Is there a Relationship Between Decel Rate of a Collision and the Impact Speed?

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MSI
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Is there a Relationship Between Decel Rate of a Collision and the Impact Speed?

Post by MSI »

Q: (This is a response to a question on the crashforum)
I am trying to estimate the deceleration rate of a truck that had an offset collision with the trailer of a semi-truck. It is presumed that both vehicles were going 60-65 MPH. I have looked into a NHTSA crash test report of a similar vehicle for the subject truck, and I have calculated the deceleration rate for that crash test, but it was conducted at 35 MPH. I have also tried to search the web, but I haven't found anything yet.
Is there a relationship between the deceleration rate of the crash time period and impact velocity of the vehicle? For example, maybe deceleration rate is linearly proportional to velocity.

A: You asked:
  • Is there a relationship between the deceleration rate of the crash time period and impact velocity of the vehicle? For example, maybe deceleration rate is linearly proportional to velocity.
Quick response: NO.

The deceleration rate of the crushing of the vehicles due to impact would be the same whether at high speed or at low speed IF the relative speed is the same. 50 mph into 30 mph crash would be the same DeltaV as 30 MPH into 10 MPH for same collision partners

It all depends on the relative speed between the impacting partners and the resulting impact speed change

So first you look at the collision partners and what parts of each interacted.
  • A complicating factor in your collision is that it was a truck into a trailer:
    • First Trucks are stiffer than most motor vehicles but what part of the front of the truck struck the trailer?
      and where?
      Second, was there any underride of the trailer?
      • Since underride collisions reduce the stiffness/accelerations in a crash (softer upper parts of a vehicle like the hood instead of engaging the frame/engine/etc)
Then if all other things are the same for relative speeds at impact, at either speed does the impacting vehicle and/or struck vehicle have the brakes on?
  • Two factors there: pitch angle and level of deceleration.
    • So first how might braking affect the pitch angle of the struck and/or striking vehicle?
      • With trucks you won't see as much pitch response as with cars. However there may be some
    • Deceleration:
      • Some deceleration prior and then during the crash depending on the vehicle (striking vs struck) might add to, or reduce from, the maximum acceleration. Braking can add or reduce the peak acceleration by approximately 0.5 to 0.8 g's or so (for trucks the maximum braking levels are generally less than for cars).
        May be a small component for higher level DeltaVs or larger part if smaller level DeltaV.
A good technical write up on this is contained in
  • See page 93-95 for a detailed discussion.
    The page 95 summary:
    "The relationships that have been derived and the associated discussion are believed to clearly establish the fact that the structural damage produced by a given speed change of a vehicle in a non-sideswipe collision is independent of the speed range that it occurred."
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