The pivot-slide model (Shegelski and Lozowski) successfully predicts the slide and curl distances of a curling rock. However, in this model, there is no dependence of the curl distance on the initial velocity, because the ratio between the pivot to sliding times is constant. A refined model is presented, in which the ratio of the pivot to sliding times depends on the stone velocity via two parameters. Confidence limits for these parameters are deduced from experimental data, which show that the pivot-slide ratio depends on the stone velocity. However, precise values of these parameters could not be obtained with this study, as more precise experiment data are needed. The refined model allows one to qualitatively explain two characteristics of the stone trajectory observed in a curling game, namely the bigger final curl with lower initial velocity and the lower curl with the effect of sweeping the ice.