Vehicle Accident Analysis and Reconstruction Methods, Second Edition by Brach Raymond; Brach Matthew;

Author:Brach, Raymond; Brach, Matthew;
Language: eng
Format: epub
Publisher: SAE International

Secondly, the coefficient of restitution for a vehicle-to-vehicle collision without braking can change significantly when braking is present because the collision conditions change [7.6].

The contact duration of a collision can change when braking exists [7.6].

The frictional impulse used in a reconstruction is not arbitrary. That is, it is possible to choose a frictional drag coefficient and impulse duration that are physically impossible to attain for conditions of a given collision.

Consider items 2 and 3 above. For the same closing velocity as a collision without braking, as brakes of the struck vehicle are applied more strongly, the frictional drag increases. This causes the vehicles to stay in contact longer. Because the frictional drag removes energy from the system, the final velocities of both vehicles approach each other, and so the coefficient of restitution decreases. Eventually, if the frictional drag becomes high enough, the struck vehicle begins to act more as a barrier (not a rigid barrier, however, because the “barrier,” i.e., the struck vehicle, has an absorbing bumper), the contact duration then begins to drop, and the restitution begins to rise. In other words, e and depend on in a relatively complex manner. The reason for this complexity is that the equations for low-speed collisions deal with external frictional impulses, not directly with forces, and the external impulses can be over-specified. Additional discussion of the characteristics of low-speed collisions can be found in the works of Heinrichs and Goulet [7.7] and Happer, et al. [7.8].

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