I was doing my usual 15 mile round, about mile or two to go, when suddenly I realised my front tyre had gone flat. It has heavy duty, mountain bike style, valves. I haven't been able to find a puncture, re-inflated the tyre and so far it's holding 40psi. I rode it for a couple of miles without any problems. Could it be that the knurled collar came loose?

  • Normally, I would say no, this should not be possible. However, this exact issue happened to my friend recently. I examined the tube under water to see if there was a micro puncture. I did not detect any. That would be the more likely explanation in your case, but I really did examine my friend's tube carefully in the sink. The knurled collar around the stem valve is something that I haven't found it to be useful at all, except on tubeless setups, and it should not have caused your issue. I'm leaving this as a comment for now, in case someone can give an alternate explanation. – Weiwen Ng Aug 31 at 17:53
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    Is your wheel tubed or tubeless ? – Criggie Aug 31 at 19:55
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    "heavy duty, mountain bike style" would normally be used to describe Schrader valves. But Knurled Collar implies Presta. What valve type do you have? – mattnz Aug 31 at 22:24
  • Did you have the little knobbie thing screwed down? It's easy for, say, a twig to briefly get in the wheel and press in the valve of a Presta, if the knobbie isn't screwed down and the cap is absent. – Daniel R Hicks Sep 1 at 0:22
  • 'Knurled collar' could that mean the knurled nut that threads on the valve stem to prevent it slipping into the rim? Might be also the nut holding Dunlop or Blitz valves in the stem. These fit the same rims as Schrader. In parts of Europe they are easier to get than Schrader tubes. In the past department store MTB came with these valves. – gschenk Sep 1 at 14:37

Possibly the valve was contaminated with a particle or dirt and was not sealing properly leading to quick but not instantaneous loss of pressure. When re-inflated the contamination was dislodged and the tube holds pressure once again.

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