Rotating Wheels - Their Impact on Wind Tunnel Test Techniques and on Vehicle Drag Results

The question of the proper simulation of wheel rotation has not so far been a major concern. Within the scope of an examination of the influence of wheels and tyres on aerodynamic drag it will be shown that their contribution to the overall drag value - whether they are rotating or not - is of about the same magnitude as the proportion of the rough underbody. Therefore the question of the importance of the simulation of wheel rotation is posed.

This paper discusses how a measurement with a better simulation can look like and what the major changes in the flow field are. In particular a new physical quantity, which has to be determined, the so-called “fan moment” is introduced. . The problems that arise in the determination of the fan moment of the wheels and hence in the required isolation of the rolling resistance, are described in detail. This is done for a test set up with full width moving belt and measurement via internal balance and sting support. An alternative set up with separate wheel drives and external balance is discussed briefly.

In a presentation of vehicle test results it is shown that the simulation of wheel rotation has a significant effect on the aerodynamic resistance. The dependence of the effect on the wheel/ tyre configuration is demonstrated in several examples. Finally, using the experimental results from a special test rig with a single wheel, the manner in which the tyre contact area and the yaw angle effects the forces at the rotating wheel is shown.