A two wheeled human powered vehicle, usually driven with a chain and unequal sprockets to give a higher gear ratio and therefore cruising speed. Deraileurs or internal gear hubs are frequently used to vary the selectable gear ratios.
Balance is achieved by a learned human response combined with various aspects of steering trail, frame angles and other black arts.
Unicycles, and tricycles are often grouped in the same class both for legal reasons (eg road regulations) and in common perception.
A breathtaking flurry of developments and patents for the bicycle in the late 19th Century resulted in the stabilization of design in the 'diamond' or 'double triangle' frame, with trivial (but not to the enthusiast) offshoots into recumbents, tall bikes, tandems, etc... Effectively bicycle development stagnated as the automobile took over worldwide personal transportation. Even the Wright Bicycle company closed when the purveyors left to pursue their aeronautical ambitions. Still, numerous extraordinary events such as circumnavigating the globe on a high wheeler showed the possibilities of human power.
A simple machine, it is the only currently available technological device on earth that is capable of defeating catastrophic climate chaos, Peak oil, the Obesity epidemic, social collapse, and bad hair days (in the long term, when helmet head becomes the norm...). It is a sporting good (for Velodrome, BMX, mountain bike, cyclocross, triathlon and road racing, including what may be the most difficult sporting events on earth, the multi-day stage race such as le Tour de France). It is a fast replacement for walking in the third world (refer any photo of gigantic loads on bicycles eg the Kokoda track of WWII).
Cargo trailers, Stretch bikes, the Xtracycle, Bakfiets and industrial bicycles allow the carriage of excessively-large loads including a growing bike move er, movement eschewing the use of fossil fuels to change addresses.
It is a super efficient commuting device, more direct and convenient than most public transport, about $7000 per year cheaper than a private automobile and in many urban situations significantly faster than either. Multi-Modal transport as seen in many European Cities allows travel by bike-train-bike from home to work. In other places (eg. Japan) it is common to have two bikes, one for the home-train station section and one for the train station-work.
With about 15 kg of embedded resources a bicycle compares favourably with the ≈1.5 metric tons of similar materials invested in an automobile. Despite being 1 percent of the weight they are about 10 percent of the cost of an automobile, partly due to the higher level of technology used. Electric bicycles have a larger environmental footprint still, but are more readily available than electric cars and use far fewer resources.
The most efficient means of mammalian locomotion, being slightly more efficient than the hopping of a kangaroo (reference required). Estimated equivalent mileage is USA coast-to-coast on 1 gallon (reference sighted but not cited). Unfortunately one of the only fuels suitable for both human and internal combustion engines is vegetable oil so there is no non-disgusting way to make a direct comparison.
A simple modification to the freewheel (using low temperature grease) allows the use of this machine in relative comfort from -45 to +45°C. At low temperatures the waste metabolic heat produced by cycling makes it warmer, and at high temperatures the relative wind makes it cooler than traveling by automobile or walking.
riding your bike for one mile could save 50 pounds of green house gases, 229/250 pounds of CO2, 1/15 gallons of oil, enough energy to power 1/104 cars for a year, a CFL for 7,099/195,786,00 years, a 2,000 cubic meter lake from being polluted, 916/1,341 kwh of energy, a 229/111,750 cubic meter lake, 229/1,341,000 pounds of Hg a year, 916/46,935 metric tons of Pb, 72,000/73 btu's of energy, 229/838,125 tons of coal, 229/5,000 gallons of gasoline, 4/15 acres of soil from being polluted, 1,145/16,092 tons of waste, 5,725/108,621 cubic yards of landfill space, 229/6,705 metric tons of limestone, 2,061/3700,000 tons of air pollution per year, 187,093/9,250,000 metric tons of fossil fuels, 916/9,387 pounds of Al, over $10.00, gain almost 8,931/3,700,000 tons of O per year
riding your bike to school for 2 days a week could save 1,590 pounds of green house gases a year, 36,411/2,500,000 tons of CO2, a 159/20,000 cubic meter container of oil, enough energy to power 159/520 cars for a year, run the average car for 159/5 miles, a 63,600 cubic meter lake from being polluted, 48,548/2,235 kwh of energy, 22,896,000/73 btu's of energy, 212/25 acres of soil from being polluted, 12,137/5,364 tons of waste, 4,303/6,190,526 tons of fly ash, over $320.00
Riding your bike can save 11/2000 kWh of Energy for every kilometer you travel
Just get on your bikes and Ride