Technical aspects

Construction
The construction of modern motorcycles has mostly standardized on the key components listed below.
Chassis
The chassis (or frame) of a motorcycle is typically made from welded aluminium or steel (or alloy) struts, with the rear suspension being an integral component in the design. Carbon-fiber and titanium are used in a few very expensive custom frames.
The chassis includes the head tube that holds the front fork and allows it to pivot. Some motorcycles include the engine as a load-bearing (or stressed) member; this has been used all through bike history but is now becoming more common.
Oil-in-Frame (OIF) chassis, where the lubricating oil is stored in the frame of the motorcycle, was used for Vincent motorcycles of the fifties, and for a while during the 1970s on some NVT British motorcycles. It was widely unpopular and generally regarded as a bad idea at the time. Today it is a widely used idea on "thumpers" (single-cylinder four-strokes) that usually have a dry-sump lubrication requiring an external oil tank. It has since gained some cache in modern (2006) custom bike world too because of the space saving it can afford and the reference to an earlier era.[7] Buell motorcycles employ a similar design - the oil is held in the swingarm, while the fuel is held in the frame.
Front fork
Main article: Motorcycle fork
A motorcycle fork is the portion of a motorcycle that holds the front wheel and allows one to steer. For handling, the front fork is the most critical part of a motorcycle. The combination of rake and trail determines how stable the motorcycle is.
A fork generally consists of two fork tubes (sometimes also referred to as forks), which hold the front wheel axle, and a triple tree, which connects the fork tubes and the handlebars to the frame with a pivot that allows for steering.
Engine
Main article: Motorcycle engine
Almost all commercially available motorcycles are driven by conventional gasoline internal combustion engines, but some small scooter-type models use an electric motor, and a very small number of diesel models exist (e.g., the USMC M1030 M1 version of the Kawasaki KLR650 and the Dutch-produced Track T-800CDI).
The displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle. In a piston engine, this is the volume that is swept as the pistons are moved from top dead centre to bottom dead centre. To the layperson this is the "size" of the engine. Motorcycle engines range from less than 50 cc (cubic centimetres), commonly found in many small scooters, to 5735 cc, a Chevrolet V8 engine, currently used by Boss Hoss in its cruiser style motorcycle.
Motorcycles have mostly, but not exclusively, been produced with one to four cylinders, and designers have tried virtually every imaginable layout. The most common engine configurations today are the vertical single and twin, the V-twin, the opposed twin (or boxer), and the in-line triple and in-line four. A number of others designs have reached mass production, including the V-4, the flat 6-cylinder, the flat 4-cylinder, the in-line 6-cylinder, and the Wankel engine. Exotic engines, such as a radial piston engine, sometimes appear in custom built motorcycles.
Engines with more cylinders for the same displacement feel smoother to ride. Engines with fewer cylinders are cheaper, lighter, and easier to maintain. Liquid-cooled motorcycles have a radiator (exactly like the radiator on a car) which is the primary way their heat is dispersed. Coolant or oil is constantly circulated between this radiator and the cylinder when the engine is running. Air-cooled motorcycles rely on air blowing past fins on the engine case to disperse heat. Liquid cooled motorcycles have the potential for greater power at a given displacement, tighter tolerances, and longer operating life, whereas air cooled motorcycles are potentially cheaper to purchase, less mechanically complex and lighter weight.
An air cooled engine contracts and expands with its wider temperature range, requiring looser tolerances, and giving shorter engine life. The temperature range of an air cooled two stroke is even more extreme, and component life even shorter than in an air cooled four stroke.

As applied to motorcycles, two-stroke engines have some advantages over equivalent four-strokes: they are lighter, mechanically much simpler, and produce more power when operating at their best. But four-stroke engines are cleaner, more reliable, and deliver power over a much broader range of engine speeds. In developed countries, two-stroke road-bikes are rare, because - in addition to the reasons above - modifying them to meet contemporary emissions standards is prohibitively expensive. Almost all modern two-strokes are single-cylinder, liquid-cooled, and under 600 cc.
