Where the motion of the side-rods is purely circular, as on locomotives driven by jackshafts or geared transmission to one driver, counterweights can balance essentially all of the motion of the side rods. Where part of the motion is non-circular, for example, the horizontal motion of a piston rod, counterweights on the wheels or drive axles cannot be made to balance the entire assembly perfectly. On a driving wheel supporting both side-rods and the connecting rod to a piston, the counterweight needed to balance the horizontal motion of the piston and connecting rod would be heavier than the counterweight needed to balance the vertical weight of the rods. As a result, a counterweight chosen to minimize the total vibration will not minimize the vertical component of the vibration.