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Where does the air go?.. Why do I have to inflate so much?(9 posts)

Where does the air go?.. Why do I have to inflate so much?rpicayo
Aug 18, 2001 5:51 PM
Ok, so this may be a dumb question, but...

I gather it is common for road tires to lose a fair amount of air overnight. I can inflate my tires to 120 and then the next day they will be about 110 and after a couple of days they are down to 90. Where does this air go? Do all tubes leak? Does the air somehow go through the rubber when inflated to high pressure? Just curious.

R
Do you use latex tubes?look271
Aug 18, 2001 6:01 PM
That seems a bit extreme.If you use latex (usually pink, or purple) tubes this would be normal, but in my experience, this is a bit much for normal butyl tubes. Perhaps you have a very slow leak?
I wonder about that too.Spoke Wrench
Aug 18, 2001 6:20 PM
The obvious answers are through the valve or through the "rubber."

My home air conditioner uses a Schrader valve and is expected to hold 200psi of line pressure year after year with no leaks. Do bicycle tires just use cheap valves that leak more or does the air excepe through the "rubber"?

Two things I do know. The higher the pressure, the faster the air leaks out. Tires filled with CO2 lose pressure much faster than tires filled with air.

Maybe one of the engineering techies can enlighten us both.
re: Where does the air go?.. Why do I have to inflate so much?mackgoo
Aug 18, 2001 6:34 PM
Very small leaks. The other day I finally took a tube out that would go from 130 to 100 in a couple of days. I filled the sink and put some air in the tube scrunched the thing up and squeezed under water. I found an area where all that squeezing produced one bubble. I patched that area and the tire holds air.
Do you use presta valves?Atombomber
Aug 18, 2001 7:49 PM
I always have to release the valve before conecting it to the pump. This process will let air out. The more time between re-fills, the stiffer the presta valve, so I let even more air out. Then when the floor pump hose is connected, the pressure is again reduced as high pressure air in the tire's tube gets transfered to the low pressure air in the pump hose, including the pressure gauge.

Test this. Pump your tires normally, screw the valve closed, and then unscrew the valve, and connect it back to your pump. You will have lost a good amount of air in that process. Road bike tires have so little air, at such a high pressure, that what you think is letting out the tinyest amount of air corelates in a huge decrease in air pressure.
You are right...Bruno S
Aug 18, 2001 8:42 PM
just by connecting the pump is enough to loose 10 psi. I also have to let some air out before I can pump air because of the locked valve. If you some how could connect a psi gage and leave it over night it would be possible to determine the real amount of air lost through leaks. I expect that to be a few psi every day even with new tubes. I pump air every 2nd ride.
leaks through the (1) valve and (2) tubenuke
Aug 18, 2001 9:14 PM
In most cases, the valves, especially presta valves, don't seal up perfectly, so many of them will leak, albeit very very slowly.

And as the other astute responder noted, there are micro punctures in your tube that also leak air.

I've seen both of these cases in many of my tubes. The most common by far is the leaking presta valve.
re: Where does the air go?.. Why do I have to inflate so much?DINOSAUR
Aug 19, 2001 7:48 AM
I noticed that the ultra light presta valve tubes, especially the long valves, have very sensitive valve stems. It is near impossible to inflate with a frame pump without damaging them, it's one of the reasons I changed over to co2. I lost two new tubes by leaking valves that only could be detected when submerging in water. When I inflate before a ride with my floor pump I am very carefull about how I use the pump, I always reach down and support the valve stem by hand when I am detaching the presta head attachment. It's normal to lose a couple of pounds or air overnight. I started using the plastic valve caps and it cuts down on air lose.
Valves, schmalves!Kerry Irons
Aug 19, 2001 5:35 PM
While you may have tiny leaks in the valves or tubes, rubber IS porous to air molecules, and so the air will permeate through the rubber and you'll lose pressure regardless. Thinner tubes lose air faster. Latex tubes lose air faster than butyl tubes. Smaller tubes lose air faster as they are stretched thinner when inflated. As a coach once told me "You're about to go out for an hour or two of vigorous exercise, and you're worried about the effort it takes to pump your tires?" As other posters noted, you lose air just putting the pump head on, but in my experience, 30 psi loss in a week would be typical.