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Why do bicycle tires/tubes lose air so quickly?

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Why do bicycle tires/tubes lose air so quickly?

Old 11-30-09, 03:06 PM
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Cross Creek
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Why do bicycle tires/tubes lose air so quickly?

I'm not a mechanic, but I do work on my own bikes and have built a few wheels and torn down a few bikes and re-built them successfully (just to let you know my experience level). My question is this--If my car, running 30-34 psi, can hold the same pressure for several weeks to a few months, why is it that my bikes (whose tires aren't all that much cheaper than auto tires, and far more expensive by the ounce) lose so much pressure that they need topping off daily? These are narrow clinchers on MTB rims and Schrader valves, but I've seen this on road bikes with Presta valves, too. If left unridden and suspended, these tires will lose essentially all their pressure in a matter of days or a few weeks at most. I've ruled out loose valve cores and leaky tubes. This just seems to go with the territory, but it seems like the state of the art could be a lot better than it is, or am I doing something wrong? Would going to one of the tubeless clincher systems work any better than tubes?
-CC
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Old 11-30-09, 03:09 PM
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Cars have a LOT more air volume in their tires, nevermind the actual pressure.

That's why road tires can't hold much air for long....mountain tires hold much longer for the most part.
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Old 11-30-09, 03:09 PM
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Think about how huge the air volume is in a car tyre vs a bike tyre. Higher priced tubes/lighter tubes will also leak air faster. Butyl < polyurethane < latex in terms of air loss over a period of time. The higher pressure being pumped into (30-34psi is not a typical road psi even for fat tyres) doesn't help either. My 700x23 road tyre with kenda butyl (cheapass tubes) only went down from 120-80 in 2 months.
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Old 11-30-09, 03:53 PM
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Also CO2 will leak out of tubes at a faster rate than regular air. So if you use a CO2 inflation-device on the road - get regular air into it as soon as possible.
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Old 11-30-09, 04:02 PM
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Originally Posted by Gene2308 View Post
Cars have a LOT more air volume in their tires, nevermind the actual pressure.

That's why road tires can't hold much air for long....mountain tires hold much longer for the most part.
It's not just the volume. A tubeless car tire is a different construction than a butyl tube. The walls are thicker in the car tire with different properties that may effect the diffusion of the gases across the barrier. Additives like silica and other materials may effect the diffusion. A bike tube is very thin and only composed of rubber. It doesn't provide much resistance to the movement of the gas through the membrane.

A higher pressure differential will move the gas across the membrane faster too.
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Old 11-30-09, 05:08 PM
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If your bicycle tires were as thick as car tires you would not like them at all.
If your car tires had as much pressure as your road bicycle tires you would not like them either.

Al
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Old 11-30-09, 07:55 PM
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Ok, so we have constant innovation in frame design/materials, drive trains, brakes, etc. And then there's the man in space, deep water diving, nuclear technology stuff. Anyone working on a tube material that doesn't leak air like a sieve?
-CC
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Old 11-30-09, 07:58 PM
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Originally Posted by Cross Creek View Post
Ok, so we have constant innovation in frame design/materials, drive trains, brakes, etc. And then there's the man in space, deep water diving, nuclear technology stuff. Anyone working on a tube material that doesn't leak air like a sieve?
-CC
You make it sound like it's a monumental problem. It's not.
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Old 11-30-09, 08:12 PM
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Originally Posted by operator View Post
You make it sound like it's a monumental problem. It's not.
+1

who gives a damn
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Old 11-30-09, 08:16 PM
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If this is such a problem, you fix it.
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Old 11-30-09, 09:57 PM
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Gee, I was just trying to learn something about why tires stillleak so badly when everything else on bicycles gets so much attention. If you guys love pumping up tires every day so much, have fun.
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Old 11-30-09, 10:04 PM
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It's good exercise.
I don't pump daily. A couple of times a week, probably.
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Old 11-30-09, 10:17 PM
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My 25mm road bike tires stay in the rideable range for just about two weeks.
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Old 11-30-09, 10:41 PM
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Why do people post the same dang topic week in week out when the word "search" is clearly listed in the tabs and menus for this website?

