18% Restitution at 35 MPH??

General Crash Reconstruction Tecnhiques
Crash tests, Pole Impacts
MSI
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18% Restitution at 35 MPH??

Post by MSI »

Sept 30 2022: this topic split off from related topic What is Restitution? (w/r to Vehicle Collisions)

Sept 2020: As part of the NAPARS facebook page presentation
  • As Posted on the Facebook NAPARS Page:
    • "NCAP report, page 2-20 shows that based on the accelerometers, the velocity change was 66.56 km/hr (41.35mph). But the impact speed was only 56.39 km/hr (35.03 mph).
      With these numbers, we can calculate the coefficient of restitution, commonly written as the Greek Epsilon, or a lower case e.
      Restitution is defined as the separation speed divided by the approach speed, both being absolute values (i.e. always positive).
      In this test that looks like this :
      • e= ABS[0-(-10.17)] / ABS[56.39-0] = 0.18. That's 18% rebound.
      That's a lot, right? Not quite superball-levels, but it is 2 or 3 times what we commonly expect in the analysis of big crashes. I think this is because we usually deal with car-to-car crashes, where both "impact partners" can plastically deform.
      Restitution for us ranges from 0 and 1, with zero being like silly putty going "splat" on the floor and stopping right there, and a "one" when the separation velocity equals the approach velocity - nothing getting lost in the exchange. But there should always be something lost, even if just a little, in the form of noise, heat, deformation, and hysteresis. As you approach zero speed change (very very small crashes) you get more and more of the impact speed back, so restitution goes up, approaching one as you approach zero change in speed.
      It [normally] drops pretty quickly to the 20-30% range for even light impacts, and down to the 5% range for highway events."
OUR COMMENTS
  • That seems high for a 35 MPH barrier crash. I tried to examine the vehicle accelerometer data from the NHTSA Test Report 10156 however it did not include any data from the 8 vehicle accelerometers.
    On careful examination of the video it appears that the tires may have participated in the bounceback from the barrier.
    The following plot shows generally at 35 MPH (~56 kph) the coefficient of restitution would be expected to be lower than 0.18.
    However some do approach that value.
    So question is:
    • Are the higher values related to a smaller front overhang and additional rebound produced by the tires being loaded/unloaded on the barrier?
    • And if so, would we see that extra amount of restitution in car to car crashes?
    Might need to do some research on that!
    restitution over a range.png
    restitution over a range.png (85.63 KiB) Viewed 6893 times
    Here is another chart of measured restitution at lower speeds
    plot of restitution.png
    plot of restitution.png (64.94 KiB) Viewed 6871 times

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