Surprises in cycling aerodynamics

Federal Lab. for Materials Testing and Research (Empa) Dübendorf Switzerland l 5 Bakala Academy Athletic Performance Center Univ. of Leuven Heverlee, Belgium Surprises in cycling aerodynamics T he greatest potential for improvement in cycling speed is aerodynamic” [1]. At racing speeds (about 54 km/h or 15 m/s in time trails), the aerodynamic resistance or drag is about 90% of the total resistance [2-5]. Most previously published studies on cycling aerodynamics aimed at reducing the aerodynamic drag of a single cyclist. Fewer publications have addressed the drag reduction due to drafting. In drafting, two or more cyclists ride close behind each other to reduce aerodynamic drag. The few published studies on drafting all confirm the large drag reduction for the trailing riders (up to 30-40%), whereas there seems to be a lack of consensus about the effect of drafting on the leading rider. In this respect, Olds [6] stated: “It has been suggested that riding close behind a leading cyclist will also assist the leading rider in that the low pressure area behind the cyclist will be “filled up” by the trailing rider. However, both Kyle (1979) and McCole et al. (1990) failed to find any measurable effect either in rolldown experiments or in field VO2 measurements.” On the other hand, Computational Fluid Dynamics (CFD) studies on human body models with simplified geometries (such as elliptical cylinders) found drag reductions for the leading cylinder up to 5% [9]. However, to the best of our knowledge, no studies have yet been published on detailed 3D CFD simulations of drafting cyclists based on realistic human body geometries. Such 3D CFD simulations were the focus of this new study, which included also wind-tunnel measurements. First, the wind-tunnel measurements were used for CFD validation for single cyclists and two drafting cyclists. Next, supported by the validation study, CFD simulations were performed for groups up to eight drafting cyclists, allowing assessment of aerodynamic drag at every position in the group. drafting is riding close behind each other to