Abstract: A support system for a boom mounted on a truck includes four outriggers mounted to the boom pedestal which can be folded against the pedestal or extended to provide a wide base for supporting the boom. Each outrigger has an inboard leg member pivotally mounted to the truck and driven by a hydraulic cylinder. An outboard leg member is pivotally mounted to the outer end of the inboard leg member and is caused to pivot relative to the inboard leg member at approximately twice the pivot rate thereof and in the opposite direction. The outriggers may be extended by a single operation of the hydraulic cylinder. The means for rotating the outboard leg member causes the outrigger not to drag along the ground until it is finally placed. Means are provided for preventing pivotal motion of the legs beyond predetermined positions.