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An underbone is a type of motorcycle that uses structural tube framing with an overlay of plastic or non-structural body panels and contrasts with monocoque or unibody designs where pressed steel serves both as the vehicle's structure and bodywork. Outside Asia, the term underbone is commonly misunderstood to refer to any lightweight motorcycle that uses the construction type, known colloquially as step-throughs, mopeds[ citation needed ] or scooters (see Scooter (motorcycle)).
An underbone motorcycle may share its fuel tank position and tube framing, along with fitted bodywork and splash guards with a scooter while the wheel dimensions, engine layouts, and power transmission are similar with conventional motorcycles.
Unlike conventional motorcycles, underbones are mostly popular in Asia and Greece. In Indonesia, the fourth most populous country in the world, and the largest country in Southeast Asia, almost half the population have a motorcycle, most of which are underbones and scooters. (120 million in 2018, compared to 16 million cars  ).
The appearance and frame of an underbone are quite different from that of a standard motorcycle, but the powertrain is functionally almost identical. The underbone engine is positioned between the rider's feet but the rear wheel is driven by a regular motorcycle secondary chain drive. Styling considerations only mean that the chain-driven nature of the machine tends to be concealed under sheet-metal covers to a greater extent than that of motorcycles.
In only one significant respect does the underbone differ from the conventional motorcycle - there is no frame member or fuel tank between the seat base and the headstock. Generally, the fuel tank is located under the seat. The underbone arrangement improves the ease of mounting and dismounting and contributes to the cross-gender appeal of the layout as an around-town shopping and commuter vehicle. The engine displacement of a typical underbone motorcycle is generally between 50 cc and up to about 150 cc, though much larger ones are available. Currently, production underbone model with the largest engine displacement is the SYM VF3i, which is powered by a fuel-injected 183 cc SOHC 4-valve water-cooled engine. 
Historically, underbones popularized semi-automatic transmissions and indicator systems. In other respects, the technical sophistication of underbones tended to lag those of larger motorcycles, but nowadays they are increasingly equipped with similar electronic ignition, fuel injection, etc.
Underbones may have spoked or alloy wheels that are spindle mounted and usually sized to fit small conventional motorcycle frames. These provide much better road-holding and braking than scooters, though it does make repairs (such as punctures) more difficult to repair.
Underbones generally features a three- to six-speed sequential gearbox, either with an automatic clutch, usually a centrifugal clutch, or a conventional manual hand clutch. Depending on whether the motorcycle has a manual hand clutch or an automatic centrifugal clutch, determines whether the motorcycle transmission is a semi-automatic (automatic clutch) or a manual transmission. It is then coupled to a sprocket and chain drive to transmit power to the rear wheel. There are some exceptions to this, such as the Yamaha Nouvo, the Piaggio Liberty, and the Yamaha Lexam. Although the similar design of an underbone motorcycle, it has a CVT transmission and therefore has no gears for the rider to select and may be considered as scooters.
The underbone concept can be seen in some of the European mopeds of the early 1950s, including the NSU Quickly and the Heinkel Perle.[ citation needed ] The spine-framed, plastic-faired Honda Super Cub is the most produced motor vehicle of all time.   Production of the Super Cub began in 1958, surpassing 60 million units in April 2008, and continuing to be made in several countries around the world as of 2012 [update] .    
In Malaysia and Singapore this vehicle is commonly known as Kapcai or Kapchai, a slang word derived from Cantonese, being a combination of the word "Cub" from the word Honda Cub and "仔" in Chinese. In Cantonese, "仔" (pronounced "jai", or in pinyin "zai") means "little" (or its derivatives, e.g. "small", "mini", etc.)[ citation needed ]. Therefore,"kapchai" literally means a "Little Cub". With Honda being a very popular brand in Malaysia, all underbone motorbikes have come to be called "kapchai". In Indonesia, it is called "motor bebek"  (literally means "duck bike"). In Greece is known as Papi or Papaki, a slang that also means duck.
A variation on the underbone concept known as the "maxi-scooter"  or "touring scooter"  is popular in the West. These are much larger than the underbones known to the Asian market and vary in size from the early Honda Helix with 250 cc  to the 850 cc Gilera GP800.  Many current versions are between 400 cc and 650 cc, including the Honda Silver Wing with 582 cc,  the Suzuki Burgman with 400 cc  or 638 cc,  the Yamaha Majesty 400 with 395 cc,  and the Yamaha T-Max 500 with 499 cc. 
Underbones of conventional size are popular in Western Europe and marketed alongside conventional scooters. They are sometimes referred to as scooters,  despite the design difference between underbones and conventional scooters.
