# Bicycle Mechanics - Why do bike tires lose air?

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View Full Version : Why do bike tires lose air?

stevebiker
08-28-12, 12:33 PM
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????

dabac
08-28-12, 12:36 PM
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.

FBinNY
08-28-12, 12:52 PM
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.

Al1943
08-28-12, 01:40 PM
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.

DiabloScott
08-28-12, 02:53 PM
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.

FastJake
08-28-12, 04:48 PM
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,

ThermionicScott
08-28-12, 04:53 PM
It's a conspiracy -- the CO2 cartridge manufacturers are in cahoots with the tube manufacturers. Leaky tubes sell more CO2 cartridges.

DiabloScott
08-28-12, 05:00 PM
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

ThermionicScott
08-28-12, 05:02 PM
So... Big Rubber is in bed with Big Gas

:lol: The scales are falling from our eyes.

Rx Rider
08-28-12, 05:08 PM
+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.

ben4345
08-28-12, 05:26 PM
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.

DieselDan
08-28-12, 06:56 PM
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.

FBinNY
08-28-12, 07:23 PM
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.

bobotech
08-28-12, 09:16 PM
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.

speedy25
08-29-12, 02:05 AM
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.

-SP

fietsbob
08-29-12, 02:20 AM
They want Equality, and will always leave their high pressure job, until,
they become equal with the outside .. . levelers ! :rolleyes:

puchfinnland
08-29-12, 02:28 AM
"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.

crackerdog
08-29-12, 11:15 AM
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.

Hendricks97
08-29-12, 11:23 AM
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????

rydabent
08-29-12, 12:49 PM
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.

FBinNY
08-29-12, 01:46 PM
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.

njkayaker
08-29-12, 02:04 PM
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).

FBinNY
08-29-12, 02:10 PM
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.

Noize4
08-29-12, 02:20 PM
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.

ThermionicScott
08-29-12, 02:57 PM
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." :rolleyes:

DieselDan
08-29-12, 04:13 PM
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.

I think the requirement has been since 2001 model year. Except for GM products, many TPMS systems are a bit over engineered with extra sensors. GM added a few lines of code to the existing ABS systems to detect drag from low tire pressure.

Ranko Kohime
08-30-12, 04:08 AM
I can vouch for no there being no in-tire system in Chevy S-10's, as late as 2003. And no apparent code in the ABS controller, either.

(Tested, drove for a week with intentionally low tires at 10 psi)

nhluhr
08-30-12, 12:31 PM
I think the requirement has been since 2001 model year. Except for GM products, many TPMS systems are a bit over engineered with extra sensors. GM added a few lines of code to the existing ABS systems to detect drag from low tire pressure.No, 2007 is the model year where all cars in the US were equipped with TPMS.
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.Each system operates slightly differently but on my Mazda for example, the TPMS activates approximately 6psi below the doorjamb specified pressure or 8psi above it. If you're driving around on a tire that is 6psi below the recommended pressure, you're going to be wearing them out badly and you are risking a blowout from heat-induced sidewall failure.

cny-bikeman
08-30-12, 02:35 PM
Well two days after the OP posted we are all just guessing. I see nowhere that we know how the OP is measuring the pressure before and after. However, given that he sees the same thing on two different bikes (4 tire/tube combinations) and presumably more than once, the only constant would be the pump and guage. If he is using a reservoir type pump a significant amount of air will go into the pump when the chuck is attached (even ignoring leakage in the process. If you pump a tire with that type pump, remove the chuck and immediately place it back on the valve the pressure will drop. The only good way to gauge pressure is with a stick guage and good technique - but even some leakage is not going to lose you more than a couple lbs of pressure.

speedy25
08-31-12, 04:57 AM
"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.

I have run across these tubes in motorcycles. I have only seen them with a Firestone brand name. I think they are all butyl instead of a blend. I also changed a motorcycle tire yesterday that had a nice tube in it. It was a "lively" rubber not like most of them. I've also had patches not wanting to stick on newer tubes.

