Motorcycle

I

Introduction

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.

II

Parts of a 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.

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A

Engine

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.

B

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.

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