Transmission
Modern motorcycles normally have five or six forward gears. Only the largest touring motorcycles and a few models that are routinely used with a sidecar or converted to tricycle configuration are fitted with a reverse gear. On a few, including the Honda Gold Wing and BMW K1200LT, it is not really a reverse gear, but a feature of the starter motor which when reversed, performs the same function. The weight of these motorcycles, in excess of 360 kg (800 lbs), means that they cannot effectively be pushed backwards in some situations by a seated rider.
Older motorcycles had various kinds of shifting arrangements. A common version before World War II was a hand operated shift lever that rose from the transmission behind the engine up to beside the fuel tank on its left or right side. Later, some motorcycles, especially British, used foot shift levers that were located on the right side of the transmission. Today, shifting on the world's motorcycles has been standardized to the left-side foot-operated shift lever.
The clutch is typically an arrangement of plates stacked in alternating fashion, one geared on the inside to the engine and the next geared on the outside to the transmission input shaft. Whether wet (rotating in engine oil) or dry, the plates are squeezed together by a spring, causing friction build up between the plates until they rotate as a single unit, driving the transmission directly. A lever on the handlebar exploits mechanical advantage through a cable or hydraulic arrangement to release the clutch spring, allowing the engine to freewheel with respect to the transmission.
The most commonly used transmission is a sequential gearbox. From neutral, either first or second gear can be selected, but higher gears may only be accessed in order - it is not possible to shift from second gear to fourth gear without shifting through third gear. A five-speed of this configuration is commonly said to be "one down, four up," and a six-speed is said to be "one down, five up" due to the placement of the gears with relation to neutral. Neutral is considered to be "half a click" from first and second gears, so shifting directly between the two gears is made in one firm movement. Neutral is not placed "below" first gear to prevent the rider accidentally selecting neutral while attempting to downshift to first gear, possibly leading to an accident.
Scooters normally have a continuously variable transmission (CVT). The CVT is a type of automatic transmission (also occasionally used in cars) that can change the "gear ratio" (gears are not generally involved) to any one of the possible undetermined settings within a given range. The CVT is not constrained to a small number of gear ratios, such as the four to six forward ratios in typical automotive transmissions. CVTs are ideally suited to 2-wheeled vehicles because of the lack of shift shock. A conventional automatic transmission might shift shock at an unexpected moment and upset the vehicle. The CVT transmission also offers maximum power efficiency, an important factor for the engines with less than 100 cc capacity typically used in scooters.
Final drive
Power transfer from the gearbox to the rear wheel is typically accomplished with a chain, which requires both lubrication and adjustment for elongation (stretch) that occurs through wear. The lubricant is subject to being thrown off the fast-moving chain and results in grime and dirt build up. Chains do deteriorate, and excessive wear on the front and rear sprockets can be dangerous. Many manufacturers offer cruiser models with final drive options of a belt or a shaft. A belt drive is still subject to stretch, but operates very quietly, cleanly, and efficiently. However, belt drives are limited in the amount of power they can transmit. A toothed belt is frequently used. A shaft drive is usually completely enclosed; the visual cue is a tube extending from the rear of the transmission to a bell housing on the rear wheel. Inside the bell housing a bevelled gear on the shaft mates with another on the wheel mount. This arrangement is superior in terms of noise and cleanliness, and is virtually maintenance free, with the exception of occasional fluid changes. However, the additional gearsets are a source of power loss and add weight.
Wheels
The wheel rims are usually steel or aluminium (generally with steel spokes and an aluminium hub) or mag-type cast or machined aluminium. At one time, motorcycles all used spoke wheels built up from separate components (see wheelbuilding), but, except for dirtbikes, one-piece wheels are more common now. Performance racing motorcycles often use carbon-fibre wheels, but the expense of these wheels is prohibitively high for general usage.
Wire wheels, a.k.a. "laced wheels," have a central hub connected to the rim of the wheel via spokes made of wire. These spokes are generally quite solid and will not easily bend as would typical wire cord. Nevertheless, they mechanically function as wires under tension, holding the rim true and providing strength to the wheel. Cast magnesium disks, produced by one-step hot forging from magnesium alloys ZK60 and MA-14, are also used for many motorcycle wheels.
