低床バックボーン式

thumb|upright=1.35|Plastic panels and covers are widely used on modern underbones e.g. Modenas Kriss series|Modenas Kriss 2. thumb|MCX Raptor, an underbone from the Philippines. thumb|Honda Winner, an underbone with 150 cc engine displacement. An underbone is a type of 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 use