MrCjolsen

Small hole on the inside, about 1-2 cm from the valve stem. It looks like a small pimple when the tube is deflated. Usually a corresponding "pimple" on the other side of the valve stem as well.

The holes do not line up with the spoke holes. Too close together.

Presta valves usually inflated to 100 psi. I check my pressure every time I ride.

Good cloth rim strips.

Nothing I can see in the rim that would cause this.

I get a lot of these and usually discover them when my bike has been sitting overnight.

FBinNY 

Often on narrow rims the inside walls of the tire at the bead are very close together creating a narrow gap between the chamber inside the tire and the smaller chamber inside the rim. As the tube inflates a section ends up laying across the gap, and then only that narrow section continues to stretch as it expands down into the rim. It's common to see stretch marks on the rim side of used tubes because of this.

You problem is probably compounded because the base of the valve is too thick to easily slide through this gap and is hanging up in the tire (I wish I knew how to attache a sketch) forcing the tube to stretch even more than normal and it's probably failing where the reinforced area ends.

You might be able to prevent the problem with a change in your installation procedure. First dust some talc on your tubes which makes it easier for them to slide within the tire. Then when you finish mounting the tire, push the valve deep into the tire to make sure the tube isn't trapped under the bead (causes blowouts) then carefully pull or work the valve down below the gap in the tire all the way until you feel it seat against the rim. That should prevent the type of local over stretch you're experiencing.

zebede

Thank you FB.

csimons

Thanks FBinNY! I just had this happen (exactly as the OP described, large hole right outside reinforced area, corresponding 'pimple' on the opposite side, fantastic cloth rim strips that seem perfect) and was racking my brain trying to figure out what could possibly have caused it (there was no 'pop', it was just flat when I checked it this morning after riding last night). Your description was very informative and helpful! Also, thanks for the advice for preventing this in the future!

canopus 

This would be one of those reasons why we always used talc inside of tires and pre-inflated (make round with a little air, not try and pump it up) the tube when mounting.