Current approaches on evaluation of linear motion (LM) guides are reaching their limits. This article describes a model that allows assessing constructive changes by simulating the trajectory of balls during their travel between rail and carriage. Basing on geometry and motion data the collision forces on the balls are being calculated. These calculated results match well to experimentally determined time dependence of carriage velocity. First simulation runs reveal direction and amplitude of forces impacting on the rolling balls. Simulations applying different geometries of the inlet section of carriage are performed. It shows that length of the inlet section plays an important role on the load of the balls, which in turn may influence the service life of a LM guide.