Aerodynamics and Cycling (Technical)

At one point almost banned by social and environmental safety national leaders, cycling, in its many forms, has become one of the most competitive international sporting events and is certainly among the top choices for transportation and recreation.  Most people today view cycling as a form of mere exercise or seasonal recreation.  However, for some, cycling is a way of life.  Paul Fournel expressed, “When you get on your first bike you enter a language you’ll spend the rest of your life learning, and you transform every move and every event into a mystery for the pedestrian” (Fournel 2003). What is it about cycling that stirs such feelings or even mania among its faithful devotees? Breath-taking landscapes, memorable sights and smells, the sound of self-induced wind challenging the rider to pedal harder, and physical and mental rejuvenation are some of the reasons.

Fig. 1 Old design of bicycle in the 1800’s.

Though the early roots of first invention designs and legal patents are blurred, there is a clear idea of the road traveled by the bicycle since its advent in the early 1800’s.  Originally known as “pedestrian’s accelerator”, “boneshaker” and “velocipede” (Norcliffe 2001), the bicycle, in its most basic form, consists of two wheels (hence the name of “bi-cycle”), a connecting metal frame, a seat, steering bars and pedals.  However, the first bike, which was invented in Germany by Baron Karl von Drais in 1817, had no pedals (Ballantine 2001).  The rider was expected to propel himself while sitting on the two-wheel frame.  This design, though more efficient than sole human transportation, was quickly and necessarily improved throughout Europe in the following decades (Norcliffe 2001).   The second half of the 19th century saw innumerable changes in the bicycle.  Pedals were co-axially attached to the front wheel.  Big (over 1 meter in diameter) front wheels were later designed to induce higher speeds (see figure 1).  Steel frames were manufactured to provide longevity and durability.  However, despite all these improvements, the bicycle still had not reached its full potential.  By the dawn of the 20th century, problems with turning, safety, cost and weight had driven engineers to design the bicycle as it is known today.

The earliest records of cycling race competitions date back to when the front wheel was almost as tall as the rider.  It was, perhaps, the desire for faster lap times that pushed bicycle engineering to new heights.  Today, there are diverse engineering and science fields involved in the design and technological development of bicycles.

Some of the aspects engineers continue to explore are air resistance, drafting, altitude, hills, rolling resistance, power transmission friction, inertia forces, and braking energy losses.  Air resistance seems to be the single most adverse resistance factor for road race cycling.  The event that revealed the importance of this fact took place in the 1989 Tour de France.  Greg Lemond was 50 seconds behind his competitor in the last stage of the race.  Unlike Greg Lemond however, his competitor did not have an aero helmet, triathlon bars and a back disc wheel.  Greg Lemond was able to beat out his opponent by 58 seconds by the end of the race (Tew and Sayers 1997, Chowdury et al. 2011).  Other similar unbelievable aerodynamic feats were repeated over the next couple of years, which caused aerodynamics to become of interest.

Aerodynamics is a subfield of fluid mechanics, a mechanical and chemical engineering field essential for the analysis of systems in which a fluid is the working medium (Fox et al. 2004).  Aerodynamics deals with the dynamics of gases, especially air interactions with moving objects(Houghton Mifflin Company 1969).  Early studies showed that in a typical training ride, wind resistance accounts for 72% (although numbers up to 90% have been reported (Brownlie et al. 2010, Gibertini et al. 2010) with faster speeds) of the force retarding the forward movement of the rider, the tires 15%, braking losses 8%, and bearing and chain losses 5% (Burke 1986).

Several attempts to improve cycling aerodynamics were made through trial and error.  Unsurprisingly, this approach was ineffective, and engineers had to step in.  There are basically two methods to analyze the aerodynamics of a bicycle: mathematically and experimentally.  In mathematics, certain fluid behavior can be predicted by solving mathematical partial differential equations.  These equations take into account momentum, energy and mass conservation laws, and often are brought together into a system of equations.  One of the most commonly used equations in fluid mechanics is called the Navier-Stokes equation (Tew et al. 1997) shown below:

This equation is a simplified one-dimensional version with several built-in assumptions, and it only represents one given particle of air.  Clearly, airflow around a cyclist involves countless particles of air—it is easy to see why a powerful computer would be necessary to solve the aerodynamics physics around a cyclist.  Engineers and mathematicians have developed different numerical methods to simplify complex systems of equations, so that a computer can solve them in real time.  One such method is Computational Fluid Dynamics (CFD), which is a field in which equations similar to the one described above are discretized by approximating a solution with a system of algebraic equations, which can then be solved on a computer Ferziger and Peric 2002).  Several computational cycling engineers, nationally and abroad, use commercially available CFD packages to solve their designs of interest.

