A planetary rover is designed to traverse rough, unpaved terrain. The all-wheel-attachment property, i.e., that each rover wheel maintains firm contact with the terrain, is considered crucial in modern rover suspension design. This paper proposes one necessary condition associating the DOF of the suspension with the number of wheels for the all-wheel-attachment property. The paper first describes an equivalent single wheel-ground contact model and then the whole rover model, through which the mobility value of 3 was calculated for the all -wheel-attachment condition. The necessary condition is finally acquired through the fact that in the all-wheel-attachment condition, mobility should equal the difference of DOF of the whole rover and constraints. Furthermore, a type synthesis is proposed for the rovers with passive all -wheel-attachment property using the necessary condition. The necessary condition was further verified and interpreted by case studies of various excellent rovers by different institutes and the simulation of newly proposed rovers.
