any two-wheeled or, less commonly, three-wheeled motor vehicle, usually propelled by an internal-combustion engine.

Just as the automobile was the answer to the 19th-century dream of self-propelling the horse-drawn carriage, the invention of the motorcycle created the self-propelled bicycle. The first commercial design was a three-wheeler built by Edward Butler in Great Britain in 1884. It employed a horizontal single-cylinder gasoline engine mounted between two steerable front wheels and connected by a drive chain to the rear wheel.

By 1900 many manufacturers were converting bicycles—or pedal cycles, as they were sometimes called—by adding small, centrally mounted spark ignition engines. The need for reliable constructions led to road motorcycle trial tests and competition between manufacturers. The original Tourist Trophy (Tourist Trophy races) motorcycle races (motorcycle racing) were held on the Isle of Man (Man, Isle of) in 1907 as reliability or endurance races. Such events have been the proving ground for many new ideas from early two-stroke-cycle designs to supercharged (supercharger), multivalve engines mounted on aerodynamic, carbon-fibre reinforced bodywork.

Motorcycles are produced with both two-stroke- and four-stroke-cycle engines and with up to four cylinders. Most are air-cooled, though a few are water-cooled. Engines are generally limited to displacements of about 1,800 cc. The smallest designs, termed mopeds (from “motor pedal”), have very small engines (50 cc) with fuel economies of as much as 2.4 litres per 100 km (100 miles per gallon). Such units are not permitted on limited-access public roads because of their low speed capability. In order of increasing power capacity and engine displacements, the other five classifications are child bikes, trail bikes, road bikes, touring bikes, and racing bikes. A subcategory of racing bikes is known as superbikes. These are motorcycles that displace more than 900 cc and in which the seat is tilted forward so that the rider is hunched over the frame, creating a more aerodynamic profile.

The motorcycle frame is often of steel, usually a combination of tubes and sheets. The wheels are generally aluminum or steel rims with spokes, although some cast wheels are used. Graphite, composite, and magnesium parts are increasingly in use because of their high strength-to-weight characteristics. Tires are similar to those used on automobiles but are smaller and rounded to permit leaning to lower the centre of gravity in a turn without losing traction. The gyroscopic effect of motorcycle wheels rotating at high speed significantly improves stability and cornering ability. Inertia and steering geometry are also significant factors. Front-wheel suspension is provided by coil springs on a telescopic fork; rear-wheel springs are often mounted on shock absorbers similar to those used in automobiles.

Transmissions (transmission) on motorcycles typically have four to six speeds, although small bikes may have as few as two. Power is normally transmitted to the rear-wheel sprockets by a chain, though occasionally belts or shafts are used.

The clutch and throttle, which control engine speed, are operated by twist-type controls on the handgrips. The front-wheel brake is controlled by a lever near the handgrip; the rear-wheel brake is engaged by a foot pedal. Except on very small machines, the front brake is usually of the hydraulic disc type. The rear brake may be disc or drum. The kick starter has been mostly replaced by an electric push-button starter.

Tailpipe emission standards for motorcycles continue to be strengthened. In 1980 the U.S. Environmental Protection Agency (EPA) first regulated new motorcycle hydrocarbon emissions, requiring motorcycles to emit less than 5.0 grams per km (0.3 ounce per mile) of highway driving. California and the European Union (EU) imposed stricter limits on hydrocarbons and added restrictions on nitric oxides (nitric oxide) and carbon dioxide. In 2006 emissions from new motorcycles sold in the United States were limited to a combined 1.4 grams of hydrocarbons and nitric oxides and 12.0 grams of carbon dioxide per km. The EPA schedule calls for a limit on combined emissions of hydrocarbons and nitric oxides of 0.8 gram in 2010. The EU reduced emissions from new motorcycles in 2004 to 1.0 gram of hydrocarbons, 0.3 gram of nitric oxides, and 5.5 grams of carbon dioxide per km; in 2007 these levels were further reduced to 0.3 gram of hydrocarbons, 0.15 gram of nitric oxides, and 2.0 grams of carbon dioxide per km. Although U.S. limits for carbon dioxide were not lowered by law, the required reductions in other pollutants effectively lowered carbon dioxide emissions in fact.

In order to meet these “clean-air regulations,” manufacturers have installed more sophisticated catalytic converters and fuel-injection (fuel injection) systems.