Bent on Speed: get the lowdown on how engineers use physics to build the fastest bikes on earth.

Sealed inside a shell-covered racing bike that's shaped like a shark and sits just barely above the pavement, Matt Weaver hopes that this race will earn him the title of the fastest person on Earth. He sweats, pedaling his machine so quickly that he would pass most cars on a highway. The bike shakes from the speed. Unlike other racing bikes, Weaver's shell is windowless. To see the road, he watches a monitor that's connected to a camera on the bike. "It's like a video game," he says.

But this is no game. When a bike going faster than a sprinting cheetah crashes, it becomes airborne--sometimes 18 meters (60 feet) or more off the ground. Then it bounces off the pavement. "It looks like a stone skipping over water," says Weaver, an engineer who holds the U.S. speed-cycling record at 126 km (78 mi) per hour--a fraction away from the world record of 130 km (81 mi) per hour.

So far, all of the top cycling speed records have been set by recumbent bikes, or bents, in which the rider's legs are stretched straight ahead, instead of below them. Racing recumbents, also called low racers, are known for their funky shell, which protects the rider's body in case of an accident, and boosts the bike's speed. Weaver plans to someday shatter the world record. But first he needs to face two big obstacles: the limits of his body and the invisible forces of air.

LAID BACK

A bicycle is a machine that runs on human power, or the work done in a given time. That's why the fastest cyclists are also world-class athletes: They use their strong muscles to crank out maximum power. "At 80 miles per hour, you're covering the length of a football field in about 2.5 seconds," says Weaver. But it takes more than just a buff body to break speed records. It also takes a super-fast bicycle.

Recumbent cycling records are more than twice as fast as the Olympic record set on an upright racing bike. "On an upright bike, 53 km (33 mi) per hour is as fast as somebody like Lance Armstrong can go," says Steve Delaire, a low-racer cyclist and engineer. One reason: On a flat road, riders on a bent bike can push against the pedals with more force (push or pull exerted on an object) than they could on an upright bicycle. That's because riders in a laid-back seat position use different leg muscles than riders seated upright. "It's like doing a chin-up with your palms turned down versus palms turned up," says Danny Too, a scientist who studies biomechanics, or how living organisms move, at the State University of New York College at Brockport. "One way is harder than the other."

To design a bent bike specially built for speed, Too experiments with the seat angle and height, and leg distance to the pedals. His goal is to max out the power that racers can transfer from their muscles to the bike. "In a certain recumbent position, you can actually pedal for a longer time and with more power [than on an upright bike], because it's a more efficient hip and knee angle," says Too. In other words, recumbent racers require less energy than ordinary bikes. But the most important reason that a low-riding, shell-covered recumbent breaks records: its crazy shape.

INTO THIN AIR

Every year at Battle Mountain, Nevada, the world's top cyclists compete for the title of fastest human. Engineers chose the site for its long, flat stretch of desert road, and also for its towering altitude--1,372 m (4,500 ft) above sea level. The air is thinner up there, which means there's less air resistance (force that acts against motion, also known as drag) to slow down bike racers.

That's important because air resistance is a cyclist's enemy--especially when a racer's tires are whirling fast. Every time a cyclist doubles in speed, (for instance, from 32 to 64 km, or 20 to 40 mi, per hour) the air resistance increases four times. To overcome that slowing force, a racer must increase their power by eight times. That means, "You have to get eight times as strong or figure out something that can go through the air eight times easier," says Weaver. That's why, to max out his speed, Weaver's high-speed bike is outfitted with a streamlined shell called a fairing.

In speed cycling, the bike's shape is everything, separating the winners from the losers. Weaver believes that perfecting his fairing will eventually help him pedal as fast as 161 km (100 mi) per hour. A good fairing has a rounded nose and tapered body. This design can push up to a half a ton of air around the bike every minute. Says Weaver: "I'd like to punch through the air, have it split apart around me, and then come back together again like I wasn't there."

NUTS & BOLTS WHAT MAKES THIS SLEEK BIKE SOAR?

HEIGHT: 41 cm (16 in.) reduces drag, or air resistance.

AERODYNAMIC, SPACE-GRADE SHELL: Protects cyclist and improves airflow over the bike.

FEW OR NO WINDOWS: They would leak air and create drag.

SOLID WHEELS: Sealed into separate compartments so no air leaks into the bike's shell.

ROUNDED NOSE: Pushes away air.

AIR DUCTS: About 400 gallons of air streams through the bike every minute to remove the cyclist's evaporated sweat.

