hey guys, I'm new here. I just recently bought a new to me, Trek 6500, the bike is awesome, is lightweight and is perfect for me. I go to the claremont wilderness mountain trail all the time, though I have one issue with my bike, IT KEEPS POPPING INNER TUBES!

When I just got the bike, I was riding around my neighborhood and pop, the rear wheel inner tube is popped, but not by a hole (i have mr.toughy), but by the valve stem, so I thought it was just an old inner tube, went to the bike store and bought a new heavy duty inner tube, after riding for 15 miles, the inner tube popped again and from the valve stem once again! So I take it to the bike shop and they check the wheel but find nothing wrong with the wheel, they replace the inner tube and filled it to approx 55-60psi (recommended for that inner tube). Guess what? Today on my mountain trail ride, the tire popped again, from the valve stem, after riding for 15-20 miles...

What is causing the inner tube popping from the valve stem?

Thanks!

-Joe