Aerodynamic Drag

What is Aerodynamic Drag

Whenever anything is moving through a fluid, drag is a major factor. Whether swimming through water or cycling through air, the effect is  similar.

If you are moving forward, the fluid flows around and by you creating a gap in your wake.  As the fluid flows in behind you, it creates a resistive drag. If you are riding with the wind, the gap is in front of you and results in a net drag in the forward direction.

How important is aerodynamic drag?

Drag is extremely important in cycling. It is what limits cycling speed on flats and descents. Elite cyclists have gone to extremes to reduce its effects. In the Eighties when cycle shoes used laces, the Tour de France riders took to taping over the laces to minimize drag.

You might think on a descent that you will simply continue to gain speed until you reach the bottom. You don’t and you actually reach a speed where your descent speed fails to increase. Hard to believe, but you can reach terminal velocity on a descent.

Why is Aerodynamic Drag a Limiting Factor?

The answer is simple and unique among cycling resistive forces. It is velocity dependent. The faster you go, the greater the drag pulling you back. At low speeds, drag is smaller than rolling resistance, but around 12 mph, it begins to become the larger of the two. It also is where wind speeds are factored in. Headwinds enhance drag, tailwinds reduce drag.

Here is the equation for computing the force. You can see that the squared portion is dependent on both your cycle speed and the speed of the wind you are riding into.

FAeroDrag = 0.5 * Coefficient of aero drag * frontal area *air density * air speed squared

FAeroDrag = 0.5 CD ρ A (Vcyclist + Vheadwinds) 2

What is the drag impact of cyclist shaping?

Cyclists are familiar with what happens when they change their riding posture thereby adjusting their exposed frontal area. The following are the four nominal positions: Drops, Hoods, Aero, and Tops.

Nominal shapes for the A factor. Upper row is drops and Hoods. Lower are Aero and Tops.

The following illustrates the impact these positions have on drag using the coefficient information on the next topic:

Impact of different A Shaping on Drag. Reagan Zogby.

What is the power expenditure against drag?

Multiplying both sides of the force equation by velocity provides the power equation for the portion of a cyclists pedaling needed to overcome aerodynamic drag. Notice net velocity includes wind.

PowerAgainstDrag =  AerodynamicDrag * NetVelocity

Next Topic:   Aerodynamic Coefficients




Cycling as seen through the eyes of elite cyclists.