The conventional simplifying assumption of conservation of momentum need not and sometimes should not be adopted. Impacts are never instantaneous, and for ground vehicles there is always some resistance to motion; so momentum and energy treatments can be made more complete by evaluation rather than neglect of tire forces and their duration.
An automotive accident reconstruction treatment is presented which includes momentum and energy changes over the estimated duration of the impact, due to the forces generated by unsteered and undamaged tires. These tire forces are computed from the stipulated average braking and from the variable slip angles found by interpolation between the iteratively determined initial and final states of motion.
The precision of the resulting momentum solution justifies also finding magnitudes and ratios of shear-plane speed and force at the impact centroids, to guide the correction of errors of observation and allow the imposition of known shear and rebound constraints.
The present scope is documentation of the necessary equations and demonstration of their application to side impacts. The use of CRASH3 absent some or all of the available refinements, and the use of SMAC as a reference, also are discussed.