Tyres (also, Tires)
Motorcycles mainly use pneumatic tyres. However, in some cases where punctures are common (some enduros), the tyres are filled with a "mousse" which is unpunctureable. Both types of tyre come in many configurations. The most important characteristic of any tyre is the contact patch, the small area that is in contact with the road surface while riding. There are tyres designed for dirtbikes, touring, sport and cruiser bikes.
Dirtbike tyres have knobbly, deep treads for maximum grip on loose dirt, mud, or gravel; such tyres tend to be less stable on paved surfaces. Touring tyres are usually made of harder rubber for greater durability. They may last longer, but they tend to provide less outright grip than sports tyres at optimal operating temperatures. The payoff is that touring tyres typically offer more grip at lower temperatures, meaning they can be more suitable for riding in cold or winter conditions whereas a sport tyre may never reach the optimal operating temperature.
Sport/performance tyres provide amazing grip but may last 1,000 miles (1,600 km) or less. Cruiser and "sport touring" tyres try to find the best compromise between grip and durability. There is also a type of tyre developed specifically for racing. These tyres offer the highest of levels of grip for cornering. Due to the high temperatures at which these tyres typically operate, use on the street is unsafe because the tyres will typically not reach optimum temperature before a rider arrives, thus providing almost no grip en route. In racing situations, racing tyres would normally be brought up to temperature in advance by the use of tyre warmers.
Brakes
There are generally two independent brakes on a motorcycle, one set on the front wheel and one on the rear. However, some models have "linked brakes" whereby both can be applied at the same time using only one control.
Front brakes are generally much more effective than rear brakes: roughly two thirds of stopping power comes from the front brake—mainly as a result of weight transfer being much more pronounced compared to longer or lower vehicles, due to the motorcycle's relatively short wheelbase. Brakes can either be drum or disc based, with disc brakes being more common on large, modern or more expensive motorcycles for their far superior stopping power, particularly in wet conditions. There are many brake-performance-enhancing aftermarket parts available for most motorcycles, including brake pads of varying compounds and steel-braided brake lines.
In 1981, BMW introduced an antilock braking system (ABS) on a motorcycle. Other manufacturers have since also adopted this technology, although Harley Davidson only offers it on some police motorcycles and not on civilian motorcycles. ABS is normally found on motorcycles of 500 cc or greater engine capacity, although it is available on motor scooters down to 49 cc.
Suspension
Main article: Suspension (motorcycle)
Modern designs have the two wheels of a motorcycle connected to the chassis by a suspension arrangement, however 'chopper' style motorcycles often elect to forgo rear suspension, using a rigid frame.
The front suspension is usually built into the front fork and may consist of telescoping tubes called fork tubes which contain the suspension inside or some multibar linkage that incorporate the suspension externally.
The rear suspension supports the swingarm, which is attached via the swingarm pivot bolt to the frame and holds the axle of the rear wheel. The rear suspension can consist of several shock arrangements:
Dual shocks, which are placed at the far ends of the swingarm
Traditional monoshock, which is placed at the front of the swingarm, above the swingarm pivot bolt
Softail style suspension, where the shock absorbers are mounted horizontally in front of the swingarm, below the swingarm pivot bolt and operate in extension.
Instruments
With dirtbikes excluded, almost all motorcycles have a speedometer, an odometer and a tachometer. Fuel gauges are becoming more common, but traditionally a reserve tank arrangement is used with a petcock (petrol tap) on the side of the motorcycle allowing the rider to switch to a reserve fuel supply when the main fuel supply is exhausted. There is not actually a separate reserve tank: The intake for the petcock has two pipes, one extending higher into the fuel tank than the other. When fuel no longer covers the longer pipe the engine will lose power/splutter and the rider switches the petcock to the "reserve" setting, which accesses the shorter pipe. Riders whose bikes lack a fuel gauge (most machines prior to the past few years) usually learn how far they can go with a full tank of fuel, and then use a trip meter if available to judge when they must refill the tank.
Fuel economy
Motorcycle fuel economy benefits from the relatively small mass of the vehicle, compared to its passengers and to other motor vehicles, and subsequent small engine displacement.
However, poor aerodynamics of exposed passengers and engines designed for goals other than fuel economy can work to reduce these benefits.
