ABSTRACT
Rollovers are chaotic events, so nonrepeatable on the whole as to take on a somewhat random nature. This is not to imply that details in the physical evidence can be overlooked. Indeed, they provide a solid underpinning to the developments of this chapter.
It is the case, however, that the reconstructionist must take a more broad-brush approach. While the vehicle deceleration along its path of travel surely varies according to whether the vehicle is airborne or in ground contact, staged rollover tests at 30 mph and above have established that the overall (average) drag factor falls into a fairly narrow band (0.38–0.50) between roll initiation and rest. This allows the roll distance to be quickly related to roll initiation speed. For lower speeds, it is advisable to account for the estimated CG heights during the roll, and thereby account for potential energy changes.
Chapter 16 describes how to lay out the rollover trajectory by matching the physical evidence on the vehicle to that on the ground. Examples are provided. Constructing the trajectory will give a preliminary indication of the number of rolls, which is subject to modification based on the roll rates discovered during subsequent analysis.
Once the trajectory is established, reverse trajectory calculations can be set up and performed in a spread sheet, much as they were for coplanar collisions as discussed in Chapters 3 and 4. The analysis is somewhat simplified because a constant drag factor—one that does not depend on crab angle or friction coefficient—may be used. However, a roll angle variable, one that requires splining, must be added. Key values of this variable are determined from the roll angles the vehicle must have in order to match up evidence on the vehicle with that at the scene. Chapter 16 includes an example of a reverse rollover trajectory analysis implemented in a spreadsheet. Physics calculations and splining operations are illustrated.
Performing reverse roll trajectory analysis produces yaw and roll rate histories. If there is a question about the number of rolls, a complete roll can be added to or subtracted from the trajectory, assuming that matchup of the physical evidence can be maintained. The effect on calculated roll rates should be dramatic, immediately indicating which is the correct number of rolls. Roll rate comparisons between staged tests and reconstruction analysis are provided.
Scratch angles are determined from the yaw orientation of the vehicle when it contacts the ground, as well as the relative translational and tangential velocities at the point of contact. Calculation of tangential velocities depends on the radial distance between the contact and the CG, which may be problematic when the shape of the vehicle markedly changes due to deformation as it rolls. Chapter 16 discusses this issue.
Finally, special considerations apply to curb and soil trips. These are also discussed.