In Southeast Asia Some manual underbone bikes are designed by cover body without side wings, full-length front shock, and sport bike look headlamp. It look more sporty and better performance than the cub underbones. For example Suzuki Raider R150, Honda Nova series, Suzuki Akira/Stinger, Cagiva Stella, and many more. In Indonesia, this type of bike are called "ayago" (ayam jago means rooster)
Underbone motorcycles often come with storage, and this may vary between markets. In Southeast Asia, there is commonly a steel basket provided as there is none under the seat, as this is the placement of the fuel tank. Some underbones have a lockable storage compartment under the seat. Some have a hook in the area between the rider's knees for a shopping bag. Another storage capacity may be provided in a top box, detachable in some cases.
The market for underbone motorcycles is dominated by Japanese manufacturers, though many of them are built in factories elsewhere, including China and Taiwan. In other cases, manufacturers have violated copyright and illegally copied the models of Japanese manufacturers. The Honda Cub, Honda Wave series, and Yamaha Lagenda series are amongst the most copied.
MZ Motorrad currently produces underbone models in Malaysia by their main shareholder, Hong Leong Group which is also the sole distributor of Yamaha motorcycle in Malaysia and Singapore. Mforce Bikes Holdings Sdn Bhd also producing an underbone motorcycle under Benelli Motorcycle.
The major underbone manufacturers are as follows:
Underbones are very popular in Southeast Asia and Greece. There is a demand for aftermarket and tuner parts. Many enthusiasts modify their underbones either for a show (such as installing small sound systems, neon lights and custom paint jobs) or for performance (like increasing the engine power and fine-tuning the suspension). Riding gear may not even extend to wearing shoes and long pants for safety. Illegal underbone drag racing has become popular in countries such as the Philippines, Malaysia and Greece, and poses safety issues for the commuting public as well as the riders themselves, as underbones offer little protection in the event of crashes. The most popular underbone for these purposes is the Honda XRM, Suzuki Raider 150 and the Honda Wave, although similar models from Kawasaki and Yamaha are also frequently used.
In Singapore and Malaysia, it is not uncommon to see old bicycles customized with parts found on an underbone motorcycle, a growing trend popular with the youth.
Underbone motorcycle racing are one of the most popular motorcycle racing class in the Southeastern Asia region, which uses a modified version of road-legal, production-based underbone motorcycle available for purchase in public, mostly manufactured by Yamaha, Honda, and Suzuki. Underbone racing popularity is mostly influenced by their wide variety of racing classes, race track limitations, and fairly cheaper cost of parts and development compared to sport motorcycle classes. Underbone racing class may be further categorized into the engine cycle, engine displacement, engine layout, fuel delivery systems, allowable modifications and restrictions, time brackets (in drag racing), and the rider's skill level. These categories are then regulated by the country's motorcycle racing governing body, from which national racing series may be organized. One of the most popular categories is the 150cc four-stroke fuel-injected class and may have engine power output ranging from 24 to 32 horsepower and can reach up to 180 km/h. Most of the motorcycle competing in this class are Yamaha T-150, Honda RS150R, and Suzuki Raider 150.
Notable underbone road racing series in Southeast Asia countries are Malaysian Cub Prix and Indonesian Oneprix and MotoPrix Series. The highest level for underbone road racing class is held by Asia Road Racing Championship under UB150 class, from which only production underbone motorcycle with a maximum of 150cc engine displacement may compete in this class, hence the name. Only Yamaha and Honda motorcycles compete in this class with teams from various Asian countries.
Underbone racing series in lower divisions in each country may also be organized to encourage new teams and riders to develop and prepare for the national racing series. In Malaysia, the underbone racing series was created in partnership with the government of Malaysia's Ministry of Youth and Culture in 2012 to encourage youth to race in a controlled track environment rather than on the streets.  It was designed to have a lower cost of entry than existing underbone series, such as the Malaysian Cub Prix.
Motorenwerke Zschopau GmbH is a German motorcycle manufacturer located in Zschopau, Saxony. The acronym MZ since 1956 stands for Motorenwerke Zschopau GmbH. From 1992 to 1999 the company was called MuZ, an acronym for Motorrad und Zweiradwerk.
Yamaha Motor Co., Ltd. is a Japanese manufacturer of motorcycles, marine products such as boats and outboard motors, and other motorized products. The company was established in 1955 upon separation from Yamaha Corporation, and is headquartered in Iwata, Shizuoka, Japan. The company conducts development, production and marketing operations through 109 consolidated subsidiaries as of 2012.
