People are often hesitant on the structural value of carbon fiber begging the question: What makes a high strength (or weak) carbon fiber bicycle frame? The following text gives an overview of distinctive properties of carbon fiber bicycles.
The inherent weakness of carbon fiber is that it is brittle. Carbon fiber composites have low elongation (typically 1-1.5%). The brittleness of carbon fiber can be seen from sudden impact forces like riding into a curb or large pot hole. These impact forces can bend the frame/fork to the point of catastrophic failure. The frame needs to be sufficiently strong to absorb impacts and transmit the force throughout the frame.
Design (or lack thereof)
The “layup schedule” or the number of layers and direction of carbon fiber is the most important aspect to building a strong bicycle. For structure, bicycles use unidirectional carbon fiber meaning that all of the fibers run in the same direction (an anisotropic material). Woven fabrics are typically cosmetic.
Unidirectional carbon fiber is 30 times stronger in the fiber direction than perpendicular to the fibers. The angle of the fiber directly affects the strength of a frame!
There are many forces applied to a bicycle while riding it and each tube resists a unique set of forces. Each tube requires a unique diameter, number of layers, and fiber directions. The true beauty of composites is that you can pick the direction of the strength. To save weight, material only needs to be added in a select number of directions. A carbon fiber frame with tubes designed with equal strength in all directions (isotropic) would weigh at least twice as much and be overbuilt! Unfortunately, frames often fail because of forces not considered when designing the layup schedule. There is always a balance of weight and strength.
There are a 101 ways to manufacture carbon fiber frames, but no matter how the frame is made air voids can be present. Air is the true enemy of composites. Air can be trapped between layers of carbon fiber during the layup process. If the air is not removed prior to the resin curing a void will form. For reference, a void content of <3% is considered acceptable in most composite industries. The void is a stress riser that enables cracks and delaminations to propagate. Whether failure occurs depends on the size and location void.
How Does Appleman Bicycles build high strength frames?
Appleman Bicycles uses high strength carbon fibers and toughened epoxy resin. The carbon fiber is preimpregnated with the resin to provide consistent resin content and low weight.
By using a multitude of angles, the layup holds tubes together while transmitting loads and forces throughout the frame. The loads are distributed along the length of the tube as well as throughout the cross-section producing an extremely lightweight and robust structure.
During my time working in the wind energy and aerospace composite industries, I’ve witnessed and developed hundreds of cure and layup schedules. Using my background, I designed new cure schedules specific for my process of building bicycles. By using heat and pressure, air is extracted from the laminate prior to the resin curing. After the air is removed, consolidation of the layers of carbon fiber is realized until the resin is cured. My process produces frames with a very low void content resulting in excellent strength, durability, and safety.