Raw carbon fiber is made with extremely thin, strong fibers. These fibers come in many different grades—or in engineering terms “modulus”— based on the material’s strength and stiffness. The higher the grade, the more it costs.
Higher grades of carbon fiber have a higher tensile modulus and are therefore stiffer. This is a good thing. However, high-grade carbon fiber can also be trickier to work with because of the need to maintain a balance between stiffness, strength and durability. It’s that balance that Felt always strives for. A frame that’s simply lighter, or stiffer, isn’t necessarily better. The goal is to balance all the elements that make a bike perform best when it hits the road or trail.
In addition to being a challenge to optimize, higher-grade carbon fiber is also more expensive. For these reasons, many bicycle manufacturers steer clear of the material. But Felt believes the investment is worthwhile. Starting with the best materials available is essential to achieving the ride quality our engineers, and our professional athletes, demand.
There are many ways to use carbon fiber in bicycle manufacturing. At one end of the spectrum is an age-old method called lug-and-tube. This is the way steel-tube frames have been built for more than a century. With this method, carbon fiber tubes are bonded to lugs, which can be made from aluminum, titanium or carbon fiber. The tubes are inserted into the lugs and the joint is glued together.
The lug-and-tube technique typically uses an overlap of extra material and therefore results in a frame that’s heavier than necessary. This type of construction also produces a “flatter” ride feel because it requires many individual parts and overbuilt intersections.
At the opposite extreme is the technique used by Felt: Dynamic Monocoque Construction. We start with sheets of unidirectional fiber (picture long, straight, black hair). These sheets can be made from fibers with different levels of modulus (60T, 40T and 30T), depending on the intended usage of the bike. They can also be mixed and matched. All of Felt’s carbon fiber bikes feature a blend of materials, which is why we call them UHC, or Ultra Hybrid Carbon.
There are three levels of UHC frames: UHC Ultimate+Nano, UHC Advanced and UHC Performance. Each targets different types of riders and serves different goals. For instance, is the bike aimed at competitive racers who favor light weight and pure performance over everything else? Or is it a sprinter’s bike, built for someone who prefers additional stiffness at the cost of a few grams?
The different types of carbon fiber can also be precisely positioned in the frame to take maximum advantage of their specific properties. For example, stiffer fiber plies may be used in areas of peak stress such as the bottom bracket shell and down tube while higher-strength fiber plies are used in areas particularly susceptible to impact.
Once the perfect blend of materials is established through CAD modeling, prototype testing and athlete feedback the final recipe is documented for production—this is called the “lay-up schedule.” The lay-up schedule details the types of materials and exactly how they are assembled to produce the frame, outlining the exact order and orientation of all the individual carbon fiber sheets, or “plies,” which are assembled by hand to create the frame. These plies come “pre-impregnated” with resin, and each is meticulously cut to the shapes needed to construct the frame.
The plies get strategically laid onto specially shaped internal molds, and once that process is complete the material is placed into a symmetrically split CNC-machined mold (think waffle iron). The mold halves are then closed and locked, and the frame gets formed with a precise amount of pressure and heat.
With high-end performance bikes such as the F1, DA, Edict, or F1X, Felt takes the process a few steps further. One of Felt’s key technological manufacturing breakthroughs is a proprietary molding process called InsideOut Internally Optimized Molding. By placing specially designed molds inside the frames during this process, Felt is able to eliminate any excess material inside the carbon fiber tubing.
“These inserts really clean up the inside of the frames,” says Felt engineer Ty Buckenberger. “The bottom bracket and other junctions are all nice and clean with no excess material inside.”