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  1. #1
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    why not carbon fiber?

    It seems that all the true touring bikes don't have carbon fiber forks. Is breakage the reason? Is this a worry for me? I bought a giant ocr3 with a carbon fiber fork to use for light touring and commuting until I can afford a true tourer.

  2. #2
    nm+
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    I did it with a suspention fork (Indy and an early SID) with no issues so a carbon fork shouldn't be an issue.
    They're not on touring bikes because a steel fork is stronger and absorbs the bumps better/equally. Weight doesn't matter that much on a tourer. the real issue is that carbon forks don't have the rack and fender eyelets we need if we want a front rack.

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    DEADBEEF khuon's Avatar
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    Quote Originally Posted by nm+
    They're not on touring bikes because a steel fork is stronger and absorbs the bumps better/equally. Weight doesn't matter that much on a tourer. the real issue is that carbon forks don't have the rack and fender eyelets we need if we want a front rack.
    It seems too many people concentrate on CF's weight (or lack thereof)... including the manufacturers. There's absolutely no technical reason why CF can't be used to make a touring fork. In general, CF is actually stronger than steels. CF can also be made more compliant than steel and can be tuned in such as way as to absorb road shocks yet still remain stiff for better handling. The fact that CF forks in general lack rack and fender eyelets is more a marketting matter than anything else. It can technically be made and will yield a superb fork.
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    CF forks are already made with mudguard eyelets and are regularly used on Audax bikes and occasionally on cyclo-cross bikes in Europe. Koga-Miyata produces a CF touring bike with CF forks. Obviously the USA is being fashionably late, again...

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    Quote Originally Posted by khuon
    It seems too many people concentrate on CF's weight (or lack thereof)... including the manufacturers. There's absolutely no technical reason why CF can't be used to make a touring fork. In general, CF is actually stronger than steels. CF can also be made more compliant than steel and can be tuned in such as way as to absorb road shocks yet still remain stiff for better handling. The fact that CF forks in general lack rack and fender eyelets is more a marketting matter than anything else. It can technically be made and will yield a superb fork.
    Carbon fibre has all those magical properties IF it is laid up perfectly with no bubbles etc.
    And the ability to withstand impacts or scratching is the worst of any material used in bike construction.

    In practice it rarely is flawless, so carbon has struggled to get out of the racer market where bikes are regularly checked over and frames are purchased with the understanding that they have a finite lifetime.
    There is literally boatloads of cheap Chinese/Taiwanese carbon kit in the road market at the moment, but the war, Airbus and large scale wind farm development has meant that there is a global shortage so prices are set to rise.

    If you are into credit card touring or smooth road cycling in Europe then carbon could be for you, but like mountain bikes, carbon will always be second to steel in the touring market.

    Reynolds 953, which can be built as light as carbon but has the durability of steel has cured me of all carbon lust.

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    Senior Member SteelCommuter's Avatar
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    Quote Originally Posted by nm+
    I did it with a suspention fork (Indy and an early SID) with no issues so a carbon fork shouldn't be an issue.
    They're not on touring bikes because a steel fork is stronger and absorbs the bumps better/equally. Weight doesn't matter that much on a tourer. the real issue is that carbon forks don't have the rack and fender eyelets we need if we want a front rack.
    Are you drawing a conclusion from an experience from a MTB suspension fork? I don't think most people will be using a carbon MTB fork for touring...

  7. #7
    Lentement mais sûrement Erick L's Avatar
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    Steel is reliable and cheap.
    Erick - www.borealphoto.com/velo

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    CF and steel are both very strong and durable. Steel is a bit heavier. But on a touring bike, weight is not an issue, and you can easily epair steel when it breaks - that's the essential thing. Alos, attaching rack and fender eyelets is trivial with steel.

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    Carbon fiber can be as strong or stronger than steel, but is prone to failure if it gets damaged at all (like scratches that penetrate the outer layer and get into the carbon fiber). It's just not really nearly as durable or reliable as steel.

