ABSTRACT
The solution of three simultaneous equations that derive from the physics model is not difficult, but it is also far from sufficient to reconstruct an accident. There are always measurement or observation uncertainties of some sort, and there are almost always missing data. So, the problem is not simply to get results, but to get results that comport with all that is known, which may include physical evidence that is not quantifiable. Fact witness testimony should be taken into account (though not necessarily relied on!).
The analysis in Chapter 26 is set up to allow the reconstructionist to check the results, which include parameters that can be examined for reasonableness. One can then adjust the parameters that are not known with complete certainty, and thereby tune the results so that all reasonableness checks are passed. Sample spreadsheet calculations are used to illustrate various topics. These include choice of roots, crash duration, selecting which vehicle is Number 1, yaw rate degradation, yaw rates at impact, crab angles at impact, approach angles, restitution coefficient, Principal Direction of Force (PDOF), energy conservation, linear momentum conservation, direction of momentum vector, impact velocities, angular momentum, force balance, and the relationship between momentum, crush energy, closing velocity, and impact velocities.
This chapter should illustrate that reconstruction is neither exact nor a science. It is engineering—in fact, reverse engineering—which not only takes full cognizance of physics and mathematics but also relies on observation, measurement, knowledge of vehicle design and construction, and the ability to deal with errors and unknowns. It is also an art, significantly shaped by experience and intuition. The analysis in this and the previous chapter was designed with the objective of placing information and tools in the hands of the reconstructionist so that he or she can deal effectively with uncertainties, and most easily apply the engineering and the art to the problem of quantifying what happened in a crash.
