A motorcycle is a two-wheeled motorized vehicle, powered by an engine. The wheels are in-line, and the motorcyle remains upright when in motion by virtue of gyroscopic forces. The rider sits astride the vehicle on a seat, with hands on a set of handlebars which are used to steer the motorcycle, and feet on a set of "footpegs" or "pegs" which stick out from the chassis.
Variations of this definition do exist: some motorcycles are equipped with footboards instead of footpegs, and sidecars and other three-wheeled variations may also be found.
A 125cc motorcycle, the Italian-manufactured Cagiva Planet.|
The motorcycle's steering is controlled by the handlebars and the rider's positioning. At speed, the gyroscopic forces cause a phenomenon known as "counter-steer" to occur, where (for instance) pushing on the left handlebar and pulling on the right will cause the bike to lean to the left, and then execute a left hand turn. Turning can similarly be executed by changing body position, which causes the bike to lean and then turn. At speeds less than approximately 10km/h turning works in a fashion similar to a car or other two-track vehicle, with the motorcycle following the direction of the front wheel.
Gyroscopic precession of the front wheel causes both counter-steer, and steering by leaning. The turning wheel rotates the effect of a force applied to the wheel by ninety degrees. So, counter-steer happens because pressing on the left handlebar applies a rightward force on the front of the wheel, and a leftward force on the back of the wheel. The wheel's motion is rotated ninety degrees: The top of the wheel (ninety degrees from the back) moves left, leaning the bike. Leaning the bike left causes the front of the wheel (ninety degrees from the top) to steer to the left.
Almost all motorcycles have a speedometer[?] and odometer and many have a tachometer. Fuel gauges are becoming more common, however traditionally a reserve tank arrangement has been used with a tap on the side of the motorcycle allowing the rider to switch to a reserve fuel supply when the main fuel supply is exhausted; this is typically done while the vehicle is in motion.
Some motorcycles include the engine as a load bearing member, however this is not common.
The gas tank is usually mounted above the engine, so no fuel pump is needed. The gas tank is made of either stamped, brazed sheet steel, or blow-molded high-density polyethylene. The wheel rims are usually steel, with steel spokes and an aluminum hub.
A fairing is often placed over the frame, to shield the rider from the wind and decrease drag. Drag is the major factor limiting motorcycle speed as it increases aa the cube of the velocity. 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.
The two wheels of a motorcycle are connected to the chassis by a suspension arrangement. The front suspension generally consists of oil shock absorbers[?], however a variety of arrangements are used on the rear. The wheels use pneumatic tires[?], generally characterised by a rounded surface, to ensure good traction while leaning as described above. Correct tire pressure and correct adjustment of suspension are essential to safe cornering, far more so than in a four wheeled vehicle, as any loss of grip will probably lead to loss of control of the motorcycle.
The front fork is the most critical part of a motorcycle. The angle of rake determines how controllable the steering is. The rake should be chosen so that precessive force from countersteer and leaning-steering slightly overbalance the leaning forces from the weight of the bike, at a speed near the running speed of a person. This is the speed at which feet can no longer be safely used to balance a bike. Another problem is that over-rake can cause a rim to dent, and begin a wobble in the front wheel, which usually causes an unrecoverable loss of control.
There are generally two independent brakes on a motorcycle, one on the front wheel, controlled by the right hand lever, and one on the rear controlled by the right foot. The front brake is generally much more powerful than the rear as the majority of stopping power comes from the front brake; rear wheels will generally lock and skid much more easily than the front. Brakes can either be drum or disc based, with disc brakes being more common on large, modern or expensive motorcycles for their far superior stopping power, particularly in wet conditions. Some manufacturers are creating motorcycles with ABS; others are creating linked brakes which actuate both rear and front brakes (althogh perhaps with different strengths) when either lever is depressed.
The motor of a motorcycle typically sits immediately under the rider's seat, between the legs. Almost all commercially available motorcycles are piston driven internal combustion engines, with typical sizes between 50cc and 1500cc. Larger motorcycles (above 500cc) on the modern market are mostly four stroke engines, but there is a sizable minority of two stroke engines on smaller motorcycles.
