Carbon Fiber Manufacturing in Bicycles. Article 2 of 3 – Carbon Quality.
A version of this article was originally published in Triathlete Magazine
Part one of this series provided an overview of the most common manufacturing processes used in carbon fiber manufacturing in bicycles and bicycle component fabrication. In part two, we will discuss the raw materials (the ingredients) that go into making the carbon fiber prepreg that is most commonly used in manufacturing carbon fiber products and how it relates to some of the common terms used in cycling marketing.
The most common form of raw composite material used carbon fiber manufacturing in the bike industry is “prepreg” – strands of carbon fiber (or other materials like glass or boron) that have been impregnated with resin to create a unidirectional tape or woven fabric. While woven fabrics of prepreg form the cosmetic patterns most people associate with carbon fiber, it is usually non-woven uni-directional tape (often hidden under a woven layer) that form the main load bearing structure of a design.
The skeletal system of a prepreg is the raw fiber. Like yarn, raw/dry carbon fiber comes on spools of intertwined carbon strands referred to as “Tow”. Tow bundles come in a variety of sizes that are categorized by the number of fibers (between 1,000 and 50,000) in the bundle. The result is the common “3K” (3,000 strand) and “12K” (12,000 strand) monichers that are attached to many carbon fiber products. The higher the “K”, the bigger the physical size of the tow and (in the case of woven prepreg) the larger the cosmetic checker pattern of the weave. However, with the exception of all but the thinnest prepregs, the tow size (“K”) has little to no impact on the quality of the carbon used. The fiber type is what matters as it directly effects important mechanical properties like tensile strength (pull strength), modulus (how much a material deflects under load) and strain rate (fiber elongation before failure). Thus, references to “K” have more to do with the aesthetics of the frame or component, while the modulus (Standard, Intermediate or High) of the fiber more directly relates to its stiffness.
Resin is the other part of prepreg. Like connective tissue, resin holds the fiber skeletal system together and must be engineered properly to allow it to function properly during the manufacturing process. Resins are temperature sensitive mixes made by combining specific ratios of liquids and solids in a heated mixer. From the instant a resin mix catalyzes, it must be kept cold (below freezing) or it will cure prematurely. While resin is rarely referred to in consumer marketing (“301 Resin” does not sound nearly as catchy as “12K”), it is crucial that the resin used meets the requirements of the end product for which the prepreg will be used.
Once blended, the resin is precisely transferred onto a roll of release paper (think Fruit Roll-Ups) using a machine called a film caster. The resulting rolls of resin paper are installed on a prepreg machine where a large loom-like machine organizes and feeds up to a few hundred tow bundles of fiber onto the resin papers in the specified arrangement. As the resin papers are loaded with tow, they are heated and compacted by rollers that help the warm resin penetrate into the fibers. The resulting sheets of prepreg are then chilled, rolled and ready to be used in production. Throughout the process, strict adherence to quality control procedures and proper documentation makes sure the end result meets specification.
Prepreg carbon fiber sheets are produced in a large number of combinations and every prepreg sheet is classified in a code that includes everything from the name of the manufacturer, to the type of fiber (modulus) used, to the fiber and resin content of the prepreg. Fiber content is expressed in “Fiber areal weight“ (Faw) and is simply the amount (in grams) of fiber in one square meter of the prepreg. If a bike company markets their product as “110”, “120”, “150”, it likely means that the prepreg used in its construction has that Faw. The lower the Faw, the thinner the and lighter the prepreg and the higher the modulus carbon fiber that is required for it to maintain stiffness. “Rc” is the resin content in one square meter of prepreg and is expressed in terms of a percentage. When a company refers to a 70/30 or 65/35 fiber to resin ratio, they are speaking of the percentage of fiber to its Rc.
In addition to bundle size (“3K”, “12K”…), Faw (“110”, “120”…) and Rc (70/30), some manufacturers refer to the grade of carbon they use by tension load capacity. For example, “800ksi” (note the small “k”) means that the fiber can withstand 800,000 pounds per square inch of pressure before it bursts. Other companies use their own “rating” system that refer to how stiff and/or how light the carbon fiber is on a relative scale (10.5, 8.5…). While some of these numbers are more telling than others, each individual number tells only a small part of the actual story as unidirectional prepreg quality is a combination of fiber type (modulus), Faw and Rc. Many frames today are also difficult to quantify as they use multi-modulus lay-up schemes that combine many types of prepreg to suit the end goals of the product and design as best as possible.
A frame is only as strong as its weakest link and the metal used in construction is an important element too. Metal is bonded into a good number of carbon fiber frames where bearings, threads and wheels contact (bottom bracket, headset, dropouts). Aluminum is the most commonly used metal in carbon frame fabrication as it is light and inexpensive. However, aluminum can galvanically corrode and requires that the builder wrap it in a material like fiberglass to minimize the likelihood of corrosion. On some of the very best quality carbon frames, titanium is used because it is very durable, corrosion proof, light and one of the purest metals to bond.
So, if most of the marketing nomenclature does not directly refer to the overall quality of the carbon, how can you determine where a bike lies in the spectrum of options? As mentioned in part one, price is frequently one of the more accurate indicators. When it comes to materials, no one in the bike industry is able to buy materials at such a significantly lower price than everyone else that they can undercut the market by offering a much higher grade product for a much lower price. The higher the quality of the carbon and metal used, the higher the price of the bike, but also the more handcrafted workmanship involved, and the more tunable and durable the ride will be. In short, with carbon fiber you almost always get what you pay for.
In part 3, we’ll explore the four different types of companies offering carbon fiber products in the cycling industry and how they relate to the fabrication methods and quality levels discussed in part 1 and 2.
Enjoy the ride and train hard and smart!
Thank you to Mike Lopez, Director of Composite Development and Manufacturing for Serotta Cycles, for his contributions to this article.
Originally published in Triathlete Magazine October 2008/Copyright © 2008