Fig. 2 ANSYS CFD image of air movement (stream lines) seen around cyclist creating pressure difference (drag). (Picture provided by Bike Tech Review)

If using an experimental approach to analyze the aerodynamics of cycling, experiments are usually carried out in a wind tunnel (see figure 3).  A wind tunnel cross section may be small enough (.5m x .25m) to test minor drag (due to clothing, for instance), or big enough (4m x 3.5m) to fit a whole bicycle (Gilbertini et al. 2010, Alam et al. 2010).  Sensors, attached to the rider and bike, are able to pick up pressure changes inside the wind tunnel and thereby measure drag (Iniguez-de-La and Iniguez 2009).  Experimental wind tunnel testing is often preferred over purely computational testing due to the limiting assumptions found in the mathematical equation solvers.  However, performing computational experiments can save a significant amount of time and money.  Often, both mathematic and experimental approaches are used in conjunction and offer comparative results.

Fig. 3 Wind tunnel experiment (picture provided by A2 Wind Tunnel)

When scientists study the aerodynamics of cycling, there are two main types of drag being considered: pressure drag and skin-friction drag.  When air hits an object (rider or bicycle), it splits and travels around the object creating a boundary layer, which is a thin film of compressed air near the surface of the object.  However, the air fails to meet back at the opposite side of the object due to a lack of energy, and the boundary layer separates from the body altogether.  This separation causes a pressure difference between the front and rear of the body thereby causing pressure drag.  The second type of drag (skin-friction or shear) occurs tangentially to the object as the air particles move around it.  This phenomenon can be observed by watching loose clothing of a rider flap with the wind.

There are three major areas of focus for drag studies: drag produced on the rider’s position, on the bike, or on the cyclist’s attire.  Two thirds of the drag experienced is due to the rider position (Kyle and Burke 1984), therefore great efforts have been focused in that area.  There are basically three positions that a rider can adopt while riding: upright position, dropped position and time trial position Burke 1986).  While the upright position provides the greatest comfort, it also induces the greatest drag.  The dropped position (20° angle relative to the horizontal) induces less drag and is generally the most used position while on a race.  In the time trial position, the rider positions his/her back almost completely parallel to the ground, with hands on the low handle bars and both pedals vertically aligned.  This provides the best aerodynamic efficiency and is mainly used during downhill rides.

Great efforts have also been made on bike design.  Lighter frames, thinner tires and aerodynamic wheel spokes are the current fields of interest for bike design.  Different wheel spoke designs—even wheel flap covers—have been tested for drag reduction because reductions of up to 50% have been observed with changes in the spoke design (Houghton Mifflin Company 1969).  While wheel flap covers have shown better aerodynamic characteristics (Karabelas and Markatos 2012), they are often undesirable during side winds.

In addition to the rider’s position and the bike itself, the rider’s attire is also important because loose clothing can generate significant drag.  This is due to the skin-friction drag phenomenon explained above.  It has been reported that tight clothing could save up to 1.17% of the rider’s finishing time in 100 meters (Brownlie 1992).  More meaningful is the drag reduction that can be obtained by use of an aerodynamic helmet.  Differences in overall drag reduction of different commercially available helmets of up to 8% have been reported (Alam et al. 2010).  Another aspect engineers have taken into account more recently is thermal comfort, which, disadvantageously, is inversely proportional to aerodynamic efficiency.  In other words, the more vents a helmet has for cooling, the less aerodynamic it is.  The third variable in the helmet design equation is safety considerations.  Certain helmets have been designed that offer great thermal comfort and good aerodynamic properties, but do not meet safety standards.

Has bicycle technology reached its end?  Or more importantly, are any further drag reductions too minimal to be worth the research effort?  Many opinions differ, but for as long as competitive cycling events still occur, researchers will continue to engineer the latest bicycle and appendages.  While the recreational bicycle rider may not be extremely concerned about which helmet or shorts to buy, it is of utmost interest for the serious race cyclist.

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.


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Articles by Cristian Clavijo.

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