The D3 Fiberlite uses a lightweight fiberglass shell with wind-tunnel proven aerodynamics and an efficient ventilation system. Deep EPS (expanded polystyrene) channeling optimises airflow to keep the head cool. The fit is aided by the contoured and anatomic 3D cheek pads and removable and washable Dri-lex liner. The 3D cheek pads also quick-release for emergency helmet removable. Thanks to the D3 Fiberlite, the most decorated helmet in professional racing is now available for riders who crave factory performance. Size Guide: X Small = 20.5-20.9\" (52cm-53cm) Small = 21.3-21.7\" (54cm-55cm) Medium = 22\"-22.4\" (56cm-57cm) Large = 22.8\"-23.2\" (58cm-59cm) X Large = 23.6\"-24.4\" (60cm-62cm) XX Large = 24.8-25.6\" (63cm-65cm)