  We present STAR's measurements of directed flow for charged hadrons in Au+Au
and Cu+Cu collisions at $\sqrt{s_\mathrm{NN}} = 200$ GeV and 62.4 GeV, as a
function of pseudorapidity, transverse momentum and centrality. We find that
directed flow depends on the incident energy, but not on the system size. We
extend the validity of limiting fragmentation hypothesis to different collision
systems.