-SP

3alarmer
09-01-12, 03:52 AM
Why do bike tires lose air?
It's Obama's fault..........................like everything else.

Uplah
09-01-12, 10:47 AM
If you don't use the bike for a week or so, do you find the tyres flat? Similarly, do you find the tyres are deflated after several rides (i.e., don't pump up after each ride)? If so, I suspect you have a slow leak - check for a pin-hole puncture or the valve itself. However, if the answer is no, then it might be what these guys have said - the way you're measuring tyre pressure...?

speedy25
09-04-12, 02:45 AM
There is no doubt there is a slow leak. Where its coming from is the REAL question. When you cant find punctures and the valve isnt leaking what do you do? If its an annoying rate of leakage, its new tube time. Maybe we need a thread about tube quality and leak rates. Between 5 bikes I have some that I have to inflate every time I ride and others that are fine for months.

-SP

DannoXYZ
09-04-12, 04:39 AM
A spray water-bottle with some soap in it will help find that slow leak. Spray it around the valve-stem first. Then all around the tyre.

Even better is to unmount the tyre and remove the tube. Then pump it up to about the same diameter as the tyre, or slightly larger. Then dunk it in a bucket/tub of water and look for bubbles.

speedy25
09-04-12, 04:50 AM
Those leaks that you can find with liquid are the easy ones. I'm talking about gases permeating the innertube at a slow rate you wont see.

-SP

DannoXYZ
09-04-12, 06:00 AM
Yeah, but 20psi in a 15-mile ride as the OP mentioned is a little excessive (45-60 minutes). A 1mm thick butyl tube will not let air permeate out that quickly. Even with the thinnest time-trial tubes, I have not seen more than 20psi in less than 24-hours.

jim p
09-04-12, 06:16 AM
This seems like a place for slime to be used. It would be nice if there was a better product which would be a very thin liquid so that only 1 ounce or less would provide all the stop leak task needed.

homechicken
09-04-12, 07:33 AM
When I was a kid in the 70's it was rare that I had to put air in my bike tires more than once every two to three weeks, and I road to school over some rough roads and through a field or two almost every day in addition to going to the store for my mom or the local toy/dime store or friends' houses. I have to top off my new bike's tires every time I ride, maybe once or twice a week. I have to do the same to my wife's. In most instances these days, cost wins over quality. No one says they make the best anything any more (at least not honestly), its "We sell more XX than anyone else in the world" . Through decades of manufacturers sacrificing quality for profit, the buying public has become conditioned to think "I know I'm going to have to replace this in a few months, or a couple of years, so what's the cheapest I can buy this for?" Buying durable goods is a constant struggle between my wife and I because of this very thing. She's driven by price, I'm driven by quality. I'll pay \$150 for something and buy it once while she'll spend \$20 on the same type of thing and have to replace it 10 times in 5 years, costing more money in the long run, and there are many more people like her than like me.

speedy25
09-05-12, 01:48 AM
See homechicken understands! :)

Another "taken for granted" item is a car thermostat. They almost NEVER failed back then, but now they are a maintenance item and can fail at any time.

So far nobody seems to have noticed what tubes are better at holding air. I'll make a note of anyhting I find when I swap tires if a certain tube does a better job. My old diamondback holds air well. I refill in about a months time about 5#. I never though to look at the tube brand when I changed tires though.

-SP

DannoXYZ
09-06-12, 04:33 AM
If you pick a touring heavy-duty thick thorn-resistant tube, you will not have to top it off any more often than once every 2-3 weeks max! Maybe just once a month.

We stocked these along with thin racing tubes in the shop where I worked for 10-years. It's really not a cost or quality issue as tubes are commodity items sold at similar price points by numerous vendors. A thin racing tube or heavy-duty touring tube costs about the same, \$0.80-1.20 wholesale depending upon QTY. Both retails for about \$5, or more for brand-name boxed packaging of the exact same item.

However, the results in air-retention is dramatic between a racing versus touring. The difference really is a matter of knowledge and experience on the part of the sales guy behind the counter and their skill at matching the most appropriate product for the customer's needs.