  A recent paper [Phys. Rev. E 87, 062114 (2013)] presents numerical
simulations on a system exhibiting directed ratchet transport of a driven
overdamped Brownian particle subjected to a spatially periodic, symmetric
potential. The authors claim that their simulations prove the existence of a
universal waveform of the external force which optimally enhances directed
transport, hence confirming the validity of a previous conjecture put forward
by one of them in the limit of vanishing noise intensity. With minor
corrections due to noise, the conjecture holds even in the presence of noise,
according to the authors. On the basis of their results the authors claim that
all previous theories, which predict a different optimal force waveform, are
incorrect. In this comment we provide sufficient numerical evidence showing
that there is no such universal force waveform and that the evidence obtained
by the authors otherwise is due to a fortunate choice of the parameters. Our
simulations also suggest that previous theories correctly predict the shape of
the optimal waveform within their validity regime, namely when the forcing is
weak. On the contrary, the aforementioned conjecture is shown to be wrong.
