Question on CRASH3, Effective Mass, Closing Speed

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MSI
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Question on CRASH3, Effective Mass, Closing Speed

Post by MSI »

Q: We are trying to come up with an adjustment for Airbag Control Modules (ACMs) that are not located at Center of Mass (COM or CG). Consider the following diagram:
        • DeltaV_at_Location.jpg
          DeltaV_at_Location.jpg (9.09 KiB) Viewed 1768 times
Our testing shows that the vehicle depicted will rotate about the right front tire. That means that an Electronic Data Recorder (EDR) located at point A will under-report the delta-V. We are interested in the delta-V at the damage centroid (point B) to get closing speed. Would it be correct to use the effective mass ratio to relate the measured delta-V at point A to the delta-V at the COM? Then, we would use a second application of the effective mass ratio to get the delta-V from COM to Point B.
If this is incorrect, could you point me in the right direction to develop a correction factor.

A: Couple of things:
Closing Speed:
  • DeltaV is not normally used with closing speed.
    From our McHenry Accident Reconstruction Book (currently out of print):
    • Closing Speed: This term is used to indicate the speed of approach of two colliding bodies immediately before contact. It may further be defined as the algebraic sum of the velocity components of two vehicles along a reference line connecting their centers of gravity, and it is generally expressed in miles per hour. For a simple example, the closing speed in a central rear-end collision is the difference between the forward velocities of the two involved vehicles.
      Closing speed, by itself, does not provide a complete measure of impact severity, since it does not include an indication of the mass ratio of the two colliding bodies. For example, with a given closing speed, a heavy truck or a freight train will obviously produce a more severe impact for its collision partner than would a subcompact automobile.
      The terms impact speed and closing speed are sometimes used interchangeably
    So I’m not sure how you’re using deltaV for closing speed.
Resolution of ACMs (Airbag Control Modules) to Center of Gravity (CG or COM):
  • Please see our paper entitled RICSAC97-A Reevaluation of the Reference Set of Full Scale Crash Tests, SAE Paper 97-0961
    which includes
    • “A new method of analyzing accelerometer data from arbitrary sensor positions, on the basis of discrete measures of the vehicle responses rather than complete time-histories, is defined”.
      That may help you resolve ACM data to the CG (since an ACM is basically an accelerometer)
    Obviously some ACM’s stop collecting data once the decision to fire occurs and also the sampling/recording rate for ACM’s are probably less than the accelerometers we were working with from those tests so some differences may be attributable to those issues.
We’ve also prepared some hand written notes on Effective Mass, Center of Percussion and radius of gyration (k**2) (k squared)
  • In the notes you’ll not that if L (the distance from the point of application of the force and the CG) is close to B (the distance from the CG to the rear wheel) then K**2 would be sufficiently close to A * B such that yaw rotations would be about a point near the right front wheel so confirming your observation

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