When computing the performance of a vehicle in longitudinal motion (maximum speed, gradeability, fuel consumption, braking, etc.), the vehicle is modelled as a rigid body, or in an even simpler way, as a point mass. The presence of suspensions and the compliance of tires are then neglected and motion is described by a single equation, the equilibrium equation in the longitudinal direction. If the x-axis is assumed to be parallel to the ground, the longitudinal equilibrium equation reduces to m¨ = x ∀i . | 23 DRIVING DYNAMIC PERFORMANCE When computing the performance of a vehicle in longitudinal motion maximum speed gradeability fuel consumption braking etc. the vehicle is modelled as a rigid body or in an even simpler way as a point mass. The presence of suspensions and the compliance of tires are then neglected and motion is described by a single equation the equilibrium equation in the longitudinal direction. If the x-axis is assumed to be parallel to the ground the longitudinal equilibrium equation reduces to mx FXi Vi where FXi are the various forces acting on the vehicle in the longitudinal direction aerodynamic drag rolling resistance traction braking forces etc. . As will be seen later Eq. is quite a rough model for various reasons. For one thing when the vehicle is accelerated a number of rotating masses must be accelerated as well this however can be accounted for easily. Other approximations come from the fact that the vehicle does not travel under symmetrical conditions particularly when the trajectory is not straight and the direction of the x-axis does not coincide with the direction of the velocity or in other words the sideslip angle 3 is in general different from zero. LOAD DISTRIBUTION ON THE GROUND Longitudinal dynamics is influenced by the distribution of normal forces at the wheels-ground contact. A vehicle with more than three wheels is statically indeterminate and the load distribution is determined by characteristics of the G. Genta L. Morello The Automotive Chassis Volume 2 System Design Mechanical Engineering Series Springer Science Business Media . 2009 185 186 23. DRIVING DYNAMIC PERFORMANCE suspensions which as seen in Part I also have the task of distributing the load on the ground in proper way. However if the system is symmetrical with respect to the xz plane all loads are equally symmetrical and the velocity is contained in the symmetry plane then the two wheels of any axle are equally loaded. In this case it is .
