Question of Trajectory Analysis Approximations

General Questions related to the Momentum Based Analysis programs
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brian
Posts: 499
Joined: Tue Jul 14, 2009 10:52 am

Question of Trajectory Analysis Approximations

Post by brian »

Q: Can we make an approximation about the rest positions depending on speed, vehicle mass or something like that? For example:We know the impact speeds, masses, coefficient of friction and some other parameters. And we want to find to where the vehicles would go after the collisions: their rest positions. I know for an accident analysis, rest positions would be a starting point but i wondered if we make an analysis from the opposite way. Thank you.

A: Your question brings up a topic which has made the rounds over the years:
Why doesn’t someone write a program that uses negative friction?
So it backs up from rest to impact?

Well that hasn’t been done yet. That would be a separate discussion.
But
what you ask about is a Trajectory Solutions Procedure. And as part of our research we have used two different approaches: A Trajectory Analysis Approximation and Simulation.
  • Trajectory Analysis Approximation:
    • One uses the vehicle properties, drag, impact/separation position and rest positions and approximates the speed required for a vehicle to travel the distance and direction from separation to rest.
      In the CRASH program you will find work my father, Ray McHenry, did on refinement of the Marquard relationships included clarification and generalization to include partially locked vehicles, to accommodate curved paths and accommodating residual linear velocity at the end of rotation, etc. for approximating vehicle separation speeds in a collision.
    • A write up of the work he performed is contained in the NHTSA CRASH3 Manual (see section 2 page 55 etc.).
    • We also include some pages on Trajectory Analysis from our book McHenry Accident Reconstruction.
    Simulation: With the speed and power of computers there is no reason not to utilize a simulation model for the spinout and/or the collision and spinout to further improve the approximation of impact speeds in vehicle collisions.
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