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At the 1991 SAE Congress, a paper entitled “Stiffness Parameters for Vehicle Collision Analysis” was published. The section that attracted the most interest dealt with various methods to relate the CRASH stiffness parameters (A and B) to the SMAC stiffness parameter (KV). The discussion immediately following the paper presentation revealed one more method (developed by Fonda) to relate the CRASH and SMAC parameters consistently with regards to crash energy. These last ten years there have been three significant developments concerning a consistent CRASH-SMAC conversion. Firstly, a greater range of software providers has developed more variations of each basic computer program. Secondly, the CRASH algorithm has been extended so that it treats rebound more like the old SMAC. And thirdly, two completely different versions of SMAC have been released which provide for a bilinear force-displacement curve like the old CRASH.

The National Highway Traffic Safety Administration maintains a large database containing data from nearly 1500 staged crash tests. A portion of that database tabulates information about the tests, vehicles, instrumented dummies, and barriers. That tabulated data has now been successfully loaded onto a desk-top Macintosh. Using a relational database program allows for fast and effective use of the database information. One application of the crash database was to investigate frontal pole barrier tests. The results from nineteen staged pole barrier tests were extracted from the database to evaluate various methods for relating the pre-impact speed to the observed crush. A common method for estimating speed involves calculation of the crush energy using the observed crush and crush parameters A and B (obtained from flat barrier tests). For frontal pole collisions, this method appears to under-estimate the impact speed by about 33%.

Two widely used computer programs developed for the analysis of vehicle collisions are CRASH and SMAC. This paper reviews stiffness parameters which are used in the application of these programs, and methods to select these parameters. The paper also introduces a rational method to select stiffness parameter KV for the SMAC program. The CRASH program expresses the vehicle force-crush relationship as FC = A + B*CR, where FC is the force per unit width, and CR is the vehicle residual crush. The “stiffness parameters,” A and B, define a linear relation with a zero-crush intercept. For collinear impacts, these parameters are used in determining crush energy, which in turn is used in determining changes in velocities of the impacting vehicles. Over the years, considerable effort has been expended by numerous researchers to determine A and B for a variety of vehicles, and a substantial body of vehicle crash test data has been developed and analyzed to this end.

A number of accidents involve a vehicle going off the edge of the road and becoming airborne. In these types of accident, the engineer is typically faced with the problem of estimating the velocity of the vehicle at launch, based on evidence defining the launch position and point of impact at the end of the flight. Two options have been available to evaluate the dynamics of such problems. The first is to treat the vehicle as a point mass, and use simple trajectory equations to define the flight path. This approach considers only the two translational degrees of freedom of the center of mass of the vehicle, and neglects effects of tire contact during launch. The second option is to carry out a full three-dimensional analysis. This approach treats the sprung mass of the vehicle as a rigid body, accounts for the four tire contact conditions, and possibly models the suspension and unsprung masses.