Causes of aerodynamic drag

Aerodynamics is the study of how air flows over objects and the forces that the air and objects exert on each other. Drag is the force of wind or air resistance pushing in the opposite direction to the motion of the object, in this case, the cyclist and the bike.

The two types of aerodynamic drag that act against a cyclist are:

• pressure drag

• skin friction drag.

Pressure drag

The main type of drag acting against a cyclist is pressure drag. It is caused by the air particles being more compressed (pushed together) on the front-facing surfaces and more spaced out on the back surfaces.

This is caused when the layers of air separate away from the surface and begin to swirl – this is called turbulent flow.

This difference in air pressure means that the air particles are pushing on the front surfaces of the bike and rider more than the back surfaces, so there is a drag force.

If the tubes of the bicycle frame are shaped more like a wing, the airflow stays more attached to the surface so that the wake left at the back is much narrower. This makes the low-pressure zone much smaller so the pressure drag will be smaller.

Pressure drag

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Pressure drag is caused by the air particles being more compressed (pushed together) on the front-facing surfaces and more spaced out on the back surfaces. This is caused when the layers of air separate away from the surface and begin to swirl – this is called turbulent flow. Note how the wing shape reduces this.

Rights: The University of Waikato Te Whare Wānanga o Waikato

This low-pressure zone occurs behind the arms, legs, head and back of the cyclists as well. It is harder to reshape those parts to keep airflow attached to reduce pressure drag.

Some things can be done to reduce pressure drag:

• Using an aero helmet to reduce the low-pressure zone directly behind the head.

• Keeping the body as low as possible so air stays attached as it flows over the back.

• Hiding cables, bottles and brake components inside or behind the frame so they are already sitting in a zone of low pressure.

• For group events, cyclists take advantage of the low-pressure zone behind other cyclists by riding closely behind. This is called draughting and can reduce the effort needed by the following cyclist by 30%.

Skin friction drag

As the layers of air move over a rough surface, the air particles in the layer closest to the surface collide with the surface. This makes the air particles slow right down (and right at the surface, they completely stop!). These particles then collide with air in layers a bit further out and make them slow down as well. As you move further away from the surface, the speed of the air particles is not affected. The region of air where the speed of the particles has been changed is called the boundary layer.

For a cyclist, the thickness of the boundary layer grows from a few millimetres to a few centimetres. The best way to reduce skin friction drag is to keep surfaces as smooth as possible. Wearing tight skinsuits makes a large difference to the speed a cyclist can reach.

Frontal area

Frontal area is the area you see if you look at a cyclist from the front. Reducing this area means that there are fewer collisions with the wind.

Ways to reduce it include using the handlebar drops or aerobars. Getting down low into a crouched position with elbows in reduces drag because there is a more streamlined shape and there is less frontal area.

Calculating drag

This equation is used to calculate the drag of an object:

FD = ½CD AρV2

• FD is the drag force.

• CD is the drag coefficient (a number that shows how streamlined a shape is). Lower CD numbers show that there is less drag, for example:

  • circular cylinder – CD = 1.2

  • square cylinder – CD = 2.0 (sharp edges are not good)

  • oval cylinder – CD = 0.6 (rounded edges are good)

  • wing shape – CD = 0.1

• A is the frontal area of the object (measured in square metres).

• ρ is the density of air (about 1.2 kilograms per cubic metre).

• V is the speed the object is travelling at (measured in metres per second – m/s).

This equation shows that, if the area is doubled, the drag will also be doubled, so it is important for a cyclist to keep their body low and to keep their arms and shoulders in close.

Cyclists with huge power and minimal drag can bike between 50 and 60 km/h during time trials. The world record for a cyclist on a non-regulation bike following closely behind another vehicle is 268.8 km/h. This shows what could be achieved if aerodynamic drag could be virtually eliminated.