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What is the Science of Cycling?

Why do road bikes have thin tires, while mountain bikes have fat tires?

What is a gear ratio? And how do gears help make the bicycle so efficient?

What are the best materials for frames? What are the best designs?

How do you stop and steer a bicycle? What forces keep the bicycle from falling over?

How does drafting help a cyclist? What is skin friction?

Why is the bicycle the most efficient way to travel? How do your muscles work?

About the logo: THE IMAGE ON THE LEFT PROVIDED BY LIBOR KIRAS & CANNONDALE BICYCLES. THE IMAGE ON THE RIGHT OF RUTHIE MATTHES PROVIDED BY PEARL IZUMI/JOHN KELLY PHOTOGRAPHY

Welcome!

The Science of Cycling is the second in a series of three planned sports science resources. These sites focus on the science behind popular spectator and recreational sports.

While the Science of Cycling is a large Web site (there are over 20 pages in six main sections, 20 audio and video clips, interactive javascripts, and lots of images), it is really only an introduction to an enormous and fascinating topic.

You'll also find information on the history and unique culture of cycling. We felt it was imporant to include these elements because these contributions will help you better understand the topic.

Our understanding of cycling was greatly enhanced by the people we interviewed. U.S. Women's cross-country champion Ruthie Matthes helped us understand the competitive aspects of the sport, while frame builder Paolo Salvagione assisted in our understanding of bicycle frames and the materials they are made of. The Exploratorium's Paul Doherty helped explain the physics and forces behind the sport of cycling. To learn more about these individuals check out the "participants" section.

The Science of Cycling was a team effort. Many thanks to Noel Wanner who assisted with the writing and created a number of the activities and sidebars. Thanks to volunteer Brian Bernard who assisted with some technical writing and formulas. Ron Hipschman provided an activity and two excellent interactive javascripts. David Barker helped create the animations found on the site.

Additional thanks to Tim Tumbleson at PowerBar for his assistance with this project. Logan Kelsey of Vertical Productions provided great action footage. Libor Karas and Bill Teel at Cannondale provided some great images and video. Julie Washnock at Pearl Izumi sent us a great image of Ruthie Matthes on short notice. Kurt Liebert and "Bicycle"--the only band in the world that tours by pedaling--helped bring music to the site. For more information on these individuals and organizations, please visit the "credits" and "references" sections.

The Wheel Page: 1 of 3

The wheel is the most crucial element of the bicycle: it allows the rider to roll over the ground with great speed and efficiency. Historians believe the wheel originated in Mesopotamia sometime around 3,500 BC. While the Sumerians did not pedal their way through ancient Mesopotamia, animal-powered wheeled chariots and carts helped haul goods and people for thousands of years. During the industrial revolution in the 19th century, advances in materials and engineering made it possible to use the wheel effectively in human-powered machines. The modern bicycle, complete with a steel frame, a chain drive, steel wheels and spokes, and pneumatic tires, would emerge in the late 1800s.

The wheels on these bicycles were made of steel but lacked pneumatic tires (with the possible exception of the boy's bike). Some bikes made for small children are still made with solid tires. This photograph was taken around 1910.

LIBRARY OF CONGRESS

On the Road

While the use of the wheel was widespread in ancient times, it did have limitations. The resistance to the motion of a wheel can vary tremendously depending on the surface on which it is traveling. A rough road is much harder to roll over than a smooth one. The Romans were aware of this and developed a massive network of paved roads. While this may have been the first time in history that roads were improved to facilitate the wheel, it certainly wasn't the last. In the United States in the 1890s, cyclists successfully lobbied for improvements in roads nationwide, and with cycling the nation's most popular sport at the time, legislators listened.

The Ordinary

When most people think about early bicycles, the high-wheelers of the late 1800s come to mind. These early models had names such as the "Ordinary" or "Xtraordinary." In England, these bicycles were also known as "penny farthings" because the large and small wheels were reminiscent of the large one-penny coin and the smaller farthing coin.

