Analysis of a Vehicle’s Longitudinal Response to an Unsteady Wind Environment

In a preceding paper an on-road investigation of the longitudinal aerodynamic response of a vehicle to ambient wind was presented. That study resulted in a frequency-dependent response function with a distinctive maximum within the range of the natural frequency of the vehicle on its suspension system. This finding raised the question as to whether the horizontal response of a car’s deceleration to wind gusts is associated with or caused by the suspension’s natural frequency. The objective of the present work is an attempt to shed some light on this question by the investigation of both deceleration and pitch angle fluctuations in additional on-road experiments. Both vehicle velocity and total airspeed in the driving direction and the pitch angle are recorded by independent data acquisition systems during a set of coastdown experiments. The deceleration data and the vehicle’s response to the airspeed fluctuations are derived by differentiation of its velocity-time history and numerical filtering techniques. The experiments reveal a cause-and-effect relation between wind gusts and pitch motion that is additionally supported by model calculations. The study results in frequency-dependent longitudinal vehicle response and pitch angle response functions. Both functions show distinctive maxima, which are clearly separated from each other. This finding implicates that the resonance in the longitudinal vehicle response is caused by an aerodynamic effect.