Glass

After two flats in two days I put the double layer in the tires. That's old tires, de-beaded and jammed into the good tires. You usually have to cut the old tires, at an angle, to fit them in there. This makes for heavier rolling weight thus you will be strong like man doing your hill repeats instead of little girly-snivelling-pussy-boy. I recommend listening to Metal during all training, especially recovery days. Keep your Madonna CDs secret and stop renting romantic comedies! HTFU!