Wednesday, January 11, 2012



Harley-Davidson Motorcycle
A popular transportation alternative to the automobile, the motorcycle is relatively compact, fuel-efficient, and maneuverable. More than 1000 motorcycle clubs exist in the United States alone. This Harley-Davidson model is a classic model that illustrates the essential features and well-balanced design characteristic of motorcycles. In addition to their functions in general transportation, sport, and leisure, motorcycles are used by police and military forces around the world.

Motorcycle, motorized two-wheeled vehicle for transporting one or two riders. Motorcycles are capable of the same speeds as automobiles and can be licensed for use on public highways. Most American states and all Canadian provinces require a special driver’s license to operate a motorcycle on public roads. Motorcycles are generally bigger, heavier, and faster than mopeds.
Motorcycles provide a convenient and relatively inexpensive alternative to automobiles. They are more maneuverable than automobiles and they deliver higher fuel economy. Depending on the size of the engine, a motorcycle may get from 19 to 36 kilometers per liter (45 to 85 miles per gallon), two to four times that of most mid-sized cars. Also, a motorcycle accelerates more quickly than an automobile does. However, riding a motorcycle requires special skill. Braking and handling demand extra caution and can be difficult on wet or slick surfaces.
Riders use different kinds of motorcycles for different purposes. Motorcycles designed for use on paved streets and roads are called street motorcycles. Street motorcycles are a popular means of transportation during summer months and in warm climates. People often use them for recreational riding as well as for commuting. Off-road motorcycles perform well on dirt or gravel roads or trails. Racing motorcycles are engineered for handling performance and increased speed.
Parts of a Motorcycle
This diagram illustrates some of the important parts of a typical motorcycle. Transmission controls, such as the clutch and gearshift levers, are located on the handlebars or foot rests, where the rider can easily operate them. The rear shock absorber and other components of the suspension system cushion the rider from bumps and jolts of the road. The exhaust pipe carries exhaust generated by the engine to the rear of the motorcycle.