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    Senior Member halfspeed's Avatar
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    Quote Originally Posted by radical_edward

    Reynolds 953, which can be built as light as carbon but has the durability of steel has cured me of all carbon lust.
    Has anyone actually built a bike with this that is not tubing manufacturer's prototype?

  11. #11
    DEADBEEF khuon's Avatar
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    CF has a reputation for notch sensitivity but this can also be mitigated through proper construction. I'm riding an all CF MTB frame that's seen quite its share of crashes. Here's a couple of recent responses of mine in the Road Cycling forum to a similar question of CF durability.

    Quote Originally Posted by khuon
    Theoretically, most CF materials have a near-infinite fatigue life. From a materials standpoint, CF can be cycled far longer than steels and titaniums at higher stresses. And they definately surpass aluminum in terms of fatigue life because aluminum has no fatigue limit. In the real world, it all has to do with the design. If the design promotes conservative stress application on the material such that applied loads do not exceed the fatigue curve then materials such as carbon fibre, steels and titanium will last forever. Aluminum because it has no fatigue limit will continue to lose strength as it's cycled so eventually it will fail. However, most designers build in an adequate lifetime for aluminum bike frames regardless of material such that it is almost impossible for one to accumulate that many cycles as to cause fatigue failure.

    The other thing that effects a bike frames useful life is overstress which can cause instant material failure. Generally speaking, CF has a very small plastic region in that the yield and UTS are spaced pretty close to one another. In other words, instead of bending, most CF will catostrophically fail. Steel on the other hand tends to have a pretty large plastic region in that once yield strength is hit, they will permanently deform before breaking. However, there are two things to keep in mind here. One is that CF is typically stronger in all aspects than most metals. And secondly, it can be engineered with a tensile modulus that will allow it to bend before reaching yield. As long as stresses are kept between the tensile modulus and yield, CF will simply bend and rebound back to shape. This can be done with metals too but they generally have a smaller region at a lower strength. The beauty in CF is that the tensile modulus of the material and by combining different types of CF layed in different directions, the structure can be micro-engineered and tuned to a much greater extent than metals.
    Quote Originally Posted by khuon
    Quote Originally Posted by bjkeen
    How do carbon fiber and aluminum and steel and titanium stack up next to abrasion damage? Meaning, how likely are they to take a deep nasty scratch from something, and how serious is that scratch,
    It really depends on the material and the exact structure. A scratch on many modern tubes used in cycling today regardless of material is something to be concerned with simply because even with metals which generally have a high abrasion tolerance, the materials are bing made thinner and thinner. In general however, steel is more abrasion resistant than titanium and titanium is more abrasion resistant than aluminum.

    However, CF is very notch sensitive. Once the weave in the layer has been compromised, it loses almost 90% of its strength. A single surface/superficial abrasion is usually nothing to be concerned with as long as it's only a clearcoat scratch. A deeper scratch however can be a problem and if the weave has been cut then that layer is done for. The good news is that most CF structures are layed up in several layers. This is done for several reasons. One is to increase not only the amount of material through cross-section padding and thus increase strength in that respect but also to add some redundancy into the material such that a failure of one layer does not compormise the entire layup. Another reason is that CF is anisotropic in nature and oftentimes the prepregs used in the layup are unidirectional as opposed to multidirectional thus it will be stiff in one direction but compliant in another. Rather than using a multidirectional weave which in actuality would limit or make it more difficult to tune the structure, the designer will specify different weave biases for different layers thus creating a more isotropic layup in the end so that it will be stiff in multiple directions.

    I run a CF seatpost on my MTB. My frame has an interrupted seat-tube and even a short-length road post is too long because it comes very close to the top of my rear shock when at extension. If I compress the shock, there is a good chance it would impact the bottom of my post. Thus I decided to trim it. The post is an Easton CT-2 which is now called the EC-70. You can see from this cross-section of the trimmed portion that there are five distinct layers.




