English, 20 pages, 16 figures, 32 references

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 all 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.
This paper includes a discussion of tire force mechanics, definitions of terms, modeling of individual tire force components and tire forces for combined braking and steering currently used in simulation software for reconstruction of accidents. The paper discusses the difference between a tire force ellipse and the friction ellipse. Equations are presented for five tire force models from three different simulation programs. Each model uses a different method for computing tire force components and combined braking and steering. Some tire force models begin with a specified level of braking force and use the friction ellipse to determine the corresponding steering force; this produces a resultant tire force equal in magnitude to full skidding for combined steering and braking.
Three dimensional surface plots of the calculated forces are presented of all of the models. This allows for a visual comparison of the combined forces over a full range of the longitudinal and lateral tire slip variables.