The job of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. In automotive suspension, a control arm is a nearly flat and roughly triangular member (or sub-frame), that pivots in two places. The broad end of the triangle attaches at the frame and pivots on a bushing. The narrow end attaches to the steering knuckle and pivots on a ball joint. Two such devices per wheel make up double wishbone suspension, while one control arm per wheel makes up a part, usually the lower part, of MacPherson strut suspension or of various other configurations. A control arm is a bar with a pivot at each end, used to attach suspension members to the chassis. When coil springs are used in both front and rear suspension, three or four control arms are placed between the rear axle housing and the frame to carry driving and brake torque. The lower control arms pivot on the frame members and sometimes support the rear coil springs to provide for up-and-down movement of the axle and wheel assembly. A-arms are control arms with two inboard pivots, giving strength. Some front end designs use control arms instead of A-arms, usually to save weight and add adjustability.