The myth of modulus
My wife’s bike has a decal on the top tube that reads “Super High Modulus,” so I am assuming this refers to the carbon fibre used. My Cervélo R5 doesn’t say anything about the material in any of the literature. Is my wife’s bike better than mine?
We are often asked what modulus our frames are made of, and the answer is both very simple and very complicated.
The simple answer is this: We use many kinds of fibres in every frame we make. But it’s too complicated to sum up in a frame sticker or fancy marketing name. Whereas the term “high modulus” has become a symbol for the idea of “light and stiff,” it fails to address other important characteristics like strength and comfort. To start with, it’s important to understand that all bicycle companies have access to the same carbon fibres as any other industry using carbon fibre. Because just three Boeing 787 Dreamliners use more carbon fibre than the entire bicycle industry in a year, no one in cycling gets special fibres made for them. (Unfortunately, in the global economic hierarchy, bicycle production does not take precedence over the production of jumbo jets.) What does differentiate one carbon-fibre frame from another, however, has everything to do with how and where in the bike’s frame and fork carbon fibres are used, and only to a lesser extent, to do with which fibres are used. One very real problem with using carbon fibre modulus as a marketing tool is that there is no consistent modulus scale. One company’s “Super High-Mod” may very well be another’s midrange.
To understand the myth of modulus, we must first think about two characteristics that direct material choice: strength and stiffness. Strength is defined here as the amount of force that can be applied to material before it breaks (fails). Often mistaken for strength, stiffness is the amount a material deforms when force is applied. A stiffer material will deform less under the same force compared to a less stiff material.
Modulus (more specifically Young’s Modulus) is the engineering term for stiffness of a material. When asked to illustrate the concept of modulus (or stiffness), we often employ some basic household items: a rubber band and a length of dry spaghetti. Using the terms above, the rubber band is very strong because is it easily bent out of shape without yielding (permanently deforming), and will return to its original shape when the force is released. A rubber band is very hard to break, but is very flexible as it takes very little force to deform it. The uncooked spaghetti noodle is the opposite. It is very stiff as it resists deformation until it ultimately snaps, suggesting that it is not very strong. Think of the rubber band as low modulus and the spaghetti as high modulus carbon fibres. The bragging rights associated with the use of high modulus fibres suggests that the bike is super stiff. However; remember what happened to the spaghetti noodle? High modulus carbon fibre may be stiff, but it is not very strong and thus — like the pasta — breaks with less force than lower modulus fibres. Simply put: fibres that are higher modulus (stiffer) are also weaker, and ones that are lower modulus generally offer higher strength (harder to break). It is also important to note that higher modulus fibres cost much more than lower modulus fibres, over 10 times more in some cases.
When a bicycle company pays better attention to the engineering details, they build a better bike. The process of choosing correct fibre modulus is not driven by marketing, rather it is the result of detailed FEA and lamination analysis, which allows engineers to understand load cases and optimize performance. Only after we know the differences between real world riding vs. tests vs. design can a much higher performance frame be produced using the right fibres in the right places. No fancy marketing names required.