Michael B. James

FRICTION APPLICATIONS IN ACCIDENT RECONSTRUCTION

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.

INJURY MECHANISMS AND FIELD ACCIDENT DATA ANALYSIS IN ROLLOVER ACCIDENTS

Rollover accidents are responsible for a significant percentage of crash injuries. Increasing seat belt restraint use is the most effectie 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. (A) For the covering abstract see IRRD 893297.

Headroom, Roof Crush, and Belted Excursion in Rollovers

Based upon a review of the literature and new test data, the human and vehicle factors leading to head-to-roof contact in rollovers are quantified and illustrated. Vehicle design countermeasures and suggested areas of research are presented. Higher and stronger roofs and improved restraints must be analyzed as a system to evaluate the potential benefits in rollovers.

Rear Stiffness Coefficients Derived from Barrier Test Data

Rear impacts in the crash test data base compiled by the National Highway Traffic Safety Administration (NHTSA) are analyzed and compared to the CRASH3 rear stiffness coefficients in this paper. The CRASH3 values do not represent the test data adequately. This is because the values were derived from limited data, and because some of the rear moving barrier test data was miscoded as fixed barrier tests. A review of the larger NHTSA data base does not support the CRASH3 assumption that vehicles of similar size (wheelbase) have similar rear stiffness characteristics. Therefore, it is important when reconstructing individual accidents to use crash test data specific to the vehicles involved. Repeated rear fixed barrier test data on four vehicles are analyzed to study the data trend at speeds below and above the NHTSA test data. Constant stiffness and constant force models are compared and a combination of the two is shown to fit available test data.

A Comparison Between NHTSA Crash Test Data and CRASH3 Frontal Stiffness Coefficients

The appropriateness of the set of eight frontal stiffness coefficients used by the CRASH3 program to estimate vehicle deformation energy (and to subsequently derive estimates of vehicle delta-V) is examined. This examination consists of constructing so-called CRASH energy plots based on 402 frontal fixed barrier impact tests contained in the NHTSAs Vehicle Test Center Data Base (VTCDB) digital tape file. It is concluded that the use of category coefficients within the CRASH3 program can result in large delta-V errors, reaffirming the inappropriateness of this program for use in individual accident reconstructions. The use of the CRASH3 category stiffness coefficients is seen to generally overestimate vehicle energy absorption for vehicles with small amounts of frontal crush and to underestimate vehicle energy absorption for vehicles sustaining large crush. Comparisons among the stiffnesses of different size vehicles indicates little or no dependence of stiffness on vehicle size and it appears that the current set of eight frontal stiffness coefficients could, for statistical purposees, be reduced to two - a set applicable to passenger cars and a second set for light trucks and vans.

Methods of Occupant Kinematics Analysis in Automobile Crashes

This chapter on methods of occupant kinematics analysis in automobiles is from a comprehensive text on vehicle and occupant responses in rollovers. The authors stress that understanding occupant kinematics is an important part of accident reconstruction, particularly with respect to injury causation. The authors describe some methods for assessing occupant kinematics in a collision, and discuss their limitations. They present a technique that is based on free-body analysis and that can be used to establish an occupants path of motion relative to the vehicle, locate the point of occupant contact, and determine the occupants velocity relative to that contact location. Establishing the occupants path of motion relative to the vehicle aids in determining the location of the second-impact and identifying contact marks, the time from initial collision to the second-impact, the occupants velocity, possible ejection portals, and occupant injury location.

A Perspective on Automobile Crash Fires

The relatively rare occurrence of injury or fatality in fuel-fed fires has received considerable attention in automotive safety rulemaking and products liability litigation. The literature related to fatalities associated with fire is confirmed by recent FARS data, and there are no reliable field data which confirm a need for further injury-reducing effect related to FMVSS 301. NHTSA has acknowledged this by removing crash fire rulemaking from its priorities plan. The police-reported crash fire data now available must be supplemented with in-depth investigation by trained teams before informed judgements can be made regarding further safety improvements with respect to crash fire injury.

Issues in seatbelt inertial release

Initial claims that seatbelt buckles might inertially unlatch in an automobile collision were based upon the observation that sharply striking the backside of a side-release buckle with some object could cause the buckle to unlatch. More recently, similar examinations have formed the basis of the hypothesis that end-release buckles might be susceptible to inertial release under certain crash conditions. This paper discusses some examples of how test data have been misinterpreted to erroneously support these hypotheses.