Carbon Fiber Grades: It’s All About Tensile Modulus

A bicycle maker purchasing carbon fiber tubing for the first time from Rock West Composites will likely inquire about the grade of the carbon fiber in our products. That’s because we offer four different grades: standard, intermediate, high, and ultra-high modulus. When it comes to grades, tensile modulus is the defining factor.

Standard modulus carbon fiber is the most commonly used grade across industries. It is used for items like sporting goods, bike frames, and general-purpose tubing, as well as aerospace applications. Standard modulus carbon fiber is rated at 33 MSI. This means the resulting material has a tensile modulus of 33 million pounds per square inch. Ultra-high modulus fiber is the least utilized and most expensive of the grades. With a modulus rating up to 135 MSI, this grade is rather brittle and is used primarily for space applications. Remember, tensile modulus is a measurement of stiffness and should not be confused with tensile strength.

Carbon Fiber Grades: It's All About Tensile Modulus
Carbon Fiber Grades: It’s All About Tensile Modulus


To make most types of carbon fiber, manufacturers start with large groups of carbon atoms that are aligned in a long plastic string. Through a pyrolyzing process that applies extreme heat to the carbon atoms, impurities are gradually burned away leaving just the carbon atoms. Modifications to the pyrolyzing process can produce higher purity strands with higher MSI ratings.

High modulus and ultra-high modulus fibers are sometimes called pitch fiber. Pitch fiber starts as a different raw material than standard or intermediate modulus fibers and uses a different manufacturing process. High modulus carbon fiber has a rating of at least 42 MSI while, ultra-high modulus is rated beginning at 65 MSI.

The downsides to creating more pure strands of carbon fiber are increased cost, increased brittleness, and decreased strength. However, the upsides outweigh the downsides for some applications. High and ultra-high modulus carbon are often used when maximum stiffness is the priority.

Modulus Carbon Fiber

Carbon Fiber Olates

Modulus Carbon Fiber


As impurities are burned away from the carbon atoms, the remaining material is reduced in size. Once fully pyrolyzed, the diameter of a single carbon fiber is a fraction of the size of a human hair. Since a fiber that small isn’t very useful, they are bundled into groups called “tows.” A carbon fiber tow is like a string made up of thousands of carbon fibers. Tow sizes are typically 1K, 3K, 12K, and 24K. The “K” refers to how many thousands of fibers are in a tow.

The weave typically associated with the “carbon fiber” look consists of 3K tows in a twill weave. 3K makes up the majority of carbon fiber materials since it can easily be woven or spread flat into a thickness that’s convenient for making laminates. If thicker layers are required, a 12K tow might be used, or for thinner layers a 1K tow is often used. You may see some materials referred to as “spread-tow” which means that the tow has been spread especially flat to reduce the thickness of the layer. An example of spread-tow can be found in the 12K plain weave on the outside of Rock West Composites’ intermediate modulus tubes.


Rock West Composites uses different tow sizes and weaves to help you identify the grade of fiber used in a tube. Tubes with a 3K twill weave are made with standard modulus fiber, and tubes with 3K plain weave are made with high modulus. As previously mentioned, tubes with spread-tow 12K weave are made with intermediate modulus fibers. 1K plain weave can be found on our ultra-high modulus tubes.

We hope this helps explain the difference between the grades of carbon fiber, and helps you spot the difference when looking for your ideal tube. Remember though, higher modulus fiber means a stiffer fiber but almost always a weaker and more brittle fiber. If you still have questions, give us a call or send us an email. We would be happy to help you select the right carbon fiber grade for your application.