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why some tires/tubes hold air forever

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why some tires/tubes hold air forever

Old 06-13-05, 11:58 AM
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dbg
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why some tires/tubes hold air forever

I'm not supplying the answer, I'm asking the question. I have some tires that have remained rock solid at full pressure for years and others that need pumping before every ride. Most fall somewhere in between. I don't mind checking and pumping before every ride, but I see my kids and others in the neighborhood riding on totally deflated tires. Clearly they need year long retention. What makes a tire/tube lose air? Why do some hold it indefinitely? What can one do to improve air retention?
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Old 06-13-05, 04:40 PM
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Originally Posted by dbg
I have some tires that have remained rock solid at full pressure for years
Lucky!
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Old 06-13-05, 05:16 PM
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At work, I have a shed with about 20 police mountain bikes inside. Generally, they all sit there pretty much unused during the winter; and they all go nearly flat.
You find enough air to keep 'em round, but usually only 15-20 pounds of pressure.

I can see very heavy butyl rubber tubes holding air for a long time, but years?
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Old 06-14-05, 08:51 AM
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Years (plural) may be a slight exageration, but I have one set that was over a year of full pressure before I moved them to another rim set. And I have several that last all summer long with less than 10% pressure loss. But invariably at least one tire on all my kids' bikes goes flat in a couple of weeks. I want THOSE tires to last all summer.

Is it a valve thing -- and being lucky to get a hardy valve? Are thick tubes better? Are expensive tubes better? I don't realy know.
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Old 06-14-05, 09:29 AM
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it can be a big difference. on my road bike maybe every two weeks I had to pump the tires. part of that was I rode 10 miles twice a day 5 days a week. I find the more I ride the longer they hold air. my bent was the same way till I got a 1" 26" specialized tube it needs air two or three times a week. so maybe some brands are better then others.
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Old 06-14-05, 09:56 AM
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There's a huge difference in tubes. Lots of high-end roadie tubes are latex, just like the stuff you'd use on your.... Anyway, it leaks. Weight-weenie guys like super-thin tubes that are hardly thicker than...Those other things.

Those of us that prefer some durability will go for a heavier tube. If you ride the roads in my area, like most, stoutness wins out over weight savings.

Sooner or later we'll have tubeless tires common for both road and MTB tires, maybe more air retention as well.
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Old 06-14-05, 10:27 AM
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Originally Posted by dbg
I'm not supplying the answer, I'm asking the question. I have some tires that have remained rock solid at full pressure for years and others that need pumping before every ride. Most fall somewhere in between. I don't mind checking and pumping before every ride, but I see my kids and others in the neighborhood riding on totally deflated tires. Clearly they need year long retention. What makes a tire/tube lose air? Why do some hold it indefinitely? What can one do to improve air retention?
Having owned over 40 cars, numerous motorcycles, a few bicycles, etc., I have yet to find these mysterious tires you allude to. Just by virute of temperature changes is it impossible for a tire to hold the same pressure for any length of time let alone "full pressure for years." I would lock up said tire, and call Guiness!
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Old 06-14-05, 11:25 AM
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well with my bikes i dont lose pressure on any of my bike my 2 year old totally stock redline single x is still full pressure with just a slight loss of air in like the last week due to my stupidity and runnin on tires they are as smooth as slicks and my road bike ive only pumped up once
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Old 06-14-05, 11:55 AM
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Damn Fick and his laws. Diffusion of air through the rubber happens due to gradients in pressure. The higher the pressure, the more driving force there is for diffusion to occur. I also believe it has something to do with the quality of the rubber/metal seal at the valve. I find a great variation in the ability to hold pressure within the same brand of tube.

In the end, retention really isn't worth worrying about. Once you buy the pump, air is free (save for the few calories spent operating the pump). It takes all of one minute to pump up before a ride.
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Old 06-14-05, 01:45 PM
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As most have stated, this seems...well...impossible. Due to factors of temperature change and the normal diffusion of the air through the tube's rubber, it's hard to imagine a tire can hold it's pressure for a couple months - let alone 12 or so of them!

No offense, it's just hard to picture. I suppose if you have some "slimed" tubes, or equivalent, and ride them everyday keeping the "slime" coating the entire tube then it could happen, it just doesn't seem right.
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Old 06-14-05, 02:13 PM
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Originally Posted by KleinRider
As most have stated, this seems...well...impossible. Due to factors of temperature change and the normal diffusion of the air through the tube's rubber, it's hard to imagine a tire can hold it's pressure for a couple months - let alone 12 or so of them!

No offense, it's just hard to picture. I suppose if you have some "slimed" tubes, or equivalent, and ride them everyday keeping the "slime" coating the entire tube then it could happen, it just doesn't seem right.
I can think of one way a bicycle tube could hold pressure for an extended period of time - the manufacturer of said tube applied for and received an exemption from the laws of physics.
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Old 06-14-05, 03:54 PM
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Originally Posted by skydive69
I can think of one way a bicycle tube could hold pressure for an extended period of time - the manufacturer of said tube applied for and received an exemption from the laws of physics.
Nah, you just keep the air pressure in your house HIGHER then the tires, any transfer is from OUT to INTO the tires. In fact, you wouldn't even need a pump, just bring the tires inside, and after a while, they would be full again. Of course, you wouldn't be able to tell untill you took them back outside. I haven't worked out all the details regarding airlock doors or how you would keep from getting the bends when you go out, but it's a small price to pay for always having tires at full pressure.

Steve
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Old 06-15-05, 08:01 AM
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Originally Posted by Mentor58
Nah, you just keep the air pressure in your house HIGHER then the tires, any transfer is from OUT to INTO the tires. In fact, you wouldn't even need a pump, just bring the tires inside, and after a while, they would be full again. Of course, you wouldn't be able to tell untill you took them back outside. I haven't worked out all the details regarding airlock doors or how you would keep from getting the bends when you go out, but it's a small price to pay for always having tires at full pressure.

Steve


I guess you'd have to have decompression chambers. You'd have to have some type of seperate one for the bike so you wouldn't lose as much pressure in the tires as you decompressed yourself.
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Old 06-15-05, 11:30 AM
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Apparently Nitrogen diffuses like 3-4 times slower than compressed Air does... and Nitrogen makes up like 78% of Air. I noticed that my tires in storage will not stay "full pressure" too long and will pretty quickly become "a little low" and stay that way for a long time before becoming "too low" at the end. Perhaps the first PSI's lost in a tire is mostly oxygen and leaves behind the Nitrogen which will disappear much slower? Anybody fill their tires with Nitrogen? I hear that some guys in the Tour de France use N2 to fill their tires, partly for this reason... not sure if it's true though.
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Old 06-16-05, 11:12 AM
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I think Sebach is on to something. After topping off a tire a few times, it seems like they hold pressure longer. Perhaps the early deflation is the oxygen leaving, so after a few top offs, you've got a much higher nitrogen content?

Sorry mentor58, I don't think it works that way. If you take a tube of air into a room with higher pressure, it will crush the tube. You would need a rigid tube in order for a higher external pressure to "inflate" the tube.
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Old 06-16-05, 12:14 PM
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http://www.innovativebalancing.com/Nitrogen.htm

From the some company in the tire business.
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