bicycle

A bicycle, also called a cycle or bike, is a human-powered, pedal-driven, single-track [单轨] vehicle, having two wheels attached to a frame, one behind the other. A bicycle rider is called a cyclist, or bicyclist.

Bicycles were introduced in the late 19th century in Europe, and by the early 21st century, more than 1 billion have been produced worldwide. The most popular bicycle model—and most popular vehicle of any kind in the world—is the Chinese Flying Pigeon [飞鸽], with 500 million produced. Bicycles parked outside an academic building at Stanford University:

  

Unicycles, tricycles and quadracycles are not strictly bicycles, as they have respectively one, three and four wheels, but are often referred to informally as "bikes" or "cycles".

The bicycle is extraordinarily efficient in both biological and mechanical terms. The bicycle is the most efficient human-powered means of transportation in terms of energy a person must expend to travel a given distance. From a mechanical viewpoint, up to 99% of the energy delivered by the rider into the pedals is transmitted to the wheels.

A human traveling on a bicycle at low to medium speeds of around 16–24 km/h (10–15 mph) uses only the power required to walk. Air drag [空气阻力], which is proportional to the square of speed, requires dramatically higher power outputs as speeds increase. If the rider is sitting upright, the rider's body creates about 75% of the total drag of the bicycle/rider combination.

In addition, the carbon dioxide generated in the production and transportation of the food required by the bicyclist, per mile traveled, is less than 1/10 that generated by energy efficient motorcars.

The global bicycle market is $61 billion in 2011. As of 2009 130 million bicycles were sold every year globally and 66% of them were made in China.

Racing bicycles (aka road bicycles) are designed for speed, and the sport of competitive road racing. They have lightweight frames and components with minimal accessories, drop handlebars to allow for a powerful and aerodynamic riding position, narrow high-pressure tires for minimal rolling resistance and multiple gears. Racing bicycles have a relatively narrow gear range, and typically varies from medium to very high ratios, distributed across 18, 20, 27 or 30 gears.

BMX (bicycle motocross) bicycles have small wheels and are used for BMX racing, as well as freestyle with tricks such as wheelies. Freestyle BMXers often ride dirt jumps and skatepark ramps, or in street-style BMXing where a rider navigates through a course of stairs and metal rails. BMX racing became a medal sport at the 2008 Summer Olympics in Beijing under the UCI (International Cycling Union) sanctioning body. In vehicle acrobatics, a wheelie, or wheelstand, is a vehicle maneuver in which the front wheel or wheels come off the ground due to sufficient torque being applied to the rear wheel or wheels, or rider motion relative to the vehicle.

A mountain bike (MTB) or mountain bicycle is a bicycle designed for off-road cycling. Mountain bikes share some similarities with other bicycles, but incorporate features designed to enhance durability and performance in rough terrain. These typically include a suspension fork, large knobby tires, more durable wheels, more powerful brakes, straight, extra wide handlebars to improve balance and comfort over rough terrain, lower gear-ratios for climbing steep grades and sometimes rear suspension to really smooth out the trail.

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