I have had three flats lately within 60 miles on both wheels of my Marin MTB, which has Continental Travel Contact tires on it. All three failures have been slit like holes at the outer edge of the round base that the valve stem is mounted on. It is where the valve stem material meets the material that the rest of the tube is made of. The failure tends to be right in the center of the tube and not on the side-wall.
I am not sure what I am doing wrong. I am carefull that the valve stem is straight when I inflate the tire and I don't use tire tools to install the tube. I have been inflating to 85 psi, which is the rating on the side of the tire but I use a compresser to inflate. I have not had these Travel Contact tires long and I think I had two more failures simular to this last year. Last year I think I was running the tires at 65 psi ( just got a new pressure gage) but the higher pressure seems to be making the problem worse. This problem seems to have started after I put these tires on, and now I am wondering if they somehow do not fit my rims.
I plan to rewrap the rims with cloth but I don't think that is causing the problem. I also noticed that the tubes were 1.25" to 1.75" width and my tires are 1.75". I thought maybe that's too close so the new tubes I got are 1.5" to 2.00".
I have had a rash of these this year using different presta tube mfr'ers with my Campy Vento rims. Rewrapping with a thicker cloth rim tape may have helped but what I did was take a old tube and cut the tube in half and then 1 inch away cross cut it again which leaves a small 1 inch wide rubber band. I then punch a very small hole through both walls in the middle and push it on the valve stem...providing in effect two plies of rubber tube. This so far has fixed the problem. In the case of the Campy Vento wheel, the interior rim hole is not precisely round and the base of the stem is getting pushed up there and causing a tear.