    Quote Originally Posted by bjkeen
    and how repairable is it?
    Most of today's bike frames and I believe all road bikes use a themoset bonding method for their CF construction. Technically this type of construction does not yield itself to being repaired. There may be some tricks you can do to some frames with debonding the epoxy at the lugs (assuming the frame uses lugged construction). In which case, the damaged section could be replaced and a replacement could be rebonded into place. But other than that once damaged, it's a goner. One-piece molded thermoset frames cannot be repaired. My CF MTB frame and a scant few others were produced using thermoplastic methods. In theory, this type of CF material can be repaired. However the technique is complicated, prone to errors and not easily accomplished by someone who doesn't have a lot of experience working with composites and CF.


    Quote Originally Posted by bjkeen
    Once a stress riser or crack has started to develop, how fast does it typically progress?
    This is a very difficult question to answer. The reason a stress riser is a problem is because the loads at that portion exceed the tensile strength of the material thus the material fails in that region. Thus to answer your question, one would have to know what the loads are imposed around and in that area of the stress riser. A stress riser without enough load to exceed the material's tensile strength won't grow the fracture.
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  12. #12
    DEADBEEF khuon's Avatar
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    Quote Originally Posted by radical_edward
    Reynolds 953, which can be built as light as carbon but has the durability of steel has cured me of all carbon lust.
    Reynolds 953 is a great material... on paper. It has the UTS (~2GPa) that's pretty close to that of low-strength carbon fibre materials (~3GPa) and 25% greater than that of typical steels tubesets such as 853 (~1.4GPa). However, I fear that much like with CF, some (many?) manufacturers will be tempted to use such thin tubesets that they will also be prone to real-world durability issues. If you find yourself interested in a frame or fork made with 953 then like with CF, you need to pay close attention to the construction process and design.

    One reason I chose Aegis for my roadbike frame was because they decided to build their bikes from intermediate modulus CF which have a UTS (~6GPa) that's three times greater than 953. IM CF is not as light as say the UHM used in the Trek OCLV bikes though. However it is more durable. And Aegis uses a substantial amount of layers. My roadbike frame weighs in around 3 lbs... not terribly light for a full CF frame but much more capable of withstanding real-world hostile environments than one made from thinner higher modulus CF.
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  13. #13
    Senior Member SteelCommuter's Avatar
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    Khuon,

    Thank you for the informative recap from other threads. Two things:

    1. My guess is that CF could be great for touring, but all previous designs have been for racing or light riding. Even a MTB has different stresses than a touring bike, which may be carrying 100 pounds in addition to the rider in the front and back. I concede I am no engineer, but I imagine that mountain bikes and touring bikes have very different design priorities and cannot be easily equated.

    2. I notice a reference to high stress tests as a source of comparison of the materials. For the purposes of a bicycle, a low stress high frequency cycle test is far more significant, isn't it? So maybe if you run into a curb 50 times at 40 mph, your CF fork, designed correctly, may not break and perhaps the steel one will. But that isn't a terribly useful piece of data, because that's not generally how I ride or break my bikes.

    Let me know if I'm mistaken. I find the discussion very interesting.

  14. #14
    DEADBEEF khuon's Avatar
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    Quote Originally Posted by SteelCommuter
    Thank you for the informative recap from other threads. Two things:

    1. My guess is that CF could be great for touring, but all previous designs have been for racing or light riding. Even a MTB has different stresses than a touring bike, which may be carrying 100 pounds in addition to the rider in the front and back. I concede I am no engineer, but I imagine that mountain bikes and touring bikes have very different design priorities and cannot be easily equated.
    It's all in the design. From a material's standpoint, it doesn't care if it's being applied to a MTB design or a touring bike design. It has certain properties and these properties are more than applicable in both domains. CF can easily be made to carry 100 pounds in addition to rider. It depends on how the structure is designed. Afterall, CF is used in tandems. A tandem is typically carrying much more weight than a loaded touring single. You are pretty much spot-on in that the previous and most current designs only apply CF towards racing and lightweight bikes. I'm not saying that you can take a current CF frame or fork that has been designed for racing and use it for purposes of touring. You have to design the structure properly. I find myself developing a mantra for bicycle frame materials: "The material is largely immaterial. To paraphrase a popular saying: 'It's the design, stupid.'"

