If you’ve followed parts one through four of this series on bicycle metallurgy, you’ve learned a lot about the physical characteristics that are important to consider when designing aluminum, titanium or steel bicycle frames. This installment takes a step outside the realm of metallurgy, and looks at the use of carbon-fiber composites in bicycle frame applications.
The Wonderful World of Composites
It’s common to use the terms carbon fiber and composite interchangeably, even though all composites are not carbon fiber. For example, both plywood and concrete are composite materials. The term composite refers to combinations of materials that result in enhanced properties not provided by the materials alone (concrete is a composite of cement, sand, gravel and water; Cheeze Whiz is air, artificial flavors and artificial colors).
In scientific terms, composites are generally acknowledged as those materials in which either particles, short fibers or long fibers are dispersed in a matrix. In the case of the Duralcan metal matrix composite that is found in the Specialized M2, aluminum oxide whiskers are dispersed in a 6061 aluminum matrix; while advanced composites – the types used to build bicycles – have continuous fibers embedded into a matrix (typically epoxy).To qualify as an advanced composite, it is generally thought that the fibers are continuous, greater than 50-percent fibers by volume, and the fiber has mechanical properties superior to fiberglass. Fibers can be carbon, Kevlar (a.k.a. aramid), boron, ceramic, silicon carbide, quartz, polyethylene … and probably others that I’m not aware of.
A Simple Lexicon
Here’s a simplified explanation of how terms will be used. A fiber is a single strand of reinforcing material. A bundle of parallel continuous fibers are bound together with a glue, or matrix. A single layer of this matrix is called a ply, and multiple plies are laid up to form a laminate. The plies can be laid up in various angles to produce different characteristics of the laminate. If you’ve forgotten about the other terms used in this series – like tensile strength and elongation – re-read the first installment of this series to reacquaint yourself with those terms, because they’ll be essential to our discussion.
The Numbers Look Good
If you look at the numbers that carbon fiber can boast, your initial thought might be that it’s crazy to build a bike out of anything else. But you astute students of the School of Bicycle Geekdom already know that numbers are not the only thing to look at – you need to check out the fine print. And get this: With carbon fiber, you need to throw most of what you’ve learned out the window.
Yes, it’s true that the potential for composite frame materials is tremendous. Unfortunately, the results of some composite bicycle-frame projects have been less than satisfactory. There are reasons for the high failure rate that composite frames have endured, but the fault is not that of the material. I know you may find this hard to believe, but sometimes even rocket scientists make mistakes. The situation is similar to what happened with titanium in the 1970s. Teledyne made some frames that failed, not because the material was bad, but because the design was bad, or the execution of the design was bad. Similar things have happened with composites, and the image of the material is not as good as it should be.
The common folly is for the designer to underestimate the complexity of the bicycle frame. Since carbon-fiber structures are not very fault tolerant (unlike metal structures), the design and execution plays an even more important role. And sometimes the fault is not in the design or execution of the structure – the fault may be a big rock coming in contact with the downtube. While the tube might not fail from such a large impact, the repercussions are usually hidden on the inside of the laminate, or within the laminate. Microcracks can then spread through the matrix, decreasing the ability of the fiber to transfer load. Metal tends to do a bit better in these situations – but you can make metal frames that break without warning, too.
It’s All in the Lay-Up
What I’m getting at is the fact that composite materials are very complex … more complex than metals. In addition to the material itself having greater complexity, the structures are not as straightforward as metal structures. As you have learned in this series, the designer of a metal structure has two variables: material choice and geometric configuration (like tube sizes, shapes and thicknesses). Those wacky composite guys not only get those same two variables, they also get to determine how the composite matrix is laid up. Bear in mind that two structures of identical geometric configuration, weight and composite material, but with different lay-up, could yield a completely different result. Not only is it possible for the obvious – like stiffness – to vary, but fracture stresses and failure modes could also vary tremendously. And the failure modes of composite structures are plentiful: exploding laminate, fibers pulling free from a matrix, first-ply-failure, matrix cracking, and delamination. And I thought designing a metal bike was tough….
Another curve ball thrown into the mix is the geometric shape of the frame. Sure you can make a frame with tubes and lugs like Trek or Giant, but you can also lay them in a shape of your own design, like Kestrel or Look. With lugs and tubes, the designer at least has metal frames with which to compare; but with a new shape, a whole new set of equations needs to be developed.
Tensile and Compressive Strength
Let’s take a look at the physical properties that have been examined with aluminum, titanium and steel frames, and see where carbon fits in (or doesn’t fit in). The way strength is measured in the laboratory is by a tensile test. In a tensile test, we use tension to pull a sample apart until it breaks. Imagine we’re pulling on a bundle of carbon fibers, doing a tensile test. It performs very well in a tensile test – actually, it performs extremely well.
