Aero basics

Aero basics

At higher cycling speeds, a significant portion of a rider's power is expended in overcoming aerodynamic drag (Martin et al. 1998).
As speed increases, wind resistance grows exponentially, meaning a small increase in speed requires disproportionately more power to maintain.  Research shows that high speeds, up to 80-90% of a cyclist's power output is dedicated to overcoming air resistance (Bassett et al., 1999; Faria, 2005). This is due to the relationship between velocity and drag, where the power required to counter drag increases with the cube of the speed (Martin et al., 1998). As a result, reducing drag through body position, equipment, or clothing can have a substantial impact on cycling performance, especially in time trials or road races where maintaining high speeds is crucial (Faria et al., 2005). 
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The aerodynamic drag force is defined as the resistance experienced by an object moving through a fluid, such as air. It is mainly caused by pressure differences between the object's front and rear surfaces and the friction of the air against its surface. The formula for aerodynamic drag force is: 
 
... which tells us that aerodynamic drag force is relative to 4 factors: 
  1. Air density (rho) 
  2. The velocity (actually, the square of the velocity) of the air relative to the rider 
  3. The frontal area (A) 
  4. The Coefficient of drag (Cd). This quantifies the drag or resistance of an object in a fluid environment, such as air or water. It represents how streamlined an object is, or how effectively it cuts through the fluid. The value of Cd is dependent on the shape, surface roughness, and flow conditions around the object. Lower values indicate more aerodynamic objects, while higher values reflect more resistance. 
In cycling, we are interested in minimising the aerodynamic drag force, so that more of the power produced by the athlete goes to increasing velocity. Which of those factors can we control? 
  1. We can’t control the air density 
  2. It would be counterproductive to reduce velocity just to reduce aerodynamic drag force, since velocity is what we’re trying to maximise overall 
  3. We can control the Coefficient of drag, by changing the shape (position) and surface roughness of the rider and bicycle 
  4. We can control the  Frontal Area, by changing the position of the rider. 
Therefore, much of the effort in aerodynamic optimisation goes into the Cd and A factors. 

References:
Fox, R.W., & McDonald, A.T. (1973). Introduction to fluid mechanics (2nd ed.). New York: Wiley. 
Martin JC, Milliken DL, Cobb JE, McFadden KL, & Coggan AR (1998). Validation of a mathematical model for road cycling power. Journal of Applied Biomechanics, 14(3), 276-291. 
Bassett, D. R. Jr., Kyle, C. R., Passfield, L., Broker, J. P., & Burke, E. R. (1999). Comparing cycling world hour records, 1967–1996: Modeling the human-machine interface. Medicine & Science in Sports & Exercise, 31(11), 1665-1676 
Faria, E. W., Parker, D. L., & Faria, I. E. (2005). The science of cycling: Factors affecting performance – Part 2. Sports Medicine, 35(4), 313-337.  

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