Tire dependence for the aerodynamics of yawed bicycle wheels

The aerodynamic characteristics of a modern road cycling wheel in cross wind are studied through force- and planar PIV measurements in the TU Delft Open Jet Facility. The performance of the 62 mm deep rim is evaluated for three tire profiles, and yaw angles up to 24 degrees. All measurements are executed at 12.5 m/s (45 km/h) freestream- and wheel-rotational velocity. The wheel's rim-tire section in crosswind is found to behave similar to an airfoil at incidence, ultimately resulting in a reduction of the wheel's aerodynamic resistance with increasing yaw angle magnitude. This phenomenon, also referred to as the sail-effect, is limited by the stall angle of the tirerim profile. The stall angle is found to depend critically on the tire's surface structure. Larger stall angles, resulting in lower resistance, are obtained if the tire profile triggers laminar-to-turbulent boundary layer transition.