McHenry Forum - Crash Reconstruction & Simulation

Q: Apart from bumpers and structures locking, are there other situations in rear-end collisions that are inelastic (i.e. the two vehicles travel together at the same velocity after the collision)? Would it be correct to say that inelastic linear collisions are not as common as rear-end collisions where the post-impact speed of the projected vehicle is greater that that of the rear vehicle?

A: For lower speed inline rear end impacts one would expect structural restitution to play a part since it acts to increase the speed of the struck vehicle while reducing the speed of the striking vehicle.
And to add to that generalized speed difference driver actions subsequent to the collision might tend to increase the relative speeds

• if for example the striking vehicle driver was braking and continued to brake subsequent to the collision.
  Also if the struck vehicle driver is knocked off the brakes as a result of the collision and so the vehicle would roll freely.

The amount of restitution in a rear-end inline collision depends on a number of factors such as relative impact speeds, vehicle and bumper types.

• For example, if the impact speed produces sufficient damage to the rear of the struck vehicle such that the drag on the struck vehicle is similar to braking (rear wheels impaired or locked due to the collision) that would reduce the separation velocity and therefore the vehicle might travel together to positions of rest.
  Alternatively if the speeds/vehicle types caused a drag on the struck vehicle due to impairment of the rotation of the wheels due to damage that would appear as a greater difference in the separation speeds.
  Under-ride and over-ride of the bumpers which may not lock however which might reduce the amount of relative separation velocities of the vehicles.

For additional information and discussion on restitution, please be sure to read our 1997 SAE paper

• "Effects of Restitution in the Application of Crush Coefficients", SAE Technical Paper No. 970960