The Mysterious Tearing Valve Stem

Our man Ed in Whistler presents an interesting case:


– Specialized 2.3 Chunder Pro tires on Mavic 721 rims. “The beads are very pliable — when the inevitable flat occurs I can remove and replace the tire with my bare hands”

– No talc. Low air pressure. “I don’t have a gauge.”

– Downhill runs in Whistler. “Thinking back though, every deflation has followed a period of heavy braking. Today’s flat occured when charging at a double jump then slamming on the anchors at the last minute; I got off the bike to take a closer look at what I’d just wimped out of, and when I picked her up again the front tyre was flat.”


– Lots of flat tires.

– The base of the valve stem is torn.


– Hard braking is making the tire turn on the rim.

– The tire is dragging the tube along for the ride.

– The stem is fixed in the rim, so the base of the stem tears.


– Talc your tubes so they can slip inside the tire. Some people claim this gives you a more supple ride.

– Get a pressure gauge, and run more air pressure. 30+ psi.

Have I said BRAAAP?

7 replies
  1. juan says:

    I get those too but not on the bend, on top of the base, So when I patch it up it blows a hole right through the patch. Is that wierd or what?

  2. Ed says:

    Good work Lee- the bike has been dragged up hill and down dale for about 7 hours over the last few days and still has air in its tyres, so fingers crossed the talc has done the track… My girlfriend Laura also thanks you for rocketing her fingertips to Internet superstardom (see pic above), and promises to rise to the occasion with a manicure next time.

  3. 29MonkeyMan says:

    IRC torque tubes with a threaded TR4 valve will also do the trick. The valve stem locks onto the rim just like a presta vavle.

  4. Chris says:

    In Juan’s case, my guess is it’s a Presta tube being used on a rim designed (or drilled out) for Schrader valves, the base of the valve pushes into the extra space and gets cut by the rim edge… once deflated/flat, it is back out of the hole. Just a theory since no one else replied to it.

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