Underbone

An underbone is a 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 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.

Unlike motor scooters, underbones are mostly popular in Asia, particularly in Southeast Asia, Middle East, India, Taiwan and China, as well as some parts of Eastern Europe (Greece) and Russia.

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.

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Wikipedia