Simulation study of oscillatory vehicle roll behavior during fishhook Maneuvers

Sustained roll oscillations were observed while performing the National Highway Traffic Safety Administration (NHTSA) fishhook (FH) test on several vehicles. This phenomenon has also been observed on several manufacturers´ high center of gravity (CG) vehicles with both solid and independent rear suspensions. Roll oscillation can be accompanied by non-convergent yaw, heave, and pitch. A study was initiated to quantify the influence of vehicle inertia, suspension, powertrain, and tire characteristics on the FH vehicle response. Design of experiments (DOE) methods were used to quantify the main effects and the significant interactions between vehicle design variables. The CarSim program was used to simulate the vehicle dynamics and the iSIGHT program was used to automate the DOE analysis. Summary findings show that tire lateral force high slip behavior influences yaw instability. This alone is not sufficient for developing diverging roll oscillations. Additionally, results show that suspension jounce travel and bumper rate, in conjunction with tire overturning moment and non-positive tire cornering stiffness, influence yaw, roll, and pitch stability. Simulation results suggest that the primary cause of roll oscillations is the transfer of some energy from the longitudinal mode into the roll and heave modes. This effect can also be influenced by other factors like the distance between the CG and the roll axis, yaw-roll cross product of inertia, for example. Also described is optimization of FH performance (minimization of wheel lift and roll oscillations) with respect to some suspension characteristics.