Where Does The "Air" Go??

[SARGE]

I've forever wondered how a perfectly good tire can lose air pressure over time. Even the tires that sit and not used. Testing them with a bubble solution never indicate a leak, duh, unless it's obvious. Nevertheless the air eventually leaks somewhere. Being without my Herbie since the wreck, I've fixed up my 2 bikes with new tubes, aired them to 60 psi, yet they eventually lose air. That's 4 new tubes with a leak somewhere. As an aside, we know the saying about "out of sight, out of mind", therefore many of us never check our spare tire but discover at the wrong time that it too is either flat or very low when we need it. Being dependent now on my bikes, I've loaded up on Co2 cartridges in case I'm out and need some air. The bicycle pump stays at the house. Where and how does air leak out of a good tire/tube???

[sgtspector]

The valve stem/valve core is usually the culprit.

[Brad the best]

do you install the tubes using a screw driver , that is usually how they get small leeks in them , perhaps they have cheep valves , or they are loose /defective , most leaks that aren't punctures happen at the valve stem , look for sharp spots on the inside of your rim . is there a piece of rubber covering up the spokes ?

you could try tightening up the valve core with a valve core tool , they are only about 99 cents . if you do buy new tubes get some Maxxis tubes and put them on using no tools , fingers only . I would check around the valve stem , there is a good chance that is were your leak is try filling it up and spraying soapy water solution there and move the valve around a bit .

[mbossman2]

Sarge, do you think the pores in the rubber might be larger or smaller than the molecules that make up air?

[KCD]

Sarge,

Unless it is "just me" that happens on my Yamaha 400, S-10, Lumina, Dodge Larime SLT. :eek:

When you get the answer please please pm me least I miss it. :D

Thanks,

Dave

[rspassey]

Sarge, do you think the pores in the rubber might be larger or smaller than the molecules that make up air?

Bingo: I remember hearing those same words in my physical science class I took my freshman year when talking about atomic size and what not. While rubber does a pretty good job when it's thick to resist the passage of air, when it is thin, it is much easier for little particles to find a way out.

[Sunset]

I think Ryan and mbossman have it. Rubber cannot fully contain air indefinitely unless it is especially thick. I use heavy duty tires on my mountain bike to fight the effects of stickers, but they will lose air no matter what I do, and yet lose less than the super thin rubber on the cheap tubes.

[usnavyretired]

"Air" has moisture in it or as the weather man/woman would say, humidity. The amount of moisture/humidity changes the volume of the air based on temps. Place a tire with 30 psi in ithe sun and several hours later, it will read higher than 30 psi, place that same tire in a freezer and it will read lower than 30 psi. Thats why aircraft use dry nitrogen vice air to inflate the tires.

[David M]

"Air" has moisture in it or as the weather man/woman would say, humidity. The amount of moisture/humidity changes the volume of the air based on temps. Place a tire with 30 psi in ithe sun and several hours later, it will read higher than 30 psi, place that same tire in a freezer and it will read lower than 30 psi. Thats why aircraft use dry nitrogen vice air to inflate the tires.

Boyle's law applies to all gases equally. In other words they all lose volume with a temperature drop equally. They all expand at the same rate irrespective of the molecular composition..whether it is oxygen, nitrogen or plain water vapor. My guess is the military does not want water vapor in tires because it condenses from a gas to a liquid at relatively high temperatures versus other gases like nitrogen which have a very low boiling point. To have a gas like water vapor condense from a gas to a water would drop the pressure because of the big drop in volume when it condenses.

Nitrogen makes more sense not because other gases do not expand or contract with temperature, (they do) but because nitrogen has such a low boiling point that they never have to worry about the gas condensing at very low temperatures. Such as when a fighter is flying at 50,000 feet.

The atmosphere is 78% nitrogen so it is plentiful and therefore cheap plus it boils at -320 Fahrenheit...so there is no way it is going to condense in a tire.

[Panama Red]

All good reasons. Another factor for us living in colder winter climates is the affect colder temps has on tire pressure. When the tires are warmer the air pressure rises. I remember in my drag racing days that we had to check the rear tire pressure as close to a run as possible in order to control the effects of the sun shining directly on one side of the car. The pressure would increase by as much as 5 pounds just sitting in the staging lanes waiting for the next run.

