Aerodynamic basics

Estimated reading: 3 minutes

What is aerodynamic drag?

Aerodynamic drag is the force of the air acting to slow down a cyclist moving through it.

It is made up of two major components – pressure drag and skin friction drag.

  • Pressure drag is related to the size and shape of the bike and rider. As you cycle forwards you hit air particles out of the way. These are compressed on impact but then space out as they pass over and past you – the difference between these two air pressures is pressure drag. This is why getting into an aerodynamic position and allowing the oncoming air to flow smoothly around you, reduces your overall aerodynamic drag and makes it easier to cycle.
  • Skin friction is related to the roughness or texture of the surface on the bike and rider, a smooth (laminar) surface allows air particles to have consistent trajectories (pathlines). A rough (turbulent) surface causes irregular pathlines. Something to note however is surface roughness can be used to keep flow attached longer (for example a textured should on a skinsuit) which increases skin friction drag, but reduces pressure drag (of which is a larger component and therefore can reduce overall drag.

How do we measure aerodynamic drag?

There are five forces acting on a cyclist. Aerosensor works by measuring or estimating forces (or power) from the rider (FR), kinetic (FK), gravitational (FG) and friction (FF) to find the fifth, aerodynamic drag (FD).

The sum of these sources (positive) and sinks (negative) plus the aerodynamic drag (sink) must equal zero:

F_{R} \pm F_{K} \pm F_{G}-F_{F}-F_{D}=0

Aerodynamic drag is comprised of the product of dynamic pressure (pDyn, directly measured by Aerosensor), coefficient of drag (Cd, a constant for a given body position/setup) and frontal area (A). In cycling, the coefficient of drag and frontal area are commonly combined as the term CdA:

F_{D} = p_{\text{Dyn}} \times C_{d}A

Combining these two equations gives you a relationship between CdA and the other bike forces:

C_{d}A = \frac{{F_{R} \pm F_{K} \pm F_{G} – F_{F}}}{{p_{\text{Dyn}}}}

This is what your Aerosensor calculates in real-time!

Why is aerodynamic drag important?

Although aerodynamic drag increases with speed, friction does not – so at high speeds aerodynamic drag dominates.

Above you can see that at 5kph just 10% of your power goes into overcoming aerodynamic drag, and at 40kph this increases to over 80% – ie 80% of your power is used only to push you through the air.

Say you’re taking part in a flat 40km TT, how can reducing your CdA (and therefore aerodynamic drag) affect your race time?

In this example, reducing your CdA by around 10% can save over 1 minute and 30 seconds. The great news about aerodynamic testing is that you can make huge strides in making yourself faster and/or saving more energy for when it matters.

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