Two stroke engines have almost twice as much power per CC of displacement as four-cycles, because they generate power on each stroke. This means that two strokes are also lighter, for the same power. Fuel-injected two-strokes even get good fuel-economy and comparably low emissions. Most two strokes inject special combustible oil into the gasoline to keep the cylinders lubricated. In California, two-strokes are generally illegal because of their poor emissions.
Fuel injection is widely available on commercially available motorcycles, but carbureators are more common. Computer-controlled engines are rare.
Two and four cylinder engines are the most common available; single cylinder engines are common on off-road bikes and small scooters. There are commercially available three cylinder designs, and even a few five and six cylinder and V8 models. Two cylinder engines are most commonly found in either a "V-twin" configuration or a "parallel-twin" configuration. Most four-cylinder engines are in-line rather than v-shaped and arranged transversely, that is, the crankshaft is at a 90 degree angle to the frame. Both water-cooled and air-cooled engines are common.
Most motorcycle engines have simpler auxiliary devices than car engines. Most notably, the ignition system and battery charging is often provided by a magneto, rather than an engine. A magneto has a special generator with a large number of turns on its coil. The gnerator directly produces the spark. Usually a secondary coil produces electricity to charge a battery. The battery charging coil's current is not as steady as a car's generator, which is why motorcycles with defective batteries have flashing headlights.
The motor is controlled by a clutch lever under the left hand in standard configurations, a throttle on the right handlebar (where pushing the wrist down increases fuel to the engine and so causes the bike to accelerate) and a gear lever on the left foot. The gear lever typically operates by downshifting when the lever is depressed, and upshifting when the lever is lifted; neutral sits above first gear and below second, so a small lift out of first causes the gearbox to change into neutral, but a large movement causes the gearbox to change into second gear. Modern motorcycles normally have five or six forward gears. Only the largest touring motorcycles and a few archaic models that are routinely used with a sidecar are fitted with a reverse gear.
The clutch is typically an arrangement of plates stacked in alternating fashion, one geared on the inside to the engine, and 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 buildup between the plates until they rotate as a single unit, driving the transmission directly. A lever on the handlebar, through a cable or hydraulic arrangment, uses mechanical advantage 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, you may select either first or second gear, but higher gears may only be accessed in order - you may not shift from second gear to fourth gear, without shifting through third gear. Internally, a rotating cam on the shift lever operates dogs on two counter-rotating shafts carrying a variety of gears. One shaft is geared to the final drive mechanism, and the other to the clutch. Operating the shift lever slides individual gears on one shaft, to intersect with a matching gear on the other. The small mass of the whole arrangement allows for extremely quick gear changes. Also, gear synchronizers typically found in passenger cars with manual transmissions are not necessary. The two shafts are always geared together (except in neutral), always spinning at a speed nearly approximating the next higher or lower gear ratio. Aided by beveled edges on the gears, shifting gears is simple for novices - no double clutching or grinding of gears. Advanced drivers can perform "full-throttle upshifts" on racing mounts, but this risks both the warranty and mechanical integrity.
Final drive from the gearbox to the rear wheel is typically accomplished with a chain, which requires both lubrication and adjustment for stretch. The lubricant is subject to being thrown off the fast-moving chain and results in grime and dirt buildup. 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. The belt is frequently toothed. A shaft drive is typically completely enclosed, the visual cue a tube extending from the rear of the transmission to a bell-housing on the rear wheel. Inside the bell housing a beveled gear on the shaft mates with another on the wheel mount, typically floating in oil in a sealed compartment. This arrangement is superior in terms of noise, cleanliness, and is virtually maintenance free. However, the additional gearsets tend to rob the system of power.
There are several ways to increase speed.
The simplest way is to pick a bike that already does what you want. It may cost a little more, but that's the price of being a hobbyist. By using factory equipment throughout, you can get factory service, which is sometimes a great time-saver.
The easiest performance improvement is to reduce the air-resistance. Adding low-drag wheels, or buying an extended fairing will often gain far more speed, for less expense and trouble than any other type of work.
The next-largest cause of loss of speed is rolling resistance. The right tires kept at the proper pressure will contribute quite a bit to both speed and safety. This is also often a better bet than engine rework. Of course, make sure you don't have a dragging brake!
