# Bicycle Mechanics - Tire/Tube question

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View Full Version : Tire/Tube question

Pooptet
09-14-10, 08:38 PM
So I inflate my wheel to the right psi, then the next morning, the psi decreases. Is this normal?

Bezalel
09-14-10, 08:43 PM
It depends by how much. All tires lose some air.

Pooptet
09-14-10, 08:44 PM
Like I inflate it to 90psi the night before, then when I check it again, it's at 83psi

Shimagnolo
09-14-10, 08:45 PM
Not normal at all.
The pressure should *increase* over night.

dscheidt
09-14-10, 08:53 PM
Not normal at all.
The pressure should *increase* over night.

I want some of your magic tubes, man.

OP: your pressure loss is a little more than usual, but it's not terribly out of line. Tubes are made of rubber, and are all more or less porous. They don't hold much air, and have a very high surface area for their volume, which gives lots of area for air to diffuse through.

cny-bikeman
09-14-10, 09:35 PM
If both tubes lose roughly the same amount of air I would not worry about it. The lighter/thinner the tire or tube the more air will be lost. In pounds of pressure you will lose more starting at a higher pressure.

Mr. Fly
09-14-10, 10:02 PM
So I inflate my wheel to the right psi, then the next morning, the psi decreases. Is this normal?

Normal.

BTW, depending on your tire size, just the act of measuring the pressure may bleed off enough air to reduce the pressure. Try pumping up to 90 psi. Remove chuck. Immediately try to measure the pressure again. This will provide you with a baseline upon to more accurately judge the pressure drop overnight.

rekmeyata
09-15-10, 12:08 AM
Depends on what kind of tubes your talking about. Thick MTB tube may loose that amount in 2 weeks, thick road bike tubes may loose that in a week, thin racing tubes will loose that in a day, latex tubes will loose at least twice of what yours does in one day.

cny-bikeman
09-15-10, 07:00 AM
One more thing - If you have a reservoir pump (gauge sits on top of a tall cyclinder) you will definitely measure a lower pressure when you reattach the pump, even immediately, as a significant amount of air goes from the tire into the reservoir when the pump is attached. It has nothing to do with the "type" of tubes, only the thickness and pressure.

constant mesh
09-15-10, 08:01 AM
First rule of all, check you tires at exactly the same temperature and humidity, then you can judge how much air escapes. Air molecules expand/shrink with temp. differences.

bikingbrit
09-15-10, 08:20 AM
Another factor: when you pump the air in it gets heated in the pump or air compressor. Over night it will cool and lose a little pressure.