What is Aerodynamic Drag and CdA

A short refresher on drag, Cd, frontal area and CdA.

Drag

Aerodynamic drag is the force that is created as an object moves through the air.

Separated flow and vortex shedding behind a cylinder section — **Figure 1.** Flow past a cylinder section. The flow does not stay attached around the back of the body: it separates, and the wake behind it sheds alternating vortices.details

The equation for drag is:

Drag Force = ½ × Air Density × Cd × Area × Airspeed²

Worth remembering that

CdA = Cd × Area

To lower CdA we want to minimise Cd and Area.

Area

Area is the frontal area: the outline of the rider and bike as the air sees them, projected onto a plane square to the airflow.

**Figure 2.** Frontal view of a cyclist. The silhouette is the frontal area.

For a racing cyclist, frontal area is usually around 0.3 to 0.4 m².

Cd

Cd represents drag from the shape of an object. Lower Cd is less drag.

Cd of a cube, a half-sphere and an airfoil — **Figure 3.** Drag coefficients of three shapes, all referenced to frontal area: cube 1.05details, half-sphere 0.42details, airfoil 0.42 falling to 0.07 as speed risesdetails.

For the same frontal area, the airfoil has around a sixth of the drag of the half-sphere.details

A cyclist is a bluff body

Cd of a racing cyclist, a car and an airliner — **Figure 4.** Drag coefficients on the same frontal-area basis: racing cyclist 0.8–0.6details, car 0.4–0.25details, airliner 0.07details.

Even the most aero-efficient cyclists do not actually have a very aerodynamic shape.

A cyclist is a "bluff body", meaning their aerodynamic wake is not much smaller than their frontal area.

Because the flow separates rather than staying attached, where it separates — and therefore how large the wake is — depends on speed and on surface finish. That is the subject of the companion entry, Variable CdA.

A note on the numbers. Every drag coefficient in this article is referenced to frontal area, and each is quoted over the range of Reynolds number appropriate to its own shape. The vehicles are quoted at their own operating speeds.

Notes, commentary and references for this page.