I've been riding for many years, and have never encountered a problem like this. On a bike trip last fall we used our mountain bikes and just before the trip put on new smoother tires and new tubes, since this was all on-road. We had three people riding, and encountered like 5-6 flat tires across the three bikes where the tube popped right at the valve stem itself. The tubes by the way are presta valves. We at first thought we had bad tubes, but then realized that the last two flats were on tubes bought at local bike shops, and could no way be the same 'lot' of the tubes we bought through bike nashbar. On some of these, we tightened the metal nut on the stem pretty snug, on others we either left it off or tightened it less. All tires were hand-pumped with a pressure guage pump. The tires were hand-installed, I've changed hundreds of tires, and never encountered problems like this. Since they were across several bikes, it couldn't be a sharp rim edge or anything like that. any ideas? Please help!
--bob
--bob