Full-scale bicycle aerodynamic evaluation methodology developed

A full-scall wind tunnel test method has been created at RMIT University. The bicycle is mounted on a platform which is itself mounted on a load cell. The load cell has 6 degrees of freedom (DoF) allowing drag, lift and side forces to be measured. The bicycle can be orientated at different yaw angles to the airflow so that the effects of crosswind can be observed. However, non-zero yaw angles increase the frontal area beyond the recommended maximum of 10 percent  solid blockage ratio for the 6 m2 RMIT wind tunnel. The platform measures 1,800 mm × 850 mm × 30 mm and is mounted just 20 mm from the tunnel floor to minimize interference. A plastic fairing is fitted to the front of the platform to further reduce flow separation. Video cameras are mounted around the cyclist to monitor any changes in body position.

Previous research has shown that aerodynamic drag causes the majority of resistance to a cyclist’s motion. For example, Kyle and Burke showed that for road bikes travelling on a level surface aerodynamics accounted for 50 percent of total drag at 13 kph (8 mph) increasing to 90 percent at 32 kph (20 mph). Other researchers have found the contribution to be just 50% at the higher end of this speed range for mountain bikes, perhaps because of considerably higher rolling resistance. Approximately 31-39 percent of the aerodynamic drag is caused by the bike with the rest caused by the rider and clothing.  The position of the rider has a significant impact on the drag.

Three positions on a road bike: Upright on top of bar, on drops and on aero (tri) bars
Road cyclist riding positions

Direct measurement of the drag force was used to determine the drag coefficient using the standard equation:

\large C_D = \frac{F_D}{1/2 \rho V^2 A}

where FD is the drag force, rho is the air density, V is the speed and A is the projected frontal area of the cyclist and bicycle. The frontal area was measured using a digital camera and image processing software. Experiments using this setup have shown standard deviations in the drag coefficient of less than one percent for a wide range of riding positions.

A major weakness of the approach documented in the paper is that the wheels of the bicycle are not rotated. The rotation of the wheels changes the flow of air and therefore affects the horizontal drag force. This is something considered by other researchers and yet not explaination for omitting it is given. There is also air resistance to the rotation its-self, something not considered by most studies.

Results

This study measured the frontal area and then calculated the coefficient of drag Cd for each riding position. It showed a very slight dependence of
Cd on velocity, which for most purposes could be ignored.

Very slight dependence of coefficient of drag (Cd) on velocity

The following values were recorded:

Cd at 20 kphCd at 70 kphArea
m2
Recreational rider in upright position1.161.130.54
Professional cyclist on tops1.041.020.411

Professional cyclist on drops
1.0210.405

Professional cyclist on aero bars
0.870.880.38

Abstract:

Aerodynamically efficient sports equipment/accessories and athlete body postures are considered to be the fundamental aspect to achieve superior performance. Like other speed sports, the aerodynamic optimisation is more crucial in cycling. A standard full-scale testing methodology for the aerodynamic optimisation of a cyclist along with all accessories (e.g., bicycle, helmet, cycling suit, shoes and goggle) is not well developed, documented, and standardised. This paper describes a design and development of a full-scale testing methodology for the measurement of aerodynamic properties as a function of cyclist body positions along with various accessories over a range of wind speeds. The experimental findings indicate that the methodology can be used for aerodynamic optimisation of all cycling sports.

Reference:

Bicycle aerodynamics: an experimental evaluation methodology
Harun Chowdhury Firoz Alam
Sports engineering. , 2012, Vol.15(2), p.73-80
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