In case of conventional rigid axle suspension, the main axle beam ensures the movement of stub axle in the horizontal plane only. In this therefore, there is no vertical deflection of the suspension and hence there is no change in effective track-rod length. However, in the case of independent suspension, the two stub axles can move up or down independent of each other due to which distance between ball-joint ends of the two track rod arms is continuously varying. On account of this a single track rod as in conventional system described above, cannot be used.
Here three-piece track rod is used, the centre portion being called the relay rod, which is connected at one end to an idler arm supported on body structure and to the drop arm of the steering gear at the other end through ball joints. The relay rod is restricted to move in horizontal plane only. Movement in vertical plane is provided by the outer portions, viz, the tie rods about the end ball joints.
