Every type of bike has a corresponding frame structure. The type of design may favor functionality, visual appeal, engine type, weight distribution or the cost of its particular model—it depends on the purpose the vehicle is intended for. To accommodate their massive powerplants, cruiser frames tend to be heavier than sportbike frames, which lean toward weight savings and performance gain. There are three basic modern frame types.
The most common type is called the cradle or tube frame. This simple design derives its strength from the triangulation of support tubing at major stress points. A center tube starts from the steering head and passes over the engine, where it splits in two around the swingarm pivot, then joins up with the cradle and wraps around the lower engine to link up with the head tube. The cradle consists of down tubes that extend under the engine and reattach to the frame near the swingarm pivot. This frame is relatively lightweight, extremely strong and easy to fabricate. The perimeter frame shown is a variation on this design. Its direct path between two load points makes the frame stiff and light and allows room for engine intake and exhaust systems.
In a backbone or spine configuration, the engine hangs from the top of the frame and acts as a structural member. This design requires the frame to be relatively heavy in order to provide adequate strength and support, but since the frame tubes don't enclose the engine, service is simple.
A pressed or stamped frame is two pieces of stamped sheet metal welded together, with strength determined by the shape of the stamping. This type of frame is the most cost-effective to produce and is used on inexpensive, small-displacement bikes. A variation is the monocoque frame, in which the body has a structural function. Instead of individual tubes, the monocoque is one large, welded, box structure. The "box" has the steering head welded to the front, and the design arches over the engine to the swingarm pivot. The airbox, cleaner and battery box can be incorporated into the main frame box, thus saving space. Scooters are classic examples.
Motorcycle frames are constantly being updated, so it's not unusual to find combinations of these designs.