The purpose of this study is to examine the rotational mechanisms of a helmeted head and the effects of whole-body kinematic behavior of a cyclist and head-helmet interaction on head rotation and brain strain in an A-pillar impact in a car-to-cyclist collision. FE simulations where helmeted and unhelmeted cyclist models configured to collide with a small sedan car model were conducted. Although the number of impact simulations was limited, rapid head rotational motion and high brain strain were observed despite wearing a helmet model due to bottoming out of the helmet liner. In contrast, a low-friction helmet could reduce the change of head angular velocity and brain strain due to a significant reduction of a moment of force. In addition, the head rotation before impact generated relatively high brain strain, which indicated importance of whole-body kinematic behavior to discuss brain deformation in car-to-cyclist collisions.