Grinder ProDescription The Challenge Gravel Grinder Pro in 700x40C is built for riders who want speed on hardpack without giving up bite when the gravel gets loose. Handmade with a SuperPoly 300TPI casing and the SmartPlus puncture protection layer this tire rolls fast feels supple and stays reliable on long mixed terrain days. The center file tread keeps momentum high while the more aggressive shoulder knobs hold tight in corners and unpredictable washboard.