In their simplest form, motorcycles consist of a gasoline engine attached to a two-wheeled steel or aluminum frame. A fuel tank sits above the engine and usually sports the name or logo of the motorcycle’s manufacturer. The front wheel and axle are attached to the frame with a fork, a two-pronged, pivoting arm. Turning the handlebars at the top of the fork causes the wheel to pivot. Power from the engine turns the rear wheel. The rear axle is connected to the frame with a shock-absorbing arm.
The engine of a motorcycle is suspended within the vehicle frame between the front and rear wheels. Like internal combustion engines that power automobiles, motorcycle engines transform chemical energy into mechanical energy by igniting a volatile mixture of fuel and air within a cylinder, causing gases to expand suddenly. The expanding gases push down on a piston, which turns a crankshaft. The crankshaft transforms the energy from the piston into rotary motion. The rotational force of the engine’s crankshaft turns other shafts and gears that eventually cause the rear wheel to rotate.
Engines with larger cylinders—or more of them—are more powerful and consume greater amounts of fuel. An engine’s displacement, or size, is expressed in terms of the number of cylinders it has and the total volume, in cubic centimeters (cc), displaced by each cylinder. Motorcycles may have single-cylinder, twin-cylinder, four-cylinder or even six-cylinder engines with displacements that range from 250 cc to 1500 cc or higher.
The cylinders in two- and four-cylinder engines may be arranged parallel to one another. Engines with this cylinder configuration, called inline engines, are usually mounted sideways in the motorcycle frame. In other engines, cylinders are canted at a 45-degree angle, in what has come to be known as a V-configuration. In two-, four-, or six-cylinder engines, cylinders may also be positioned horizontally opposite one another. The horizontal configuration produces less vibration than V-configurations or inline configurations do. It also lowers the center of gravity of the engine, improving motorcycle handling.
Motorcycle engines are also distinguished by the number of movements, or strokes, a piston makes per cycle. In four-stroke engines, the piston moves four strokes, igniting on the third stroke and expelling the spent gases on the fourth. Two-stroke engines have a simpler design that enables them to fire in two strokes. However, two-stroke engines burn a combination of oil and fuel, thus producing more pollutants than four-stroke engines, which separate the oil and the fuel. Some motorcycles have single cylinder two-stroke engines, but all motorcycle engines that have multiple cylinders are four-stroke to reduce exhaust emissions.
Ignition and Fuel Delivery System
An engine’s ignition system controls the spark that ignites the fuel in a cylinder. Smaller displacement engines for off-road use typically have a kick-starter, a starter crank activated with the rider’s foot. Larger displacement engines and those designed for street use have an electric starter activated by turning a key in the ignition and pressing a starter switch.
Riders regulate motorcycle speed with a twist-grip on the right handlebar called the throttle. Twisting the grip backwards opens a throttle valve in the engine, increasing the amount of air and fuel that enters the cylinders. In older motorcycle engines, twisting the throttle increases the amount of fuel and air pulled into the carburetor, a device that mixes the fuel and air before it is delivered to the cylinders for combustion. Many motorcycles built after 1990 have fuel injection systems instead of carburetors. A fuel injection system uses computer-controlled fuel injectors to spray measured amounts of fuel into each of the engine’s cylinders.
Most motorcycles have a manual, five-speed transmission. The transmission consists of a series of gears, shafts, and other parts that control the forward motion of the motorcycle and enable it to maintain high cruising speeds. To change gears, the rider activates a clutch lever on the left handgrip to disengage engine power from the transmission, then shifts a foot-operated gear lever on the left side of the motorcycle. Some motorcycles have automatic transmissions, which eliminate the need for a clutch and manually shifted gears.
On most motorcycles, the transmission delivers engine power to the rear wheel via a drive chain. Chains stretch with age and require periodic adjustment. In some motorcycles, a cogged rubber drive belt or an enclosed drive shaft replaces the drive chain. Belts and drive shafts do not require adjustments and operate more quietly than chains.
The front and rear wheels on a motorcycle each have a brake. Most motorcycles have a disc brake in the front and a disc or drum brake in the rear. On most motorcycles, the rider activates the front brake with a hand lever on the right hand grip. The rear brake is operated by the right foot pedal.
Frame and Suspension System
Motorcycles have a steel or aluminum tube or box frame. Some have full frames that encompass the engine, while others have partial frames to which the engine is attached. Motorcycles with partial frames tend to be lighter in weight than those with full frames.
The frame also supports the suspension system, a collection of springs and shock absorbers that helps to keep the wheels in contact with the road and cushions the rider from bumps and jolts. The front wheel and axle are mounted on a telescoping fork with internal shock absorbers and internal or external springs. The suspension on the rear wheel and axle consists of either a pair of shock absorbers or a single shock located within the frame.
Seats and Accessories
The seats on most motorcycles are located behind the gas tank and are designed to carry one or two passengers. Some seats have small cargo compartments underneath or behind them to carry small items. Add-on hard plastic boxes or leather pouches called saddlebags may be installed on either side of the rear wheel or over the rear fender to increase cargo carrying capacity. Large motorcycles may also be fitted with a trailer hitch to pull a small trailer. A sidecar is an add-on accessory that can be installed to carry an extra passenger. The sidecar has its own wheel for added support and may have an enclosed seating compartment for all-weather riding.
Manufacturers produce many kinds of motorcycles, each specially designed for different riding conditions. Motorcycles intended for use on paved roads and highways are called street motorcycles. Off-road motorcycles are designed for riding on dirt roads and trails. Racing motorcycles are generally lighter and more powerful than other kinds of motorcycles.
Street Motorcycles
Street motorcycles are intended primarily for everyday riding. They have all the required safety equipment for use on public highways, such as lights, mirrors, a horn, and a muffler. Their tires have a tread pattern that provides good traction on both dry and wet surfaces.
There are two chief kinds of street motorcycles: sport touring motorcycles and cruising motorcycles. Sport touring motorcycles typically have wind guards, or fairings, around the headlight and engine to enhance styling and reduce drag; short, straight handlebars; and a seat shape and position that causes the rider to lean forward over the gas tank. In contrast, cruisers have no fairings around the headlights or engine and have deep handlebars and a seating position that allows the rider to sit upright. Most cruisers also have a V-configuration engine. During the 1950s, 1960s, and 1970s, customized cruisers called choppers were popular. Choppers have lengthened front forks and high handlebars.
Police officers typically ride custom-built cruisers with large 1200 cc engines. Police motorcycles are usually equipped with two-way radios, flashing lights, and police emblems so that motorists can easily identify them as official police vehicles. Many also have a windscreen or front fairing and extra cargo compartments for carrying emergency gear.
Off-Road Motorcycles
Motocross Racer
Motorcycles used in motocross racing are modified to improve off-road performance and handling. The motorcycle pictured here has a light, powerful engine; large wheels with knobby tires for improved traction; and flat, wide handlebars to maximize the racer’s control in bumpy terrain. A racer’s gear is equally important. It must protect against injury without trapping body heat or overly restricting movement. Racers wear ventilated helmets, plastic body armor, heavy gloves, flat-soled boots, and knee pads.

