Aerodynamics of Cycling - Part 1
Air Pressure Drag
Written by Benjamin Chee, last edited 4/1/2022
Air resistance is ignored in most introductory physics classes. However, it is too important to be ignored in reality, especially in the world of cycling.
What is air resistance? Say air flows in a straight line when not interacting with any objects. When a cyclist interrupts the airflow, though, it displaces a certain amount of air. It is this act of displacing air that causes air resistance.
The force of air resistance is made of the air density, frontal surface, area drag coefficient, and speed of the cyclist. It is hard to change the density of air because it is an aspect of nature, so to go faster, cyclists can only change their frontal surface area and drag coefficient (surface friction). In this three-article series, we will be discussing the two types of air resistance, air pressure drag and surface friction, and the ways for a cyclist to decrease this air resistance to go faster.
When air encounters the front of a cyclist, its path is disturbed; the air essentially hits a wall–the cyclist–so there is a force opposite the direction of motion exerted on the cyclist by the stopped air. This force against the front of the cyclist is the first of two main components of air resistance. The “backward” force is determined by the total frontal area, or their frontal cross-section, of an object. A parachute, for example, has no air resistance when it is crumpled in a ball and not in use. But when it is deployed, it has a very large surface area to catch as much air as possible, slowing down a falling object because of air particles pushing against the enlarged surface.
If a cyclist could decrease their frontal cross-section, there will be fewer air particles pushing against the cyclist, allowing cyclists to go faster for the same energy spent. In addition, this displaced air causes more air particles to be in front of the cyclist compared to behind it, creating a higher air pressure in front of the cyclist. Since air wants to go from high to low pressure, the air creates a slight backward pull on the cyclist, but nowhere near as significant as the force on the front of the cyclist.
This physics concept matches my experiences as a novice cyclist. I cycle to school on a regular basis, encountering this invisible force every time. On days where there is no wind, for example, I can go up to 30 mph downhill. If there is a stiff headwind, however, this speed can decrease to a crawling 15 mph. Assuming everything else about me and my environment is the same, including my frontal surface area, my weight, and tire-road conditions, this decrease in speed is solely due to air resistance. ​
With a strong headwind, there are more air particles in front of me for me to push aside in the same amount of time compared to if there is no wind. With more air particles having to be pushed aside in the same time interval, a larger force opposes my direction of travel, slowing me down because more of my energy is spent fighting air resistance than making myself go forward.
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On one windy day, however, I was riding downhill and decided to tailgate a school bus. Don’t worry, I knew what I was doing (I think). I was somehow able to go 35 mph even though on every other windy day I can only go 15 mph. How is this possible? The reason is because the bus does the work of displacing the air for me, and before the airstream can reform behind the bus, I slip through in the little “bubble” behind the bus where there is low-pressure air both in front and behind me. The air conditions are similar to if there was no wind at all!
Why could I not go faster than 35 mph? The speed limit for the road is 35 mph. So, with careful management of air resistance, a little help from gravity from going downhill, and a conveniently placed bus, a little kid like me on a bicycle can go as fast as the cars do. Physics is magical after all!
References:
https://interestingengineering.com/when-cycling-becomes-a-drag-the-aerodynamics-of-cycling
http://slocyclist.com/whats-the-average-speed-of-tour-de-france-riders-in-mph/
https://www.welovecycling.com/wide/2020/04/15/how-to-fight-aerodynamic-drag/
https://www.simscale.com/blog/2017/07/cycling-aerodynamics/
https://www.princeton.edu/~asmits/Bicycle_web/bicycle_aero_old.html#intro
https://www.youtube.com/watch?v=eNgeMjWRtrc&t=174s
Image Sources
​https://www.youtube.com/watch?v=5rN7UkmEKdU
https://blogs.nasa.gov/spacestation/2020/08/02/astronauts-wake-up-prep-crew-dragon-for-splashdown-today/
https://www.rohrerbus.c