But what about the compressive behavior of carbon? Not too good by itself, kind of like a bowl of spaghetti. You need some kind of adhesive to bond the fibers together, and give the material compressive as well as tensile strength. The matrix connects this whole disorganized mess of fibers by transferring the load between the fibers and between the plies. Since the matrix and the fiber combine to make up the composite, we’ll look at them together to give comparative results.
Density and Modulus
At the risk of being accused of comparing apples and oranges, I’m going to give you some guidelines for a generic carbon fiber lay-up. Bear in mind that there are many different ways to look at this, and I’m only making a comparison for the sake of continuity in the series. The density of your laminate is in the neighborhood of 0.056 pounds per square foot, which is about 60 percent of the weight of aluminum, our previous lightweight winner. The modulus of a generic not-very-high-zoot carbon fiber is about 30 to 33 MSI, or about 10 percent higher than that of steel, previously the stiffest of the three materials we’ve looked at. So you can see we’ve got some stiff, light stuff here.
When we throw the epoxy into the mix, things start to get interesting. A well-made laminate will have 62- to 65-percent fibers by volume. The Rule of Mixtures says that the modulus is proportional to the percentage of fiber in the matrix, since virtually all of the resulting mechanical properties come from the fiber. In other words, the matrix transfers the load to the fibers. So if we start with 30 MSI modulus, with only 65 percent of the matrix contributing, we end up at about two thirds of that, or 18 to 21 MSI for our modulus. Still not too shabby: density one third of titanium, and modulus about 25 percent higher.
This modulus measurement is only in the zero-degree direction though (that’s the direction parallel to the fiber in the ply), and as we know, bicycles get varying stresses applied from varying directions. That matrix does a good job of holding together those fibers, so they don’t buckle under the combined loading. Let’s rotate the ply so that the modulus is measured perpendicular to the lay-up of the fibers. Now our modulus reads a pathetic 1.5 MSI or so, essentially giving us the modulus reading of the epoxy. Yuk! What’s worse, the modulus drops off precipitously between zero and 30 degrees, giving low results almost all the way to 90 degrees. This matters because bicycle tubes (or structures) are subjected to torsional loads as well as longitudinal ones. What’s the answer? Add layers of plies that are at different angles (often 45 degrees) to the initial zero-degree layer. The result is an overall modulus of approximately 10 to 14 MSI, still not too shabby. Again, these are generic numbers for the sake of a simplistic comparison.
What is extremely cool about the ability to lay-up a laminate, is that you can dictate the exact characteristics you want your tube or structure to have. Stiff in torsion, soft in bending. Soft in both, stiff in both. You determine the characteristics – the material doesn’t dictate them. This phenomenon is called anisotropy, and you just can’t do it with metal.
The Weak Link – Elongation
Now for the bad news: carbon’s weak link is elongation. Elongation is your safety net, but with carbon it’s low, low, low. Depending on lay-up, it’s possible to get some elongation out of carbon. For example, there is a scissoring of layers in the 45-degree plies, but in general we’re dealing with a material that doesn’t have an overabundance of ductility. Composite designs are not meant to permanently bend. And when they fail, they fail all at once, so designers build in a big safety net. This is similar to what the aluminum designers do, in order to overcome the low elongation of that material.Most manufacturers are very secretive about their lay-ups, so getting good info isn’t always easy. Reading through the Trek technical manual yields numbers for the specific modulus of that company’s lay-up, which measures the modulus divided by the density. Backing these numbers out yields an 8 MSI modulus for the Trek OCLV lay-up.
The strength of the advanced composites is very high. The zero-degree strength for even a standard carbon unitape (the building block of the laminate) is 300 KSI or better. Looking at the big picture, the strength of the laminate still ends up way above 100 KSI, and this is at very low density. Trek’s specific strength numbers yield actual ultimate values of about 115 KSI. Take a look at the 8 MSI modulus and 115 KSI strength that Trek claims for its laminate, and compare to aluminum. The carbon has about 20 percent lower stiffness, but is 40 percent lighter, and the strength is roughly double, while still being 40 percent lighter. Very impressive numbers.
A Brilliant Future
What’s the future of advanced composites? Their reputation is definitely on the rise. These days, most of the hideously ugly carbon projects have gone away, and there are several very successful carbon production lines happening. The two biggest players at this point are most likely Trek with its OCLV bikes, and Giant, which markets its bikes under several different brand names as well as its own. My guess, after polling a few people in the industry, is that there are probably two to three times as many carbon-fiber bikes sold in the world today as there are titanium bikes. Surprising perhaps, when you consider all the hype that titanium has received. But when you look at how inexpensive a frame from Trek or Giant is, this starts to make sense.
The future of composite bikes will likely parallel my prediction for aluminum rigs – that the material advances will be a lot less significant than our process and execution of making these promising materials work to their best advantage.
Steve Levin, the engineering manager of Schwinn, gave Scot Nicol considerable input for this article. Thanks, Steve.
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