In comparison to street motorcycles, off-road motorcycles have narrower, lighter frames, increased ground clearance, and sophisticated suspension systems. Most are powered by single cylinder, two-stroke or four-stroke engines with displacements ranging from 250 to 500 cc. They often have a kick-starter rather than an electric starter to reduce weight. Large diameter tires with a knobby tread pattern provide increased traction, and a large gear on the rear wheel multiplies engine torque for climbing hills. Straight, wide handlebars provide extra leverage. If used strictly for off-road riding, the cycle may not have lights, mirrors, a horn, or a muffler. Most off-road motorcycles have flexible plastic fenders and fuel tanks to minimize damage in the event of a fall. Riders typically wear extra protective gear, including chest, knee, elbow, and shin protectors.
Racing Motorcycles
Road-Racing Motorcycle
Road-racing motorcycles are designed for high-speed racing on special paved tracks. They have powerful engines, wide tires for good traction, and stiff suspension systems to improve handling in corners.

Racing motorcycles are specially designed for closed circuit tracks or road racing. Most motorcycles designed for road courses have fiberglass or carbon fiber fairings around the front and engine to improve aerodynamics. They have stiff suspension systems to improve handling agility on curves, and their engines are modified to produce more power. In Europe, two-person racing cycles with sidecars are popular.
Motorcycles used for drag racing are often lengthened to improve high-speed handling stability. Bars extend behind the rear wheel to prevent the front wheel from lifting off the ground during acceleration, which would cause the cycle to flip over. A wide rear tire with no tread maximizes tire contact with the track surface. In some types of motorcycle racing, alcohol or nitromethane fuel may be used instead of gasoline to boost power. Many racing motorcycles can reach speeds as high as 390 km/h (242 mph) from a standing start in a quarter mile.
The First Motorcycle
In 1885 German engineer Gottlieb Daimler mounted an internal combustion engine of his own design into a wood-framed vehicle. The vehicle had four wheels, including two round stabilizing wheels (much like training wheels), which technically disqualify it as a bicycle. Nonetheless, historians consider Daimler’s vehicle to be the world’s first motorcyle. Daimler’s son Paul became the first motorcyclist on November 10, 1885, when he rode his father’s invention for nearly 10 km (6 mi).

The invention of the motorcycle closely followed the development of the bicycle and the four-stroke gasoline engine. German inventor Gottlieb Daimler created the first motorcycle in 1885. He attached a four-stroke gasoline engine to a wooden bicycle frame and added two extra side wheels for stability, similar to training wheels on a bicycle. Daimler’s engine turned the rear wheel with a pulley.
Various models were introduced in Germany, France, and Britain in subsequent years, each attempting to turn the motorcycle into a practical means of transportation. In 1903 American inventor William Harley, his neighbor Arthur Davidson, and Davidson’s brothers Walter and William built the first Harley-Davidson motorcycle. A year later Harley-Davidson began manufacturing motorcycles for sale. In 1909 Harley-Davidson introduced the first V-Twin engine, which had two cylinders arranged in a distinctive “V” angle. The engine, which produced a deep, rumbling sound, soon became the classic American motorcycle engine.
By 1914 the basic layout of the modern motorcycle was established. This layout featured an engine located between the front and rear wheels and a chain to transfer power from the engine to the rear wheel. During World War I (1914-1918), motorcycles proved a reliable source of transportation for American and European military forces negotiating war-damaged roads and carrying communications to forward positions. Following the war, the use of motorcycles spread rapidly in Europe and the United States.
Through the 1950s most of the motorcycles in North America were manufactured by Harley-Davidson or by British manufacturers, such as the Birmingham Small Arms Company (BSA), Norton, and Triumph. But during the 1960s and 1970s, Japanese manufacturers, including Honda, Kawasaki, Suzuki, and Yamaha, began to introduce motorcycles that had improved engines and suspensions capable of outperforming their American and English counterparts. Powerful inline four-cylinder 750 to 1200 cc engines in Japanese motorcycles soon dominated the street motorcycle market, while their 250 to 500 cc two-stroke engines virtually ruled the market for off-road motorcycles. The highly competitive Japanese manufacturers soon forced most of the English companies out of business. Harley-Davidson almost went out of business as well, but the company was saved from bankruptcy by a special U.S. import tax placed on Japanese motorcycles over 750 cc.
In the 1970s, new pollution emission regulations made many two-stroke engines obsolete. To improve emissions performance, many manufacturers replaced the traditional air-cooled engine with liquid-cooled four-stroke engines. Electronic ignition also replaced contact point ignition systems to improve reliability.
In the 1970s many U.S. states and Canadian provinces adopted helmet laws that required people to wear protective headgear when riding a motorcycle. The helmet laws proved very unpopular with many cyclists and were overturned in some states. The controversy continues today over who should decide whether riders wear helmets. Advocates of helmet laws believe that the government has the right to compel riders on public roads to wear helmets. Opponents argue that this decision should be left entirely to the individual.
In the 1980s some high performance motorcycles with “turbocharged” engines were introduced. A turbocharger uses hot exhaust gases to spin a small turbine blade. The spinning blade pushes more air and fuel into the engine, boosting its power output significantly. In the 1990s fuel injection replaced carburetors on some engines for easier starting, improved fuel economy, and reduced emissions.

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