All Terrain Vehicles (ATVs) have unique design features, including low pressure tires, a solid rear axle (i.e., no differential), and a relatively high center of gravity compared to wheel track width, that exert significant influence on their lateral/directional handling and stability properties. In addition, rider weight is a reasonable proportion of vehicle weight and weight shift is used as an additional control means in combination with steering, throttle and braking. This paper describes a nonlinear, time domain simulation analysis of the transient lateral/directional response properties of ATVs with rider control. The simulation is derived from earlier automotive applications. A description of the analytic model and computer simulation are given along with validation comparisons of instrumented vehicle field test data and computer simulation runs. Simulation analysis of limit performance maneuvering situations is given to illustrate the lateral/directional handling and stability issues of oversteer and spinout, and rollover.