Challenge's Grifo XS 33 Open Tubular (Clincher) tire grips great on grass, pavement, and tacky dirt, plus it rolls fast, too! Its center file tread mean easy speed while its side knobs grip great. The XS is built on the same supple and light 260tpi hand-made SuperPoly casing as Challenge's renown tubulars. It boasts an anti-puncture strip under the tread and polyester in the sidewalls for extra durability. The white-tread version features a softer rubber for better grip in cold temperatures.