Figure 1: From Dr. Andrew Coggan’s “The individual pursuit: Demands and preparation.”
To understand aerodynamics, we first have to understand how air flows around a body moving through it (as pictured below).
As the body passes through the air, it pushes to the left and right (as with a bicycle), or up and down (as with an airplane wing). The air presses on the body, and the component of this pressure that faces aft is called pressure drag.
Air, like fluids, has viscosity. The air molecules that come into contact with the body stick to the surface; they are stationary with respect to the body. As the body continues through the air, other air molecules pass by the stuck molecules as they flow around the body in layers, following parallel paths. This is the laminar boundary layer. The viscous nature of air creates a shear force, or friction drag.
At some point on nearly all bodies, laminar flow cannot be maintained and the air molecules tumble and mix instead of flowing smoothly. The transition point is where this turbulent boundary layer begins.
Due to its mass, moving air has inertia and therefore cannot easily follow high-curvature surfaces. Under these conditions air separates from the body, creating low pressure regions that "pull" on the surface, contributing to more drag.
If the trailing edge of the body fails to guide the air neatly back together again, there is a “stalled flow” — a region of recirculating air that has low pressure. This low pressure again pulls on the surface, adding still more drag.
Understanding air flow around a body helps explain how drag is created, but how does the measured drag change in response to variables such as shape, frontal area, air density and speed?
In the formula above, “FD” is the drag force we want to reduce. The "half" is a constant required to make the formula work in real life. “V” is the velocity we want to maximize. “Cd” is the coefficient of drag, the inherent drag related to any particular shape. “A” is the frontal area, the front-view silhouette of the body. Bicycle engineers can't change most of these variables; in fact, the only parameters affecting drag that we control are shape and frontal area.
Any shape has its own Cd, and for the range of bike speeds it does not change. But different shapes have different effects on drag. For example, a circular shape has roughly 24 times the drag of the Cervélo-developed TrueAero tube shape pictured below. (TrueAero shapes, it should be noted, are products of an uncompromised focus on aerodynamics and speed.)
Remember, however, that most round-tube bikes don't present the tubes at right angles to the airflow — the seatpost and downtubes, for example, are presented at an angle, meaning the air hits a shape that’s closer to an ellipse, which has only about four times the drag of a TrueAero shape. Clearly shape matters a lot.
The other parameter we control is frontal area. This is intuitive: Drag goes up for bigger things. This is reflected in the drag equation: Double the “A,” double the drag.
So that's how drag is created, and how we can engineer our designs to generate less drag.
READ MORE ABOUT AERODYNAMICS IN OUR ENGINEERING FIELD NOTES:
AERO IN THE PELOTON: During WorldTour events there’s always a lot of talk about the aero models the pros are riding. Does it really make that much of a difference since they're usually drafting?
WEIGHT VS. AERO: Which is faster: A more aerodynamic or a lighter bike?
SLOW VS. FAST RIDERS: Everyone knows aerodynamic drag increases the faster you go, so an aero bike is important for very fast riders. But what about the rest of us?
ONE MORE METRE: In a 2,000-metre sprint finish, is it better to be on a Cervélo R5 or S5?
WHAT ABOUT WIND TUNNELS: How does testing in wind tunnels help in the design of aero bikes, and which tunnels do you use?
THE SCIENCE BEHIND LEAD-OUT TRAINS: Top sprinters rely on their lead-out trains or a specific lead-out technique. Learn if this is that just about strategy, or is there science behind it?