What is Carbon Fiber?

There’s a lot to know about carbon composites, but we’re going to keep it relatively simple. Here are the basics.

Carbon Fiber

Carbon fiber starts as a sheet that looks exactly like fabric. It comes in several different patterns, two of the most commonly used in cycling are weave and unidirectional. Weave is most commonly used on ‘nude’ finishes because of its attractive look. Weave fiber is available in different filament counts ranging from 1,ooo filaments per strand to 12,ooo. These bundled filaments, known as tows, are commonly referred to as 1K, 3K, 6K, 12K etc. They are also available 0 degree (axial), +/- 45 degree and 90 degree (transverse) configurations. Laying these plies on top of each other can create a quasi-isotropic structure, (covered in more detail below). Unidirectional carbon fiber has all of its tows oriented in one direction, which makes it easier for handling and layup with certain fabrication and repair jobs.

Unlike metals, which have a specific strength that cannot be altered beyond tube butting, the stiffness properties of carbon fiber can be emphasized or deemphasized during the construction of a frame. By orienting fibers either unidirectionally to handle specific loads or in a quasi-isotropic configuration for overall strength, the way a carbon composite bike rides can differ completely from one builder to another. It’s what enables a carbon composite frame to be stiff as an i-beam laterally yet compliant vertically for a smooth ride. That’s the inherent beauty of carbon fiber – there are far fewer construction limitations than metal.

Composites, Resins and Prepreg

Technically, all bikes made of carbon fiber should be called carbon composite, because the carbon fiber would not hold together without the matrix, or resin, that is used to bond the plies of carbon fiber together. The resin is what allow each fiber to transfer load to the next, creating rigidity and dissipating stress across the frame.

Pre-impregnated carbon – known as pre-preg – is carbon fiber that comes from the supplier pre-impregnated with resin. It is frozen until ready for use, and is cured at elevated temperatures between 120 – 200 degrees Fahrenheit. Pre-preg is what most builders use, as it is far easier to handle and less messy than carbon fiber wetted with resin.

Modulus

In marketing mumbo-jumbo, you’ll hear manufacturers talk about modulus, and how their bikes only use ‘high modulus’ carbon fiber. But what is modulus? Modulus is used to grade different types of carbon fiber, and it refers to tensile modulus or Young’s Modulus, which is the relative stiffness of an isotropic elastic material. So now what the hell is an isotropic elastic material? Think of a carbon tube that has five plies of fiber, each of the plies oriented in a different direction so that all five plies have lines that intersect, much like an asterisk (*). Each of these plies transfer load out away from the center of the asterisk equally, making for a quasi-isotropic formation. Since carbon fiber is elastic in that it can absorb shock, tensile modulus measures how much elasticity carbon fiber has.

This is commonly referred to by engineers as “Modulus of Elasticity” and is a driving force behind the designs of manufacturers like ENVE Composites. Modulus of Elasticity is calculated by dividing stress by strain. Stress is a force applied over a unit area such as PSI or pascals (Pa). Strain is how much a material deforms when stress is applied, and is calculated by how much deformation occurs as compared to the structure’s original dimensions. Imagine when you hit a bump on your bike, how much the frame deflects from and returns to its original dimensions; that’s Modulus of Elasticity.

Therefore, the carbon is ranked as low, standard, intermediate and high modulus depending on its modulus of elasticity. Here is a quick run-down on the different grades:

Grades                                   Modulus of Elasticity (MSI – million pounds/square inch)

Low Modulus                                           below 33
Standard Modulus                                     33 – 34
Intermediate Modulus                               42 – 43
High Modulus                                               55+

All modulus of carbon fiber can come from the same basic strand, the difference is like an onion. The outermost layer of a carbon strand has lots of microscopic ridges and valleys, so by peeling off a layer, you get a more refined fiber filament, which is standard modulus. Peel off another, and you get intermediate. One more and you are at the densest, most refined type of carbon fiber, high modulus.

The drawback with high modulus fiber is that although it is the lightest form of carbon fiber with the greatest stiffness characteristics, it is far more expensive and brittle than low modulus, can shatter very easily, resulting in a cracked carbon frame. Therefore, true high modulus carbon fiber is rarely used in bike frames, especially mass-produced frames made overseas. It may say ‘high modulus’ on the frame, but in reality, it is more of a standard modulus. Otherwise it would be double the cost. Additionally, because of its brittle nature, high modulus carbon fiber usually needs to be reinforced with a tougher material like boron to resist damage upon impacts, making for an even more expensive bicycle frame.

This all brings us back to the repair of carbon frame damage. With all of the above information in mind, we strive to ensure that your repair is as close as possible to the original construction of your frame.

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