Sensitivity of System Boundary Conditions on the Migration of Low Frequency Modes Controlling Longitudinal Vehicle Response

Vehicle driveability is increasingly used as a key measure in media evaluations, and is refined aggressively to differentiate and position the product within its market segment. This is a highly complex system level issue, and encompasses the non-linear interactions between the driveline, suspension and powerunit mounting hardware. The driveability character of the vehicle has typically been tuned through calibration in the later stages of development. Through the use of physical prototypes, such activities have typically been performed on the basis of subjective assessments, to achieve a balanced compromise with other vehicle attributes such as ride, handling and refinement. This paper introduces a model-based approach to facilitate design and detailed analysis early in the product development process, thereby reducing reliance on physical prototypes and the need to implement late design changes. A detailed non-linear mathematical model has therefore been developed in order to characterise the low frequency, longitudinal behaviour of a prototype, four-wheel drive vehicle both in the time and frequency domains. In conjunction with full vehicle test measurements, the analytical model has been validated and then used to investigate a low frequency, fore-aft vehicle oscillation issue that was identified specifically during part throttle pullaway events in cold climate testing.