[mbossman2]

its called Boyle's Law (I think)

[jgis19]

"Air" has moisture in it or as the weather man/woman would say, humidity. The amount of moisture/humidity changes the volume of the air based on temps. Place a tire with 30 psi in ithe sun and several hours later, it will read higher than 30 psi, place that same tire in a freezer and it will read lower than 30 psi. Thats why aircraft use dry nitrogen vice air to inflate the tires.
Exactly, I buy the tires for my cars from Costco & they inflate the tires with nitrogen and they never lose air,at least between tire rotations.

[Stuey]

PV~T. The pressure times the volume is related to the temp. As the temp goes up, the pressure and/or volume must increase. Tires are relatively ridgid and while the volume might increase a litte bit, it takes less energy for the air pressure to rise than it does to expand the balloon.

I agree with what you guys have been saying, but some air might escape through the valve too, no?

Also, it's not too difficult to attach a moisture trap/filter in-line to an air compressor, so I doubt that moisture content is even partially responsible.

[jgis19]

PV~T. The pressure times the volume is related to the temp. As the temp goes up, the pressure and/or volume must increase. Tires are relatively ridgid and while the volume might increase a litte bit, it takes less energy for the air pressure to rise than it does to expand the balloon.

I agree with what you guys have been saying, but some air might escape through the valve too, no?

Also, it's not too difficult to attach a moisture trap/filter in-line to an air compressor.
Hence the use of nitrogen. As it was explained too me,nitrogen is less affected by the heat variances. And yes pressure can escape through the valve stem,which is why I have them replaced every 3000 miles at rotation.

[Brad the best]

Hmmm . my tires on my DH bike havent lost one pound of pressure in 6 months , i guess its because there thick meen maxxis DH tubes . and i filled them with my compressor which has a moisture trap . the nitrogen thing is big , it makes alot of sense

[David M]

People...all gases expand at the same rate. If this were not the case then Boyle's law would have a different coefficient for each type of gas.

The reason Nitrogen is more effective is because Nitrogen migrates 3 to 4 times slower through rubber than Oxygen. This has to do with Frick's law which relates to how molecules disperse through a membrane...(even though N2 is a smaller molecule than O2) Try getting the public to understand Frick's law and the molecular composition of the atmosphere. So Costco has to sell a bogus concept (a different rate of expansion) in order for people to "understand" why they should pay more for N2. More accurately, it is a different rate of deflation.

Another reason is that you don't want water vapor in there because of its relatively high boiling point...compared to N2.

Another good reason for N2 is that it is not an oxidizer.

[Stuey]

Also, air = ~78% nitrogen.

[jgis19]

But as I remember there are about five different laws of gases & none are specific too nitrogen! most of these laws originated in the 1700's & I feel quite certain new discoveries have been made since then.
Anyway this discussion has gotten quite silly now as I am sure Sarge isn't going to fill his bicycle tires with nitrogen. All I know it works for me. I buy my tires for my cars at Costco as they carry Michelin's at a decent price and the tire rotations are included in the price of the tires and they don't charge extra to fill them with nitrogen & my tires don't lose any pressure between tire rotations & I am also anal enough to take the tire pressures between tire rotations to see if it works.
By the way being retired I drive about 3000 miles in six months & my wife drives about 3000 miles in a year,certainly enough time for leakage & temperature variations.
Just my 2 cents.

[Brad the best]

i believe most car tires would loose air from pin hole leaks , imperfections in the tire or from hitting curbs , also perhaps the valve leaks a little air . a slight leak around the bead maybe . tires leak , there are several reasons why it happens . its why pumps and compressors were created . it sounds like you have a leaky valve sarge . thats just my guess . or thin cheng ching tubes (or w.e that cheep japanease brand that is every were is called)

we could always go into how the valve could be leaking , expansion and contraction of the different types of metals inside . perhaps the outside is expandin at a greater rate then the core , maybe its not conducting heat between the two (core and stem) so they don't expand at the same rate . and don't forget about the actual seal . they are a type of compression seat made of some type of rubber . maybe its shrinking when the temperature rises and creating a leak , or maybe the spring is worn out or broken . is it a presta or schrader ?

i would pretty much ignore it or go buy some 5$ a peice maxxis dh tubes .maybe try tightening up the valve cores , i beleive the external spring type are more likely to leak , the little internal ones are better . i personally hate presta's , schraders are more common

how much pressure are you running in the tire's and how wide are they ? 2.0? or are they skinny road tires. i forget what the average bike's tire width is .

