Bicycles are one of the world's most popular modes of transportation, with some 800 million bicycles outnumbering cars by two to one. Bicycles are also the most energy-efficient vehicle—a cyclist burns about 35 calories per mile (22 calories per km), while an automobile burns 1,860 calories per mile (1,156 calories per km). Bicycles are used not only for transportation, but for fitness, competition, and touring as well. They come in myriad shapes and styles, including racing bikes, all-terrain bikes, and stationary bicycles, as well as unicycles, tricycles, and tandems. History
As far back as 1490, Leonardo da Vinci had envisioned a machine remarkably similar to the modern bicycle. Unfortunately, da Vinci did not attempt to build the vehicle, nor were his sketches discovered until the 1960s. In the late 1700s a Frenchman named Comte de Sivrac invented the Celerifere, a crude wooden hobby horse made of two wheels and joined by a beam. The rider would sit atop the beam and propel the contraption by pushing his or her feet against the ground. In 1816 the German Baron Karl von Drais devised a steerable hobby horse, and within a few years, hobby-horse riding was a fashionable pastime in Europe. Riders also discovered that they could ride the device with their feet off the ground without losing their balance. And so, in 1840, a Scottish black-smith named Kirkpatrick Macmillan made a two-wheel device that was operated by a treadle. Two years later he traveled as many as 40 miles (64 km) at a stretch during a record 140-mile (225 km) round trip to Glasgow. A couple decades later, a Frenchman, Ernest Michaux, designed a hobby horse that utilized cranks and rotating pedals connected to the front axle. The Velocipede, made with wooden wheels and an iron frame and tires, won the nickname of the "boneshaker." The 1860s proved to be an important decade for bicycle improvements with the inventions of ball-bearing hubs, metal-spoked wheels, solid rubber tires, and a lever-operated, four-speed gearshift. Around 1866 an unusual version of the Velocipede was created in England by James Stanley. It was called the Ordinary, or Penny Farthing, and it had a large front wheel and a small rear wheel. The Ordinaries were soon exported to the U.S. where a company began to manufacture them as well. These bicycles weighed a hefty 70 pounds (32 kg) and cost $300—a substantial sum at the time. By 1885, another Englishman, John Kemp Starley, created the Rover Safety, so called since it was safer than the Ordinary which tended to cartwheel the rider over the large front wheel at abrupt stops. The Safety had equally sized wheels made of solid rubber, a chain-driven rear wheel, and diamond-shaped frame. Other important developments in the 1800s included the use of John Boyd Dunlop's pneumatic tires, which had air-filled inner tubes that provided shock absorption. Coaster brakes were developed in 1898, and shortly thereafter freewheeling made biking easier by allowing the wheels to continue to spin without pedaling.
The frame consists of the front and rear triangles, the front really forming more of a quadrilateral of four tubes: the top, seat, down, and head tubes. The rear triangle consists of the chainstays, seatstays, and rear wheel dropouts. Attached to the head tube at the front of the frame are the fork and steering tube. During the 1890s bicycles became very popular, and the basic elements of the modern bicycle were already in place. In the first half of the 20th century, stronger steel alloys allowed thinner frame tubing which made the bicycles lighter and faster. Derailleur gears were also developed, allowing smoother riding. After the Second World War, bicycle popularity slipped as automobiles flourished, but rebounded in the 1970s during the oil crisis. About that time, mountain bikes were invented by two Californians, Charlie Kelly and Gary Fisher, who combined the wide tires of the older balloontire bikes with...