Temporal evolution of the photosphere expansion velocity of SN 2013dx (large circles) measured directly on the spectra from the position of the minimum of the Si iiλ6355 absorption line. For comparison we report the photospheric velocities of other core-collapse Ic SNe (1994I, small circles; 2002ap, stars; 2003dh, triangles; 2006aj, small filled squares), as derived from spectral models (Sauer et al. 2006; Mazzali et al. 2002; 2003; b). For SN1998bw we report both the velocities derived from direct measurements (crosses, Patat et al. 2001) and those derived from models (large open squares, Nakamura et al. 2001).