ABSTRACT
The most common type of low-speed impacts involves the front of one vehicle crashing into the rear of another that is stopped with the driver’s foot on the brakes. Chapter 25 discusses the implications of braking by the struck or striking vehicle in such crashes. Generally, tire scrub by the struck vehicle would not come into play. Tire marks associated with the impact phase, or other forms of evidence, could indicate otherwise, of course.
When a vehicle is struck in the side, its tires can be expected to scrub sideways on the road, regardless of the degree of braking. Even if tire forces are not present for the striking vehicle, they exist for the struck vehicle. Whether they can be neglected in the reconstruction becomes a question of crash severity and the relative magnitude of impact forces.
Chapter 25 describes two low-speed impact tests conducted by Exponent, Inc. The tire scrub was estimated by applying a friction force over the distance the vehicle slid sideways during the crush phase, as determined from acceleration data. The estimate was consistently high, probably due to overestimates of the distance, so further attempts to quantify tire scrub were discontinued.
A third side impact test of a similar vehicle was performed for the National Highway Traffic Administration. To characterize its structure, the usual crash plot was drawn between a fictitious point at an assumed a no-damage threshold of 2.0 mph ΔV, and a data point corresponding to the actual test. Adding the low-speed test data to this plot showed that their Energy of Crush Factor (ECF) and crush energy would be seriously overpredicted. Clearly, the fictitious point at 2.0 mph ΔV was incorrect and should be jettisoned.
Doing so and just using the actual data points, while still including tire scrub, resulted in a much better fit to the data. Excluding tire scrub gave a fit very nearly as good. The crash plot showed a no-damage ΔV of 0.35 mph, which is recommended as a fictitious data point all characterizing all side structures in which data exist for only a single high-speed test.
For low-speed impacts, it is not unusual for the vehicle to be repaired prior to inspections by reconstructionists. Crush data may not exist, but either estimates of or actual repair costs do. This chapter proposes a methodology through which crash severity may be estimated using repair costs for front, rear, and side impacts.