The pedals were attached directly to the front wheel of the high-wheelers. The larger the front wheel on an "Ordinary," the farther the cyclist would travel with each turn of the pedals. Exploratorium Senior Scientist Paul Doherty explained, "Every time the pedals would go around once, that whole giant front wheel would go around once. So, for one cycle of the bicyclist's legs he might go 140 inches (3.556 meters), a tremendous distance forward." This made pedaling up hills quite difficult, but allowed for great speed on the flats.

BICYCLE INSTITUTE OF AMERICA This image shows an "Ordinary" bicycle. What is somewhat out of the ordinary is that the cyclist in the photograph is a woman. Although cycling became quite popular with women in the late 1800s there were still social taboos associated with it.

The Exploratorium's Paul Doherty talks about the early high-wheeled bicycles.

While the high-wheels were quite efficient, they were also dangerous: the cyclist was very high off the ground and perched precariously over the front wheel. So, while the high- wheelers broke new speed and distance records, they quickly gained notoriety for the dangers involved in riding them. The slightest obstacle in the road could result in a nasty head-first fall. "Headers" or "taking a header" were common terms used to describe an all-too-frequent problem. With a high center of gravity and narrow tires made of solid rubber (which occasionally could roll off their rims), high-wheeled bicycles were designed for speed, not for safety.

The Safety Bicycle

The safety bicycle that was developed in the 1880s closely resembles the bicycles of today. The rider is suspended on a metal frame between two wheels of equal size. A chain drive mechanism connects the pedals to the rear wheel. The stability and comfort of the design was superior to the high-wheelers, and so earned the "safety" its name.

Click on the image to view a QuickTime clip (3.1 megabytes).

LIBRARY OF CONGRESS

The image shows stills of a saftey bicycle and rider from an 1899 Thomas Edison film.

Spokes

Even the earliest bicycles used spokes of one sort or another. In fact, even in ancient times many chariots and animal-drawn carts used spokes. A spoked wheel can be made as strong as a solid one and have only a fraction of the weight.While early spoked wheels were almost always made out of wood, the bicycle wheels and spokes of today are made out steel or aluminum or occasionally more exotic materials such as carbon composite or ceramics.

Minimizing the weight of the wheels is extremely important in bicycle design. Why does weight matter? Each time you push the pedals, you have to accelerate the weight of the wheel both forward and around its center. In other words, the wheel undergoes angular and straight motion simultaneously. You can see this when you ride--the front tire of your bicycle rotates while it moves forward along with you and the bike.

The Bicycle Craze of the Late 1800s

In the America of 1867 most people got around by on horse or on foot. That was the year that the first factory-produced bicycles arrived on American shores from Europe. Though the first high-wheeled "Ordinaries" were quite expensive, costing about $100-150 (at a time when median annual incomes hovered around $450), they were destined to become the rage. The appeal of speed was strong, and bicycles offered mobility to young people and to women, an unprecedented revolution in social freedom.

The craze caught fire with the production in 1887 of the Victor Bicycle, a machine with two identically sized wheels and a chain drive much like a modern bicycle. By 1885, over 400 bicycle factories were working non-stop to keep up with skyrocketing demand. In 1895, Americans bought 2 million bikes, one for every 27 people in the country. Cycling "academies," clubs, and professional races sprang up across the land.

But in 1902, Henry Ford introduced his "Tin Lizzy" automobile, and the bicycle craze was quickly replaced with an obsession with the car. The 1960s saw the beginning of a resurgence for the bicycle, and in 1984 Americans bought 14 million bikes, compared with 10 million cars.

Tangential & Radial Spokes

There are many different ways to spoke a bicycle wheel. Most bicycles have tangential spokes, meaning that the spokes do not connect from the hub to the rim in a straight line, but at an angle. There are many different patterns of tangential spokes. Occasionally bicycles will have completely radial spokes. These spokes go straight from the hub to the rim of the tire. Wheels typically have tangential spokes. The way in which the wheels are spoked determines how they will perform.

A bicycle wheel with tangential spoking.

"You can spoke the front wheel completely radially, but the rear wheel had better not be spoked radially. There is no way to convey the twist of the wheel out to the rim to drive you forward," Exploratorium Senior Scientist Paul Doherty explained. Tangential spoking helps transmit the torque from the hub out to the tires.

