As the body of a car is crushed, it exerts a force that is approximately proportional to that crush. The SMAC program assumes that at any point on the interface between the two vehicles, the pressures exerted by the two surfaces must be equal and opposite. Units for pressure are in pounds/inch2 (2-dimensional). The pressure is assumed to be proportional to the depth of crush. In finding equilibrium of crush forces, the depth of crush on each vehicle is adjusted by ∆ρ until the pressures on the two vehicles are within an allowable error of λ. For solution stability it is necessary that λ> Kv∆ρ for both vehicle 1 and vehicle 2 (Kv1 & Kv2). In m-smac a warning message and adjustment of λ will occur for inputs which do not permit solution stability.