An underbone is a small motorcycle built on a frame consisting mostly of one large diameter tubular component.
The stylistic difference between a motorcycle and an underbone is considerable and they are sometimes known as "step-throughs" or even marketed as scooters in the West.
The fuel tank position and the open-frame design, along with the splash guards often fitted, give the underbone an appearance similar to a scooter, however the wheels, engine position, and power transmission are based on the design of conventional motorcycles.
Underbones are popular worldwide, but in East Asian and South East Asian countries in particular they dominate the low cost market segment with their scooter-like ease of use, their appeal to both sexes, and their motorcycle-like dependable handling properties.
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The appearance and frame of an underbone is quite different to that of a motorcycle, but the powertrain is functionally almost identical. The underbone engine is positioned between the rider's feet (perhaps a bit lower than that of a motorcycle) 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. The underbone arrangement improves 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 size of a typical underbone is generally between 50 cc and up to about 150 cc, though much bigger ones are possible.
Historically, underbones led the way in automatic transmissions and indicator systems. In other respects, the technical sophistication of underbones has tended to lag those of larger motorcycles, but they increasingly have similar electronic ignition, fuel injection etc.
Underbones have spoked wheels which are spindle mounted, similar to those of conventional motorcycles - this provides much better road-holding and braking than scooters, though it does make the (increasingly rare) punctures more difficult to repair.
The underbone concept can be seen in some of the European mopeds of the early 1950s, including the NSU Quickly and the Heinkel Perle. The spine-framed, plastic-faired Honda 50 is the best-selling powered vehicle of all time. Production of the Super Cub began in 1958 and was still in production in April 2008 when the 60 millionth was built.[1]
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.). Therefore,"Kapcai" 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"[2] (from the Indonesian word for duck).
A variation on the underbone concept known as the "maxi-scooter"[3] or "touring scooter"[4] 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[5] to the 850 cc Gilera GP800.[6] Many current versions are between 400 cc and 650 cc, including the Honda Silver Wing with 582 cc[7], the Suzuki Burgman with 400 cc[8] or 638 cc[9]), the Yamaha Majesty 400 with 395 cc[10], and the Yamaha T-Max 500 with 499 cc.[11]
Underbones of conventional size are popular in Western Europe and marketed alongside conventional scooters. They are sometimes referred to as scooters,[12] despite the design difference between underbones and conventional scooters.
Underbone motorcycles often come with storage, and this may vary between markets, eg in Southeast Asia there is commonly a steel basket provided. Some underbones have lockable storage compartment under the seat. Some have a hook in the area between the riders knees for a shopping bag. Other 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 share holder, Hong Leong Group which is also the sole distributor of Yamaha motorcycle in Malaysia and Singapore.
The major underbone manufacturers are as follows:-
Underbones are very popular in South East Asia. There is a demand for aftermarket and tuner parts. Many enthusiasts modify their underbones either for 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 and Malaysia, 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 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.
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