Motorcycle frames are usually made from welded aluminium, steel or alloy, carbon-fibre is used in some expensive or custom frames. The purpose of a motorcycles frame is to act as a base onto which all the various components can be bolted to. The engine generally sits inside the frame, the rear swingarm is attached by a pivot bolt (allowing the suspension to move) and the front forks are attached to the front of the frame. The frame can also help to protect the more sensitive parts of a motorcycle in a crash.
Buell, one of the motorcycling world's greatest innovators, uses the frame as a fuel tank on many of it's models like the XB12S Lightning.
The various types of frame commonly used include.
Single cradle frame. The single cradle is the simplest type of motorcycle frame, and looks similar to the first ever motorcycle frames. It is made from steel tubes that surround the engine with a main tube above and other, smaller diameter tubes beneath. If a single cradle becomes double at the exhaust, as frequently occurs, it is referred to as a split single cradle frame. Single cradle frames are usually found in off-road motorcycles.
Double cradle frame. Double cradle frames are descended from single cradle frames. They consist of two cradles that support the engine one either side. Double cradle frames are commonly used in custom motorcycles and simpler road bikes. They offer a good compromise between rigidity, strength and lightness, though they have now been technically surpassed by perimeter frames.
Backbone frame. Far from the most desirable frame around, the backbone frame comprises a single, wide main beam from which the engine is suspended. The backbone frame allows for great flexibility in design, since it is concealed inside the finished motorcycle. The engine just seems to hang in mid air. It is simple and cheap to make, and is used mainly on naked and off-road motorcycles.
Perimeter frame. Motorcycle racing research has shown that major advantages are to be gained in terms of rigidity by joining the steering head to the swingarm in as short a distance as possible. Flexure and torsion are dramatically reduced. This is the concept behind the perimeter frame. Two robust beams descend in the most direct way possible from the steering head to the swingarm, passing around the engine. The earliest perimeter frames were made from steel, but the need to improve rigidity to weight ratios led most manufacturers to adopt aluminium instead. Aluminium is now by far the most common road bike frame material and the aluminium perimeter frame is the most popular frame for modern supersports motorcycles.
monocoque frame. The monocoque frame is used nearly exclusively on competition bikes and is very rarely found on road-going bikes. Monocoque frames act as a single piece unit that functions as seat mounting, tank and tail section. Though they offer certain advantages in terms of rigidity, monocoque frames are heavy and generally not worth the effort.
Trellis frame. The trellis frame rivals the aluminium perimeter frame for rigidity and weight. A favorite of Italian and European manufacturers it has proved a great success in racing and competition. The Trellis frame uses the same principles as the perimeter frame, and connects the steering head and swingarm as directly as possible. The frame is made up of a large number of short steel (or aluminium) tubes welded together to form a trellis. The trellis frame is not only easy to manufacture but extremely strong as well. The frame pictures is from the Suzuki SV650S.
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