Motorcycle frame

A motorcycle frame is a motorcycle's core structure. It supports the engine, provides the hinge points for both front and rear suspension, and supports the rider and any pillion or luggage. Also attached to the frame are the fuel tank and battery. At the front of the frame is found the head tube that holds the pivoting front fork, while at the rear there is a pivot point for the swingarm. Some motorcycles include the engine as a load-bearing stressed member; while some others bikes do not use a single frame, but instead have a front and a rear subframe attached to the engine.

In the early days, motorcycles were little more than motorised bicycles, and consequently frames were tubular steel. While the use of steel tubing is still common, in modern times other materials such as titanium, aluminium, magnesium, and carbon-fibre, along with composites of these materials, are now used. As different motorcycles have varying design parameters (such as cost, complexity, weight distribution, stiffness, power output and speed), there is no single ideal frame design, and designers must make an informed decision of the optimum choice.

In Europe and the USA, steel tubing was the default material until recent times. All the major manufacturers (AJS, Ariel, BSA, Matchless, Norton, Sunbeam, Triumph, Velocette, BMW, DKW, Ducati, Moto-Guzzi, Harley-Davidson and Indian) used steel tubing.

The motorcycle engine is suspended from a single spine.

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

Examples

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

The motorcycle engine is held in place within a pair of separate cradles. The Norton Featherbed frame was the classic example, but many "duplex" frames actually have a single spine beneath the tank.

Also called Beam or twin spar, two beams wrap around the engine to joint the steering head and swing arm in the shortest distance possible for better rigidity. Beams are usually made of pressed metal (steel/aluminium). The trellis frame employs the same concept but uses welded members to form a trellis instead of pressed metal.

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Wikipedia