To the casual observer, a motorcycle's frame is simply that -- a skeleton that supports the weight of the engine, rider and just about everything else. However, terms such as steering head, swingarm, downtubes and twin-spars can make a frame much more complicated than it may appear to be. While there are just as many type of frames as there are motorcycle designs, understanding the basic anatomy of a motorcycle frame will pay dividends during your regular maintenance chores.
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Motorcycle frames are typically built to highlight the purpose of the motorcycle. Sport motorcycles often use a lightweight aluminium twin-spar frame to reduce flex within the frame, thus aiding maneuverability. Cruisers and most motorcycles built between 1960 and 1980 use a variation of a single backbone, cradle-style frame made from tubular steel.
Motorcycles such as cruisers or choppers are often built on a single backbone frame, which uses a single large diameter tube to connect the motorcycle's seat rails to the steering head. A pair of downtubes, extending out from below the steering head, form a cradle to support the engine. Tubular seat rails are welded onto the end of the backbone to support the rider. Sport motorcycles use a twin-spar-type frame, wrapping around the sides of the engine without the use of downtubes. The engine itself is used to strengthen the frame. Newer sport motorcycles use a detachable subframe to support the rider.
The steering head is the tubular structure at the front of the frame, providing a pivot point for the front fork triple-clamp assembly. The triple-clamp is composed of an upper and lower unit, with the handlebars mounted directly to the upper triple-clamp. The steering head contributes to the motorcycle's steering geometry by providing the rake -- or angle -- of the front fork, directly affecting the motorcycle's stability at high speeds. A relatively short 24-degree rake angle, as used by Honda's 2003 CBR600RR sport bike, will allow the motorcycle to change directions easily, but reduces straight-line stability at high speeds. Alternatively, a cruiser with a long 39-degree rake will be very stable in a straight-line, but will be harder to turn.
At the rear of the motorcycle is a pivoting fork called a swingarm. The swingarm carries the rear wheel and is supported by either a monoshock or twin-shock absorber set-up to reduce the impact created by changing road surfaces. A monoshock set-up is typically mounted at the front of the swingarm, connecting directly to the motorcycle's frame, below the seat rails or subframe. Twin-shock setups place the shock absorbers on both sides of the swingarm and mount directly to the motorcycle's seat rails. Rigid frames, as used by early motorcycles and custom choppers or bobbers, eschew the use of a rear suspension set-up. Instead, the rear wheel is mounted solidly to the motorcycle's frame.
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