Not only would a radially spoked rear wheel be less efficient than one spoked tangentially--it would be significantly weaker. A bicycle wheel needs to be able to handle a variety of forces. Besides holding up the weight of the cyclist, a wheel must withstand the forces of pedaling and braking and the jarring effects of the road surface. The benefit of radial spoking has to do with the stiffness of the wheel (less deformation makes the wheel slightly more efficient).

The Exploratorium's Paul Doherty talks about different spoking patterns.

Tension not Compression

It's easy to think of the spokes as columns supporting the wheel and helping it retain its shape. But, the "support" that the wheel receives is created by pulling the spokes towards the center of the wheel (tension) rather than pushing out from the center (compression). If you've had the occasion to hold a spoke that was removed from a wheel, you've probably noticed how flimsy it is. You could bend one in half without too much effort. However, if you tried to pull one apart you would not be able to. The "pulling" of the spokes toward the center of the hub is what gives the bicycle wheel its strength.

Bicycle maker Paolo Salvagione discusses how bicycle wheels get their strength.

So just how strong are bicycle wheels? "Wheels, from what I remember, can hold about 400 times their own weight on a regular basis and they won't collapse until roughly 700 times their own weight, which makes them one of the strongest man-made structures on the planet," explained bicycle maker Paolo Salvagione.

Pneumatic Tires

The pneumatic (or air-filled) rubber tire is something we take for granted today. Almost every type of bicycle wheel has a pneumatic tire on its rim. The development of the pneumatic tire was an important landmark in the development of the modern bicycle. Prior to its invention in 1888 by John Boyd Dunlop, bicycling was a bumpy and somewhat uncomfortable experience. Tires were made out of leather (and later solid rubber) attached to a wood or metal rim. The air-filled tire brought with it a smooth, comfortable, and stable ride. It's no surprise that it also helped make bicycling more popular.

Paul Doherty talks about the difference between "road" and "mountain" tires.

Are your tires fat or thin?

Depending on the type, your bicycle has either fat tires or thin tires. Most road bikes and touring bikes have thinner tires, while mountain bikes have big fat tires. Each type of tire has been adapted for the surfaces they ride on.The road tires are inflated to 100 or even 120 PSI (pounds per square inch). A firm thin tire on the asphalt surface won't flatten much. The less the tire flattens out on the bottom, the less surface area is in contact with the road. Less contact in this case means less friction, and more speed. This is why keeping tires properly inflated is so important.

Wide and fat mountain bike tires flatten out more on a hard asphalt surface. However, on a dirt trail, a mountain bike tire "floats" on top of the rough surface. A thinner road tire would cut deep into the dirt, forcing the cyclist to pedal her way out of a hole.

It is easy to imagine a pneumatic tire "flattening out" on the bottom as it rotates. But surprisingly, steel train wheels on a steel rail experience the same effect. The temporary flattening-out of the wheel, as well as sinkage on the contacting surface, is what leads to "rolling resistance." This term is used to describe how much energy is "lost to the road" as a wheel moves forward. Tires with low pressure traveling on soft ground tend to have higher rolling resistance. This one of the major reasons why road racing is a faster sport then mountain biking.

Although you can't tell from this image of a locomotive, a steel wheel will "flatten out" on a steel rail. All wheels must overcome this "rolling resistance."

This mountain bike tire is made of natural rubber, which some believe grips the trail better than synthetics.

Tire treads

The treads of mountain bike tires can affect performance. Rough or "knobby" treads grip dirt trails better, but create greater friction on smooth roads. Smooth tires grip smooth roads better, with less resistance, but slip on dirt trails. Mountain bike tires manufacturers produce a variety of different patterned nobby treads. While cyclists have different preferences, there has been little scientific support for one tread performing better than another.

Drives & Gears Page: 1 of 3

The development of the chain drive helped make the bicycle that we know today possible. The chain drive eliminated the need to have the cyclist directly above the wheel. Instead the cyclist could be positioned between the two wheels for better balance. With the advent of gears, the cyclist could also pedal more efficiently. Riders enjoyed increased speed and easier riding up steep grades.

A modern chain drive and derailleur system.