On the engine, one can keep the air filter and chain clean, use high-quality lubricants and fuel with precisely-tuned spark plugs, mixture and timing. This is obvious, but often neglected.
Try the tips above first before engine modifications. None of them will mess up the bike's basic design, so they're pretty safe and sane. Remember, the factory engineers aren't stupid, just on a budget.
More improvement can be had by precision-balancing the turning parts, slightly loosening the piston/cylinder-wall gap (within tolerance!) to reduce friction, and polishing the intake and exit ports and manifolds, which are usually rough castings that slow the flow of air and fuel. Changing sizes of ports and valves will probably reduce performance unless done as part of a redesign by trained engineers.
Another way to increase performace is to have a tuned exhaust system. This helps evacuate the engine rapidly, and permits a longer power-stroke. However, be wary! Many production bikes already have tuned exhausts! A "custom" tuned exhaust will often operate only at a narrower range of engine RPM-- great for racing, but literally a drag on the street.
With great care, an engine can be helped to "sprint" by injecting small amounts of nitromethane. Nitromethane adds power because it doesn't need air to burn. Engines using large amounts need precise mixtures, or configurable timing and carburation. It's very easy to blow the gaskets or burn the valves of your expensive, custom-tuned engine with careless nitromethane injection.
Road motorcycles are motorcycles designed for being ridden on the road. They feature smooth tires, and engines generally in the 250cc and over range. Most are capable of speeds in excess of 100 km/h, and many of speeds in excess of 160km/h.
Road motorcycles are themselves broken down into several sub-categories.
These motorcycles mimic the style of American machines from the 1930s to the early 1960s, such as those made by Harley-Davidson, Indian, Excelsior and Henderson, even though they have benefited from advances in metallurgy and design. The riding position places the feet forward and the hands up, with the spine erect or leaning back slightly. Cruisers are less suitable for high-speed riding, and are often used to signal adherence to an alternative lifestyle, the most extreme form of which is found in motorcycle gangs such as the Hell's Angels. Choppers are extreme cruiser configurations where the handlebars rise to a level above the riders shoulders with very long forks. They are notable for their extreme looks and equally extreme handling characteristics.
These motorcycles are visually similar to machines used in motorcycle racing and are generally only a few years behind the real racing machines. The riding position places the feet towards the back, the hands low and the spine inclined forward.
Touring motorcycles are characterised by wind protection for the rider (in the form of a fairing or windscreen) and the ability to carry some amount of luggage (usually in the form of panniers and/or a topbox mounted towards the rear of the motorcycle). Although any motorcycle can be so equipped and used to tour with, specialised touring motorcycles such as the Honda Goldwing have become increasingly popular. Sport tourers are a hybrid form between sporting bikes and tourers and allow long-distance riding at higher speeds - the first example of this type of motorcycle was the BMW R100RS. Another hybrid is the custom tourer, which combines cruiser and tourer characteristics - the original form of this type is the Harley-Davidson Electra-Glide.
Also known as the "naked" bike, this is the basic form of the motorcycle. No longer as popular as in earlier days, when this was the only form of motorcycle commercially available. This style of motorcycle is however seeing a resurgence as at the end of 2000, with many manufacurers releasing new models with minimal fairings.
Scooters are similar to motorcycles and are also designed for being ridden on the road. They are characterized by small wheels, small (generally less than 125cc) engines, and a step-through configuration allowing the rider to ride with both feet on a running-board and knees together.
The moped is a hybrid between the bicycle and the motorcycle, being equipped with an engine (usually a small two-stroke engine, but occasionally an electric motor) and a bicycle drivetrain, and motive power can be supplied by the engine, the rider, or both.
Off-road motorcycles are motorcycles designed for being ridden in rougher terrain. They are also known as "dirt bikes" and "trail bikes". An off-road motorcycle will typically have suspension with more travel than a road bike, higher ground clearance and hence a higher centre of gravity, and a small (less than 500cc) single cylinder motor.
Competitive dirt bikes are optimized for speed trials, enduro (long distance racing), hill climbing, or timed-trials. Some authorities think that the competition that best reflects real-life needs is the timed trials, because they require a balance of maneuverability, speed, light weight and reliability.