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Old 08-28-12, 11:33 AM   #1
stevebiker
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Why do bike tires lose air?

My tires lose 20 pounds after a simple 15-mile ride on a paved bike path. It's easy riding. But they always lose air. My girlfriend's bike tires also lose air. All bike tires constantly leak air, right?

I have Bontreger Hardcase Race Light tires. There's an inner tube, which I would think would keep air in. Unlike car tires, which don't have inner tubes, yet never lose air.

Why????
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Old 08-28-12, 11:36 AM   #2
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A bike tire/ tube is just a fraction of the thickness of a car tire, and it runs on a higher pressure. Air simply seeps through the rubber. The same happens to your car, but at such a slow rate that you'll be well challenged to notice it.
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Old 08-28-12, 11:52 AM   #3
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A bike tire/ tube is just a fraction of the thickness of a car tire, and it runs on a higher pressure. Air simply seeps through the rubber. The same happens to your car, but at such a slow rate that you'll be well challenged to notice it.
+1, the tubes and tires are porous to air and so they bleed. If you've had a balloon go soft in a day or two you've seen the same effect.

The rate of bleed depends on the wall thickness and exact material, and on the pressure which acts as a driving force. But the rate of pressure drop depends on one other factor, which is the ratio of surface to volume, As the size increases the ratio decreases so it'll take longer for a fatter tube (of the same material and thickness) to lose the same amount of pressure.

That's why narrow road tires need to be topped off every day or two, while HP mtn tires might go for a week.
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Old 08-28-12, 12:40 PM   #4
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Unlike car tires, which don't have inner tubes, yet never lose air.
Car tires do lose air, but with a larger volume and lower pressure they lose at a much slower rate than bike tires. Insufficient air pressure is the most common cause of tread wear on cars and trucks.
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Old 08-28-12, 01:53 PM   #5
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My tires lose 20 pounds after a simple 15-mile ride on a paved bike path.
All bike tires lose air... but that's way more than you'd get from normal permeation, it's a sign of a pinhole leak somewhere. I'd be really surprised if both your tires and both your GF's tires all lost the same amount of pressure in the same ride. I'd also be really surprised if any individual tire didn't lose the same amount of air in the same amount of time whether you were riding it or not.

Or it could be that you're introducing error when you measure the pressure loss.
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Old 08-28-12, 03:48 PM   #6
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Or it could be that you're introducing error when you measure the pressure loss.
+1

By the time you burp the valve (if applicable) and stick the pump head on you've lost at least 10 pounds on a narrow road tire.

If you're really losing 20 pounds every 1-2 hours (15 mile ride...) then your tires would be completely flat in a day and you have a slow leak to address,
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Old 08-28-12, 03:53 PM   #7
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It's a conspiracy -- the CO2 cartridge manufacturers are in cahoots with the tube manufacturers. Leaky tubes sell more CO2 cartridges.
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Old 08-28-12, 04:00 PM   #8
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It's a conspiracy -- the CO2 cartridge manufacturers are in cahoots with the tube manufacturers. Leaky tubes sell more CO2 cartridges.
So... Big Rubber is in bed with Big Gas
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Old 08-28-12, 04:02 PM   #9
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So... Big Rubber is in bed with Big Gas
The scales are falling from our eyes.
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Old 08-28-12, 04:08 PM   #10
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+1
By the time you burp the valve (if applicable) and stick the pump head on you've lost at least 10 pounds on a narrow road tire.
If you're really losing 20 pounds every 1-2 hours (15 mile ride...) then your tires would be completely flat in a day and you have a slow leak to address,
so glad you mentioned that, I was going crazy wondering why some tubes "needed" that and others don't. I was blaming my pump for the issue. giant tubes especially need a burp.

+1 on the leak.
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Old 08-28-12, 04:26 PM   #11
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Gah, no, you guys have it all wrong!

It's the Bicycle Tire Air Fairies, stealing your air! The more you have protecting your air the harder it is for the Bicycle Tire Air Fairies to take it. There just isn't a such evil little fairy for cars.
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Old 08-28-12, 05:56 PM   #12
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Most people never deal with air pressure in their car tires as oil change shop routinely check tire pressures and top off as needed. Then most of those same people raise he!! with bike mechanics because they don't have this air pressure "problem" with their cars.
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Old 08-28-12, 06:23 PM   #13
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Most people never deal with air pressure in their car tires as oil change shop routinely check tire pressures and top off as needed. Then most of those same people raise he!! with bike mechanics because they don't have this air pressure "problem" with their cars.
You're giving the oil change shop rats too much credit, and also car owners. Most people never notice auto tire pressure drop, it's a slow process and they just drive arounf with soft tires, not know or caring.

