Carbon Fiber Manufacturing Techniques in Bicycles, Article 1 of 3

A version of this article was originally published in Triathlete Magazine

Q:   I am looking to buy a new carbon bike and I am wondering how price relates to what I get? I’ve seen $800 frames and $7000 frames that are both made of carbon. Being the same material, how different can they really be? I am just looking for a good value and want to take advantage of all that carbon can offer.

Allen, via e-mail


A:  The common perception is that frames and parts built of the same material are all pretty similar. This is a huge mistake, as all frames and components are quality dependent and carbon fiber is arguably the most quality dependent material of all. The “why” of this is more complicated than it may initially seem and is easier to understand when broken down into parts, each covered by its own article:

  1. Carbon fiber manufacturing techniques commonly used in the bike industry.
  2. Carbon fiber material and quality grading.
  3. The four different types of bike companies and how they employ the manufacturing techniques and carbon grades.

This article covers common carbon fiber manufacturing techniques used in the bike industry.

There are two general forms of carbon manufacturing techniques used in frame construction:

Bladder and Foam Core Carbon Fiber Molding:

Bladder molding basically consists of taking carbon and resin substrate, putting it into a preset mold (usually made of aluminum or steel), exerting pressure from the inside with an inflatable bladder (think blowing up a balloon in a jar), and curing. During curing, the carbon and resin harden and hold the shape of the mold. Foam core molding uses a similar process, but heat activated foam applies the internal molding pressure instead of a rubber bladder.  In foam core molding the foam core will remain inside the carbon fiber structure where it can offer further structural support, additional vibration/noise damping quality and can increase repairability.

Some form of molding can be found in almost every frame on the market. Many mass produced frames use a monocoque bladder mold where the entire frame is molded at one time. Other designs use a combination of molded lugs that are bonded or wrapped with other molded, roll-wrapped or filament wound tubes (see below) to create a frame.

The primary benefits of molding are that any shape and carbon lay-up an engineer designs can be built and, once the molds are paid for, production costs are minimal. Mass production is feasible as bladder molding is much like baking a cake – you mix the ingredients (lay-up the matrix), put it in the oven (the mold), and bake (pressurize and cure). The primary drawbacks to one-piece monocoque molded frames are that the shape and geometry of the frame can be changed little to none once the mold is made and it can be difficult to apply consistent pressure to the entire structure during molding. Inconsistent pressure can lead to weak areas of uneven carbon fiber compaction known as “voids”.

2)  Roll-Wrapping and Filament Wound Carbon Fiber:

While roll-wrapping and filament wound construction are different processes and have some different limitations and benefits, they are often used to construct similar structures. In each case, carbon fiber is rolled or wound around a steel or aluminum mandrel in the shape of the end product. The carbon is wrapped with heat shrinking tape to create pressure and is then cured (pressurized and heated until hard). Once cured, the tape is removed and the hardened part can be sanded to its final diameter and shape.

Once wound or rolled tubes are created, they are often bonded and/or compression molded to bladder molded or wrapped lugs to create a frame. This is similar to how metal frames are built, except that instead of metal and torches, carbon fiber, pressure and heat are used.

Roll-wrapping and filament winding are more time consuming than bladder molding and can be difficult to use for mass production. However, when done properly, they allow for great tunability and customization in both ride quality and frame geometry while minimizing voids.

Material and Process Quality Counts:

Arguably more important than the manufacturing technique a builder uses, the care and skill applied during the engineering (the recipe) and the assembly (the baker) of the frame, in combination with the quality of the carbon fiber used (the ingredients), are crucial to the end result. This is where some frames become the cycling equivalent of Twinkies, while others become a decadent triple layer cake – it all depends on the company’s approach to materials and process.

Like all ingredients, carbon needs to be stored and maintained properly to maintain quality. When it comes to raw materials, a very small percentage of bike builders use a “RTM” (Resin Transfer Mold) based system where dry carbon fiber is injected with resin during the actual molding process. However, most builders use “Pre-preg” carbon. In its raw form, pre-preg is carbon fiber that has been pre-impregnated with resin and is stored in sheets that are ready for the builder to cut, lay-up, shape and cure into the end product. Prior to use, pre-preg carbon fiber needs to be stored in a controlled environment that is below 0 degrees Celsius to prevent premature curing of the resin. In addition to proper storage, if the assembler does not use care to keep the work area, cutting tools or molds contamination free, the final product will be compromised.

Ingredients only become a cake after proper mixing and baking and carbon structures only achieve their potential when the resin and carbon fiber are layed-up and cured ideally. It is quicker and less expensive to minimize engineering and limit heat and pressure treatment processes when building with carbon. While a rideable frame may still result, a poorly engineered frame that is improperly treated will not be as strong or ride the same as a frame that is well engineered and properly treated. Frequently, part of what you pay for in a higher grade carbon frame is not only better grade materials, but also the assurance that the frame was engineered and manufactured using refined and proven processes that minimize contamination and maximize the integrity and ride quality of the materials.

In the end, an aerospace grade frame only results if aerospace grade materials, aerospace grade engineering and aerospace grade assembly processes are all used simultaneously. If one frame/bike costs more than another, it is likely because it was built using higher grade materials and/or more refined and controlled processes. Shop intelligently and remember, the best made bike in the world will not do much for you if it does not fit you well. Have a professional fit and rider matched bike search done first and then consider the quality and ride of only those options that fit you well.

In the next article, we will bite into carbon fiber quality, grading methods, and the murky marketing that often surrounds the subject.

originally published March 2008/Copyright © 2008

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