Crash tests of vehicles by striking deformable barriers are specified by Government programs such as FMVSS 214, FMVSS 301 and the Side Impact New Car Assessment Program (SINCAP). Such tests result in both crash partners absorbing crush energy and moving after separation. Compared with studying fixed rigid barrier crash tests, the analysis of the energy-absorbing behavior of the vehicle side (or rear) structure is much more involved. Described in this paper is a method by which analysts can use such crash tests to determine the side structure stiffness characteristics for the specific struck vehicle. Such vehicle-specific information allows the calculation of the crush energy for the particular side-struck vehicle during an actual collision – a key step in the reconstruction of that crash. Based on fundamental principles of physics and engineering, this method provides transparency in the calculation of side structure stiffness parameters for the particular vehicle of interest. Sample calculations are presented. Utilizing widely available computational tools and publicly available test data, the method was applied to a representative sample of recent-model vehicles. The results of this study show that variability in side structure stiffness appears to be real – not an artifact of computational method or data uncertainty – and not related to vehicle size or weight, for example. Independent tests of comparable vehicles, while few in number, produce remarkably similar stiffness results. Supported by sensitivity analyses, various simplifications are presented that in most cases will allow a good approximation to more rigorously-derived results.