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Old 07-05-20, 09:55 AM
Mad bike riding scientist
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Originally Posted by merlinrkd View Post
Feeling dumb about this. Been fixing flats tires for 50+ years but this one isn't getting resolved. Recent 90's Conley MTB. Very clean. Deore components. Came with very recent Kenda tubes and Komfort tires. Am losing 2psi a day out of the rear. Pumped the wheel to 65 psi and submerged it in 6" of water in my laundry utility sink. Don't see any bubbles. Watched it for quite a while. Pulled the tube and pumped it up to about twice the tire size. Not a bubble. I'd like to pump the tube up more. Is there a general rule on how big an unconfined tube can be safely expanded?
2 psi per day is about the average for the diffusion rate of air out of a bicycle tube. You wonít see bubbles because the air is coming out at the molecular level through the bulk rubber. The diffusion isnít linear so you lose more at higher pressure than at lower pressure. But, generally, I expect to have to pump up bicycle tires on a daily basis.

Bottom line: If the tire goes flat overnight, thatís a puncture. If it only drops a little (<10 psi) overnight, thatís just diffusion and you shouldnít worry about it.
Stuart Black
Gold Fever Three days of dirt in Colorado
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