This paper presents a compliant structure with minimally actuated legs in order to improve the locomotion properties while rolling fast on unknown and irregular terrains. The robot's four wheels are attached to the main body through two successive pivoting segments. While first elements are passively maintained parallel, with the additional support of a specific wheel speed control, horizontal elements are linked to the body through the equivalent of angular series elastic actuators (SEA). The postural control then relies on a projected force servoing, allowing the robot to adapt freely to the ground geometry while maintaining the balance of the chassis.