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  1. #1
    merckxxx
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    carbon fiber modulus

    what is the significance of the modulus of carbon fiber?? I knbow it has to do with how many threads of carbon fiber per inch in the cloth that is use to mold the object (frame, crank etc..) but I dont know what that means in terms of high number heavy and stiff????? low number cheap but light (and flexable)??

    what should a rider who is around 200lbs and considering racing be looking for in terms of the carbon material in the frame ??/

    I would like to consider a carbon frame for my next bike but I dont know what to consider in the way of strength vs ultralight???
    thanks

  2. #2
    Long-time Curmudgeon DrPete's Avatar
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    I can speak semi-intelligently about CF modulus as it applies to Pinarello's frames because I was shopping for them.

    Basically, as the modulus goes up, the strength to weight ratio goes up. So, for a frame like the Paris Carbon, they use the 46K carbon and the frame weighs in at 900g and change. The F4:13 is built to a very similar spec in terms of stiffness and ride quality, but because it uses a lower-modulus carbon they made the frame heavier by $200 (and Cheaper by $900) to yield similar strength and ride characteristics. The F3:13 is also built to a similar spec as the F4:13, in fact the same molds are used, but again, cheaper, lower-modulus CF, so the weight is higher and the price lower. The F3:13 also gives up the pretty CF weave, so it gets a paint job instead of just clear coat and paint accents.

    I have to throw in my shameless plug for Competitive Cyclist here. Give them a call, discuss your riding style, current bike, and what kind of riding/racing you want to do and they'll give some good advice. FWIW, I'm the same weight and 6'2, and the bikes they recommended highly for me were the Cervelo R3 and the Pinarello Paris Carbon/F4:13. I got the F4:13, and in addition to the amazing ride, the total weight for a Chorus/Fulcrum Racing 1 bike is about 16lb.

    Here's a much more eloquent explanation from their website:

    Quote Originally Posted by Competitive Cyclist
    A critical distinction between the Paris and the F4:13 is the composition of the carbon itself. You'll see a downtube decal on the Paris that reads "Carbon 46HM3K." The key here is the number 46. This signifies that the carbon used can withstand a pulling force of 46 tons per square mm. The F4:13 is made from 30 ton per square mm carbon. This "elongation factor" speaks directly to the weight and to the ride characteristics of a bike. The quality of the 46HM carbon allows Pinarello to use less material in building a frame with optimum stiffness and durability. You can build a frame of equal stiffness and durability from 30HM carbon, but it requires more material so it'll weigh a bit more. This is the central reason why a medium sized Paris Carbon will weigh in at 950g, but an F4:13 is more in the realm of 1100g.

    The "3K" designation specifies the finish of the frame itself. The Paris' 3K finish means that each visible square of material contains 3 carbon "strands". The analogy between carbon fiber and expensive dress shirts is useful here -- the more tightly you weave a fabric, the higher its quality. According to Pinarello the tighter weave goes beyond aesthetic concerns. A more tightly woven carbon provides superior strength -- again, allowing the builder to use less material -- one more reason why the Paris weighs so little.
    "Unless he was racing there was no way he could match my speed."

  3. #3
    Peloton Dog patentcad's Avatar
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    >>I can speak semi-intelligently <<

    This gives you a 50% advantage over most BF members at any given time, myself included.

  4. #4
    Senior Member
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    I work in materials r&d and am only barely versed in carbon fiber. That being said, I probably have had more experience in CF composites than most here (I think that there are a few aerospace guys who can speak to this stuff much better than I). As far as I know, there isnt a specific CF expert in the forum. The reason that I say that is I am barely versed is that the depth of understanding required to make a leading quality carbon composite part is literally a lifelong pursuit.

    To make a claim like a higher modulus fiber creates a stiffer frame is true, but highly simplified. Low modulus carbon is actually the most common fibers used in bikes. For high end, the CF is actually an intermediate CF and often only used in chain stays and areas like that. True HM carbon fiber isnt used in the bike industry, except for perhaps in small quantities in the most specialized locations, same as the intermediates, but even less widely used. Generally speaking, the higher the modulus, the lower the tensile strength of the fiber. Finding the right trade off is key. A super high modulus material is a ceramic. They are also very brittle because they are so stiff. Not a good choice for a bike. The true name of the game, in terms of weight is to have a fiber with adequate tensile strength, very high modulus and very high toughness (ability to withstand energy input - not break). That way, you can keep backing off of the amount of fiber (and thus weight) while still keeping the frames integrity. This is exactly what the Pinarello info that Dr Pete posted is talking about. But, the higher the modulus fiber you put into the mold, the more brittle it is. Therefore, the raw materials that are used in the frame have to be chosen closely. And this is just focusing on weight. When you throw in other factors, such as ride quality (infinite stiffness would suck on a bike), workability of material, availability, layup design, and then finally price, it is basically impossible to select a bike from a property table of the carbon fiber. There are just far too many variables to throw in to be able to extract a bike's ride characteristics from a CF material property data sheet.

    I know that the above is not very clear, but I could ramble on for days about all the ins and outs, and again, I am only a novice in understanding. Composites are an artform, and are infinitely more complex than tube properties, due to all the added variables in composite construction.

    I know that was really not answering your question, so here is the more direct answer:

    1) There isnt a relationship between fiber size and modulus. A high mod fiber can be made coarse or fine. Finer fibers are more difficult and more costly to spin. Their advantage is that they will make a more uniform part, since they will have more fibers to spread variation across and the bond with the resin will be better due to more surface area. They are also more workable, as the bundles will be easier to flex (think steel cable vs steel rod of same size).

    2) To get a better idea of what fibers are in premium frames, take a look at this link. This is the best composites for dummies guide around. It also has a list of fiber properties in the materials section. The intermediate modulus fibers will often be in premium framesets, but not always.

    http://www.netcomposites.com/education.asp

    3) The construction of a high quality, high strength frame will be unidirectional carbon fiber (just about every frame is built this way). The cloth that you often see is an asthetic, which does theoretically provide better vibration dampening, but that is debatable.

    4) Practically speaking, at 200 lb, staying away from an uber-light frame might be a good idea. However, any carbon frame of any reputation will more than likely be plenty stiff for you. Try to find some testing for BB stiffness, as this is the best overall lab measure of frame stiffness (not really the whole story, but a good number). The BB stiffness, not normalized for weight, will be the theoritical winner.

    The absolute bottom line is that any reputable frame will probably be an amazing ride. The super-high price point frames, such as Pinarello, Scott, Trek (ugh!), et al will all be well engineered and perform well. Even the commodity carbon frames made in good plants in Taiwan (ADK, Martec, etc) will be well designed and constructed.

    Note: I edited to make the above more coherent from the original, now that I am not at work.
    Last edited by jamiewilson3; 01-05-07 at 07:07 PM.

  5. #5
    merckxxx
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    whew.... ... that is some big words..

  6. #6
    DocRay
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    Quote Originally Posted by jamiewilson3

    4) Practically speaking, at 200 lb, staying away from an uber-light frame might be a good idea. However, any carbon frame of any reputation will more than likely be plenty stiff for you. Try to find some testing for BB stiffness, as this is the best overall lab measure of frame stiffness (not really the whole story, but a good number). The BB stiffness, not normalized for weight, will be the theoritical winner.
    200lb is not a problem, few manufacturers have any stated weight limits for frames. Ultralight frames like the CR-1 or R3 tests stronger than much heavier frames by impact testing or EFBe fatigue testing. These frames are also stiffer in the head and BB than most heavier frames.

    Heavier=stronger is no longer true. Heavier=cheaper to produce is more likely.

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