2013 Giro Aeon Helmet plastic Measures out to be approximatley 55-59 cm, 21.75-23.25 inches Featherweight webbing with Slimline buckle In-Mold polycarbonate shell with EPS liner Thermoformed SL Roll Cage reinforcement Roc Loc 5 Fit System and 24 Wind Tunnel vents, internal channeling