Motorcykelfordon

Motorcykel, vilket som helst tvåhjuligt eller, mindre vanligt, trehjuligt motorfordon, vanligtvis drivet av en förbränningsmotor.

Historia

Precis som bilen var svaret på 1800-talets dröm om att själv driva hästvagnen skapade uppfinningen av motorcykeln den självgående cykeln. Den första kommersiella designen var en trehjuling byggd av Edward Butler i Storbritannien 1884. Den använde en horisontell encylindrig bensinmotor monterad mellan två styrbara framhjul och ansluten med en drivkedja till bakhjulet.

År 1900 konverterade många tillverkare cyklar - eller pedalcykler, som de ibland kallades - genom att lägga till små, centralt monterade gnisttändningsmotorer. Behovet av pålitliga konstruktioner ledde till provning av motorcykelkörning och konkurrens mellan tillverkare. De ursprungliga Tourist Trophy-motorcyklarna hölls på Isle of Man 1907 som pålitlighet eller uthållighetstävlingar. Sådana händelser har varit den bevisande grunden för många nya idéer från tidiga tvåtaktscykelkonstruktioner till överladdade, multivävliga motorer monterade på aerodynamisk, kolfiberarmerad karosseri.

Komponenter

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 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.

Emissions standards

Tailpipe emissions 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 and carbon monoxide. 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 monoxide per km. The EPA decreased the limit on combined emissions of hydrocarbons and nitric oxides to 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 monoxide 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 monoxide per km. The EU did a further emission reduction in 2016 to 0.17 gram of hydrocarbons, 0.09 gram of nitric oxides, and 1.14 grams of carbon monoxide, with a further reduction to 0.1 gram of hydrocarbons, 0.06 gram of nitric oxides, and 1 gram of carbon monoxide planned for 2020. Although U.S. limits for carbon monoxide were not lowered by law, the required reductions in other pollutants effectively lowered carbon monoxide emissions in fact. In order to meet these “clean-air regulations,” manufacturers installed more sophisticated catalytic converters and fuel-injection systems.