Carbon Revolution-one piece carbon fiber wheel

Carbon Fiber Material Explained

Steel, aluminum, titanium and carbon fiber all differ from each other in strength, stiffness, weight, fatigue resistance, corrosion, etc. For example using aluminum or titanium in the same design dimensions as a traditional steel section would reduce weight but would produce excessive flexibility. So non-ferrous metal designs typically have larger section dimensions than steel ones to gain rigidity.
Metal parts usually do not fail due to a single catastrophic load but because of small, repeated stresses (called "fatigue"). Steel and titanium have defined minimum fatigue limits - if the stresses are smaller than these limits, these smaller forces generally don't shorten the fatigue life of the frame. Aluminum has no such specific endurance limit, so each stress cycle, however small, takes the material that much closer to fatigue failure. This sounds worse than it is, however - designers realize this limitation and attempt to "over build" their wheels for a lifetime of use.
Metals are equally strong and stiff in all directions (a property called "isotropy"). Once the cross section geometry of a metal design is determined to meet strength or stiffness requirements in one plane, an engineer lacks the freedom to meet varying demands for strength or stiffness in any other plane. In metal designs, by setting diameter and wall thicknesses to meet bending standards, this automatically determines torsional and lateral bending stiffness. Metal designs are just variations on a single theme compared to composites. Composites consist of reinforcing fibers that are embedded in a matrix material. These advanced composites make structures that are as strong and rigid as metal ones of equal size, but weigh much less. Furthermore, until the matrix material is hardened by a chemical reaction or heat, the resin-soaked fibers can be molded or formed into virtually any shape. Unlike isotropic metals, composites are anisotropic - their strength and stiffness is only realized along the axis of the fibers which can be arranged in any desired pattern. Thus, to absorb the variable stresses seen by wheels on road, composite designs can use multiple layers with different fiber angles for each. This puts strength only where it is needed while minimizing weight.
The Benefits of Carbon
A wheel is a considerably complex structure with performance characteristics that include: lightness, rigidity, durability, and shock absorption. The metallurgical composition of a metal design can't be varied through out the geometry. In contrast, composites can be infinitely varied through out the structure. Some of the variations include: different fiber angles, different plies, different plythicknesses, and different combinations of materials. So the properties of the end product made from composites can be tailored toprecise specifications. It is also easier to customize a composite design for varying degrees of stiffness than it is to customize ametal design.
Each design 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. Figure 1 shows the specific stiffness of several materials. 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 parts 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 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. Titanium is roughly two-thirds the weight of steel, one-half as stiff, and about 60 percent as strong as steel. The carbon fiber composites 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 ride and handling. 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 vibration 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 vibration 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 wheels.