A semi-automatic transmission is a multiple-speed transmission where part of its operation is automated, but the driver's input is still required to launch the vehicle from a standstill and to manually change gears. Most semi-automatic transmissions used in cars and motorcycles are based on conventional manual transmissions or sequential manual transmissions, but use an automatic clutch system. However, some semi-automatic transmissions have also been based on standard hydraulic automatic transmissions with torque converters and planetary gearsets.
The Suzuki GSX-R is a series of sport bikes made by Japanese manufacturer Suzuki. Current models are the GSX-R125 and GSX-R150 since 2017; GSX-R600 which was manufactured from 1992 to 1993, and then since 1997; the GSX-R750 since 1985; and the GSX-R1000 since 2001.
Syarikat Motosikal dan Enjin Nasional Sdn. Bhd, or known as Modenas for short is a Malaysian national motorcycle company producing various small motorcycle models below 400cc targeted for local market and export. The company's headquarters and factory are located at the small town of Gurun, Kedah, Malaysia.
The Honda Super Cub or Honda Cub is a Honda underbone motorcycle with a four-stroke single-cylinder engine ranging in displacement from 49 to 124 cc.
A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.
The six main types of motorcycles are generally recognized as standard, cruiser, touring, sports, off-road, and dual-purpose. Sport touring is sometimes recognized as a seventh category or integrated with the touring category.
The Malaysian Cub Prix Championship is a national-level underbone or moped racing series for motorcycles with displacements from 100 to 150cc. The tournament is a grassroots developer with the main objective of discovering and developing Malaysian motorcycle racing talents.
The Yamaha T135 is an underbone manufactured by Yamaha Motor Company since 2005. It is known as the Spark 135/135i in Thailand, Sniper/MX 135 in the Philippines, Jupiter MX 135 LC in Indonesia, 135LC in Malaysia, Exciter 135 in Vietnam, and Crypton X 135 in Greece. It is powered by a 134.4 cc (8.20 cu in) single-cylinder engine.
The Yamaha Nouvo is a CVT underbone manufactured by Yamaha Motor. It was introduced in April 2002 for Southeast Asia markets and In 2004, Brazil, where it was renamed the Yamaha Neo. Nouvo is from the French word Nouveau, which means new or fashionable.
Yamaha Lagenda is a series of underbone motorcycle produced by Yamaha for the Southeast Asian market. It was introduced for the 2000 model year replacing the SRX 100. A more performance-oriented version of the Lagenda is marketed as ZR.
Doni Tata Pradita is an Indonesian motorcycle racer who raced in the 2013 Moto2 World Championship. In 2008, he raced for Yamaha Pertamina Indonesia Team in the 2008 250cc Grand Prix World Championship. Doni is the first Indonesian rider ever to participate in a 250cc Grand Prix World Championship race & remains the only Indonesian rider to score points in intermediate class so far.
A scooter is a motorcycle with an underbone or step-through frame and a platform for the rider's feet, emphasizing comfort and fuel economy. Elements of scooter design were present in some of the earliest motorcycles, and scooters have been made since at least 1914. Scooter development continued in Europe and the United States between the World Wars.
A motorcycle transmission is a transmission created specifically for motorcycle applications. They may also be found in use on other light vehicles such as motor tricycles and quadbikes, go-karts, offroad buggies, auto rickshaws, mowers, and other utility vehicles, microcars, and even some superlight racing cars.
Thai Honda Manufacturing Co., Ltd. is the official manufacturer and distributor of Honda motorcycles in Thailand. Its headquarters are in Samut Prakan. Honda Motorcycle Thailand plays a significant role as a center in distributing Honda motorcycles to Asian and European markets.
The Honda Winner is an underbone motorcycle from the Japanese manufacturer Honda. It was launched in April 2016 in Vietnam. It was also launched in May 2016 in Indonesia as the Supra GTR. In June 2016, the bike was launched in Malaysia as the RS150R. The engine is shared with the 2015 Sonic 150R, 2015 CB150R (StreetFire), and 2016 CBR150R. With the engine producing 12 kW, it makes the Winner as the fastest and most powerful 4-stroke underbone model ever offered by Honda, along with the Sonic.
The FIM Asia Road Racing Championship is the regional motorcycle road racing championship for Asia, held since 1996.
The Cagiva C593 was a racing motorcycle made by Cagiva, which was used in the 500cc class of Grand Prix motorcycle racing during the 1993 season. The name is formed by an amalgamation of words and letters, namely the "C", "5" and "93". The "C" stands for the company (Cagiva), the "5" stands for the class the company races in as well as the engine capacity (500) and the "93" stands for the season the bike raced in (1993). The bike replaced the C592 model used in 1992 and was replaced by Cagiva's last model, the C594.