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The determination of appropriate friction coefficient values is an important aspect of accident reconstruction. Tire-roadway friction values are highly dependent on a variety of physical factors. Factors such as tire design, side force limitations, road surface wetness, vehicle speed, and load shifting require understanding if useful reconstruction calculations are to be made. Tabulated experimental friction coefficient data are available, and may be improved upon in many situations by simple testing procedures. This paper presents a technical review of basic concepts and principles of friction as they apply to accident reconstruction and automobile safety. A brief review of test measurement methods is also presented, together with simple methods of friction measurement to obtain more precise values in many situations. This paper also recommends coefficient values for reconstruction applications other than tire- roadway forces.

Vehicle accident reconstruction methods based on deformation energy are argued to be an increasingly valuable tool to the accident reconstructionist, provided reliable data, reasonable analysis techniques, and sound engineering judgement accompany their use. The evolution of the CRASH model of vehicle structural response and its corresponding stiffness coefficients are reviewed. It is concluded that the deformation energy for an accident vehicle can be estimated using the CRASH model provided that test data specific to the accident vehicle is utilized. Published stiffness coefficients for vehicle size categories are generally not appropriate. For the purpose of estimating vehicle deformation energy, a straight-forward methodology is presented which consists of applying the results of staged crash tests. The process of translating crush profiles to estimates of vehicle deformation energies and velocities is also discussed.

Among the several data recorded by a typical motor vehicle’s event data recorder (EDR) prior to, during and after a crash event, are sampled time histories of longitudinal and lateral components of delta-v. The delta-v components are not measured directly but are calculated by numerically integrating the outputs of two perpendicular accelerometers contained within the EDR box. As currently designed and implemented a typical EDR does not measure yaw velocity or track vehicle heading during the impulse phase of a crash. Without this yaw information to orient the accelerometers relative to the fixed ground, the delta-v values calculated by the EDR through direct integration of its measured acceleration components should not be interpreted as representing absolute changes in vehicle velocity, especially in cases where the yaw velocity is high. EDR-calculated delta-v components must be adjusted to account for the yaw motion that occurred during acquisition of the data.

Rollover accidents are responsible for a significant percentage of crash injuries. Increasing seat belt restraint use is the most effective way to reduce rollover injuries. Injuries to restrained occupants are also of interest. It has been suggested that head/neck injuries are caused by roof crush, and that modification to roof structures and seat belt systems would lead to a substantial reduction in severe rollover injuries. Field accident data and rollover testing are used to evaluate the relationship between roof crush, seat belt design, and severe rollover injuries.

Field accident data suggest that a significant number of occupants involved in rear impacts may be positioned at impact other than in the “Normal Seated Position” - the optimum restraint configuration that has been used almost exclusively in published seat testing. Pre-impact vehicle acceleration from braking, swerving, or a prior frontal impact could cause an occupant to be leaning forward at the instant of the collision, creating a situation where the vehicle “ride-up” potential would be limited. No rear impact tests involving yielding, production-type seats with forward-leaning dummies are found in the literature. Thirty rear-impact sled tests with a forward-leaning, “Out-of-Position” Hybrid III dummy are presented. Tests were performed with a calibrated seat set in either the rigidified or yielding configuration and with the dummy either unbelted or restrained by a production three-point belt system. Test speeds ranged from 5 to 20 mph.