How Motorcycles Work

The layout of the modern motorcycle was established by 1914 and has remained fundamentally unchanged ever since. The overall structure and function of a motorcycle is quite simple. It includes a gasoline engine, which converts the reciprocating motion of pistons into rotary motion, just like the engine in a car. A transmission system transmits this motion to the back wheel. As the back wheel turns, it propels the motorcycle forward. Steering is accomplished by turning the front wheel via the handlebars and by leaning the bike to one side or the other. Two hand levers enable the rider to operate the clutch and the front brake, while two foot pedals enable him to change gears and control the rear brake.

Motorcycle Engine


Inside a typical bike engine
Motorcycle engines work the same way that car engines do. They consist of pistons, a cylinder block and a head, which contains the valve train. The pistons move up and down in the cylinder block, driven by explosions of a fuel-air mixture that has been ignited by a spark. Valves open and close to allow the fuel-air mixture to enter the combustion chamber. As the pistons move up and down, they turn a crankshaft, which transforms the energy from the pistons into rotary motion. The rotational force of the crankshaft is transmitted, via the transmission, to the rear wheel of the motorcycle.

Motorcycle engines are generally classified by one of three characteristics: the number of cylinders they possess, the capacity of their combustion chambers or the number of strokes in their power cycles.

The Revolution, a Harley-Davidson V-twin engine

Cylinders

Motorcycle engines can have between one and six cylinders. For years, the V-twin design was the engine of choice for motorcycle engineers in America, Europe and Japan. The V-twin gets its name from the fact that the two cylinders form a V shape, such as the classic Harley-Davidson V-twin shown below. Notice the 45-degree angle in the Harley-Davidson V-twin -- other manufacturers may vary this angle to reduce vibration.

The V-twin is just one way to accommodate two cylinders. When the cylinders are oriented so that the pistons oppose each other, the result is an opposed-twin design. Parallel-twin engines have their pistons placed side by side in an upright position.

Today, the most popular design is the four-cylinder, which runs more smoothly and at higher revolutions per minute (rpms) than a comparable twin. The four cylinders can be placed in a row, or they can be arranged in a V-shape configuration, with two cylinders on each side of the V.


Capacity

The size of the combustion chamber in a motorcycle engine is directly related to its power output. The upper limit is about 1500 cubic centimeters (cc), while the lower limit is about 50 cc. The latter engines are usually found on small motorcycles (mopeds) that offer 100-miles-to-the-gallon fuel economy but only reach top speeds of 30 to 35 miles per hour.
Motorcycle Transmission

A simple transmission
A motorcycle engine can create an enormous amount of power, which must be delivered to the wheels of the vehicle in a controllable way. The motorcycle transmission delivers power to the rear wheel through a series of structures that include the gearset, the clutch and the drive system.

Gearset

A gearset is a set of gears that enable a rider to move from a complete stop to a cruising speed. Transmissions on motorcycles typically have four to six gears, although small bikes may have as few as two. The gears are engaged by shifting a lever, which moves shifting forks inside the transmission.


Clutch






The job of a clutch is to engage and disengage power from the engine crankshaft to the transmission. Without the clutch, the only way to stop the wheels from turning would be to turn off the engine -- an impractical solution in any kind of motorized vehicle. The clutch is a series of spring-loaded plates that, when pressed together, connect the transmission to the crankshaft. When a rider wants to shift gears, he uses the clutch to disconnect the transmission from the crankshaft. Once the new gear is selected, he uses the clutch to reestablish the connection.

The belt drive on a Buell Lightning

Drive Systems

There are three basic ways to transmit engine power to the rear wheel of a motorcycle: chain, belt or shaft. Chain final-drive systems are by far the most common. In this system, a sprocket mounted to the output shaft (i.e., the shaft in the transmission) is connected to a sprocket attached to the rear wheel of the motorcycle by a metal chain. When the transmission turns the smaller front sprocket, power is transmitted along the chain to the larger rear sprocket, which then turns the rear wheel. This type of system must be lubricated and adjusted, and the chain stretches and the sprockets wear, requiring periodic replacements.

