What a control arm actually does
A control arm is a wishbone or A-arm that locates the hub relative to the body. It is not the ball joint alone. It is not the sway bar, and it is not the strut.
People point at the ball joint and call that the corner. Or they point at the strut and say that locates the hub. Or they think the bar across the car is the arm.
Those yellow A’s are the control arms — wishbone, A-arm, same part. Inner pivots sit on the body or the subframe. The outer end meets the hub, usually through a ball joint. As the wheel goes up and down the arm swings and sets camber and the track. It is not the ball joint alone. That joint is the hinge at the tip. It is not the sway bar. It is not the strut.
On a MacPherson corner the lower arm still sets the bottom. The strut is the tower. Two wishbones do the locating without that tower. The diagram paints the lower wishbone cyan on an independent rear — same job, different axle.
