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  1. #1
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    Carbon or aluminum?

    It's time for a new bike. I recently sold my Nomad 2, which I liked a lot, but I foolishly bought a medium when I should have been on a large. My mistake. If someone said that I HAD to buy another Nomad I wouldn't be too sad but I am curious by nature and I want to try something different. I think I've narrowed it down to two DW link bikes, the Mojo HD or the Pivot Firebird. My riding is probably 70/30% trail/bike park. I ride local CT trails and head up to Highland bike park in New Hampshire several time a season. I have never ridden a carbon bike. I am not hard on my bikes, I'm not as aggressive as I was in my younger years but I want something that I can take to the bike park and have some fun with. Any input would be appreciated.

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    aluminum is tougher typically, you dont make a carbon frame to be tough, you make it be lighter, thats what people pay for. you CAN make a tougher carbon bike, but they wont.
    that said, carbon is repairable in your garage, aluminum is not repairable at all. but, typically with a long travel bike, you will break before the frame does.

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    The above reply is completely wrong. A carbon frame will be stiffer, stronger, better fatigue life, and lighter than a comparable Al frame. No real drawbacks if you can afford it, especially if you're not planning on slamming into rocks all the time.

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    Quote Originally Posted by Dichotomous View Post
    aluminum is tougher typically, you dont make a carbon frame to be tough, you make it be lighter, thats what people pay for. you CAN make a tougher carbon bike, but they wont.
    that said, carbon is repairable in your garage, aluminum is not repairable at all. but, typically with a long travel bike, you will break before the frame does.
    Probably the most inaccurate statement I've read on MTBR. It's completely opposite of the truth.

    To the OP: You should post on the Pivot and Ibis site (I have the OG Mojo). You're going to get biased reviews but if you keep that in mind, there is still good info there. Obviously try demoing if you can.

    If you're going by specs and opinions alone, I would say the FB is more "DH" oriented than the HD. IMO, the carbon vs. AL debate shouldn't be a debate or rather, shouldn't be a concern to you. I'd toss out that difference and concentrate on which bike in terms of ride, sizing, cost, aesthetics (yes, it matters ), customer service makes more sense to you.

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    depending on how carbon is built there can be plenty of drawbacks. but I guess I am sorry to be the one to post something other than carbon is the greatest creation ever and everyone should buy carbon everything.

    as the owner of several carbon item, all of which have started getting brittle and cracking and also knowing that typically the reason for carbon is weight (otherwise you'd use chromoly for even better stiffness and stength) then the bike will be built to weight.

    notice how in my post I referance how carbon CAN be tougher? CAN but rarely is in this application.

    you will pay considerably more for the same bike in carbon, and it will be lighter, but I would be suprised if tests showed it stiffer or stronger.

    go ahead and refute my statements, the "everything carbon" people always do. I am a fan of it myself, but there are times when its not needed, is worse, and its always more expensive.

  6. #6
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    Agree- I use to think that carbon was light and weak- after one ride on a mojo hd it changed my mind- the only drawback with carbon is that you may not notice the damage till it fails- other than that it's better in nearly every way to alu- oh and Price- lol

  7. #7
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    Quote Originally Posted by Dichotomous View Post
    aluminum is tougher typically, you dont make a carbon frame to be tough, you make it be lighter, thats what people pay for. you CAN make a tougher carbon bike, but they wont.
    that said, carbon is repairable in your garage, aluminum is not repairable at all. but, typically with a long travel bike, you will break before the frame does.
    This post might have been accurate if it were made 8-10 years ago.

    The HD is in fact made to be strong rather than light, even the SL and SL-R are made with strength as the number one priority, weight savings second. Same with the Nomad Carbon and most of the newer carbon frames out there. It's incredibly stiff and strong. The main problems(and the bad reputation) come from replacing aluminum bits with carbon and using the same exact design. You have to design carbon fiber parts differently.

    OP, you wont go wrong with either bike. I'd test ride them both and see which one fits better.

  8. #8
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    On my MTBR I have broken every carbon part I have purchased except for my carbon steering tube spacers.
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  9. #9
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    Quote Originally Posted by Dichotomous View Post
    you will pay considerably more for the same bike in carbon, and it will be lighter, but I would be suprised if tests showed it stiffer or stronger.
    Prepare for surprise!

    When it was released, the Santa Cruz Blur LTC was not only stiffer & stronger than the AL version, it was stiffer & stronger than their AL V10. IN SC's own words it's, "The strongest bike we have ever built, bar none."