The Friction Drive
The friction drive is another transmission found in some motorcycles. A friction drive is a type of continuously variable transmission, or CVT, in which the variation in gear ratios comes about as a disc connected to the engine (the driving disc) rotates across the face of a second disc connected to the rear wheel (the driven disc). By varying the radius of the contact point between the two disc surfaces, different gears can be achieved. Continuously variable transmissions have a long history of use in motorized vehicles, with variable friction transmissions appearing in motorcycles in the early 1900s.

Belt drives are an alternative to chain drives. Early motorcycles often used leather belts, which could be tensioned to give traction using a spring-loaded pulley and hand lever. Leather belts often slipped, especially in wet weather, so they were abandoned for other materials and designs. By the 1980s, advances in materials made belt final-drive systems viable again. Today's belts are made of cogged rubber and operate much the same way as metal chains. Unlike metal chains, they don't require lubrication or cleaning solvents.

Shaft final-drives are sometimes used. This system transmits power to the rear wheel via a drive shaft. Shaft drives are popular because they are convenient and don't require as much maintenance as chain-based systems. However, shaft drives are heavier and sometimes cause unwanted motion, called shaft jacking, in the rear of the motorcycle.

Motorcycle Chassis
The motorcycle chassis consists of the frame, suspension, wheels and brakes. Each of these components is described briefly below.

Frame

Motorcycles have a frame made of steel, aluminum or an alloy. The frame consists mostly of hollow tubes and serves as a skeleton on which components like the gearbox and engine are mounted. The frame also keeps the wheels in line to maintain the handling of the motorcycle.

This Harley-Davidson Softail has a swingarm rear suspension.

Suspension

The frame also serves as a support for the suspensionsystem, a collection of springs and shock absorbers that helps keep the wheels in contact with the road and cushions the rider from bumps and jolts. A swingarm design is the most common solution for the rear suspension. On one end, the swingarm holds the axle of the rear wheel. On the other end, it attaches to the frame via the swingarm pivot bolt. A shock absorber extends upward from the swingarm pivot bolt and attaches to the top of the frame, just beneath the seat. The front wheel and axle are mounted on a telescoping fork with internal shock absorbers and internal or external springs.

A tubeless front tire
for touring motorcycles


Wh­eels

Motorcycle wheels are generally aluminum or steel rims with spokes, although some models introduced since the 1970s offer cast wheels. Cast wheels allow the bikes to use tubeless tires, which, unlike traditional pneumatic tires, don't have an inner tube to hold the compressed air. Instead, the air is held between the rim and the tire, relying on a seal that forms between rim and tire to maintain the internal air pressure.

Tubeless tires are less likely to blow out than a tube-type tire, but on rough roads, they can be a problem because even a small bend in the rim can cause a deflation. Tires come in a variety of designs to match the needs of terrain and driving conditions. Dirt bike tires, for example, have deep, knobby treads for maximum grip on dirt or gravel. Touring bike tires, made of harder rubber, usually provide less grip but last longer. The tires of sportbikes and racers (generally steel-belted radials) deliver astonishing gripping power, especially considering the small area that is in contact with the road surface.

Parts of a disc brake

Brakes

The front and rear wheels on a motorcycle each have a brake. The rider activates the front brake with a hand lever on the right grip, the rear brake with the right foot pedal. Drum brakes were common until the 1970s, but most motorcycles today rely on the superior performance of disc brakes. Disc brakes consist of a steel braking disc, which is connected to the wheel and sandwiched between brake pads. When the rider operates one of the brakes, hydraulic pressure, acting through the brake line, causes the brake pads to squeeze against the disc on both sides. Friction causes the disc and the attached wheel to slow down or stop. Brake pads must be replaced periodically because the pad surfaces wear away after repeated use.

Seats and Accessories
Seats on motorcycles are designed to carry one or two passengers. They are located behind the gas tank and are easily removable from the frame. Some seats have small cargo compartments underneath or behind them. For more storage, saddlebags -- either hard plastic boxes or leather pouches -- can be installed on either side of the rear wheel or over the rear fender. Large motorcycles can even tow a small trailer or pull a sidecar. The sidecar has its own wheel for support and may have an enclosed seating compartment to accommodate a passenger.

