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I recently went to get my bike ready for a ride, and I noticed right away that my rear tire had completely deflated. I took off the wheel, and took out the tube and checked it for puncture but didn't find anything. So, I put the tube back in and inflated the tire. As usual I had the tube inflated to 120psi, the max for the tire.

I went on a ride, it did not go flat on me. I put my bike away, and lo and behold the next time I looked it was completely deflated again.

What causes an inner tube to deflate on its own?

It also gets very hot in my garage. Does heat play a part?

I assume a new tube will do the trick. Is there a reason to worry, or can I safely ignore the problem and just inflate the tube every time?

  • Air leaks out of tubes over time -- even a "perfect" tube with a perfectly-sealed valve will "diffuse" air through the rubber. A tube may hold pressure just fine for a 2 hour ride, but be nearly flat 48 hours later. And some tubes (especially expensive exotic ones) are just leaky. A 120 PSI tube would generally be expected to need reinflation every 48 hours, and every 12 if an exotic variety. – Daniel R Hicks Oct 7 '15 at 11:43
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    I don't know about 48 hours. I have 120psi tyres and only need to pump them every month or so, even though they get ridden every day, and the tubes are old and full of patches. But maybe if that's true if you spend big bucks on lightweight tubes. – stib Oct 7 '15 at 12:29
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    Sure a butyl tube may need re-inflation every 48 hours to maintain it at exactly 120 psi, but it will stay at lower pressures for far longer. So if the tube is "completely flat" within a few days or even weeks, you've got an actual leak somewhere. – z7sg Oct 7 '15 at 12:42
  • @stib - I use plain old standard neoprene tubes, and my 100 psi tires are down to about 80 after a week. I suspect that your "120psi tyres" are down to 80 or lower after a month. – Daniel R Hicks Oct 7 '15 at 23:12
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Sometimes the puncture in an inner tube is very small and the air leaks out very slowly. This is called a slow puncture. The classic technique for finding a slow puncture is to immerse the partially inflated inner tube in water and watch for the appearance of any bubbles.

In my experience even the slowest punctures usually get bigger somehow - I don't know how but I prefer not to wait until this happens so I'll usually fix the tube or replace it.

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    I found a slow puncture, and patched it. It's now good to go. Thanks. – leaflifelayf Oct 7 '15 at 20:46
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Even a new tube will deflate over time - it is incredibly difficult to fully seal the tube/valve perfectly so air will leak out, mostly around the valve seal.

An older tube may well have worn rubber in the seal, around the edge of the valve stem where it joins the tube, or possibly the tube has worn thinner. All these will allow air to leak away.

And the higher pressure your tyre is at, the faster it will lose pressure.

With a brand new 90psi tube on my bike, I'd expect to have to pump it every week when I want to ride it. And that is with no holes, punctures or tears.

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