body - dependent on both the diffusion coefficient,
D, and the solubility coefficient, S, ie, permeability coefficient, P = D.S). Flynn
et al. cite Berthalot as postulating,
at the beginning of the nineteenth century, that the flow of mass
by diffusion (ie, the flux), across a plane, was proportional
to the concentration gradient of the diffusant across that plane.
In the mid-1800's, Fick
introduced two differential equations that quantified the above
statement for the case of transport through thin membranes. Fick's
First Law states that the flux, J, of a component of
concentration, C, across a membrane of unit area, in a
predefined plane, is proportional to the concentration differential
across that plane (see note), and is expressed
Fick's Second Law states that the rate of change of concentration
in a volume element of a membrane, within the diffusional field,
is proportional to the rate of change of concentration gradient
at that point in the field, as given by:
where t = time.
If you're 'contental', why are you citing UK web sites?
We have wasted enough time. I'm sorry I took the time to register and post.
|regardless of reason, LFR is correct||DougSloan|
May 13, 2003 6:26 AM
|Empty and refill when you get home.
BTW, if a tire worries me enough to want to carry a spare, I won't use it.
|Sorry, but I don't buy that.||MR_GRUMPY|
May 13, 2003 5:13 AM
|First, a CO2 molecule is bigger than either an O2 or an N molecule, so it should leak out slower than either of them.
Secondly, I've never been able to get more than 90 pounds out of a 12g cart, in a 23mm tire.
Third, CO2 is actually better for a tube than O2, because it won't react with the rubber.
|Ficks Law has nothing to do with molecule size||Mel Erickson|
May 13, 2003 5:48 AM
|It's simply the difference in concentration of a gas on either side of a membrane. Nature likes equilibrium and strives to attain it. Pure CO2 inside a tube and regular N2 O2 in the atmosphere sets up a disequilibrium. To equalize the concentrations of the gases the CO2 is pulled out. Molecule size has nothing to do with it. I've often gotten more than one full fill out of a CO2 cart but certainly nowhere near enough to fill a second tube to riding condition. One fill is the practical limit. CO2 may be better for the tube but it won't last long. I won't be refilling my tubes with CO2 each day to preserve them. At that rate I could replace my tube each week instead and save money over buying CO2 carts.|
|I always thought that "Flick" ........||MR_GRUMPY|
May 13, 2003 6:09 AM
|had something to do with snot on your finger.
We're talking theory here, not practice. Really, how much faster will CO2 migrate out of a tube than a mix of O2,N, and CO2 ???????
|It's permability differences, not "disequilibrium" causing the pressure loss||Continental|
May 13, 2003 6:09 AM
|See post above, and if you are really a geek, see the link above. CO2 actually dissolves in the rubber and passes through the rubber much more easily than oxygen or nitrogen. The "disequilibrium" is a much much smaller effect than the permablity differences.|
|Ow..., you're making my head hurt..||MR_GRUMPY|
May 13, 2003 6:20 AM
|It would be interesting to inflate two tires to 125 pounds. One with just air, and the other with CO2, and check back every 24 hours to check the leakage.|
|I've done that||Mel Erickson|
May 13, 2003 7:57 AM
|Every time I have a flat and fill the spare with CO2. Now we have a test because the tire that didn't flat is filled with regular old air and the flatted tire (new tube, not patched) is filled with CO2. The pressures are close enough for this test and the CO2 filled tire will have lost much more air in 24 hours.|
|Second fill got me home||pitt83|
May 13, 2003 6:10 AM
|After a flat, I changed the tube. When filling, the cold of the expanding gas froze the presta valve. I didn't let it thaw and snapped it. Oh crap! No patches to fix the original and only one cartridge with me.
So, I remember a tip from bicycling magazine. I put the original flatted tube back it after tying up the hole. My only 1/2 spent CO2 cartridge had enough left to make the tire rideable. I didn't want to weigh down that wheel, so I rode standing for the 25 miles home. Hurt like hell, but I made it.