Motorcycle Frames Explained - The Frame Game

Cast Aluminum
The latest method of frame construction is to use large cast sections rather than extruded beams. Clever computerized design lets engineers put metal where it's most needed (not where it's redundant) so parts are lighter but still very stiff. Further refinements in casting technology include the use of pressurized filling molds, which make stronger, more consistent cast sections to save even more weight and improve stiffness.

Engine mounting
Early chassis designs treated the engine and frame as two separate entities with very little integration of their designs. But, starting with the Honda CBR900RR of 1993, design teams began to cooperate more which allowed better synergy between the two.

When you think about it, an engine is a pretty solid lump of metal, so why not use it to stiffen up the frame? It's not as simple as that of course-using the engine as a structural part causes other problems with vibration and maintenance, but these have been overcome. Modern bikes like Ducati's 1198 use the engine to help the frame do its job better.

Aluminum Beam
Aluminum is the material used in almost every modern sportbike frame, and it's not hard to see why. Aluminum is one-third the weight of steel, so you can use three times as much of it in a component while weighing the same as a steel part. More material in a component is important because it allows a stiffer shape. So, a part with the same weight can be made that much more rigid.

Early aluminum frames like the original GSX-R750, were made from extruded rectangular-section tubes, welded together with cast aluminum sections for the steering head pivot and swingarm pivot plates. As bikes got more powerful, the aluminum tubes got larger with thinner walls which made them stiffer, yet also lighter. These beams were curved around the engine to try to make as straight a line as possible between the steering head and swingarm pivot.

Carbon Fiber
Twenty years ago, many people predicted that carbon fiber would be the frame material of the future. Extremely light, yet also very stiff, carbon beat aluminum hands down on paper. But in practice it's yet to make any impact in mainstream design. Boutique bike builder Bimota used carbon fiber swingarm plates on its SB8R, and Ducati is using a carbon fiber frame for its 2009 MotoGP bike, but the problems of making production frames at a reasonable price have kept carbon out of road bikes (for the time being).

Stiffness vs. Strength
It's easy to get confused with stiffness and strength. Strength is a characteristic of a material such as steel or aluminum, which describes the material's resistance to fracture. Steel is stronger than aluminum: It's harder to break a piece of steel than a piece of aluminum.

Stiffness, on the other hand, is a characteristic of a structure, and describes its resistance to deformation. Need proof? Roll this magazine up into a tube, sit it on the table end-on, and pile books onto it. It will hold an amazing amount of weight compared with an unrolled magazine. The material is the same, so the strength is the same. But the cylinder shape is extremely stiff, making it into a viable structure.