    Quote Originally Posted by SteelCommuter
    2. I notice a reference to high stress tests as a source of comparison of the materials. For the purposes of a bicycle, a low stress high frequency cycle test is far more significant, isn't it? So maybe if you run into a curb 50 times at 40 mph, your CF fork, designed correctly, may not break and perhaps the steel one will. But that isn't a terribly useful piece of data, because that's not generally how I ride or break my bikes.

    Let me know if I'm mistaken. I find the discussion very interesting.
    You are largely correct. Typical road cycling usually involves stresses far below shock-impacts seen in say MTBing and thus things like UTS and yield do not directly come into play. What does come into play is fatigue life. A fatigue curve (S-N) charts a material's fatigue life. It depicts stress vs number of cycles and relates the decay of tensile strength as a function of how many times the material is cycled. In most ferrous metals (such as steels) and titanium the curve decays to a plateau or limit. This is known as the fatigue limit and basically says that after a certain number of cycles, the decay stops and as long as you cycle the material at stresses below that tensile strength, you will never fail the material. Aluminum however has no such limit and will continue to decay. Assuming that the average road stress remains constant, you will eventually fail an aluminum structure. The key of course is to design the structure so that loads are kept well below this curve. This is why aluminum frames are so overbuilt. CF also exhibits a fatigue limit and thus can be cycled indefinately below a certain stress level. However, the decay curve of CF is typically well above that of steels and titaniums thus the amount of stress can be higher.

    I am not saying that CF is every bit a substitute for steel. There are other factors that go into a bicycle design than just weight. While I am a proponant of CF, I do find that the applications sometimes are just silly and done so for the sake of doing so which is not necessarily a bad thing. Other times, CF is used merely for asthetics and serves no more purpose than to act as high-tech wicker. But if a design is well thought out and brings out what I think are the more important qualities of CF (flexibility in design, tunability of ride quality, accomodates radical shapes) then I believe CF has been applied effectively. That said, my next bike will most likely be steel not because I'm disenchanted with CF but just because I like variety and want a steel bike. Previous to CF, I rode steel bikes and wish to get another.
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  15. #15
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    Quote Originally Posted by khuon
    Reynolds 953 is a great material... on paper. It has the UTS (~2GPa) that's pretty close to that of low-strength carbon fibre materials (~3GPa) and 25% greater than that of typical steels tubesets such as 853 (~1.4GPa). However, I fear that much like with CF, some (many?) manufacturers will be tempted to use such thin tubesets that they will also be prone to real-world durability issues. If you find yourself interested in a frame or fork made with 953 then like with CF, you need to pay close attention to the construction process and design.
    I doubt that 953 will be used by any other than boutique frame builders for a long time. Independent Fabrications say that it is twice as hard to work as titanium. It will be very hard to find a disreputable frame builder willing to work in 953. Still, one can dream....

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    The reason you don't see any carbon frokss on touring bikes is simply that they don't have the "braze-ons" for mounting front racks and fenders and carbon forks don't like silly clamps.

    I'm sure that if they used the latest cyclocross forks as a basis and the manufacturers put on the proper fittings they would make a GREAT touring fork.

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    Fuji, Koga-Miyata and Thorn all do touring forks with low-rider mounting points. The forks are quite a bit heavier than race forks and there is little experience on how they cope with the harsh life of loaded touring.

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