Originally Posted by
specialmonkey
Valve was tight.
There was a hole in the tube ~ 90° away from the valve (doh!).
It was a brand new tube and the hole was on the seam, on the rim / tape side. I didn't see or feel anything there.
I patched this tube and will keep in saddle bag for commuting.
I replaced it with another new tube, fingers are crossed.
On narrower tubes, the reinforced rubber pad surrounding the valve stem goes about half way around the tube, and hardly stretches at all. So as the narrow tube expands to fill the volume of the larger tire, half as much rubber ends up having to do all the stretching needed for the tube to inflate to the tire's cross-sectional diameter, which is apparently enough to cause any additional stress concentration (such as a seam, or at the pad's periphery) to perforate.
If the perforation is indeed facing the rim channel, then the pad surrounding the valve stem never settled into the rim channel, so on an older tube there will be tell-tale stretch marks in the thinner rubber evident there.