One direction that carbon-fiber bikes have taken is a mimicry of the conventional metal-tubing bicycle. These are essentially the same as a bonded aluminum bicycle, but with carbon-fiber tubes substituted for some or all the aluminum tubes. The idea of joining tubes in a triangulated structure is a familiar one that is logical for a designer to begin with. It reduces the number of unknowns because it allows the carbon-fiber frame to be modeled after a successful design. It also allows production of different sizes and angles by simply creating new lugs to join the carbon-fiber tubes. Traditional cyclists are likely to appreciate this approach as well. While forced to deal with the strangeness of carbon fiber, they are not asked at the same time to accept a completely new idea of what is a bicycle. However, putting both rigidity and ride comfort into a diamond frame made of metal has always resulted in a compromise. The diamond frame is a triangulated structure that stiffens up vertically as efforts are made to stiffen it laterally. In any way that carbon fiber is used, these attributes can be realized through good design, because they are intrinsic to the carbon-fiber material. The question is the degree to which these benefits are realized.
Several other, less-traditional, approaches to building frames with carbon fiber exist. Foam-core and bladder-molded frames are sometimes referred to as "one-piece" molded structures. By "one-piece" it's meant that the frame is molded as a single complete unit. Some indeed are, while others are molded in several pieces and then glued together to give a one-piece look. These processes can be complex, but they generally give engineers freedom to place carbon fibers wherever they wish. Extensive seams resulting from the overlap of material required by the molding process can sometimes create weak areas in the frame. Extremely careful attention must be paid in both the design and manufacturing of these frames to ensure proper quality control.
Another method is the use of a high-pressure lamination process. Here, a lug-less frame is created where the structural members forming the frame are carbon fiber tubes that are melded together with an epoxy-impregnated carbon fiber fabric. Gussets are integrally and simultaneously formed with the joining of the tubes. Along with increased design freedom, eliminating the dependency on lugs also eliminates the inherent weakness found in the tube and lug joints of many other designs. In turn, fine tuning of the frame's ride characteristics is possible since the same material is used in the tubes as in the tube transition areas. The fiber flow between the tubes is continuous which allows for an even distribution of stress flow throughout frame, virtually eliminating fatigue problems.
