Sept 2020: As part of the NAPARS facebook page presentation
- Today: An NCAP Report, Part5, Restitution
they included- NHTSA Crash test #10156
- which was a 2018 Chevrolet Traverse 4D SUV 35 MPH barrier crash.
They calculated the coefficient of restitution for the 35 MPH barrier test at 18%!!
THAT is outside the normal range of the structural restitution of motor vehicles at that speed range.
so....why?- A sidenote: A comment posted to the Facebook NAPARS post on this topic brought to our attention an irresponsible video posted up by a software vendor which purported to include an 'example of negative restitution!!'...
- IMPOSSIBLE!
- IRRESPONSIBLE!
- Please see our response to the comment and the video:
to which the correct answer is
NO!!
- A sidenote: A comment posted to the Facebook NAPARS post on this topic brought to our attention an irresponsible video posted up by a software vendor which purported to include an 'example of negative restitution!!'...
- which was a 2018 Chevrolet Traverse 4D SUV 35 MPH barrier crash.
- NHTSA Crash test #10156
- 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.
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."
- "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).
- 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?
Here is another chart of measured restitution at lower speeds