The D3 Fiberlite uses a lightweight fiberglass shell with wind-tunnel proven aerodynamics & an efficient ventilation system. Deep EPS (expanded polystyrene) channeling optimises airflow to keep the head cool. The fit is aided by the contoured & anatomic 3D cheek pads & removable & 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)