The frame is typically made from welded aluminium or steel (or alloy) struts, with the rear suspension being an integral component in the design. Carbon fibre, titaniumand magnesium are used in a few very expensive custom frames.
The frame includes the head tube that holds the front fork and allows it to pivot. Some motorcycles include the engine as a load-bearing stressed member; this has been used all through motorcycle history but is now becoming more common.
Oil-in-Frame (OIF) chassis, where the lubricating oil is stored in the frame of the motorcycle, was used for Vincent motorcycles of the 1950s, and for a while during the 1970s on some NVT British motorcycles. It was widely unpopular and generally regarded as a bad idea at the time. Today it is a used on some “thumpers” (single-cylinder four-strokes) that usually have dry-sump lubrication requiring an external oil tank. It has since gained some cachet in the modern custom bike world too because of the space savings it can afford and the reference to an earlier era.
Modern designs have the two wheels of a motorcycle connected to the chassis by a suspension arrangement, however ‘chopper‘ style motorcycles often elect to forgo rear suspension, using a rigid frame.
The front suspension is usually built into the front fork and may consist of telescoping tubes called fork tubes which contain the suspension inside or some multibar linkage that incorporate the suspension externally.
The rear suspension supports the swingarm, which is attached via the swingarm pivot bolt to the frame and holds the axle of the rear wheel. The rear suspension can consist of several shock arrangements:
- Dual shocks, which are placed at the far ends of the swingarm
- Traditional monoshock, which is placed at the front of the swingarm, above the swingarm pivot bolt
- Softail style suspension, where the shock absorbers are mounted horizontally in front of the swingarm, below the swingarm pivot bolt and operate in extension.
A motorcycle fork is the portion of a motorcycle that holds the front wheel and allows one to steer. For handling, the front fork is the most critical part of a motorcycle. The combination of rake and trail determines how stable the motorcycle is.
A fork generally consists of two fork tubes (sometimes also referred to as forks), which hold the front wheel axle, and a triple tree, which connects the fork tubes and the handlebars to the frame with a pivot that allows for steering.