  10. #10
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    Quote Originally Posted by Dichotomous View Post
    depending on how carbon is built there can be plenty of drawbacks. but I guess I am sorry to be the one to post something other than carbon is the greatest creation ever and everyone should buy carbon everything.

    as the owner of several carbon item, all of which have started getting brittle and cracking and also knowing that typically the reason for carbon is weight (otherwise you'd use chromoly for even better stiffness and stength) then the bike will be built to weight.

    notice how in my post I referance how carbon CAN be tougher? CAN but rarely is in this application.

    you will pay considerably more for the same bike in carbon, and it will be lighter, but I would be suprised if tests showed it stiffer or stronger.

    go ahead and refute my statements, the "everything carbon" people always do. I am a fan of it myself, but there are times when its not needed, is worse, and its always more expensive.
    Think about what you're saying and go re-read your OG message.

    you dont make a carbon frame to be tough, you make it be lighter, thats what people pay for. you CAN make a tougher carbon bike, but they wont.
    How you qualify that is beyond me. Physically, carbon is lighter. Doesn't mean that bike manufacturers sacrifice strength for weight. There are plenty of bikes that are built using carbon with the exact goal of being stronger than the same AL frame. You may argue that they aren't physically stronger but you don't even acknowledge the fact that manufacturers are able to manipulate carbon is such a way to make it stronger and lighter than AL.

    carbon is repairable in your garage,
    Ridiculous

    as the owner of several carbon item, all of which have started getting brittle and cracking and also knowing that typically the reason for carbon is weight (otherwise you'd use chromoly for even better stiffness and stength) then the bike will be built to weight.
    Which items? I call BS.

    you will pay considerably more for the same bike in carbon, and it will be lighter, but I would be suprised if tests showed it stiffer or stronger.
    Carbon is stronger and stiffer than AL (all else being equal). That is it's physical property. The manufacturing process will affect the strength, weight, durability of the finished product. Not all carbon bikes are built the same just like not all AL bikes are built the same. Either can be "bad" depending on how it's manufactured/engineered. Blanket statements like "Carbon is bad" is just as ignorant as saying "AL is bad".

  11. #11
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    Quote Originally Posted by Dichotomous View Post
    aluminum is tougher typically, you dont make a carbon frame to be tough, you make it be lighter, thats what people pay for. you CAN make a tougher carbon bike, but they wont.
    I totally disagree. For quite a few years now, CF has been proving itself at least as strong as AL counterparts for similar applications, be it frames, rigid forks, bars, and seatposts. I've had an Easton Monkeylite CF bar get twisted so hard it bent a DH stem (fell off a car on the highways), but the bar did not break. Plenty of CF "AM" frames have been proving perfectly tough over the past few years.

    I generally do not pay the price difference for CF, but I would not worry about the toughness as long as it is the appropriate application for the part (just like with Al).

    To the OP: I would not worry much about the CF vs AL aspect of your decision, there are other, more important factors. Having demoed some Pivots, I want one so bad I might have to sell a kidney to get one.
    15mm is a second-best solution to a problem that was already solved.

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    Quote Originally Posted by Eh-ron View Post
    Prepare for surprise!

    When it was released, the Santa Cruz Blur LTC was not only stiffer & stronger than the AL version, it was stiffer & stronger than their AL V10. IN SC's own words it's, "The strongest bike we have ever built, bar none."
    would SC come out and tell you that its anything but? how many bikes would they then sell. pull your head out of the sand. plus, if weight was NOT the main issue, they would have made it stronger out of aluminum, just more of it, they wanted to use a more expensive material with more hype than plain old boring aluminum. as far as price, show me the same bike in carbon and aluminum where the carbon is cheeper, stronger, lighter......

    again, I am NOT against carbon, I love it, even though it has failed me on many items, and failed others too. I am still going to homebrew myself a full squish MTB out of it. I am not saying carbon is worse. my goodness people dont read posts. I LIKE CARBON. I just dont think its the end all and be all and final word for everything, there are alternatives and sometimes they are better.

  13. #13
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    Barring getting hit by a truck, attacking it with a power tool or having a massive crash, the carbon frame will be much tougher. In five years time a carbon bike will be as strong as it was when new, an alloy frame will not.

    I ave no experience at all of Pivot bikes (hard to get in the UK), but Mojos are built tough more than they are built light. You would have to try quite hard to snap one.

