I have had to replace 3 tubes in the last 2 months. The valve stem keeps getting torn/sliced where it meets the tube. Could it be rubbing where the stem goes thru the rim and cutting it? I was going to go rider last week , pulled the bike out and noticed it was flat. I put air in and i could hear it leaking. Then when i wiggles the valve stem it would get louder depending on which direction i wiggled it. This is starting to piss me off......I run the rear tire at 12-14 psi, and always make sure the valve stem it tight.