There's a big difference between car and bike tires and it isn't the tires. Most car owners don't care about cars. They just want to get from place to place in comfort listening to a decent stereo. But bike owners are different, at least the ones we see on the forum. They're actually dialed into their bikes, and concerned about every creak or click, and notice things like soft tires.

Also soft tires are harder on the engine, something that's easier to notice when you're the engine.
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Old 08-28-12, 08:16 PM   #14
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Originally Posted by stevebiker View Post
My tires lose 20 pounds after a simple 15-mile ride on a paved bike path. It's easy riding. But they always lose air. My girlfriend's bike tires also lose air. All bike tires constantly leak air, right?

I have Bontreger Hardcase Race Light tires. There's an inner tube, which I would think would keep air in. Unlike car tires, which don't have inner tubes, yet never lose air.

Why????
I have the same exact tires as you do. Mine are 700x32c sized. I lose about 10 pounds over a month in them. I don't know why but I lose very little in those tires.

On my other bikes say with 700x23c sized tires, I will lose about 20 pounds within a week or so.
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Old 08-29-12, 01:05 AM   #15
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I used to be in rubber chemical research and did a lot of testing on componds.

Tubes are made from Butyl rubber. Its typically impervious to air and holds pressure well. Tubeless tires have a ply of butyl so they can hold air, becuse the other compounds in the "carcass" cant hold air.

I thnk the older product was made a LOT better than what we have now. Materials in todays product seem to be poor. All you can try is another set of tubes and see what leaks the least. Dont overlook the obvious like a leaky valve. That you can do something about.

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Old 08-29-12, 01:20 AM   #16
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They want Equality, and will always leave their high pressure job, until,
they become equal with the outside .. . levelers !
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Old 08-29-12, 01:28 AM   #17
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"old tubes are better"

this is interesting, I have a bike from the 50's and inside tubes are some super soft red tubes, dont know the material but they hold air just fine, 60 year tubes super soft- something has changed.
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Old 08-29-12, 10:15 AM   #18
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You just need fatter tires. I lose about 5 lbs a month in my tires on my Xtracycle, they are 1.95 running at 60 psi with extra thick inner tubes. I don't care about weight on the Xtra.
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Old 08-29-12, 10:23 AM   #19
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I ride my Pugsley just about every day, and checked the tires this morning after about a week and a half. I was running 11 in the front and 13 in the back. This morning, it was 7 in the front, and 17 in the back????
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Old 08-29-12, 11:49 AM   #20
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Also some tubes just hold air better than others. We are told that there are less than 5 manuf world wide of bike tires. However I have found that Bontrager tubes seem to leak down less than other brands.
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Old 08-29-12, 12:46 PM   #21
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Also some tubes just hold air better than others. We are told that there are less than 5 manuf world wide of bike tires. However I have found that Bontrager tubes seem to leak down less than other brands.
Some things are repeated so often that people start believing them as fact. The 5 tire factory myth is one of these.

There are many more than 5 tire/tube makers. It doesn't matter if you count multiple factories owned by one company as one, there are still plenty more than 5 or 10. However it is true that there are more brands than actual makers. Bontrager is one example. They don't have a factory and their tires and tubes are made by one (or more) of the major producers.
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Old 08-29-12, 01:04 PM   #22
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My tires lose 20 pounds after a simple 15-mile ride on a paved bike path.
Slow leak (or you are letting air out when you check).
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Old 08-29-12, 01:10 PM   #23
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Slow leak (or you are letting air out when you check).
+1, normal bleed is 5-20#s per day or less. Significantly more than that is a leak. Most of these stable, slow leaks are at the valve.
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Old 08-29-12, 01:20 PM   #24
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I call shenanigans on modern cars riding around with flat tires, unless the owner is just lazy. Most all cars made in 2007+ have TPMS, and even if the tires are uniform, will warn you at 29psi or so. I typically need a cold winter to lose appreciable tire pressue, and never lose a measurable amount for months at a time in my car.
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Old 08-29-12, 01:57 PM   #25
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I call shenanigans on modern cars riding around with flat tires, unless the owner is just lazy. Most all cars made in 2007+ have TPMS, and even if the tires are uniform, will warn you at 29psi or so. I typically need a cold winter to lose appreciable tire pressue, and never lose a measurable amount for months at a time in my car.
Nobody said "modern."
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