A mathematical model to describe tractor overturning and impact was based on the force-deflection relationships of the vehicle and ground structures at their points of contact. The model was used in a computer simulation, which showed good agreement with full scale overturning experiments. The simulations helped to explain aspects of behaviour including tyre bounce and slip during overturning, multiple roll dynamics and the ways in which energy is dissipated during impact. Dangerously large deformations of roll over protective structures are shown to be possible in certain, rather uncommon conditions.