A tear at the base of the valve is typically caused by the inner tube moving inside the tire relative to the hole. If you are running inner tubes with Schrader valves, you need to run the tire with more air to keep the tube valve centered under the hole. If you're using Presta valve-equipped tubes, you need to be careful to run adequate air pressure, a rim that supports Presta valve use, and a locking nut on the valve to help keep it seated.