The driving experience of an electric powered wheelchair (EPW) can be disturbed by unpleasant dynamic and kinematic effects of the caster wheels, particularly during maneuvers in narrow rooms and direction changes. In order to minimize this nasty behavior, we propose in this article a kinematic model of the wheelchair taking into account the effects of the caster wheels. The orientation of the caster wheels has been measured for different configurations: initial orientation, wheelchair velocity and user mass. The repeatability of the motions has been studied, and from these tests, their behavior has been modelled. This model has been then used to determine the wheelchair trajectory by using a more realistic kinematic. We have thus obtained a model allowing us to calculate more precisely the spaces that can be reach by powered wheelchairs, which are useful information to enhance control laws of their driving assistances.