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This is the start of a three-part series all about carbon fiber. Part I looks at the basic properties on carbon fiber, Part II focuses on designing carbon wheels, and Part III on manufacturing with carbon.

Zipp is famous for designing and making deep section carbon-fiber bicycle wheels. But what exactly is carbon fiber? What are properties of carbon fiber? What is resin? Beyond bikes, what other industries use carbon fiber?

For starters, this is how Merriam-Webster defines carbon fiber:

Noun: a very strong lightweight synthetic fiber made especially by carbonizing acrylic fiber at high temperatures

also  a material made from such fibers embedded in a resinous matrix

Carbon fiber is the main material we work with at the Zipp factory in Indianapolis, so we wanted to spend some time talking about it. This episode is the first of a three-part series all about carbon fiber. In this opening episode, Zipp Design Engineer Manager Tess Denning discusses the basics of carbon fiber and what makes it both challenging to work with but ideal for high-performance components. Tess also talks about how Zipp’s engineering team literally went back to school, taking a graduate level course in advanced composites to provide a fresh perspective on their work at Zipp. Below are edited excerpts of our conversation with Tess, but for the full story listen in the link below:

The Zippcast · All About Carbon Fiber, Part 1
To start at the most basic, what is carbon fiber?

It’s a material that’s known for its high strength-to-weight ratio. It’s stronger than steel when you look at it on a weight basis. There are other properties, too. You have less problem with corrosion. … That’s why people like to use it, but as far as what it is, you start with petroleum as the very basic building block. There are companies that make some precursor from that, so basically different styles of molecules. For the products that we’re using, the carbon fibers, those molecules go through several steps. They start stripping off the hydrogen, some of the other atoms that are there, and what you’re left with is a long chain of tightly bonded and tightly packed carbon atoms. It’s the way those atoms are arranged and the fact that they are bonded to one another that gives them that high-to-weight ratio. Other materials such as steels and alloys, those have also a crystalline, that helps give them their strength, but they are lacking that bonding. Carbon is superior in that it has both the arrangement of atoms but also has that bonding to help it out. That’s kind of the magic in it. 

What exactly is the resin used with carbon fiber?

You could think of it as the glue that’s holding the carbon fibers together. Picture a string of carbon fiber. It’s strong if you’re pulling on the string, but when you line multiple strings up together there’s nothing to hold them. If you pull on them, they would just separate. The Resin is just the glue, or that matrix, that’s holding those fibers together.

Zipp carbon cut table
At Zipp, we buy our carbon fiber already applied with our proprietary resin, correct?

Yes, that’s called a prepreg. It comes in rolls, or just sheets that are wound up in rolls. The carbon fibers are just running down the length of that roll, and you have the resin that is holding those fibers together.

What are the different types of carbon you could use for Zipp wheels or different applications?

The unidirectional carbon is the common one. It’s typically the highest strength you can get out of the material because whenever you start to weave it, you’re starting to crimp the fibers. “Uni,” also, you can get in lighter weights. That’s one of the advantages of using uni. But within woven, there are different styles, the pattern of the weave: there’s satin, there’s twills, there’s plain weave. Different rim manufactures will you use different ones. We primarily unidirectional and the plain weave in our rims.

How would you describe a sheet of carbon? How does it feel to handle? 

The sheets are from 6000th or 12,000th of an inch. Pretty thin, but you have to remember that those carbon fibers are tough to cut. It will wear out blades very quickly.

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Tess talks Zipp

The first thing to think of is what are the industries or applications where the benefits of using the material offset the higher upfront costs. In sports, as humans we’re powering the equipment, so it’s looking for that competitive advantage or that energy savings.

–Tess Denning on reasons certain industries use carbon fiber

How can you position the unidirectional carbon to increase strength?

With unidirectional, you only have strength in that one direction. So, in certain areas you’ll need strength in multiple directions. So, that’s why we have to start stacking together plies so that we get strength in each of the different directions that we need.

A few years ago, several Zipp engineers participated in a graduate-level composites course at a local university. How was that experience?

It was a challenging experience. It was one of those things that we wanted to do, but at the same time you know you’re signing up for basically going back to school while working. Fortunately, it was a great course. We were fortunate enough to find a professor locally. We just laid out all the things we want to learn, and he came up with a tailored course for us. It’s always good to get back to fundamentals. It’s been a long time since I was up doing calculous problems late into the night! IT was good to go through that accelerated learning again. Some of the things we wanted to learn, there are common things you learn on how to do analysis of beams of different materials. It is a little more difficult when you’re trying to model a composite beam, but then when you get into something like a rim that is not just straight like an I-beam, you have all this extra geometry. How do you model that? Also, when you start to model things, what are the correct boundary conditions? … When you’re setting up and trying to simulate how the part will behave under different loads.

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Do you consider your job more math or more science?

Good question! I would say a mix of both. It’s a mix of book learning or equations, but also too it’s a big mix of hands on. There are some things you’ll never learn in a textbook. Between math and science, I am going to say almost equal—it just depends on the problem you’re dealing with at the time.