CRASH program sensitiviy for rotation < or > 360 degrees?

[MSI]

Q: I have been told both by the police that for their WinCRASH reconstruction they have my car rotating 360 degrees before stopping. It did not do that but they say if it did not it would produce a faster speed, not a slower one. My uneducated brain thinks that to spin a car would take more weight and impact speed than if it did not spin. Which is correct?

A: In general, for a given travel distance from separation to position of rest, a vehicle which rotates 360 degrees will generally require a lower speed than a vehicle which rotates only 180 degrees. Of course, generalities are where you can get into trouble!
The theory is that when a vehicle rotates, It spends part of the time in a longitudinal position (little drag) and part of the time in a lateral position (full drag) and the rest of the time in between (somewhere between little and full drag). So if a vehicle rotates twice in a given distance, the amount of time spent in the lateral position can be less than for a single rotation. However of course the starting and ending angles for the rotation have an affect (did the rotation start and end in a lateral (full drag position?) The amount of spin is a function of the impact configuration, and a greater offset of the collision point from the vehicle center of mass generally will produce more rotation. (there is that generality thing again!)

Using CRASH or CRASH clones, the calculations are computed by answering one question:
Did the vehicle rotate more or less then 360 degrees?.
The CRASH algorithms can be extremely sensitive to the assumptions of impact position and headings and post-impact drag. The CRASH program, upon which these programs are based, was developed as a pre-processor for the more sophisticated SMAC program (which is marketed as WinSMAC, EDSMAC, msmac, smacFX). With CRASH for certain impact configurations the assumptions for the heading angles at impact can dramatically affect the resulting impact speed calculations. Likewise, the higher the assumptions for the post impact drag factors, the higher the speeds at impact.
So if you are sure of your travel speed and that the police results are dramatically different than your travel speed, then be sure to have the techniques used by the police tested for proper inputs and sensitivities.