Aerodynamics of Cycling - Part 3
Surface Friction
Written by Benjamin Chee, last edited 5/14/2022
The second aspect of air resistance is the friction of air particles on the cyclist and the bicycle. Surface friction is the second of the two main sources of air drag, so this is another key way to decrease air resistance.
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Undisturbed air flow travels in straight lines, also known as laminar flow. Since the pathways of air are undisturbed, there is minimal air resistance. However, when the air gets disturbed by a cyclist, the smooth, laminar flow turns into turbulent flow which causes air resistance.
The turbulent air particles rub against all surfaces of both the rider and the bicycle. Since no surface is perfectly smooth, the bumps in the surfaces, such as skin and clothing, cause small vortices to form, generating a substantial amount of air resistance. To decrease the number of vortices formed, most professional cyclists wear tight-fitting skinsuits to decrease this frictional force and allow the riders to go faster with the same amount of energy spent.
The gear of professional cyclists designed to decrease the frontal air resistance is a smaller bicycle and aerodynamic helmets. Look at the small frontal profile of the cyclist in the Tour de France. This is also why a racing bicycle has as few components as possible–it will not have any excess parts to increase the amount of surface that air particles can rub against. However, these tubes cannot be too small, few, and weak such that they break easily in any collision because then it would hamper, not help, the cyclist go faster and win a race.
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Another object aiding cyclists is their special helmets. Their aerodynamic helmets are shaped to both decrease the frontal surface area as well as easily redirect air over the head, decreasing the frontal air pressure while also not completely stopping the airflow. The gentle sloping shape of the helmets helps the cyclist cut through the air, decreasing the surface friction caused by air particles.
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The helmet here also helps to create a seamless transition from the head to the back. Drag occurs when air particles rub on uneven surfaces of the person. That friction between the surface molecules and the air particles creates surface friction. So if the rider is able to create a smoother surface, the air molecules are less turbulent, so fewer vortices and surface drag are created.
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All these are ways for cyclists to travel faster. However, going faster means larger drag force as there are more air particles interacting with the cyclist through the frontal surface area and surface friction per unit time. If velocity increases, air resistance increases.
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This is the formula for air resistance. The drag force, or air resistance, is equal to half of the air density times the frontal surface area times the drag coefficient times velocity squared.
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Assuming air density, the frontal surface area, and the drag coefficient remain constant, the drag force is proportional to velocity squared.
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Power is the amount of energy spent, work (W) over time.
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With some physics and algebraic manipulation we find that...
Power is equal to the force multiplied by the velocity. Recall that force–the drag force–is proportional to velocity squared. So, power is proportional to velocity cubed
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What this means is that if you double the speed of the cyclist, the drag force quadruples and the energy needed to maintain the same speed increases eight times. This demonstrates why decreasing air resistance, or the drag force, is so important–at faster speeds, up to 90% of the energy spent is fighting air resistance, so if cyclists can decrease air resistance and go faster for less energy spent, it helps them win the race.
In cycling, you need a lot of power from leg muscles and a lot of aerodynamic efficiencies to increase speed. Physics, even the invisible aspect of air resistance, is instrumental in this sport, so much so that almost everything is designed around this one physics principle. Turns out the saying “ignore air resistance” should be ignored.
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
https://www.youtube.com/watch?v=3Iz7ZMALaCY
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