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Many accident reconstructions rely on the use of friction factors for the analysis of vehicle speeds. Measurement of the friction factor, or coefficient of friction, at the accident site is usually an important step in achieving a more accurate estimate of the friction factor at the time of the accident. Over the years several on site test methodologies have emerged within the accident reconstruction community. However, little has been published which compares the data and results from the different methods. This paper presents a comparison of some methodologies. A g-analyst1 accelerometer, a VC•20002 accelerometer, and a bumper chalk gun3/radar gun4 are compared for locked wheel friction values under different speed and road surface conditions. Data from the two on board systems are recorded simultaneously. Measurements are made for several stops at each of the speeds and two road surface conditions.

In the vast majority of impacts involving light vehicles, traditional impulse-momentum collision models can be used to analyze the mechanics of two colliding vehicles. However, these models cannot handle the multiple degrees of freedom associated with articulated (pin-connected) vehicles. In addition, collisions involving one or two articulated vehicles may not satisfy the basic assumptions of these traditional collisions models. In particular, the assumption that impulses of external forces (such as tire-road friction) are negligible compared to the impulse developed over the crash surface may not be valid. The large masses, long dimensions, the presence of the pinned joint, or all of these factors, may necessitate special considerations and more flexible model capabilities. This paper lists the assumptions that underlie the application of the principle of impulse and momentum to a planar collision between rigid bodies.

Various vehicle dynamic simulation software programs have been developed for use in reconstructing accidents. Typically these are used to analyze and reconstruct preimpact and postimpact vehicle motion. These simulation programs range from proprietary programs to commercially available packages. While the basic theory behind these simulations is Newton's laws of motion, some component modeling techniques differ from one program to another. This is particularly true of the modeling of tire force mechanics. Since tire forces control the vehicle motion predicted by a simulation, the tire mechanics model is a critical feature in simulation use, performance and accuracy. This is particularly true for accident reconstruction applications where vehicle motions can occur over wide ranging kinematic wheel conditions. Therefore a thorough understanding of the nature of tire forces is a necessary aspect of the proper formulation and use of a vehicle dynamics program.