Fahrrad-Laufräder - Aerodynamic investigations on bicycle wheels

  • Czaputa, Klaus, (Co-Investigator (CoI))
  • Meile, Walter (Co-Investigator (CoI))
  • Brenn, Günter (Principal Investigator (PI))
  • Kramer, Thomas, (Co-Investigator (CoI))

Project: Research project

Description

The aerodynamic drag of bicycle wheels usually amounts a considerable portion to the overall drag of the complete system athlete-bicycle. This holds although the projected front area (perpendicular to the incoming flow) of the wheels is much smaller than that of the athlete. One reason may be found through the fact that, due to the rotation, the velocity of the upper part of the wheels relative to the air is much larger than that of the other parts of the system, and the drag is proportional to the square of the velocity.
During the development of new wheels, the manufacturer XeNTis has chosen a very innovative way by a four-spoke design via covering the actual carrier spokes with low-drag profiles made from carbon fibre composites. This should lead to considerably reduced drag in regions of high velocities when compared to the usual cylindrical or flat spokes. On the other hand, these profiles are oriented inversely to the flow in regions of lower velocities, thus leading to larger drag. Altogether, it may be expected that the aerodynamic drag is smaller above the rotation axis of the wheels and larger below the axis. Therefore, the rotating motion of the wheel should be supported by this design. This expectation is strongly supported by preliminary wind tunnel tests with the wheel freely mounted in the air stream. In these tests the air velocity was increased stepwise with the wheel initially at rest, and a wind-induced rotation started at a certain air velocity. After the acceleration period, a constant rotation speed was achieved, which depends on the angle of attack.
Previous publications of wind tunnel tests usually specified the drag of the rotating wheel in the air stream, but do not give the moment necessary to sustain the rotation.
The present project aims at providing basic data for the evaluation of bicycle wheels with respect to their aerodynamic behaviour. For a correct comparison, several wheel designs available on the market will be incorporated (disc wheel, four-spoke wheel, and various conventional spoke designs).
Wind tunnel tests are performed at flow conditions comparable to velocities achieved in street races. The resulting forces and moments are measured with a six-component wind tunnel balance. The rotation of the wheel corresponding to the driving speed is achieved using a device for supporting and driving the wheel. This device is mounted on the wind tunnel balance.
Furthermore, in a second project phase numerical simulations for optimizing the XeNTiS product will be carried out, where the localisation of drag inducing regions and the interaction of tires, rims and spokes will be of major interest.
StatusFinished
Effective start/end date1/03/0631/12/06