Driving a Motorcycle




Many states require that motorcycle riders wear helmets. Helmets perform two functions in a crash. The outer shell, which is constructed of fiberglass or injection-molded plastic, distributes energy from an impact across a wider area. An inner lining made of polystyrene absorbs most of the shock of the impact.

Driving a motorcycle is very different from driving a car. Since motorcycles are two-wheeled vehicles, they topple over when ­they stop moving. A moving motorcycle is affected by gyroscopic forces that are unique to two-wheeled machines. As a result, new motorcyclists must develop the skills necessary to handle their machines and must be licensed before they can drive their motorcycles on the street. In particular, motorcycle riders must master the art of steering, braking and changing gears.

Steering

Steering a motorcycle at low speeds is a straightforward process. The rider simply turns the handlebar in the direction he wishes to go. This only works at speeds below five miles an hour. If a motorcycle is traveling any faster, the rider must use a different kind of steering, known as counter-steering. This type of steering may seem counterintuitive. That's because motorcycle riders must push the handlebars to the left to make the vehicle turn right and vice versa.

Here's how it would work out on the street. Imagine that you're riding a motorcycle on the interstate. In front of you, blocking the right half of your lane, is a wreck or some other obstacle. If you're a novice rider, you might be tempted to push on the right side of the handlebar, thinking this will turn the bike to left. In reality, this will steer the bike to the right, directly into the obstacle. Instead, you should push on the left side of the handlebar, which directs the front wheel to the right but steers the vehicle to the left.

Figure 1: The gyroscope is spinning on its axis.
Figure 2: A force is applied to try to rotate the spin axis.
Figure 3: The gyroscope is reacting to the input force along an axis perpendicular to the input force.


Why does a motorcycle work this way? The motorcycle's wheels act like gyroscopes and create gyroscopic forces when moving at speeds above five miles an hour. One of the most interesting effects related to a gyroscope is a phenomenon known as precession. When a force is applied perpendicular to a gyroscope's axis of rotation, the resulting motion is perpendicular to the input force. This motion is called precession, and it's what causes the steering in motorcycles to be counterintuitive. It's also why instructors often arm their beginning riders with a simple mnemonic: "Push left, turn left. Push right, turn right."

Braking

Stopping a moving motorcycle requires that the rider use two brakes -- one on the front wheel, controlled by the right hand, and one on the rear wheel, controlled by the right foot. Both brakes should be used at the same time, although the front brakes are more powerful and will typically provide 70 to 90 percent of the total braking force. New riders often fear using the front brake, but it should be applied every time a motorcycle is slowed or stopped. Many accidents are caused by riders braking incorrectly. According to the California Highway patrol, locking up the rear brakes is a factor in the majority of motorcycle crashes.


Changing Gears

Early motorcycle clutches were operated by a foot pedal in the same way that automobile drivers use clutches. This was awkward and dangerous because it required that the rider's left foot be off the ground when the bike came to complete stop (at an intersection, for instance). British designers solved this problem with a hand-operated clutch. Today, hand-operated clutches and foot-operated shifters are standard on all models.

Braking Advice

When a motorcycle experiences a rapid deceleration, weight shifts to the front wheel. This makes the back of the bike lighter and can result in the rear wheel locking up and skidding. In this situation, riders should simply keep the rear brake applied and focus their eyes on the horizon where they want the bike to go. The bike will continue to skid, but in a controllable manner with little fishtailing.

When the front wheel locks up, riders should ease off the front brake. If they don't, the front wheel can tuck under the bike, causing a fall. The best way to avoid a front lockup is to use a technique called "staged braking." In staged braking, the rider progresses through four stages, with each stage corresponding to a greater amount of pressure applied to the front brake:
Stage one has the rider applying the brake just to the point where there is the slightest friction between the brake pads and disc.
In stage-two braking, the rider progresses to stage one, then continues to apply a steadier force.
By stage four, which is usually reserved for emergencies that require rapid deceleration, the rider bears down on the brake as hard as possible, but only after progressing through the other stages.

This kind of progressive braking will serve motorcyclists in all driving situations and will usually prevent a front lockup.

