Bicycles

How do a bike and its rider stay up?

In the Colorado desert in 1992, a speeder was clocked going 110 kilometers
(68 miles) per hour. No ticket was issued to the driver. A lapse in law
enforcement? Not at all.

The Dexter Hysol Cheetah, an experimental bicycle, had just broken the world record for human-powered speed. The Cheetah incorporated many new innovations in bicycling-a recumbent seating angle, aerodynamic fairing, and the latest lightweight materials. The result was a milestone in the annals of two-wheel transportation.

Two principles of physics explain how a bike works. First, angular momentum, the same force at work in a gyroscope, makes wheels want to keep turning in the same direction and position as they have been. So as your bike wheels spin underneath you, they're actually helping you stay upright as their angular momentum resists changes in the bike's upright position. Second, because of the way bicycles are constructed, inertia swings the upper part of a bicycle away from the center of a turn, even as the front wheel dips into the turn, keeping the bike in an upright position.

Bicycles have undergone few design changes since they were first invented. The earliest known bicycle design dates back to the 1490s, when a student of Leonardo da Vinci sketched a vehicle which looks remarkably like today's bicycle. The first functioning velocipedes of the 19th century also strongly resembled today's bikes, with two wheels of the same size. The old-fashioned bicycles most people think of with enormous front wheels and tiny back wheels were actually invented later. Called "ordinaries," these bikes were fun and fast, but quite unstable and dangerous.

Most innovations in biking happened as a result of the energy crisis of the '70s. Many of these innovations have already been incorporated into competition-class bicycles, such as aerodynamic carbon-fiber frames. Other improvements include wheels that attach on only one side, two-wheel drive for more traction, and tension wires that offer extra stability for less weight. Recumbent bicycles are also becoming popular, in part because the rider's lower center of gravity makes the bike more stable. Not only are you less likely to fall off-it's a shorter distance to the ground if you do!

Connections

1. What needs to happen for more people to use bicycles as daily transportation?
What would help you make more trips on a bike?

2. The front wheel of an "ordinary" could be up to 1.5 meters
(5 feet) tall, with the rider perched on a seat on top. How would this
affect the stability of the ride when you compare it to a regular bicycle?

**aerodynamic** designed to minimize wind resistance

**angular momentum** tendency of a spinning object
to keep spinning with the same orientation

**center of gravity** point around which the entire
mass of an object can equally balance

**fairing** bubble structure surrounding nonaerodynamic
surfaces to reduce air drag

**gears** bicycle's driving system using cogs with teeth connected by
a chain

**inertia** tendency of an object in motion to resist
changes in direction or speed

**recumbent bicycle** bicycle slung low to the ground,
with wheels far apart and a rider seated in a prone position between them

**two-wheel drive** drive assembly which diverts
the pedaling force to both the back and front wheels

- A new bike harnesses the best of all possible wheels. (1992, Feb 29) The New York Times, p. 52.
- Ballantine, R. & Grant, R. (1992) Richards' ultimate bicycle book. New York: Dorling Kindersley.
- Dream machines. (1992, Feb) Bicycling, pp. 56-58.
- Evolution of a dream: From Da Vinci to Daedalus. (1988, Dec) Bicycling, pp. 68-74.
- Fisher, L. (1992, Aug 5) Shifting a bike with centrifugal force. The New York Times, p. D4.
- Korten, Tristram. (1991, Jan 21) Pedal power. Design News, pp. 111-114.
- McGurn, J. (1987) On your bicycle: An illustrated history of cycling. New York: Facts On File Publications.
- Posth, M.A. (1993, Oct) The world's fastest bike. Popular Science, pp. 78-80.
- Sullivan, K. (1992, July 12) Laid-back bikers love this cycle. The Washington Post, p. B1.
- Thisdell, D. (1992, Sept 12) For sale: The world's fastest bicycle. New Scientist, p. 23.

**Additional resource**

NEWTON'S APPLE Show 501 (high-speed bicycles). GPN: (800) 228-4630.

Get in the Right Gear!

Find out how to bike more efficiently by calculating gear ratios.

The main driving action in a bicycle happens because of a gear system. Calculate the gear ratios of your bike, make predictions based on your calculations, and test them with a ride.

**Materials**

- one multigear bicycle

- pencil and paper

- measuring tape

- bike helmet

1. Stand the bicycle up. Count the number of teeth on the largest chain
ring (the circle with the teeth that's connected to your pedals). Then
count the teeth on the biggest and the smallest cogs on the back wheel.

2. Calculate the gear ratios for the biggest and smallest back cogs. The
ratios will be for your highest and lowest gears. The formula for calculating
gear ratios is:

gear ratio = number of teeth on the back cog

number of teeth on the front

The ratio for your highest gear should be smaller than for the lowest
gear. That's because the gear ratio measures how many rotations of the
pedals you need to turn the back wheel once. So it makes sense that the
higher the gear (with a greater number of teeth on the front gear and/or
a lesser number on the back gear), the less often you have to turn the
pedals to make the wheels go around.

3. Pedaling
produces torque (rotary force) in the front gear, which is transmitted
to the rear wheel. Compare the two gears by riding the bike first set at
the highest gear and then set at the lowest. Do you have to push harder
to start the bike in the higher gear? Which gear ratio is better for starting?
Knowing the lowest and highest gear ratios, can you roughly predict the
ratios for different gear settings in between? Don't forget your helmet!

4. With your gear ratios, figure out how many times you would need to pedal
in high versus low gear to get from one end of an area to another. First,
measure how long a distance you will cover. Then figure the circumference
of your wheels, using this formula:

circumference = diameter of wheel x pi

Now work backwards from your distance measurement to figure out how many rotations of your wheel it will take to travel from one end to the other:

= distance you want to cover

circumference of wheel

Use gear ratios to calculate how many times you would need to move your feet around to cover the distance. Remember, gear ratios are the relationship between pedals and wheels.

**Questions**

1. Which gear ratio makes you move your feet more?

2. Which gear is more efficient for biking on level ground-high or low?

You probably don't consciously know where your center of gravity is, but
you've used it to steer your bike. To find your center of gravity, lie
on the floor on your stomach with your arms at your side. Bend your elbows
into your belly and try to balance on your hands. Think of your body as
a seesaw, with your elbows at the center. It may take a while, but eventually
you'll find it!

A heavy load on a bike makes it harder to start and stop due to inertia.
See if you can borrow a friend's saddlebags for your bike. Try loads of
books in different weights. What is the highest gear you can start in without
any books at all? How quickly does this change as you add weight?

Compare a touring bicycle and a mountain bike. What design choices were
made to adapt mountain bikes to the rocky trails and steep inclines of
off-road biking? What are the features of a touring bike that make it suitable
for racing?

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