Aerodynamics1 is the study of how air flows over objects and the forces that the air and objects exert on each other. Drag2 is the force3 of wind or air resistance4 pushing in the opposite direction to the motion of the object, in this case, the cyclist and the bike.
The two types of aerodynamic drag5 that act against a cyclist are:
- pressure6 drag
- skin friction7 drag.
Pressure drag
The main type of drag acting against a cyclist is pressure drag8. It is caused by the air particles9 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 flow10.
This difference in air pressure11 means that the air particles12 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
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.
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 pressure13.
- 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 collide14 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 area15 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 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 metres16).
- ρ is the density17 of air (about 1.2 kilograms18 per cubic metre19).
- 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 power20 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.
Related content
These articles provide additional valuable information related to aerodynamic drag:
- Forces and speed
- Rolling resistance
- Faster bikes
- Wind tunnel testing of cyclists
- Creating new cycle helmets
Understanding about aerodynamic drag also relates to:
Related activities
Investigating airflow over shapes activity requires students to investigate their own drag and rolling resistance21 on bikes.
In this activity, students learn how to make an aerofoil22 and to make and fly paper planes, helping them to learn about aerofoil wing shape.
- aerodynamics : The study of how air flows over and through objects and the forces generated by the flow.
- drag: Sometimes called air resistance or fluid resistance, drag refers to forces that oppose the relative motion of an object through a fluid (a liquid or gas).
- force: A push or a pull that causes an object to change its shape, direction and/or motion.
- air resistance: The forces that oppose the relative motion of an object through air. Also known as drag.
- aerodynamic drag: The force that acts against the motion of an object as it moves through the air. Also known as wind resistance or air resistance.
- pressure: The force per unit area that acts on the surface of an object.
- skin friction: The opposing force caused as particles of air (or another fluid) move over the surface of an object. Collisions cause the air particles to change speed.
- pressure drag: A type of drag caused by the difference in air pressure between the front and back surfaces of an object as it moves through the air (or other fluid).
- air particles: The structural components of air. It includes gas molecules such as oxygen and nitrogen as well as larger dust, pollen and ash particles.
- turbulent flow: The swirling motion of air (or other fluids) caused as an object moves through it. Turbulent flow is created when the airflow separates from the surface of an object.
- air pressure: The force exerted by the weight of a column of air over a given surface area.
- air particles: The structural components of air. It includes gas molecules such as oxygen and nitrogen as well as larger dust, pollen and ash particles.
- low pressure: In aerodynamics, low pressure is formed over the top of an aerofoil (wing) as it moves through the air because the air particles become 'stretched' out as they travel faster over the upper surface towards the wing.
- collide: When two or more objects, including particles, briefly come into contact with each other.
- frontal area: The area of an object as seen from front on. This frontal area is one of the variables that affect drag. Doubling frontal area will result in twice as much drag.
- metre: The base unit of length in the International System of Units (SI).
- density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- kilogram: The base unit of mass in the International System of Units (SI).
- metre: The base unit of length in the International System of Units (SI).
- power: 1. The rate at which work is done (defined as work divided by time taken). 2. Mechanical or physical energy, force or momentum.
- rolling resistance: The force that opposes motion as a tyre (or other rolling object) rolls over the ground. Rolling resistance is caused as energy is converted into heat energy.
- aerofoil: A specially shaped structure designed to produce lift for flight. It usually refers to a wing that is curved and narrows to a sharp trailing edge.
