The Aerodynamics of a Small Car Overtaking a Truck

The influence of a large truck on the aerodynamics of a small passenger car in an overtaking manoeuvre on the motorway was considered, many years ago, during the 1970's, to be a potential problem for the vehicle aerodynamicist. The concern never became significant as vehicle architecture evolved and car weights increased. The current drive for improved fuel economy is advocating that a considerable reduction in vehicle mass is desirable and therefore it may be time to readdress the significance of the truck passing manoeuvre. A quasi-steady experiment has been undertaken at small model scale to examine the aerodynamic characteristics of a small car in proximity to a large truck. Measurements at yaw were included to crudely simulate the effects of a crosswind. The wind tunnel data is presented and the limitations of the experimental procedure are discussed. An estimate of the increased aerodynamic input on a car in a real world overtaking manoeuvre with relative velocity between the two vehicles is introduced. The data is compared with the steady state yaw characteristics of the car alone to relate truck passing with the general concern of crosswind sensitivity. Under the same crosswind conditions the truck passing manoeuvre is shown to be a more extreme, but rarer event.