Motor graders have a longitudinal main frame which carries a circle mounting bar that pivots on a horizontal axis at a connecting end and that has a carrying portion remote from its connecting end. A circle rotates on a vertical axis on the mounting bar carrying portion, and a grader blade is mounted on the circle. The present improved circle mounting bar is a box-like beam with internal vertical webs, and the beam increases in width from its connecting end toward a carrying portion of box-like cross section which is a separate part integrally welded to the beam. The shape of the beam in plan is intermediate between an isosceles triangle with its apex at the connecting end and a parabola with its apex at said end so as to give optimum uniformity of stress, throughout the length of the mounting bar, to both vertical and lateral forces.