The chassis of a motorcycle includes the frame and suspension, along with the front forks, of the vehicle.
Main article: Motorcycle frame
The frame is typically made from welded aluminium or steel (or alloy) struts, with the rear suspension being an integral component in the design. Carbon fibre and titanium are used in a few very expensive custom frames.
The frame 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 motorcycle 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 1950s, 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 used on some “thumpers” (single-cylinder four-strokes) that usually have dry-sump lubrication requiring an external oil tank. It has since gained some cachet in the modern custom bike world too because of the space savings it can afford and the reference to an earlier era.
Buell motorcycles employ a similar design the oil is held in the swingarm, while the fuel is held in the frame.
Main article: Suspension (motorcycle)
Plunger design suspensions, as on this BSA Bantam, were superseded by the swinging arm
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.
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.
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).
Opposed twin engine on a Ural
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 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, though two firms Megola and Redrup put radial engined motorcycles into production.
A Sok 1000 V-twin engine
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 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, becausen addition to the reasons aboveodifying them to meet contemporary emissions standards is prohibitively expensive. Almost all modern two-strokes are single-cylinder, liquid-cooled, and under 600 cc.
In November 2006, the Dutch company E.V.A. Products BV Holland announced that its diesel-powered motorcycle, the Track T-800CDI, achieved production status. The Track T-800CDI uses a 800 cc three-cylinder Daimler Chrysler diesel engine. Other manufacturers, including Royal Enfield, had been producing diesel-powered bikes since at least the 1980s. Also, Intelligent Energy, a British alternative-fuel company, is developing a motorcycle powered by a detachable hydrogen-powered fuel cell, which it calls an Emissions Neutral Vehicle (ENV). According to reports, the vehicle can sustain speeds of 50 mph (80 km/h) while making virtually no noise, and can run for up to four hours without refueling.
The transmission on this 1921 ABC motorcycle is located behind the engine and shifts by a long hand-operated lever on its right side.
A typical 5-gear, foot-shift transmission on an HD Sportster
Modern motorcycles (excepting scooters) all change gears (of which they will increasingly have five or six) by foot lever. The weight of the largest touring motorcycles (sometimes in excess of 360 kg or 800 lbs) is such that they cannot effectively be pushed backwards by a seated rider, and they are fitted with a reverse gear as standard. In some cases, including the Honda Gold Wing and BMW K1200LT, this is not really a reverse gear, but a feature of the starter motor which when reversed, performs the same function. In earlier times pre WWII, hand-operated gear changes were common, a lever was provided to the side the fuel tank. British and many other motorcycles after WWII used a lever on the right but today gear-changing is standardised on a foot-operated lever to the left.
All two-wheelers use a sequential gearbox. On a motorcycle either first or second gear can be selected from neutral, 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 would be known as “one down, four up” because of the placement of the gears with relation to neutral. Neutral is to be found “half a click” away from first and second gears, so shifting directly between the two gears can be made in a single movement.
Traditional scooters still have manual gear-changing by a twist grip on the left hand side of the handlebar, with a clutch on a lever also rotating. Increasingly they may be fitted with a continuously variable transmission or CVT instead, a kind of automatic transmission (as once used in the DAF car) that is stepless. Step-through motorcycles often have a three-speed foot change, but the clutch is automatic.
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.
A shaft final drive is housed within a rear swingarm of a BMW R1200GS
Power transfer from the gearbox to the rear wheel is accomplished by different methods.
Chain drive uses sprockets and a roller 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. In a chain drive the power is transmitted into the real wheel via a cush drive. Virtually all high performance racing motorcycles use chain drive.
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 added weight. A shaft-equipped motorcycle may also be susceptible to shaft effect.
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 used spoke wheels built up from separate components, 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.
A wire wheel and pneumatic motorcycle tyre on a Ural
Wire 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.
Main article: Motorcycle tyres
Motorcycles mainly use pneumatic tires. 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. Because of 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.
Front disk brakes with an ABS sensor ring on a BMW R1200RT
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 brakeainly as a result of weight transfer being much more pronounced compared to longer or lower vehicles, because of the motorcycle’s short wheelbase relative to its center of mass height. This can result in brake dive.
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.
Most road motorcycles have an instrument panel, usually consisting of speedometer, odometer and 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.
Bicycle and motorcycle dynamics
^ “Chassis”. motorcycle-glossary.com. http://motorcycle-glossary.com/chassis/36. Retrieved 2007-07-03.
^ Martin Peacock (3 April 2006). “Oil In Frame Triumphs – A Potted History”. The Vintage Motorcycle Club, South Durham Section. http://www.vmccsouthdurham.co.uk/content/view/22/51/. Retrieved 2007-05-16.
^ “The first commercially-available diesel motorcycle”. Gizmag. November 20, 2006. http://www.gizmag.com/go/6493/. Retrieved 2007-06-28.
^ “Diesel motorbikes”. Journey to Forever. http://journeytoforever.org/biodiesel_bikes.html. Retrieved 2007-06-28.
^ Earth Talk (July 1, 2007). “How Much Pollution do Motorcycles Generate?”. Environment, Health News Digest. http://www.healthnewsdigest.com/news/Environment_380/How_Much_Pollution_do_Motorcycles_Generate.shtml. Retrieved 2007-07-02.
How Motorcycles Work by Bill Harris
Motorcycle Glossary – definitions of motorcycle terms
Categories: Motorcycle technology | Motorcycle safety