This paper deals with the simulation of off-road robots while taking into account the mechanical behavior of the locomotion system and its interaction with its environment. This interaction is studied and discussed for different behaviors of wheel-soil contact. The analysis considers phenomena of slips, of soil shear deformations, of soil compaction and wheel elastic deformation. Wheel-soil contact models are expressed by analytical relationships which link each contact force components (radial, longitudinal and lateral forces) to relative displacements (radial displacement, longitudinal slip ratio and side slip angle). These laws are then coupled to the dynamic equations of the mechanical system in order to characterize the behavior of the whole system. These models are implemented on a graphic simulation system which provides a basic tool for the design of the mechanical system, for the path planning and for the definition of control schemes.