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Innertube Porosity?
Steve Sr. writes:
I was wondering if anyone has any information on the porosity of various kinds / brands of inner tubes. By porosity I mean the very slow leakdown rate that occurs with sound tubes over days and weeks. You know... The reason you are always topping up your tires to prevent pinch flats. My current interest is any differences between the regular and "lightweight" road tubes of the same size. I am also wondering if "brand" makes any difference as well. I doubt that anyone has numbers but you could buy one and see if you can put up with its inflation demands. That is what counts isn't it? I never liked daily pumping of tubulars with latex tube especially because we did that with a Silca frame fit pump. It is one of the things that inspired me to design a two stage pump. Jobst Brandt |
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#2
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Innertube Porosity? = Le Cycle to the rescue (as usual)
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#3
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Innertube Porosity? = Le Cycle to the rescue (as usual)
I use extra-heavy thorn-resistant tubes and lose only about 1% of the pressure per day, in my 110-lb. road bike tires. This is less than half of the loss with standard tubes. Obviously, if you have a puncture, the loss of pressure is much greater, so the TR tubes save me about 99% of the pumping I would be doing without them. I have averaged 2 flats a week with thin tubes and 2 a year with the TRs. Since I got both TR tubes and Kevlar-belted tires on one bike, I've had 0 flats with it. Another thing is, that the amount of loss goes up greatly with higher pressure tires. Despite having TR tubes, I have to pump up my 110-lb. tires about twice as often as the 65-lb. ones on my mountain bikes. Steve McDonald |
#4
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Innertube Porosity?
I notice the opposite effect, which I find curious. That is, a completely deflated inner tube, with its valve (presta) tightly closed, partially reinflates after several days if it is not tightly compressed. That is, when I attempt to roll it back up I have to again loosen the valve to let out some air. What causes this? Is it leakage at the valve? This occurs on more than one brand of butyl inner tube. Joe |
#5
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Innertube Porosity?
Joe Riel wrote:
I notice the opposite effect, which I find curious. That is, a completely deflated inner tube, with its valve (presta) tightly closed, partially reinflates after several days if it is not tightly compressed. That is, when I attempt to roll it back up I have to again loosen the valve to let out some air. What causes this? Is it leakage at the valve? This occurs on more than one brand of butyl inner tube. Joe Tubes lose air because they are under high pressure relative to the atmosphere, which causes diffusion from the tube back to its surroundings. If you push out all of the tube and seal it, now the atmosphere is at a higher pressure than the tube. So it would make sense that air would diffuse back into the tube, right? -- Paul M. Hobson Georgia Institute of Technology ..:change the f to ph to reply:. |
#6
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Innertube Porosity?
"Paul Hobson" wrote: (clip) So it would make sense that air would diffuse back into the tube, right? ^^^^^^^^^^^^^^^^ Doubtful. It would be virtually impossible to squeeze a tube enough to force ALL the air out. Since the walls are flexible, the pressure in the tube will be equal to the atmospheric pressure outside it. Possibly the tube is put away cold, and then warms up afterward, causing the air inside to expand. Could it be that the rubber "outgasses," releasing a little air after the tube is deflated? |
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Innertube Porosity?
Leo Lichtman wrote:
"Paul Hobson" wrote: (clip) So it would make sense that air would diffuse back into the tube, right? ^^^^^^^^^^^^^^^^ Doubtful. It would be virtually impossible to squeeze a tube enough to force ALL the air out. Since the walls are flexible, the pressure in the tube will be equal to the atmospheric pressure outside it. Possibly the tube is put away cold, and then warms up afterward, causing the air inside to expand. Could it be that the rubber "outgasses," releasing a little air after the tube is deflated? well yeah. I didn't mean that one would create a vacuum. But if you fold a tube, push it flat, then seal the valve, it'll be under a negative gage pressure. That's all I was trying to say. The temperature-driven scenario is definitely plausible - especially if you have to change a tube on the road this time of year. No clue on the outgassing though. \\paul -- Paul M. Hobson Georgia Institute of Technology ..:change the f to ph to reply:. |
#8
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Innertube Porosity?
Joe Riel wrote:
I notice the opposite effect, which I find curious. That is, a completely deflated inner tube, with its valve (presta) tightly closed, partially reinflates after several days if it is not tightly compressed. That is, when I attempt to roll it back up I have to again loosen the valve to let out some air. What causes this? Is it leakage at the valve? This occurs on more than one brand of butyl inner tube. Joe diffusion/porosity works both ways. elasticity of the tube will create a [minimally] lower pressure inside an evacuated tube, so there's no reason air won't migrate to reestablish equilibrium. |
#9
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Innertube Porosity?
Paul Hobson wrote:
Leo Lichtman wrote: "Paul Hobson" wrote: (clip) So it would make sense that air would diffuse back into the tube, right? ^^^^^^^^^^^^^^^^ Doubtful. It would be virtually impossible to squeeze a tube enough to force ALL the air out. Since the walls are flexible, the pressure in the tube will be equal to the atmospheric pressure outside it. Possibly the tube is put away cold, and then warms up afterward, causing the air inside to expand. Could it be that the rubber "outgasses," releasing a little air after the tube is deflated? [snip[ The temperature-driven scenario is definitely plausible - especially if you have to change a tube on the road this time of year. No clue on the outgassing though. If you change a tube, it probable has a hole in it. Sorry (I'm still on vacation) \\paul |
#10
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Innertube Porosity? = Le Cycle to the rescue (as usual)
Sandy wrote: From its November 2005 issue : Winner overall : Décathlon butyl (1,5€) Loser overall : Challenge latex (10€) INDEFINITE PRONOUN ALERT! What is "its"? -- Tom Sherman - Fox River Valley |
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