I have a DRZ400S that I commute to work on and have had four flats in the last year, two on the front, two on the back and each time the tube has torn right at the valve stem. Needless to say I am getting tired of fixing flats, i could understand if I am offroad and going places where I shouldn't be but on the pavement, really? I am running a MT21 Perrelli on the front (previously had a 606 but didn't like the offroad ability or how it wore, it also had a flat once same MO) and a 606 Dunlop on the rear. I am running various heavy duty tubes with same result. Has anyone else had this issue? I have about a 3 mile trip to work on asphalt and ride in temps from 35 - 100F. All the offroading I have done and had no flats, always on the pavement. I typically run about 18-20 PSI on the front and 15-18 PSI rear. Any ideas?