| There’s a lot of hype about carbon fiber bikes these days, and with that hype comes a lot of confusion. Just what is high modulus fiber and what does it mean for you? Is a higher modulus always better? What’s the difference between monocoque and tube-and-lugged or tube-to-tube construction? And which one is better? It’s a fact: all carbon bicycle frames are not created equal, even if they’re built with exactly the same type of carbon cloth. Just like people, it’s not what’s on the outside that counts; it’s what’s on the inside. From design, to fiber, to resin, to lay-up schedule, to testing, to control of the manufacturing process; if there’s any cheating or pretending going on, it will show in the ride. Every time. So here’s some carbon hype-busting info, from the inside out. |
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Carbon fiber is just that, extremely thin fibers composed of carbon atoms bonded together in microscopic crystals that are aligned parallel to the long axis of the fiber. These fibers are twisted together to form a yarn, which is then woven into a fabric in any one of many different weave patterns. Because the density of carbon fiber is much less than that of steel, aluminum or titanium, yet has extremely high tensile strength, it is ideal for applications requiring low weight, like for our Xenith, XCR and Dakota bike frames.
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Fiber filaments are rated by their tensile strength and by their modulus (stiffness). Code words are T-1000, T-700, M50, M40, or M30. These attributes don’t always correlate with each other. Some carbon is higher in tensile strength, but lower in modulus or stiffness or vise versa. To optimize performance, reduce weight and increase durability, we have to balance fiber strength with fiber modulus, a challenge that requires FEA computer modeling, incessant machine testing and rigorous field testing of prototype after prototype to hone and fine tune the ride until that elusive sweet spot is found. It almost always means we will have used multiple types of fiber in any given frame to achieve that goal, and that’s what we mean when we refer to Omniad (primarily one type of fiber), Dyad (two types of fiber) and Triad (three types of carbon fiber) lay-ups on our frames and forks.
But the carbon fiber fabric is not the only hero here. Resin, which binds the fibers together, is as critical to the weight and strength of the structure as is the carbon fiber. Too much resin binder and you have a heavy, dead feeling frame. Too little and you risk fiber separation and failure. The ideal binder will be of sufficiently low viscosity to thoroughly coat all fibers with the least amount of resin utilized, allowing a higher concentration of carbon fiber in the yarn, resulting in a stronger structure. That binder will also need to possess high impact resistance, without deadening ride qualities. Where many manufacturers source their carbon fiber pre-impregnated with stock resin binders, we source our carbon fiber from the most trusted names in the business, Toray, Toho and Mitsubishi and then secure resin binders to our specification from specialists in that field. For 2010, we’ve specifically focused on finding resin binders that enhance the impact resistance of our frames, to better preserve your frame in the event of a crash or a fall. |
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We’ve designed, built and ridden both carbon fiber monocoque frames as well as tube and lug carbon frames, and frankly, our monocoques were always lighter, more durable, and simply rode better. The overlap of tube and lug in a lugged frame always yielded extra weight and always served to concentrate stress, since material flow was discontinuous at the bonded joint. We think this disruption in stress flow at the lugged joints also contributed to a deader feel in the frame. The beauty of a monocoque is that is it completely unified, and stresses are distributed over a greater portion of the frame structure, instead of being concentrated at the joints. Which allows us to design a lighter, stiffer, stronger frame and one that rides with a snap and liveliness feels that like it has a life of its own.
But a monocoque’s structural integrity relies not simply on the carbon fibers or resin binders selected, it’s in the lay-up schedule, the master plan for the location of each and every carbon ply that makes or breaks the monocoque. We start with FEA computer modeling. Finite Element Analysis software visualizes where structures bend or twist and simulates the distribution of stresses and displacements. It allows us to design, refine and optimize the materials and lay-up of that material in our frames and forks before cutting molds and burping prototypes. It’s rarely absolutely perfect the first time. But it gets us to prototypes for machine and field testing so much faster, with greater reliability in “almost there” results, that we can’t fathom how we ever designed without it. |
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That said, it’s still just Step One in the process, and it’s at this point that the PC geek squad hands it over to the factory engineers who then relentlessly cycle test for fatigue on every single frame size, with deflection tests for stiffness at every point of the frame. If Chief Engineer Dr. No says a prototype doesn’t pass (and his nickname should indicate how often they haven’t), then it’s back to the lay-up room for some material massaging and ply re-arrangement.
