Bern Brentwood Summer Helmet with Visor 23 7/8 - 24 3/8 Meets Safety Certification ASTM F 2040, CPSC and EN 1078 The Lowest Profile Head Protection offering doesn't look like a spaceship landed on your head like other brands do. Fit system at back of the head customizes to the shape of the head, keeping it stable and comfortable. Separate Winter liner can be purchased to convert to either cold weather bike riding, or for skiing/snowboarding