Originally Posted by Calfee Design
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.