I wouldnt blame the rubber , your bug probobly didn't loose as much air . im gonna put 5$ on its the valves.

we cant blame the air , sarge states he usues co2 cartriges to fill them up , i wonder if co2 expands as much as air .bike tires aren't really affected much by temperature there isnt that much volume of air . they usually fluctuate very little

[Brad the best]

I did some research this sounds lodgical

Years ago, a few tube manufacturers, under pressure to reduce the weight of their inner tubes began looking for ways to make a lighter tube. First they did a little market use analysis and determined that the worst time for a tube to lose air is when it's being used. As a tube is being spun while in use, they realized that due to the laws of centripetal acceleration, the air presents the greatest pressure on the outer wall of the tube. This lead to the obvious answer, they could shave the thickness of the inner wall!

By doing this, they've created tubes that are lighter, and yet, just as effective at holding air while being used. When the tube is not spinning, it of course, loses air at a faster rate, as the air is no longer constrained by centripetal forces, and presents more pressure to the inner wall (the thinner one).

if you believe that b.s your nuts .

there are to many explenations , i have wasted half an hour searching for an explenation .

what happens when you ask a bunch of nerds a simple question?

you get a complicated answer that doesnt really answer the question .

[mbossman2]

Hence the use of nitrogen. As it was explained too me,nitrogen is less affected by the heat variances. And yes pressure can escape through the valve stem,which is why I have them replaced every 3000 miles at rotation.

Nitrogen is used as it it inert, it can not and will not support combustion or explosive oxidation

[Redfallon]

Sarge's point about checking your spare is valid. I drop mine every six months and top it up and put a little grease on the cable which winches it up (SUV). Dropped it over the weekend and it was at around 20PSI, when I normally fill it to 45PSI (tires spec around 38, but I carry a gauge with me and if I ever had to use the spare I could quickly drop it down to 38 if it hadn't already leaked down below that amount :rolleyes: ).

[HAL9000]

We took this up in Physics years ago in high school. Rubber is not a solid material. Under a powerful microscope, it looks like a bunch of strands of material randomly woven together. There are molecules in "air" that are small enough to escape out between the gaps between the strands. Nitrogen molecules are the largest in the air composition and can still escape, but because of their physical size, it is much more difficult, therefore the tires don't leak. Thicker tubes mean more strands for molecules to work their way though, less chance for leakage or slower leakage.

[David M]

Sarge..ask a simple question and get 15 different answers. It kinda makes you wonder if you were better off before asking. :)

[Litespeed]

Sarge, on a side note to keep your tubes running longer without the biggest culprit on bike tubes, which is pinch flats, is next time you change a tube clean out the area on the tire and tube then lightly dust the inside of the tire with some baby powder. This lessens the very harsh interaction between the tube and tire. The rubbing of those two componenets together will cause many cracks in your tubes. Have fun riding in Dallas, been trying to get more poeple commuting here and I think the community is slowly growing. Be safe too. It is very dangerous here. Dallas, one of the largest cities in the US with out any bike lanes. Makes for scary bike riding.

Let me know how it goes with riding or it you need any tips or routes mapped out for you around these areas.

[David M]

Just wondering...why don't they make tubeless bike tires? It is leakage from the spokes or trying to make a seal against the rim...or something else?

[HAL9000]

Sounds like way too much trouble for a bike tire... sealing around spokes, valve stem, sealing around the rim... a stiff enough rim for the initial installation...

[mbossman2]

sealing around the spokes (at least for a motorcycle) is a real PITA but it can be done. One company that does is is: http://www.wheel-works.com/.

Motorcycles have much more substantial tires than bikes do so I doubt that you could build the necessary strength into a bike tire to support a tubless install.

[Cricket]

Just wondering...why don't they make tubeless bike tires? They do...they're called UST (Universal System Tubeless) tires. Been around for a couple of years.

:) Cricket

[Litespeed]

Just wondering...why don't they make tubeless bike tires? It is leakage from the spokes or trying to make a seal against the rim...or something else?


That UST tire started on mountain bikes to prevent the common problem I mentioned earlier which is pinch flats. There still needs to be some perfection in the road wheel system as the pressure and set up is a little more difficult. But I haven't had too many problems with the UST wheels and tires. I hate changing out tubes.

[HAL9000]

Cool... never thought something like that would exist for a bicycle.... seems like too much of a PITA to set up initially.

[SARGE]

Sarge..ask a simple question and get 15 different answers. It kinda makes you wonder if you were better off before asking. :)

I appreciate all the knowledge presented. My question was indeed answered. :D