Motorcycle Mechanic

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Motorcycle Engine

 

Inside a typical car 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.

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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.

Next, we'll examine the motorcycle transmission.

Motorcycle Transmission

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.

belt drive on a Buell Lightning
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.

The other components that make a motorcycle a motorcycle are part of the chassis.

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.

Harley-Davidson Softail
This Harley-Davidson Softail has a swingarm rear suspension.

Suspension
The frame also serves as a support for the suspension system, 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.

tubeless front tire
      
A tubeless front tyre
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 tyres, don't have an inner tube to hold the compressed air. Instead, the air is held between the rim and the tyre, relying on a seal that forms between rim and tyre to maintain the internal air pressure.

Tubeless tyres are less likely to blow out than a tube-type tyre, but on rough roads, they can be a problem because even a small bend in the rim can cause a deflation. Tyres come in a variety of designs to match the needs of terrain and driving conditions. Dirt bike tyres, for example, have deep, knobby treads for maximum grip on dirt or gravel. Touring bike tyres, made of harder rubber, usually provide less grip but last longer. The tyres 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.

disc brake
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.

Next, we'll see what it's like to drive a motorcycle.

Driving a Motorcycle

Helmet Head
motorcycle helmet

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.

gyroscope
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 asprecession. 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 Types

Motorcycles come in many different styles, each offering design and performance characteristics to accommodate specific riding conditions. Let's take a look at the common categories of motorcycles.

touring and cruising motorcycles
Examples of touring motorcycles (left) and cruisers (right)

Street Bikes
Street motorcycles come with all of the necessary equipment to be street-ready. They have lights, mirrors, a horn and a muffler. Their tires have a tread pattern that provides good traction on both wet and dry roads. Street motorcycles generally come in two forms -- touring motorcycles and cruisers. Touring motorcycles are specially designed for long-distance travel. Their most distinctive features arefairings, aerodynamic wind guards that wrap around the headlight to enhance styling and reduce drag. Touring bikes also come with other long-distance amenities, such as saddlebags and a comfortable passenger seat. Cruisers, which typically have no fairings, offer a more laid-back look. They are built with swept-back handlebars, low seats, and casual riding positions with forward-set footpegs.

005 Yamaha FZ6

2005 Yamaha FZ6, a "naked bike"

Sportbikes
Sportbikes are designed to handle well at high speeds and on winding roads. They offer multi-cylinder engines to produce more power, aluminum alloy frames, stiff suspensions to improve handling, high-grip tires and powerful brakes. Instead of sitting up straight, sportbike riders lean forward over the gas tank to reduce wind resistance.

"Naked Bikes"
Naked bikes offer the performance of sportbikes without the aesthetics. In most cases, they are stripped of any unnecessary bodywork. Because they're often the product of bike customizers who want a "road warrior" appearance, naked bikes are also called streetfighters, especially in Europe.

1978 Suzuki GS550

1978 Suzuki GS550, a typical UJM

Traditionals
Also known as standards, traditionals look and handle like an archetypical model known as the Universal Japanese Motorcycle (UJM), built primarily in the 1970s. The UJM was an all-purpose, do-everything bike, and today's standards offer the same versatility and straightforward design.

Off-road Bikes
Off-road motorcycles include both motocross bikes and dirt bikes -- machines designed to handle jumps, bumps and other obstacles found on closed racing courses or woodland trails. Off-road motorcycles have narrower, lighter frames, increased ground clearance and advanced suspension systems. They also have a kick-starter to reduce weight and tires with a knobby tread pattern for increased traction. Because off-road bikes usually don't come standard with lights, mirrors, a horn or a muffler, they aren't street-legal.

Motocross bikes

Motocross bikes in action.

Dual-purpose
Dual-purpose bikes, also known as dual-sports, are street-legal motorcycles that offer some off-road capabilities. Like dirt bikes, dual-purpose machines are lightweight and durable. Like standards, they offer great versatility for newcomers and long-time riders alike. The dual-purpose motorcycle falls somewhere in between a dirt bike and a street motorcycle. For example, dual-purpose bikes feature specialized tires that work on both dirt and pavement.