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 fiber used. My Cervélo R5 doesn’t say anything about the material in any of the literature. Is my wife’s bike better than mine?
That’s a great question…
Recently we have seen bikes advertising 100%High Modulus, Super High Modulus, Aerospace grade Carbon… and list goes on and on. The truth is that we are now often being asked what modulus our frames are made of, and the answer is both very simple, and very complicated.
The easy answer is this: We use all kinds of fibers in every frame we make.
The answer most people are looking for is far 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 fibers as any other industry using carbon fiber. Because just three Boeing 787 Dreamliners use more carbon fiber than the entire bicycle industry in a year, no one in cycling gets special fibers 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 fiber frame from another however, has everything to do with HOW and WHERE in the bike’s frame and fork carbon fibers are used, and only to a lesser extent, to do with WHICH fibers are used. One very real problem with using carbon fiber 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.
Basic lesson on carbon fibers
To understand the myth of modulus, we must first think about two characteristics which direct material choice. They are Strength and Stiffness. Strength is defined here as the amount of force that can be applied to material before it breaks (fails). Often mistakenly interchanged with strength, think about stiffness as the amount a material deforms when a given 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 explain the idea 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 however, 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 fibers. The bragging rights associated with the use of high modulus fibers suggests that the bike is super stiff. However; remember what happened to the spaghetti noodle? High modulus carbon fiber may be stiff, but it is not very strong and thus--like the pasta--breaks with less force than lower modulus fibers. Simply put: fibers 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 fibers cost much more than lower modulus fibers, over 10 times more in some cases.
When engineering a frame, there are a series of factors to consider, and each of these affect fiber selection for any given location on the frame. Stiff fibers may be required to resist bending (flex), strong fibers may be employed to resist failure (breaking), and the cost of fibers chosen impacts the overall production cost. Different areas of the bicycle require different mixtures of fibers. The front of the bike, for example, experiences different forces on different planes than does the bottom bracket junction. This is the same story for the down tube, the seat tube, the drive side chain stay as opposed to the non-drive side chain stay and so on. Because each of these locations demands a unique mixture of stiffness and strength to resist the numerous unique load cases, at Cervélo we utilize location specific laminations (layers of fibers at different angles) of high strength and high stiffness fibers to achieve the desired performance.
The basics of lay ups
For most of us who have been around a while, the image we have of carbon fiber is a sheet of woven fibers often seen through the clear coat of earlier bicycles. Although still around and sometimes used for their high strength, woven fibers are less stiff than the unidirectional fibers used more commonly in newer frames. Using layers of unidirectional fibers (all the fibers in one layer are aligned in the same direction) allows engineers to make the most of the high stiffness and light weight of carbon composites, however doing so requires much more detailed knowledge about composite materials. It is easier to design a frame using all woven material, but it will not be as stiff or light as one designed properly with unidirectional material. For frame engineering, knowing which fibers to place where and what directions to put them in is critical when attempting to reduce weight, maintain strength and still provide a stiff bike which is comfortable to ride. An advantage of composites is you get to place each fiber where it belongs; the challenge is that you have to place each fiber where it belongs! Here is where good engineering takes over from simple material choice. The key is that we need to select different types of fibers and carefully position them in the correct locations and orientations to best exploit their properties. Using advanced engineering software tools like Finite Element Analysis (FEA) and Ply Draping software allows us to better understand exactly how each layer of carbon fiber is working and if it is being used properly. Of course, all of these tools would be worthless without the in-house engineering knowledge to understand how they work and correlate them with real world testing. The more critical the relationship between strength, stiffness, and weight becomes, the greater the number of precisely placed and carefully chosen layers are required. This is one of the reasons that, currently it is only possible to make a frame as light and stiff as the Cervélo R5ca in our Project California facility. There, we have the ability to very precisely control the ply size, shape, and position tolerances. (Yes, there is no bicycle frame as light and stiff as the Cervélo R5ca.)
Figure 1: woven carbon fibers often used as top sheet
The long and short of carbon fiber engineering
When a bicycle company pays better attention to the engineering details, they build a better bike. The process of choosing correct fiber 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 fibers in the right place. No fancy marketing names required.
Figure 2: FEA results comparing different layups under head tube impact load