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Aerodynamics and Bicycling | |
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Introduction
The bicycle has been around for over 100 years. One of the first bicycles was the "high wheeler". This bike had a large front wheel and a very small rear wheel. The larger the front wheel the faster the bike. This design was limited to tall riders and was not popular very long. By 1880, the modern bicycle appeared and was known as the safety bicycle. In the 1930's a strange new bicycle design appeared out of France. The rider of this bicycle sits in a low easy-chair position (recumbent). This creates much less drag. This bicycle was so efficient (less drag) that an unknown rider was able to defeat the best riders of the day, as well as break most existing speed records. This lead to a furious debate about whether the "velocar" was a bicycle or not. In 1934, the UCI (Union Cyclist International) passed a series of rules against the use of recumbent bicycles. The velocar proved that aerodynamic drag (wind resistance) plays an important role in cycling. In fact, at 8 mph (3.5 meters/second) the aerodynamic drag of a bicycle and rider is greater than the rolling resistance (wheels on the ground). At 20 mph (11 m/s), the aerodynamic drag is more than 80% of the total drag. So, how does one improve upon the aerodynamics of a standard bicycle and rider? There are several areas for improvement. The most important area is the rider's position on the bike. Aerodynamic improvements can also be made for the frame, wheels, and individual components (parts) of the bicycle. Aerodynamics The rider causes 65% to 80% of the drag. Therefore, the rider's position is very important. The importance of rider position has been known for a long time. The crouched racing position and the drop handlebars have been used since the 1890's. Wind tunnel, and other testing, have shown that proper body position can reduce drag by 31% over an upright riding position. New handlebars have allowed riders to achieve an optimal aerodynamic position. This position is head down, back straight. This still allows for efficient pedaling. In this position drag is reduced by 25%. A helmet can also help to decrease the aerodynamic drag, by about 2%. In fact, modern aerodynamic helmets result in a lower drag even for a bald bicyclist! Therefore, the right helmet not only protects your head, but can help you win a race. Bicycle Frames Since the early 1980's, there have been advances in the aerodynamics of bicycle frames. The first step was to use oval tubing, instead of round. This helps to streamline the frame by reducing "stream separation" (the airstream comes together sooner behind the rider, thereby reducing drag). Another trick is to add fairings (aerodynamic structures) to the seat tube. Thanks to modern materials, some of the frame tubing has been eliminated (removed) to reduce drag. Usually, the crossbar is eliminated as well as the chainstays. Another bicycle has eliminated the seat tube. To decrease the aerodynamic drag still more, the Olympic riders use bicycles which are custom fitted for their body. Wheels In the late 1890's, the role of wheels in the production of aerodynamic drag was known. At that time, a company in England produced a solid disk and a four-spoked aerodynamic wheel. These wheels are capable of reducing the overall drag of a bicycle and rider by about 5%. However, these original aerodynamic wheels were heavier than typical spoked wheels. Therefore people continued to use spoked wheels. A typical bicycle wheel is made of a hub, a rim, and 32 or 36 spokes. As the wheel rotates, the air flow separates causing turbulence, which in turn increases drag. Aerodynamic rims help to decrease the drag. Solid disk wheels and three-spoke wheels eliminate (get rid of) the pressure drag with typical wheels. Today's lighter materials make these more usable. The drag of these aerodynamic wheels decreases or increases depending on the crosswind. The crosswind can cause drag or lift. Drafting Reducing aerodynamic drag is especially important in bicycle road races. Road races usually include many riders. The riders in the middle of the group experience less drag than the front riders. When riding in a pack, the riders in the front use 30% to 40% more energy than the riders in the middle of the pack. When a rider "breaks away" from the pack he must time it carefully to win. As soon as he leaves the pack he takes the full brunt (force) of the wind. This means the others may catch up soon. However, by taking turns a rider at the front of the pack may drop back to rest up. This method allows the pack to maintain a higher average speed than an individual rider. The benefits of riding in a pack can also be extented to tandem bicycles (bicycles with two or more pedaling positions). Tandem bicycles average 10% faster than an individual rider. In the late 1800's, individuals would use multiple-rider bikes to pace behind when attempting speed records. (The lead bike would "break" the wind allowing the single rider to take advantage of less drag and therefore greater speed. This is called "drafting"). Soon, riders were drafting behind motorized vehicles to increase their speed. Some riders take drafting to extreme speeds. In 1896 a rider exceeded 60 mph by riding behind a specially shielded railroad car. By the 1980's riders were going over 140 mph! The current world record is just under 167 mph! In order to achieve these speeds, the riders use special bicycles which must be moving over 60 mph before the rider can pedal under his own power! Summary Although many changes have been made to bicycles in the last 100 years, they still "look" a lot like the original safety bike. These changes have decreased aerodynamic drag, but when riding in a pack, the advantages gained by drafting are more important than any advantage gained through the use of aerodynamic equipment.
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