Early Drives

Leonardo Da Vinci is credited with developing the idea of the chain and cog in the 15th century. However, it took nearly 400 years for the idea to become a practical aspect of bicycle design. For a chain drive to be effective it needs to transmit power efficiently from the rider's legs to the back wheel. It also must be designed so that pedaling resistance is within a comfortable range for the cyclist. The development of stronger materials and other technological and engineering advances made this possible. By the 1880s, the chain drive was commonplace.

The Benefits of Gears

A chain drive alone (without gears) is effective on flat surfaces and going downhill. However, when it comes to headwinds, hill climbing, and even starting on a bicycle without gears--the cyclist has to stand on his pedals and strain while pedaling at a very low rate. Gears allow the cyclist to pedal at a comfortable and efficient rate while traveling either uphill or downhill or with a headwind or a tailwind.

On the old high-wheelers, the pedals were attached directly to the wheel. One turn of the pedals equaled one turn of the wheel. Gears allows the cyclist to change that ratio. For steep hills, we choose a gear that lets us turn the pedals many times to turn the wheel just once; on flats or downhills, we might choose a gear that turns the wheel many times for each turn of the pedals.

PAUL de VIVIE, alias "Velocio"

One of the greatest developers and proponents of the derailleur was the Frenchman Paul de Vivie (1853-1930). A passionate advocate of cycling, he rode his first high-wheeler at the age of 28, and soon had sold his silk business and started a bike shop. He also founded the magazine Le Cycliste in 1887, where he wrote under the nom de plume "Velocio," touting the joys and benefits of cycling. A tireless inventor, he was convinced that geared bikes, then an oddity, were the future of cycling. Though existing gear-changers were awkward and unreliable, Velocio was undeterred. He spent much time inventing various gearing schemes. In 1905 he tested a two-speed derailleur called the Cyclist. Through his efforts in both engineering and publicity, the derailleur was ultimately perfected, and a relatively dependable version was produced by Tullio Campagnolo in 1933.

Count your teeth

Paul Doherty, Senior Scientist at the Exploratorium, explains the gear ratio on your bicycle. With this method you simply count the number of teeth in front chain ring and then the number of teeth in one of the rear chain cogs. In the example Paul created, the front chain ring had 54 teeth and the rear cog had 27. "That means every time I pedal around once on the front chain ring, the chain goes around 54 teeth. That means a 27-tooth (rear) cog goes around twice (rotating the rear wheel twice)." This would provide a ratio of 2-to-1. If the rear cog had 11 teeth, the ratio would be closer to 5-to-1 and so on.

Rear Cog Front Cog

This animation demonstrates the rotation of the front and rear cogs of a bicycle. Notice that for each turn of the larger front cog the rear cog turns twice. The animation also produces an interesting optical illusion where the "spokes" on the front cog appear to be moving backward.

The Exploratorium's Paul Doherty talks about gear ratios.

Penny Farthing & French Twist

Paul explained that the method for counting teeth was only one way of demonstrating gear ratios. In England, the gear ratios are converted into the diameter of the large front wheel of a high-wheeled bicycle, called the "Penny Farthing." Paul's earlier example used a 2-to-1 ratio. To convert this, the diameter of the rear wheel would be multiplied by two. A gear with a 2-to-1 ratio and a 27-inch-diameter rear wheel would be considered a 54" gear.

In England, the Penny Farthing bicycle is still used today to measure the gear ratios of safety bicycles.

In France they use the metric system. They take the circumference of the wheel in meters and multiply it by the gear ratio. Again using Paul's example, given a 2-to-1 ratio and a tire with a circumference of 1.5 meters, the result would be 3 meters. (Unlike the English system, this method tells you how far you have traveled, in this case 3 meters.)

Chain Drive Activity

The chain drive is what connects the pedals to the rear wheel. It allows the power you apply to the pedals to be transferred to the rear wheel, moving the bicycle forward.

What You Need:

You can explore the concept of the chain drive quite simply. You'll need the following items:

Thread spools: a pair of the same size, and one of a larger or smaller size

A flat wooden board

Nails

Rubber bands

To Do:

Mount the two spools onto the board with the nails, far enough apart that the rubber band will have to stretch to connect them, and loosely enough so that the spools can turn easily. Then connect the two spools with the rubber band.