Motorcycle History
Motorcycl­es evolved from the "safety" bicycle, a bicycle that offered many advantages in stability, braking and ease of mounting. The essential features of a safety bike included:
Spoked front and rear wheels of the same size - roughly 30 inches in diameter (compared to the "ordinary" bicycle, which had a 48-inch front wheel and a 30-inch rear wheel)
A chain-driven rear wheel
A front chainwheel roughly twice as large as the rear sprocket
A low center of gravity
Direct front steering

1901 Hendee Single

The first bicycle to provide all of these features and gain market acceptance was the Rover Safety, designed by John Kemp Starley in 1885. After the Rover pattern took over the market, safety bicycles were simply called "bicycles."

It didn't take long for someone to take the user-friendly safety bicycle design and strap on an internal combustion engine. The first to do so successfully was Gottleib Daimler, who is credited with building the first motorized bicycle -- or motorcycle -- in 1885. Daimler's motorcycle included a single-cylinder Otto-cycle engine mounted vertically in the center of the machine. It also had one wheel in front, one wheel in back and a spring-loaded outrigger wheel on each side for added stability. Its chassis consisted of a wooden frame and wheels with wood spokes and iron rims. Such designs were called "boneshakers" because of the rough, jarring ride they delivered.

The next notable motorcycle was designed in 1892 by Alex Millet. Millet incorporated the basic safety bicycle design, but added pneumatic tires and a five-cylinder rotary engine built into the rear wheel. The cylinders rotated with the wheel, while the crankshaft formed the rear axle.

The first Harley-Davidson was basically a motorized bicycle.

The Hildebrand & Wolfmueller was the first successful production two-wheeler, patented in Munich in 1894. More than 200 vehicles made it onto the road. Hildebrand & Wolfmueller decided to cool their parallel-twin engine with water, which required a water tank and radiator. Their solution was to build the coolant system into the top of the rear fender.

In 1895, DeDion-Buton introduced an engine that would revolutionize the motorcycle industry by making mass production possible. The DeDion-Buton engine was a small, light, high-revving four-stroke engine that could generate half a horsepower. Although DeDion-Buton used the engine in its motortricycles, motorcycle manufacturers around the world copied and used the design.

American production motorcycles were also based on the DeDion-Buton engine. The two most famous American motorcycle manufacturers to incorporate the DeDion-Buton engine, however, were the Indian Motorcycle Company and Harley-Davidson.

Motorcycles Go to War

A Harley-Davidson military motorcycle
When World War I started in 1914, the automobile did not own the roads. Motorcycles filled the gaps as dependable, reliable vehicles. In the war, their utilitarian nature was put to good use. American and European armies used motorcycles extensively to gather reconnaissance, deliver messages and, in some cases, engage in combat. In 1917, roughly one-third of all Harley-Davidson motorcycles produced were sold to the U.S. military; in 1918, that figure rose to 50 percent. By the end of the war, it is estimated that the Army used some 20,000 motorcycles -- most of them Harley-Davidsons.

Carl Oscar Hedstrom and George M. Hendee founded the Hendee Manufacturing Company in 1900 with the goal of producing a "motor-driven bicycle for the everyday use of the general public." In 1901, they rolled out the Single, a 1.75-horsepower motorcycle that could reach a top speed of 25 miles per hour. They also decided to roll out a brand-new trade name for their motorcycles. That name was Indian, and it was the world's best-selling motorcycle until World War I.

Founded by William S. Harley and Arthur Davidson in 1902, the Harley-Davidson Motor Company went on to produce the most influential machines of the industry. Its first models used the basic DeDion-Buton layout and borrowed heavily from chassis designs already employed by other motorcycle manufacturers, including Indian, Excelsior and Pope. The Harley-Davidson eventually made its presence known with its sturdy, strong and durable machines. In 1908, Walter Davidson, riding what came to be known as the Silent Gray Fellow, scored a perfect 1,000 points at the 7th Annual Federation of American Motorcyclists Endurance and Reliability Contest. Soon after, Walter Davidson, Arthur's brother, set the FAM economy record at 188.234 miles per gallon. By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world.