DID YOU KNOW?

* "When you put a fairing on [an upright bike] you can [only] get up to 97 kilometers (60 miles) per hour," says David Gordon Wilson, a professor of engineering at MIT. "On a recumbent bike you're in a better position to make a [torpedo] shape, like you see on high-speed birds and dolphins." That's why a recumbent bicycle, with a fairing, can get up to such high speeds.

* In 2003, Damjan Zabovnik, of Slovenia, broke the European speed record by tiding a racing recumbent at 110 km (68.21 mi) per hour--facing backward.

CRITICAL THINKING:

* Compare an upright bike to a recumbent one. What are the pros and cons of each type of vehicle?

CROSS-CURRICULAR CDNNECTIONS:

HISTORY: Research and create a historical time line on the development of human-powered vehicles.

RESOURCES

* Learn more about cycling at the International Human Powered Vehicle Association Web site at: www.ihpva.org/

* For an in-depth technical manual on bike physics, engineers refer to Bicycling Science, Third Edition, by David Gordon Wilson, MIT Press, 2004.

It's Your Choice:

Which of the following is NOT true about low racers?

(A) They run on human power.

(B) They are streamlined,

(C) They are less efficient than upright bicycles.

(D) They are low to the ground.

2 One force that slows a racing recumbent is called

(A) Drag

(B) Power

(C) Watts

(D) Fairing

3 Every time a racer doubles in speed, the air resistance increases--times.

(A) Two

(B) Four

(C) Six

(D) Eight

1. c. 2. a 3. b.

DIRECTIONS: Select the term that best completes each of the following sentences. Circle the correct answer.

1. (Biomechanics, Biophysics, Physiology) is the study of how living organisms move.

2. (Force, Efficiency, Resistance) is the push or pull exerted on an object.

3. Cycling speed records have all been set by (upright, recumbent, three-wheeled) bikes.

4. A recumbent biker compared with an upright cyclist can pedal for a longer time with (more, less, the same amount of) power. That's because of the (less efficient, more efficient, directly proportional) hip and knee angle.

5. The fairing improves a cyclist's speed by lowering (air resistance, gravitational pull, wind speed) and reducing the amount of (power, drag, propulsion).

1. Biomechanics

2. Force

3. recumbent

4. more, more efficient

5. air resistance, drag

HANDS-ON ACTIVITY

FLOATING-FISH COMPASS

In "Which Way Is North?" (p. 12) you learned about Earth's weakening magnetic field and the possibility of a pole reversal in the distant future. But where are the magnetic poles now? To find out, follow the directions below to create your own compass. This simple "floating-fish" compass is very similar in design to the ones used in China more than 1,000 years ago.

You Need:

1 aluminum pie tin * water * scissors * 1 tall Styrofoam cup * marker * 1 bar magnet * 1 large sewing needle * school glue

To Do:

1. Fill a pie tin with water and place the tin on a flat surface.

2. Use scissors to cut out the fish shape on this page (see above).

3. Place the fish shape on the outside of a Styrofoam cup. Position the fish so that its head points toward the cup's rim, and its tail points toward the cup's base.

4. Use a marker to trace the outline of the fish onto the cup. Then, cut out the Styrofoam fish, and set it aside.

5. Hold the bar magnet in one hand with the north end of the magnet pointing up.

6. Carefully hold the needle in the other hand with the needle's eye pointing down. Gently stroke the needle vertically--from eye to tip--along the magnet for approximately 200 times. (Note: For the compass to work, you must always stroke the needle in the same direction. Do not stroke back and forth.)

7. Set your Styrofoam fish on a flat surface. Then, place one drop of school glue onto the center of the fish.

8. Lay the needle in the middle of the glue, with the eye of the needle pointing toward the head of the fish and the sharp end pointing toward the tail.

Allow the glue to dry.

9. Gently place the fish--with the needle-side up--in the middle of the pie tin.

10. Observe carefully as your fish compass spins. It will stop spinning when it locates the magnetic poles.

Conclusions:

1. Which end of the fish is pointing toward the North Magnetic Pole? (Hint: Opposite poles attract. You magnetized the eye of the needle by rubbing it toward the south pole of the magnet.)

2. Suppose Earth's magnetic field reversed. How would the behavior of the fish compass change?

Don't Stop Now!

Research to create a historical time line, highlighting five items in the development of navigational devices.

1. The head of the fish is pointing toward the North Magnetic Pole.

2. If Earth's magnetic field is reversed, the fish would face the direction that is opposite of the direction it is facing today.