A variety of spool sizes makes this activity more interesting.

Notice the red triangles which are drawn on the tops of the spools. The marks make it easier to follow the movement of each of the "gears."

Explore how turning one spool now causes the other spool to turn. Do they turn at the same rate? In the same direction?

Other possible explorations:

1) Mark a point on both spools and rotate them. Try using one small and one large spool. How does the rotation of one relate to the other?

2) Put one twist into the rubber band, so that it forms a figure-8 between the spools. Does this affect the rate of rotation? Does it affect the direction of rotation?

3) Look at a bicycle's chain and gears. How is the spool contraption similar to a bicycle? How is it different?

Gear Exploration

The gears of a bicycle make pedaling more efficient, allowing the cyclist to travel faster and more easily handle steep grades and other obstacles.

What You Need:

To try this activity you'll need the following materials:

A bike with gears

A piece of chalk or masking tape

Paper and a pencil

A bicycle, masking tape, paper, pencil, and a little curiosity are all you need to find out how the gears of a bicycle work.

To Do:

Shift the gears so that the chain is on the smallest cog in the front, and on the largest cog in the back.

Mark the top of the rear tire with the chalk, or with a piece of tape.

Note the position of the pedals. Have someone hold the bike upright as you turn the pedals one full revolution, so that the pedals return to their original position.

How many times did the rear tire revolve? Write down the number of revolutions. (Note: you may have to have a friend control the rear tire with his hand, so that the tire does not spin freely past the point that the pedals pushed it to.)

Now try the largest gear in front combined with the smallest gear in the rear. How many times does the rear wheel revolve for one turn of the pedals?

Which of these combinations would be better for climbing a hill? Which would be better for a sprint on a flat road? (You can test your guesses later by riding the bike!)

Experiment with the intermediate gear ranges. Make a chart of the number of rear wheel revolutions each combination of gears produces for one pedal revolution. Why do you think bikes have evolved to have more and more gears?

The Campagnolo Story

One of the first and most renowned bicycle parts makers was the Campagnolo family. Tullio Campagnolo (1902-1983) founded the Campagnolo Company, which has made quality parts for over 50 years. Tullio is credited with perfecting the modern parallelogram derailleur, and inventing the quick-release mechanism for wheels. Legend has it that Campagnolo, a pro racer, was leading a race through Italy's snowy Dolomite Mountains when he punctured a tire on a descent. Fumbling with frozen fingers trying to loosen the heavy wing nuts on his wheel, he was passed by a score of riders. Infuriated by this experience, he designed a hollow-axle quick-release mechanism which is used on almost every bike today.

The quality of Campagnolo component sets, or "grouppos," has become legendary, along with their cost. Though some questioned whether the sets were worth the price, afficionados would use nothing else. Cost-conscious alternatives were often referred to as "Cramp-and-go-slow" as opposed to the dependable, smooth-shifting Campagnolo. And though the component market has been dominated in the past decade by the giant Japanese company Shimano, "Campy" still finds its niche at the high end of the performance scale.

Cadence Gears make it possible for riders to maintain the cadence (or rate of pedaling) that makes them the most efficient. While there are many opinions as to what exactly is the optimal cadence for bicycling, everyone seems to agree that cadence is important. According to Paul Doherty, "The human body delivers the most energy for pedaling in the most efficient way at certain cadences. I tend to keep my cadence between 60 and 90 (cycles) per minute." Paul went on to mention that most recreational bicyclists pedal too slowly, expending energy inefficiently in higher gears.

Professional cyclists have very high cadences. Road racers' cadences can vary between 75 and 120 revolutions per minute. In mountain biking it is a somewhat different story. U.S. Women's cross-country champion, Ruthie Matthes explained, "In mountain biking sometimes you'll be going up such a steep climb, maybe 50 RPMs (is used) and then going down a very fast descent sometimes you won't pedal at all, you're just balanced on your bike, no brakes, and going as fast as you can. In mountain biking there is a wider range and variety of cadences used."

Ruthie Matthes talks about cadence.

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