3634. Mass Properties and Automotive Crash Survival

Paper

Abstract

Problems in dynamics may be solved by any one or more of three basic methods: force and acceleration, work and kinetic energy, impulse and momentum; these are just different ways of looking at a common underlying reality1. The method(s) used to investigate a particular dynamics problem depends upon the specific nature of the problem. Problems involving that most severe form of automotive longitudinal deceleration, crashing, are no exception. Even at the most elementary level, as represented by the previous equations, the unifying role of mass properties is evident. Notable in the basic formulae of all three methods for the solution of problems in dynamics is the common parameter m (mass). However, this represents just the tip of the iceberg ; at the detailed level representative of actual engineering problems the full role played by mass properties is often revealed to be far more complicated than that indicated by such simple basic equations. For instance, an automobile traveling at a particular velocity will possess a certain amount of kinetic energy which must be dissipated for the vehicle to come to a stop. The dissipation can be controlled and orderly as in the case of braking a car to a stop at an intersection, or it can be somewhat more violent as in the case of a collision with a concrete abutment. In both cases the outcome is directly dependent upon the magnitude of the kinetic energy involved. Initially the mass properties involvement seems to be very simple: the kinetic energy of any body of mass m moving at a velocity V is expressible as; to come to a stop that energy must be dissipated through the work done by a deceleration force F times the distance d traveled during the deceleration. However, the kinetic energy possessed by an automobile is much more than would be indicated by a simple determination of its mass m from its weight. Many components of an automobile possess not only translational kinetic energy, but rotational as well. Thus the simple mass m is not the appropriate value needed for kinetic energy determination; there is a greater value me , termed the effective mass . The calculation of me involves the rotational inertia of such components as the wheels, tires, brakes, shafts, bearings, etc. Thus not only the mass of the automobile as a whole, but that of various components, come into play when calculating the amount of kinetic energy which, in turn, determines the magnitude of the deceleration forces required to affect a complete stop in a certain distance. When the deceleration is a matter of braking, certain other vehicle mass properties come into play: the vehicle longitudinal, lateral, and vertical CG. When the deceleration is a matter of crashing, then the vehicle mass density and mass density distribution also have significance. The purpose of this paper is to make explicit the exact role that all the mass properties play in determining the automotive deceleration performance during a crash. This has a direct bearing on the survivability of a crash, which can be enhanced through thoughtful mass properties engineering.