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A bike frame is a considerably complex structure with performance characteristics that include: lightness, rigidity, durability, and shock absorption. Aluminum and titanium frames have become popular because they challenge steel frames in at least two areas of performance - lightness and corrosion resistance. But, at the high end of the industry, composites will likely eclipse frames made from any metals in all performance areas.
The metallurgical composition of a metal tube can't be varied over the length of the tube. In contrast, composites can be infinitely varied over the length of the tube. Some of the variations include: different fiber angles, different plies, different ply thicknesses, and different combinations of materials. So the properties of the end product made from composites can be tailored to precise specifications. It is also easier to customize a composite tube for varying degrees of stiffness than it is to customize a metal tube. Additionally, the tooling cost for metal tube production is several orders higher than that of composite tube production.
Composite tubes are typically formed around a mandrel (a metal dowel, typically steel, that is later withdrawn) by either "filament winding" (winding strands at various angles), "roll wrapping" or "braiding." Another method called "pultrusion" pulls fibers through a heated die that melts a thermoplastic matrix. Each manufacturer has its own special number of layers and orientations of fibers to create its desired combination of strength, weight, and stiffness. This is the beauty of carbon fiber: with metals the choices are much more limited, but with carbon fiber they are almost limitless.
Tailoring of a bicycle frame is not new; it's been done with steel frames for years through the butting process, where tubes are thickened at the joints to handle stress and thinned out in their long center spans to reduce weight. What if the size and shape of each tube are matched precisely to the predicted loads of pedaling and road shock? What if the material could be distributed precisely where it is needed. What if the rigidity of each tube, through some complicated shaping or milling process, varied from one plane of bending to another or from one end to another? The frame could be built to be rigid to lateral pedaling loads, but fine-tuned in the vertical plane for compliance to road shock. Shaping and milling a metal frame in this manner would be nearly impossible. But, composites can be relatively easily molded into structural members with complex cross sections.
Figure 1 shows the specific stiffness of the four main materials used in making bicycle frames. Specific stiffness is defined as tensile modulus divided by density or simply, the stiffness to weight ratio. One might ask: "If carbon fiber has such a high stiffness to weight ratio, why aren't carbon fiber frames lighter than they are?" The answer is that carbon fiber has a huge advantage in tension but in practice, it is difficult to direct all the stresses imposed on a structure. It is up to the designer to take this into consideration and to do their best to load the fiber in tension.
Composites can be molded into structural members with complex cross sections with relative ease. They also have some very impressive mechanical properties. The 6061 and 7000 series aluminum used in bike frames is roughly one-third as heavy as steel, one-third as stiff, and, at best, is about 80 percent as strong as the 4130 cro-moly steel used in most bike frames. Titanium is roughly two-thirds the weight of steel, one-half as stiff, and about 60 percent as strong as steel. The carbon fiber composite most used by bicycle manufacturers is less than one-quarter the weight of steel, but it is about as stiff (which makes it almost four times as stiff on a weight-to-weight basis), and it is roughly four times as strong in tension. Carbon fiber also has a better fatigue life than steel, titanium, or aluminum, and the resins typically used to bond the fibers offer extremely good vibration damping.
Vibration and shock damping are two important factors that affect the cyclist. However, they are two of the least understood subjects in materials science. There are so many variables involved - including how atoms in a material absorb and dissipate vibrational energy, how the structure is built, what type of paint and plating are applied - that it is hard to predict how a structure will react to vibrational input. Composite's vibration damping is far superior to any metal, which is why it is the preferred material for race car springs and high performance airplanes. The smooth ride quality is one of the first things people notice about carbon fiber bicycle frames.
Sophisticated finite element analysis programs and laminated-plate theory help define the properties of a composite structure. An inherent difference between composites and metals is that composite products are constructed in layers, or plies, of directional material. Interfacial adhesion and the potential for delamination (separation) under shear or compressive loads must be considered when analyzing an advanced composite design. This information is essential when addressing the variable requirements of a bicycle.
Composites differ from metals in that they don't carry loads equally in all directions, but bear loads best in tension. A composite is similar to a bundle of strings soaked in a layer of glue or resin. The bundle can bear more weight, and flex less, if pulled from end to end or flexed like a diving board than if compressed or loaded transversely. The changing face of the bundle's performance occurs because the real strength of the bundle comes from the string, not from the resin. The primary function of the resin is to lock the fibers in place, transfer loads among fibers, protect the fibers from environmental forces, and give the structure impact strength. The directional nature of the fibers' load-bearing abilities changes the rules of structural design.
| Comparison of Materials Used in Bicycles | |||
| STEEL | TITANIUM | ||
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| ALUMINUM | CARBON FIBER | ||
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