Aerodynamics and Bicycling page 1
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Introduction

bicyclist


The bicycle has been around for over 100 years. One of the first bicycles was the ordinary or high wheeler bicycle. This bike had a large front wheel and an extremely small rear wheel. The speed with which one could travel on this bicycle depended on the size of the front wheel. The larger the wheel, the faster the bike. This design was limited to tall riders and this bicycle was short lived. By 1880, the modern bicycle appeared and was known as the safety bicycle.

In the early 1930's a strange new bicycle design appeared out of France. The rider of this bicycle sits in a low easy-chair position. This translates to a much smaller frontal area and, therefore, less drag. This bicycle was so efficient that an unknown rider was able to defeat the best riders of the day as well as break most existing records. This lead to a furious debate about whether the velocar was a bicycle or not. In 1934, the UCI (Union Cycliste International) passed a series of rules against the use of recumbent bicycles.

The velocar proved that aerodynamic drag plays an important role in cycling. In fact, at 8 mph (3.5 m/s) the aerodynamic drag of a bicycle and rider is greater than the rolling resistance. When the speed increases to 20 mph (11m/s), the aerodynamic drag is more than 80% of the total drag. So, how does one improve upon the aerodynamics of a standard bicycle? By looking at a bicycle and rider, there are several areas for aerodynamic improvement. The most important area is with the rider position. Aerodynamic improvements can also be made for the frame, wheels, and individual components.

Aerodynamics

The rider accounts for 65% to 80% of the drag. Therefore, the rider's position is very important to the overall aerodynamics of a bicycle and rider. 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. Still, advances have been made through the use of testing techniques such as coast down tests and the wind tunnel. Coast down testing uses the velocity at the end of a ramp of known geometry to determine the drag. By utilizing coast down testing, researchers have shown that proper body position can reduce drag by 31% over an upright riding position.

The most aerodynamic position is obtained by using the hill-descent position where the hands are on the center of the bars. In this position the elbows are tucked in and the chin is on the hands. Also, the cranks are at 90 degrees and the knees are squeezed into the top tube. While being very aerodynamically efficient, the hill-descent position doesn't allow for efficient riding. For efficient riding, the drag is reduced by 25% by assuming a crouched position with a flat back.

New handlebars have allowed riders to achieve an optimal aerodynamic position while still allowing for efficient pedaling. These handlebars allow the rider to achieve the same effect as the hill descent position while still allowing the rider to pedal efficiently.

A helmet can also help to decrease the aerodynamic drag that a bicyclist encounters. An aerodynamic bicycle helmet reduces the drag by approximately 2% over a rider with no helmet. In fact, modern aerodynamic helmets result in a lower drag even for a bald bicyclists. Therefore, the right helmet not only protects your head, but can also give you a competitive edge in a bicycle race.

Bicycle Frames

Since the early 1980's, advances in the aerodynamics of bicycle frames have also taken place. The first step was to utilize oval tubing. This helps to streamline the frame to reduce separation downstream of the tubes. Another trick is to add fairings to the seat tube. This fills in the gap between the tube and the wheel which, in turn, decreases the pressure drag.

Thanks to modern materials, some bicycle manufacturers are eliminating some of the tubing to decrease the drag. Usually, the crossbar is eliminated as well as the chainstays. Another bicycle has eliminated the seat tube which they claim reduces the amount of separation behind the rider, thereby reducing the drag. It is important to note that when designing an aerodynamic bicycle, the combination of rider and bicycle must be examined together. Therefore, 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 importance of wheels to 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 significantly heavier than typical spoked wheels. Therefore, the more common spoked wheels were used.

A typical bicycle wheel is made of a hub, a rim, and 32 or 36 cylindrical spokes. As the wheel rotates, the flow separates behind the spoke which increased the level of turbulence behind the wheels. This results in a significant amount of drag. Aerodynamic rims help to decrease the drag by reducing the length of the spokes. Solid disk wheels and three-spoke aerodynamic wheels eliminate the pressure drag associated with typical wheels.

The drag of these aerodynamic wheels decreases further with a slight crosswind. This is due to the production of lift by the wheel. As the lift force is perpendicular to the wind direction, the lift can either increase or decrease the drag.

Components

Little can be done to improve on the aerodynamics of individual components such as the derailleur and brakes. By cleaning up the protruding parts and using smooth curves as opposed to sharp corners, modest reductions in drag can be obtained. Another important source of drag comes from the brake and derailleur cables. In order to eliminate the associated drag, cables are routed through the bike frame and handlebars whenever possible. When cables must be exposed, they are placed either immediately in front of or behind the bicycle frame. When taken as a whole, the overall drag reduction for a complete bicycle will be larger by utilizing aerodynamic components.

Drafting

In individual events, every effort is made to reduce the aerodynamic drag of the bicycle and rider. In road races, aerodynamics still play an important role, but in a different manner. When an individual breaks away from the pack, he must be careful to time the breakaway so that he can finish the race before the pack catches him.

When riding in a pack, the riders in the front consume 30% to 40% more energy than the riders in the middle of the pack. This is also the case for the individual rider. However, by taking turns a rider at the front of the pack may drop back to rest up. By taking turns at the front, the pack may maintain a higher speed than the individual rider.

The benefits of riding in a pack can also be extended to tandem bicycles. On average, tandem bicycles are 10% faster than an individual rider. In the late 1800's, individuals would use multiple-rider bikes to pace behind when attempting speed records. This soon evolved into pacing behind motorized vehicles. If you've ever seen the Tour of France or any other bicycle race, you've probably seen individual riders trying to pace behind motorcycles. This gives them an advantage in a breakaway as they greatly reduce their aerodynamic drag and, hence, 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 shy of 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

The bicycle has been around for a little more than a century. In that time, the general design of the bicycle has changed little from the safety bicycle. Still significant advances have decreased the aerodynamic drag a rider must overcome. While some of these ideas have been around as long as the bicycle, the proper materials to take advantage of the aerodynamic knowledge has only appeared recently.

While it is obvious that the aerodynamic drag facing riders can be decreased by using the proper equipment and riding techniques, how does this translate to an advantage in a race. By reducing the drag by even 1%, a rider can possibly gain more than a five foot advantage per mile at 30 mph. However, this is only important in individual events. When riding in a pack, the advantages gained by drafting outweigh any advantage gained through the use of aerodynamic equipment.

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