Once Dr.No has given his go, then it’s time for us to do the hard part of our job. Ride! We ride and record, ride more and record more. Then we send the frames on to our pro riders, for more evaluation and comment. The process is dull, at least the part is where we have to ride back to the office and record more, but the beauty of carbon fiber is that it can be so easily tuned by manual manipulation of the plies. Almost like the head of a drum or the string of a guitar. Ride quality, the balance between stiffness and resiliency, the ability to feel the road, these are all attributes we’d love to be able to quantify absolutely/positively for the geek squad so that everything that burped out of their PC’s and downloaded into our first molds was spot on. But we’re not there yet. (And frankly, we’re kinda glad we’re not. It might mean less rides on weekdays.)
The thrill of the test rides, the high technology of FEA simulation and the carbon fiber material itself often overshadows the skill and artistry of the workers who accurately apply small squares, rectangles and triangles of carbon fiber according to the schedule our engineers assign. But it is their skill and precision that accounts in great part for the ride called Xenith or XCR or Dakota. We don’t see their work because it’s hidden beneath a cosmetic layer of 1K, 6K or 12K weave that’s hand finished for hours before it’s painted and clear coated, but it’s no less skillful and significant than the precise TIG-weld beading on our aluminum frames or the brazing work on the investment cast lugs of our steel frames. |
These carbon fiber swatches are laid up on a silicone mandrel, one section of the frame at a time, with each carbon fiber ply and each layer of plies interwoven and overlapped so that when fully cured, the monocoque is fully unified (hence the name, monocoque). As the lay-up for each section is complete, the silicone mandrel is removed, then each section is joined to the other sections, completing the monocoque. Expandable air bladders are run through the frame, the frame is placed in a steel mold and the mold in an oven, bladders are pressurized, the oven is heated to melt and disperse the resin, and then the whole thing is cooled to harden and cure.
As important as every component of this process has been so far, compaction is where it’s at as far as carbon fiber structural integrity is concerned. If the interior design has constrictions that bind bladders or the bladder material doesn’t sufficiently sustain air pressure, fiber wash or wrinkling in the fiber and pooling of resin is likely. While this is not unusual in most carbon fiber frames today, it represents unnecessary additional weight and a possible stress riser. That’s why we’ve taken monocoque molding technology to the next level with our Near Net Molding technology and featured it on our 2010 Xenith SL, Xenith Team and the new Dakota dXC Team models. NNM is a patent pending process that uses both air bladders and a polystyrene pre-form core that recedes as the oven heats, assuring an interior that is “near net” in finish. Meaning: an inside that is as smooth and pristine as a baby’s butt, and your assurance that every gram of resin has been compressed, every length of fiber has been flattened and aligned.
After hours of hand finishing, before heading on to the painters and clear coaters, EVERY frame is weighed to make sure it’s neither resin rich nor resin deficient. We also measure the stiffness of each frame
in 6 critical areas as a check on lay-up production. Each deflection test must fall within 5% of the standards our machine and field testing have established. This weighing and stiffness deflection testing guarantees every single frame we produce meets all Jamis manufacturing protocol and will deliver the ride qualities we defined and demand.
If this is all starting to sound like the sort of hype we promised to dispel, forgive us. We know we’re on to something and we just want to share it. If you need some credible, objective insight and feedback to verify our Xenith claims, just check out the video review of the Xenith SL by cycling legend Frankie Andreu for Cyclist Village. Or better yet, head on down to your Jamis dealer for a test ride. It’s all hyper-bull until you click in and put it down. |
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