An underbone motorcycle 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.
Underbone may also refer to a class of lightweight motorcycles that use the construction type, known colloquially as step-throughs, mopeds or scooters.
An underbone cycle may share its fuel tank position and tube framing, along with fitted bodywork and splash guards with a scooter while the wheel size, engine position, and power transmission are like those of conventional motorcycles.
Underbones are popular worldwide, especially in the developing world.
The appearance and frame of an underbone is 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 under the riders seat. 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 available.
Historically, underbones led the way in semi-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 can have spoked or alloy wheels which are spindle mounted, usually the size fitted to small conventional motorcycles. These provide much better road-holding and braking than scooters, though it does make the (increasingly rare) punctures more difficult to repair.
Unlike majority of scooters, underbones generally features a manual gearbox, either with a standard clutch (like any conventional motorcycles) or an automatic clutch (semi-automatic). This is then coupled to a chain drive to direct power to the rear wheel. There are some exceptions to this, such as the Yamaha Nouvo, the Piaggio Liberty and the Yamaha Lexam, which although of underbone design, has a CVT transmission and therefore has no gears for the rider to change.
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 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
Other names and derivations
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" (from the Indonesian word for duck). In Greece is known as Papi or Papaki, a slang that 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.
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 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:
- China: Haojue, Zongshen, Lifan, LUOJIA
- Malaysia: Modenas, Momos Motor (rebadge of Cagiva), Naza Bikers (rebadge of Zongshen and ZXMCO) and Demak.
- Germany: MZ Motorrad
- India: TVS Motors (underbone versions are available in Indonesia and the Philippines)
- Indonesia: Kanzen, Suzuki, Yamaha, Honda, Binter rebadge of Kawasaki
- Italy: Cagiva, Piaggio
- Japan: Honda, Yamaha, Suzuki, Kawasaki
- Philippines MCX Motor (Phils.), Blaze Motortech, RACAL, Sunriser
- Taiwan Kymco, SYM Motors
- Thailand: Tiger Motor Co. Ltd
Underbones are very popular in Southeast 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, 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.
An underbone motorcycle 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.