Why do I irritate people?

Why do I only have three friends in life...wife, son, and my bike?

Why did I post?

Why am I saying "nevermind"?

=8-)
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Old 11-30-09, 10:54 PM
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CC, I don't know what your particular tire/tube problem is; but I've been running 55-60psi in my mtb tires for years, and I have to top them off maybe 1x/month.

I have noticed that the longer your bike SITS, the faster the tires will lose air. A bike ridden daily will retain air in the tires better -- that's been my experience.

If you're really that uptight about it, consider a couple things -- Slime tubes or, if you want to be creative, coat the inside of your tires with automotive fix-a-flat before you re-mount them. Pretty effective sealer for the rubber, and is compatible with Stan's, for you tubeless users out there.
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Old 11-30-09, 11:05 PM
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because your friends walk by and tap your valve core to piss you off.
duh.
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Old 12-01-09, 12:46 AM
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Old 12-01-09, 12:47 AM
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Originally Posted by Cross Creek View Post
Gee, I was just trying to learn something about why tires stillleak so badly when everything else on bicycles gets so much attention. If you guys love pumping up tires every day so much, have fun.
They already have a fix for that. It's called an "air compressor".
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Old 12-01-09, 12:47 AM
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Originally Posted by mrrabbit View Post
Why do people post the same dang topic week in week out when the word "search" is clearly listed in the tabs and menus for this website?)
Well, at the moment, Search isn't working.
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Old 12-01-09, 12:54 AM
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Has anyone else noticed that patched tubes need topping up more than new tubes?

Or do I just suck at patching...
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Old 12-01-09, 01:04 AM
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Yep.




I mean, option 2.
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Old 12-01-09, 03:10 AM
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Anyone compared the thickness of an auto tyre compared to a bike tyre & tube lately??? It also seems auto tyres have some sort of slick coating on the inside; might just be mould release compound. But I'm sure it helps to seal in the air.
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Old 12-01-09, 06:15 AM
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SLIME tubes don't leak. Had some on my mountain bike, only needed a top-up at 4-6 months.
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Old 12-01-09, 06:54 AM
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Originally Posted by Cross Creek View Post
Ok, so we have constant innovation in frame design/materials, drive trains, brakes, etc. And then there's the man in space, deep water diving, nuclear technology stuff. Anyone working on a tube material that doesn't leak air like a sieve?
-CC
Do you have latex tubes? While those give a more supple ride and perhaps some improvement in puncture resistance, they seep air at a much higher rate than butyl tubes. It's normal with latex to lose 50% or more of riding pressure overnight. This is unlikely, since if you had latex you'd know based on the price - you'd be complaining about having been gouged by the LBS.

I've also found the seepage rate with super light butyl tubes (notably the Performance Forte Lunar Lights, around 40 grams for a 700c x 25 mm) to lose pressure nearly as fast as latex.

If you don't have either of these and you're losing 50% per day, I think you have a leak that you haven't found. You might try a heavier tube such as a Michelin Airstop. Somewhere I read that these hold up better than many others. You might need to learn more techniques for finding leaks, or stop re-using old tubes (guilty!). Carefully check the inner surface of your tires for debris like a single fine steel wire from a disintegrated steel belted radial or truck tire. That stuff has weird effects.

My clincher wheels tend to stay at a riding pressure for around a week. I think that's what's normal. Your situation sounds like there's a problem, or your tubes are too light for practical use.

Last edited by Road Fan; 12-01-09 at 07:00 AM.
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Old 12-01-09, 07:15 AM
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Thorn-resistant inner tubes have much, much thicker rubber and I only have to top up every 6-8 wks with daily riding.

Still, I'm with the OP on this, why are tires/tubes the most unreliable part of the modern bike? (Yes, I know Conti Gatorskins, Specialized Armadillos, and Schwalbe Marathons exist . . . ) The fact is, tube technology sucks, just one example is the incidence of cut valves. Specifically, Bontrager has for several years been selling Presta valve tires that have inherently defective valves which separate from the rubber after a few weeks of riding. You guys who fetish-ize changing tires and and pumping tubes -- and thus showing your devotion to the sport -- probably don't give a dang.
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