Motor Scooters - Underbone

Underbone

An underbone is a motorcycle built on a chassis consisting mostly of a single large diameter tube. An underbone differs from a conventional motorcycle mainly by not having a structural member connecting the head stock to the structure under the front of the seat and by not having a fuel tank or similarly styled appendage in the space between the riders knees. Underbones are commonly referred to as "step-throughs" and appeal to both genders in much the same way as scooters.

Underbones are often mistaken for scooters and are sometimes marketed as such. However, an underbone does not have a footboard, and is therefore not a scooter.

The engine of an underbone is usually fixed to the chassis under the downtube, while a scooter usually has its engine mounted on its swingarm. As a result, underbone engines are usually further forwards than those of scooters. A typical underbone therefore has a more central centre of gravity than a typical scooter. Furthermore, having an engine mounted on the swingarm gives a typical scooter more unsprung mass than a typical underbone. These factors give a typical underbone better handling than a typical scooter.

The engine of an underbone typically drives the rear wheel by a chain of the kind used on a conventional motorcycle. This final drive is often concealed by a chain enclosure to keep the chain clean and reduce wear. The final drive of a scooter with a swingarm-mounted engine runs in a sealed oil bath and is shorter.

An underbone is usually fitted with near full-size motorcycle wheels, which are often spoked. Scooter wheels are usually small, and made from pressed steel. In both cases, more recent examples often have cast alloy wheels. The bigger wheels of an underbone allow more ventilation and better cooling for the brakes than the smaller wheels of a scooter.

While the engine and suspension layouts described here for scooters and underbones are typical, they are not rigid definitions. There have been scooters with fixed engines and chain drive, and there have been underbones with swingarm-mounted engines. A twenty-first century example of variance from the typical scooter layout is the Suzuki Choinori, which had both its engine and its rear axle rigidly bolted to its frame.

• Some atypical scooters and underbones

• 1956 Heinkel Tourist. This scooter had a frame-mounted engine and a swingarm with an integral chain enclosure.

• 1980 Honda NC50. This underbone had its engine mounted on its swingarm.

• Suzuki Choinori. Introduced in 2003, this scooter has no rear suspension. Both its engine and its rear axle are bolted to its frame.