Originally Posted by
cyccommute
2 psi per day is about the average for the diffusion rate of air out of a bicycle tube. You won’t see bubbles because the air is coming out at the molecular level through the bulk rubber. The diffusion isn’t 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, that’s a puncture. If it only drops a little (<10 psi) overnight, that’s just diffusion and you shouldn’t worry about it.
I wouldn't accept a tube that leaks 2 psi overnight on a mountain bike. On a road bike, with skinny tires at 100 psi, I'd exepect that. I run michelin airstop tubes, mostly, my bike with 700X50 tires loses about 2 psi a week (40 psi front, 50 rear). That's checked inside, so temperature isn't an issue. I would expect to be able to find a 2psi overnight leak with leak detection fluid, but maybe not in a sink.