saturnsc2

everybody knows bicycle tires tend to lose air pretty fast especially if the bike isn't used. i wonder if you can see the air loss from the tires if you hold them underwater. has anyone tried this? any air loss would bubble underwater obviously. it's strange how they lose air if not used faster than if used. it seems to me it should be the opposite..

Bobcat

My MTB tyres seem to stay up better than road tyres--likely because they are pressurized to only 30-50 psi, as opposed to 90-120. Most of my leaks lately are from the tube seams.

But yeah, I wonder just where the air loss is comin' from. Seams, valve, or through the tube wall?

LandLuger

I believe the air spontaneously diffuses into a infinitely unique, parallel universe.

enduro

The air loss is coming from everywhere! Diffusion is a powerful thing.

You could keep your bikes in a pressurized container at 120psi or whatever pressure you run, but even then you'd have to reinflate once in a while. Every time you'd go for a ride the tires would lose a tiny amount of pressure from diffusion. Since the tires do not have rigid walls, when you'd put the bikes back in the pressure chamber they would not "reinflate"

Can't beat entropy.

EDIT: Actually, even in the pressurized chamber the tubes would lose air. Since they are elastic, they will only reach an equilibrium when they return to their unstretched/uninflated dimensions. It would not go totally flat though.

Bike Nurse

You'd also assume tube rubber to be slightly porous. One would also assume the air leaking from these miniscule holes would only be visible under a magnifying microscope and with the help of timelapse photography ... hey give it a go .... I'll be waiting on the edge of my seat with excited anticipation for your results!

ken cummings

I have had a tire go 4 months of active use without needing air. Any diffusion underwater might be slow enough for the gas atoms to go directly into solution in the water and not form bubbles. If I need to pump up the next AM I change tubes and patch the leak. If it need air every 3 or 4 days and I find no leaks I get a new tube.

cyccommute 

You probably couldn't see anything happening even under the most powerful magnification currently available. Although tunneling electron microsopes can see metal atoms and some carbon molecules, those are huge compared to gas molecules. The pores in the rubber are also incredibly small, just larger than the gas molecules, but it's not like they are tunnels that run in straight lines from the inner wall to the outer wall of the tube. The route is going to be very convoluted.

The actual mechanism is probably for single gas molecules to pass through the rubber all the time. It's not going to be a mass exodus like even a small puncture but just a steady drain at a very slow rate.

enduro

It's pretty much impossible to make a barrier that is totally impenetrable to gas molecules. You can certainly come close enough that it is impenetrable for all practical purposes. Not with a thin butyl tube though.

The maximum vacuum obtained in ultra-high-vacuum systems is sometimes limited by diffusion of air through the stainless steel walls of the chamber.