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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. ==Design== 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Underbone」の詳細全文を読む スポンサード リンク
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