Driving stability of passenger vehicles under crosswinds

Licentiate thesis, 2021

Passenger cars are a vital part of modern society, giving people the freedom of flexible travel. As technology advances, customers increase their demands for future products. The automotive industry must, therefore, adapt to society's requirements of energy-efficient travel, where developing low drag vehicles is key. However, if not designed with care, streamlined bodies of low drag might impair driving stability. In addition, raised customer demands of perceived control and stability elevate the research needs on driving stability in crosswinds.

Vehicles travelling on open roads are always exposed to the changing crosswind conditions. Most road vehicles have the aerodynamic centre of pressure located at the front half of the vehicle, making them sensitive to these crosswinds. Strong winds and sensitive vehicle designs may degrade the perceived level of driving stability by drivers and passengers. In extreme winds, this can even cause accidents. Furthermore, the aerodynamic loads increase with flow velocity, deteriorating the driving stability performance at higher speeds.

The assessment of driving stability in the development of a new vehicle is often done at the test tracks during late design phases when prototype vehicles are available. However, the current demands of faster development times require robust virtual methods for assessing the stability performance in early design phases. The goal of this thesis is, therefore, to find virtual simulation tools for assessing straight-line driving stability, and to gain more insights on the interdisciplinary physics between aerodynamics and vehicle dynamics.

By conducting experimental on-track measurement, it was confirmed that crosswinds deteriorate the driving stability and that the vehicle motions of lateral acceleration and yaw velocity correlate with the drivers' subjective assessment. These motions were combined into a proxy measure for stability, later used for objective assessment in the numerical simulations. The numerical study employed a coupled simulation methodology between aerodynamics and vehicle dynamics. It was shown that a 1-way coupling was sufficient for passenger vehicles in normal wind conditions.
Furthermore, the aerodynamic loads, including the yaw moment overshoots during transient gust events, could accurately be predicted by a quasi-steady model accounting for the phase delay between axles when driving into crosswinds. An extensive parametric study highlighted the aerodynamic yaw moment coefficient and the longitudinal centre of gravity position as the two most influential vehicle parameters. In addition, the suspension characteristics revealed potential in improving the driving stability performance under crosswinds.