Ricta Wheel Dynamics Tom Asta Geo Naturals Pro Wheels in 52mm Slim shape with Ricta's 101a high performance urethane for maximum speed, smooth roll, and fewer flat spots. Wheel Diameter (Millimeters) 52.1 Wheel Durometer/Hardness 101a Road Surface (Millimeters) 18.3 Wheel Shape Slim Wheel Width (Millimeters) 30.3