This paper deals with the complete shaking force and shaking moment balancing of planar in-line four-bar linkages with constant input speed. A new solution is discussed, which is carried out without counter-rotations. The complete shaking force and shaking moment balancing is obtained by using a coupler link, which is a physical pendulum and by adding a class-two Assur group with prescribed geometrical and mass parameters. Two types of Assur group are examined: two links group with three revolute joints (RRR) and two links group with two revolute and one prismatic joints (RRP). Numerical simulations carried out using ADAMS software validate the proposed balancing technique and illustrate that the obtained six-bar mechanism is reactionless, i.e. the sum of all ground bearing forces and their moments are eliminated.