Unidirectional carbon fiber is the most common type of carbon fiber used in bicycle construction. As its name implies, a sheet of unidirectional carbon has its fibers oriented in one way, or in parallel (as in, “one direction”). To help visualize the characteristics of unidirectional carbon fiber, think of a wine glass. If you drop it on a hard floor, it will shatter because it’s so thin, stiff, and brittle. Now think of a plastic cup. You can drop it and nothing happens because of its flexible properties. Carbon follows similar rules. As you increase the modulus, the stiffer and more fragile the fibers become. And if you go in the other direction, the fibers are not as stiff, but more compliant and durable. As such, there is no such thing as a bicycle frame constructed completely from high-modulus carbon fiber, at least not one that would be safe or pleasant to ride.

It would likely break apart the first time you hit a big enough pothole. High-modulus carbon fiber can be tricky to work with and there must be balance between stiffness, weight, strength, and durability. Also remember that not all unidirectional carbon fiber is created equal—there are countless varieties and versions created by various composite manufacturers around the world, each one with different weights, durabilities, vibration resistance, and other attributes that can greatly affect how a completed structure will perform. Think of different types of carbon like paper: Plain, white sheets of paper are intended for computer printers; thicker, colorful construction paper is ideal for crafts; and thick-stock, supple, and durable paper is best for archival printing and bookkeeping. Every type has its unique properties and ideal uses.

Through extensive Computer Aided Design (CAD), Finite Element Analysis (FEA), and prototype testing by Felt’s professional riders, our engineers are able to ascertain which fiber types to use. The goal is to precisely position each piece on the frame to take maximum advantage of its specific properties. In some areas, we may use intermediate modulus material because it has the potential to offer an ideal balance of stiffness and strength. In other areas we opt for high-modulus material for additional stiffness. TeXtreme is used in many models, as well, often as a way to add strength and durability without excessive weight. Every frame design behaves differently, and every frame size requires a unique lay-up. That's why we've been creating size-specific frames for decades, to ensure that every cyclist shares the same riding experience.

The blending of these materials can vary greatly. For example, stiffer fiber plies are typically used in areas requiring maximum performance, such as the bottom bracket and down tube. Higher-strength fiber is best for areas more susceptible to impact. The same attention is applied to changes in frame size. As tube diameters and intersections change, so will ply orientation and material specifications. These determinations take months, even years to finalize considering all the variations. Once the ideal blend of materials and ply orientation is determined, the lay-up schedule is finalized, outlining every detail of the construction process. Then the frame is produced in the factory and made ready for real-world test-riding before being mass-produced for the marketplace.