Considered to be one of the most exhilarating and efficient human-powered vehicles ever invented, the bicycle continues to find its way into the lives of many Americans and people throughout the world. Cycling sporting events have exponentially increased in popularity since the day German inventor, Baron Karl von Drais, put two wheels on a wooden frame in 1817 (Ballantine 2001). Since then, the bicycle has undergone several modifications. Once engineering technology caught up with the public’s need for speed, cycling was transformed from a field of design guesswork to a scientific research analytical field involving a diverse number of disciplines.
There are many factors that hinder a bicycle from reaching maximum velocities. Among these are transmission friction, air drag, rolling resistance and inertia forces. The most significant of these is air drag. 70% – 90% of the resistance experienced by a rider in a high-speed race is due to drag (Brownlie 2010). Therefore, aerodynamic studies are of paramount importance for better bike design, and several studies have been conducted within the past two decades.
In the most basic definition of air resistance, drag occurs due to a pressure difference between the front and rear of an object, which is in movement relative to its medium (air). When air hits the rider, it is brought to a stop, creating what engineers refer to as the stagnation point. At this point, the air splits and moves in opposite directions following “streamlines”, which contour around the rider. The air sticks to the body of the rider as it moves around him/her. However, due to a lack of energy, it separates from the body somewhere around the rider’s back. This separation creates a low-pressure field in the rear of the rider. This pressure difference between the front and the rear causes a force, which pushes the rider backwards, called drag.
There are two methods to study aerodynamics around a bicycle: mathematically and experimentally. In the first method, engineers and scientists solve complex systems of equations, which predict air behavior around the rider. Since some of these equations are too difficult to be solved by hand, computers programs are often used. Others study drag experimentally. They put a rider on a bike inside a big tunnel (generally referred to as a wind tunnel), which blows air at high speeds while the rider stays still. Sensors attached to the rider are able to measure pressure differences and compute drag. Both methods have advantages and disadvantages, and both are often used in unison.
Different components have been individually tested for drag such as helmets, loose clothing, rider’s position and wheels. Studies have shown that even seemingly small factors, such as loose clothing, can increase drag significantly during a race. Bicycle scientists continue to try to find the ultimate rider’s position, the best helmet and the most appropriate clothing to help cycling athletes further improve their performance.
By: Cristian Clavijo, University of Utah
Cristian Clavijo is a native of Peru, moved to the US in the 8th grade, and is now a Masters student in Mechanical Engineering at the University of Utah. As an advocate for the Hispanic underserved population in Utah, Clavijo is involved in educating children and parents on how basic scientific and medical knowledge can help them progress and become collaborators of their communities. He plans to pursue a PhD in Mechanical Engineering next year.
Ballantine, R. 2001. Richard’s 21st-century bicycle book. The Overlook Press, New York, New York, USA.
Brwonlie, L., P. Ostafichuk, E. Tews, H. Muller, E. Briggs and K. Franks. 2010. The wind-averaged aerodynamic drag of competitive time trial cycling helmets. Procedia Engineering 2:2419-2424.
Articles by Cristian Clavijo.