Underbone

Plastic panels and covers are widely used on modern underbones eg Modenas Kriss 2.

MCX Raptor a motard type underbone

An underbone (or step-through) 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 often 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 wheel size, 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.

Design

The origin of underbone design: the Honda Super Cub

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 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.

Transmission

Generally underbones feature a manual (foot gearchange with hand operated clutch) or a semi-auto manual (foot gearchange only) transmission. This is then coupled to a chain drive to direct power to the rear wheel. There are some exceptions to this, such as the Piaggio Liberty 150 which although of underbone design, has a CVT transmission and therefore has no chain or gears for the rider to change.

Origins

The Heinkel Perle was introduced in 1954, four years before the Honda Super Cub

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 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.

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).

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.

Storage on underbones

Yamaha Nouvo's helmet storage compartment.

Underbone motorcycles often come with storage, and this may vary between markets, eg 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 do 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.

Underbone manufacturers

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 other China brand.
• 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, RUSI, Sunriser
• Taiwan Kymco, SYM Motors
• Thailand: Tiger Motor Co. Ltd

Underbone culture

Underbones parked in Hanoi

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.

See also

• Loh Boon Siew