Motorcycle chassis

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A motorcycle frame includes the head tube that holds the front fork and allows it to pivot. Some motorcycles include the engine as a load-bearing (or stressed) member; this has been used all through motorcycle history but is now becoming more common. The rear suspension being an integral component in the design. Frames have been made from titanium, aluminium, magnesium, steel and carbon-fiber along with a combination of these materials.

The frame provides the supporting skeleton to which the components are attached. A frame must be light, strong and rigid. Although often hidden from view, made of simple materials and manufactured using simple techniques, the frame contributes as much to the "personality" of the motorcycle as any other component. To a large extent, frame geometry determines the motorcycle's handling characteristics.

Frame Materials

Frames are traditionally made of steel, aluminum alloy or chrome alloy. Each has its appropriate use. Aluminum alloy is one-third the weight of steel, but also one-third the strength. There is no inherent weight-to-strength advantage for aluminum or steel. Steel is used in traditional triangle frames, where the tubes can be connected in a series of triangles. Steel has properties that make it great for this purpose because it is strong and resistant to fatigue. But the tubed design does not withstand super-bike racing stresses. Aluminum can be designed in boxed tube sections that have greater stiffness for a given weight compared to steel.

Steel

Examples

Norton Featherbed frame Early Honda CBR600F

Aluminium

Examples

Honda VFR750

Aluminium and carbon-fibre

Examples

Bimota SB8K (composed of two aluminium alloy beams and carbon fibre plates)

Carbon Fibre

Bimota TESI 3D

Titanium

Examples

1971 Titanium Husqvarna

Magnesium

Examples

Elfr 1986

Magnesium and Aluminium

Examples

Aprilia ETV 1000 Caponord (aluminium alloy welded on to aluminium-magnesium alloy castings)

Frame Types

Modern frames on standard production motorcycles can be grouped into three basic types: tube, pressed steel and monocoque. Tube frames are historically the oldest and now the most common. Tube frames are inexpensive to manufacture yet are light weight and offer good strength and high stability at highway cruising speed. Like bicycle frames, tubed frames come in welded and "tube and lug" varieties. The welded frame is self-explanatory. The tube and lug variety fits the tubes into manufactured joints, much like a tinker toy.

Frames may be either "cradle" or "spine" designs. In the cradle design, single or double downtubes support the engine in the classic bicycle "diamond" design. In a spine design, the engine literally hangs down from the frame. Pressed steel or fabricated sheet metal has been used to create "backbone" frames. These frames consist of stamped sheet metal that is welded together, as shown in the accompanying figure. It is econom-ical, but heavier and less rigid than tube designs. This makes the pressed steel frame ideal for small, inexpensive motorcycles. Although through extensive engineering and use of modern materials, pressed steel frames are also well suited for larger applications.

Pressed frame

The frame is mass-produced by sheet metal pressed or stamped into shape. Typically a single-cradle structure is used.

Examples

Single cradle

The motorcycle engine is held in a single cradle with a single spine.

Half-duplex cradle frame

The motorcycle engine is held in a double cradle with a single spine and single downtube.

Examples

(Full) duplex cradle frame

The motorcycle engine is held in a full by two separate cradles, normally with a single spine.

Examples

Double cradle or perimeter frame

Two cradles follow the perimeter

Examples

Spine or backbone frame

The motorcycle engine is suspended from a single spine.

Examples

Beam

Twin beams join headstock to swingarm pivot in as straight and short a line as possible.

Examples

Monocoque

Monocoque frames are an extension of the pressed steel frame. Monocoque frames incorporate many bike components (e.g., fuel tank, rear fender, seat pedestal, engine mount) into cast sections that com-prise the frame structure. Scooters are the classic example. Supports structural load using the external skin of the frame.

Examples

Honda NR500

Trellis

Similar to the beam frame, connecting the steering head and swingarm pivot point directly as possible. The frame is made up of a large number of short steel (or aluminium) tubes welded together to form a trellis.

Examples