New CRASH Stuff

By Badger, Joseph | Law & Order, May 1999 | Go to article overview

New CRASH Stuff


Badger, Joseph, Law & Order


If you're reconstructing an emergency vehicle accident or any sort of traffic crash, you'll want to know about McHenry Accident Reconstruction. If you've ever heard of Simulation Model of Automobile Collisions (SMAC) or Calspan Reconstruction of Accident Speeds on the Highway (CRASH) computer programs, then you've heard of Raymond R. McHenry. McHenry developed those programs and he wrote the popular User's Manual for the CRASH Computer Program.

Raymond and his son Brian published their first edition of McHenry Accident Reconstruction in October, 1998. They produced a revision later that year and another revision in January 1999. It's a 147-page text on everything from the Abandonment of A, B, and G (referring to stiffness coefficient letters; not the alphabet) to yawing.

They're not really forsaking the traditional A and B stiffnesses. In my email "conversation" with Brian McHenry, he explained, "That section is in our review of NHTSA `refinements' to CRASH. In our 1986 and 1997 papers on including restitution in damage analysis, the beauty of our proposal (at least to us) was that we retained the A & B stiffness coefficients while adding a definition of the restitution properties of the vehicles (the Ki, K2, Rho & Gamma.) It would have been easier for us to just abandon A & B but we felt it important to retain the current body of information."

The various topics discussed in the text are not divided into numbered chapters, but there are separate headings such as Cornering Stiffness, Trajectory Analysis, Damage Analysis, Crush Coefficients, Occupant Trajectory, Pole/Tree Collisions, Rollovers, Speed Change at Occupant Positions and 3D Reconstruction Analysis.

The McHenrys explained, "With the increasing capabilities of computers, there is a growing inventory of commercially available, special purpose computer programs that deal with a wide variety of technical topics. This trend is clearly present in accident reconstruction. This book is a collection of useful information related to the theoretical bases of reconstruction calculations."

The text is not, however, an instruction manual on how to do reconstructions. It does not teach you how to interpret the massive amounts of physical evidence found at a crash site. This comes with years of practical knowledge learned in the field.

Early in their Trajectory Analysis discussion they talk about the workhorse of accident reconstruction, Conservation of Momentum. They refer to spinouts on flat, uniform surfaces. "In simple spinout motions, the actual distances traveled to rest can be approximated, with reasonable accuracy, by straight lines between the separation and rest positions. The separation velocities can be estimated on the basis of the total work done by each vehicle against tire-terrain friction forces between separation and rest." We often refer to "separation velocities" as the "post-impact speed."

Note use of the term "simple spinout motions." But too many accidents aren't simple. The book states, "With freely rotating wheels, the linear and angular velocities of a vehicle are decelerated alternately as the heading direction changes with respect to the direction of motion." Wouldn't it be nice if sheet metal was strong enough so that fenders and bumpers didn't collapse around the wheels and lock them up? "In that situation the linear velocity is decelerated, generally at different rates, during ... the spinout motion. The amount and location of the wheel drag on a vehicle directly affects its behavior."

Under "Yaw Inertia," Table 2 shows conversion factors for changing such values as slug-feet into kilopond-meter-second (both terms squared, of course). The authors define "radius of gyration" and present the equations for calculating both it and moment of inertia.

For readers who use computer programs (such as CRASH3 and CRASH-based software) you'll want to carefully read the Damage Analysis section.

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