Fuel economy varies greatly with engine displacement from a low of 29 mpg U.S. (8.1 L/100km) reported by a Honda VTR1000F rider[8] to 107 mpg U.S. (2.2 L/100km) reported for the Verucci Nitro 50cc Scooter.[9] A specially designed Matzu Matsuzawa Honda XL125 achieved 470 mpg U.S. (0.5 L/100km) "on real highways - in real conditions."[10]
Dynamics
Main article: Bicycle and motorcycle dynamics
Motorcycles must be leaned in order to turn. This lean can be induced by a method known as countersteering, in which the rider turns the handlebars in the direction opposite of the desired direction of turn. In other words, press forward on the handgrip in the desired direction — press right to go right, press left to go left. This concept is counter-intuitive and often very confusing to novices — and even to many experienced motorcyclists.[11]
The primary effect of countersteering is to shift the front tyre contact patch such that a lean is initiated. Other methods allegedly used for turning a motorcycle, such as shifting weight to the side or the pegs, only help leaning the body of the motorcycle towards a direction; thus turning the front wheel in the opposite direction due to the preservation of angular momentum, and eventually resulting in countersteering.
Short wheelbase motorcycles, such as sport bikes, can generate enough torque at the rear wheel and enough stopping force at the front wheel in order to flip longitudinally. These actions, especially if performed on purpose, are known as wheelies and stoppies respectively.
Additions
Various additions may be attached to a motorcycle or come as an integral part of a motorcycle from the factory.
Fairing
A plastic or fibreglass shell, known as a "fairing", is placed over the frame on some models to shield the rider from the wind, aid in aerodynamics and protect engine components in an accident and also from theft. Drag is the major factor that limits motorcycle speed, as it increases at the square of the velocity, with the resultant required power increasing with the cube of velocity. As can be seen from the streamlined appearance of new performance motorcycles, there is much aerodynamic technology included in the design. Unfortunately, since the 1958 FIM ban on "dustbin" fairings no major manufacturer has been bold enough to overcome the effect of the turbulence caused by the spinning front wheel, which prevents the motorcycle from cutting a clean path through the air. The "dustbin" fairing can improve aerodynamic performance without unacceptably compromising the rider's ability to control the machine, although with a weight penalty.
In the absence of a fairing or windshield, a phenomenon known as the windsock effect occurs at speeds above 100 km/h, where the rider becomes a major source of drag and is pushed back from the handlebars, tiring the rider. However, these motorcycles still effectively push their way through the atmosphere with brute force. A cabin cycle, which has a hull that wraps around the basic cycle frame, solved the problem of aerodynamics by isolating driver from outside air.
Modern fairings on touring and sport-touring motorcycles dramatically improve a rider's comfort and attention on long rides by reducing the effect of the wind and rain on the body. They also help keep a rider warm in cold weather, reducing hypothermia. Heated hand grips, and even heated seats, also improve rider comfort in cold weather. Motorcycles from a number of manufacturers now have electric windshields, which raise and lower the windshield with the push of a button to the optimum height for conditions.
Heated hand grips/seats
As motorcycles lack climate control or proper protection from the wind, some manufacturers offer heated seats or hand grips to relieve the discomfort of wind chill during night riding or the colder months. They can also be added on as after market accessories and are powered by the bike's battery.
Luggage rack
A common addition to many bikes is an attachment onto which bags or other luggage can be fastened. This removes the need for rider backpacks and is generally a more secure and safe way to add carrying capacity to a motorcycle.
Sidecar
A sidecar is a one-wheeled device attached to the side of a motorcycle, producing a three-wheeled vehicle. Early sidecars were intended to be removable devices that could be detached from the motorcycle. Sidecars gradually superseded forecars and trailers. The forecar comprised a two-wheeled attachment attached to the front of the motorcycle; the trailer was just that, leaving the passenger to be hauled along behind the vehicle. In neither case could rider and passenger converse easily – this may be why early sidecars were often called 'sociable' attachments.
Trailer hitch
A trailer hitch or tow hitch is a device mounted on a motorcycle that enables it to tow a motorcycle trailer, usually in order to haul additional gear.
Trunk
A motorcycle trunk is a storage compartment in the vicinity of the seat, other than panniers or saddlebags. A trunk mounted above and to the rear of the seat is also called a top box.