    Although, if it was my money I'd be giving Doc at Superco a call, steel FS bikes are marvellous things

    Quote Originally Posted by Dichotomous View Post
    aluminum is tougher typically, you dont make a carbon frame to be tough,.
    Please stop talking rubbish.

  14. #14
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    as many above have already mentioned, carbon fiber provides much more strength compared to aluminum without having to add on more weight. and, i don't know what kind of shop dichotomus has in his garage...but, it must be awesome since he can repair carbon fiber frames. haha.

    actually, the major drawbacks of carbon fiber is when it fails, it's typically catastrophic and unrepairable. just like your typical thread, once carbon fiber is "cut," it's hard to put them back together. still, there are plenty of shops who specializes in carbon fiber repair...or you can visit dichotomus's garage.

    other than strength, one of the mechanics from lbs said that over time, aluminum will stretch and fatigue, while carbon will maintain its core structure and strength. i think his statement has some merit, but i don't think you really have to worry about fatiguing your aluminum frame over the lifetime of the frame.

    personally, i am leaning towards a carbon bike for strength and looks. good luck!

  15. #15
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    Quote Originally Posted by saki2mi View Post
    i don't know what kind of shop dichotomus has in his garage...but, it must be awesome since he can repair carbon fiber frames. haha.
    It's much easier to repair Carbon than most people think, a small hole or crack can be patched at home provided you buy the right kit to do it.

    If you're feeling really ambitious, you can weld and heat treat (small) alu parts like linkages and swingarms at home too. You just need a tig welder, an oven, a bath tub and a really understanding wife!

  16. #16
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    carbon itself is not what they make frames from, if it were they would be brittle and shatter easily. its a cloth, which only has tensile strength. combined with a resin or plastic or epoxy (or concrete on buildings and bridges, but we wont go there for now).
    so saying carbon is stronger is just silly. saying anything is stronger is just silly. carbon can be MADE stronger, and sometimes is. Al is a metal and uniform(ish) in its density, and has similar properties all around, carbon is a composite, built of several items. its basically strong strings oriented in plastic or resin. the strings keep the resin from pulling itself apart. you cannot ever say a blanket statement of "carbon is stronger" because it very much depends on the design, the materials used, if there is fiberglass and kevlar in there as well.

    things they make out of carbon that are better than aluminum in many ways: sailing ship masts, gold clubs (technicall steel alloy is their competeition), tennis rackets, airplanes now.

    gosh, my post said that most aluminum frames are stronger, I still stand by this, there are a few they make now at very large price increases that are stronger in carbon, but the majority of carbon out there is not built to be as impact resistant.

    carbon does have a "small" problem of UV resistance, it has none, and will fade and get brittle, like the very expensive hood of my car, which after subjected to snow and summer sun and wind decided to crack in half. thats a non structural part even. so paint it you say, good, now you can't see if that rock you hit with your bike (these are bikes that get ridden and people try to progress on right? rock garden impacts should be expected) that left a small scratch, but would have left a small dent in a metal frame, well is that a structural issue with the fibers under the paint that you cant see, will it fail completely next time? did you know that most composite structural items are considered done after severe impacts? also, carbon doesnt often fail catastrophically, it takes a more effort to fully break if its even reasonably built. I've seen aluminum snap like a twig before, carbon usually rips and tears, it doesnt bend break though.

    also, carbon sucks for abrasion resistance, cables will eat through it, chainsuck will eat it fast.

    carbon IS easy to make, comparitively. a foam form or better yet a half mould and a guy with gloves to lay cloth is cheeper and less skilled than a skilled welder to work with AL. and used correctly can be very good, if not the best.

    I'm sure the santa cruz vp10 carbon and ibis mojo hd are great bikes. the frames cost what now?

    you 13lovers will have to forgive me inserting reasonable doubts and explainations of a materials disadvantages. you all forgot to see where I wrote lots of nice things about how good carbon is, oops, I said something bad about it.

  17. #17
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    Quote Originally Posted by Fix the Spade View Post
    It's much easier to repair Carbon than most people think, a small hole or crack can be patched at home provided you buy the right kit to do it.

    If you're feeling really ambitious, you can weld and heat treat (small) alu parts like linkages and swingarms at home too. You just need a tig welder, an oven, a bath tub and a really understanding wife!
    yeah, at worst you can throw a few more layers on top to shore it up, epoxy and carbon fabric, wet it and lay it and squeeze it, let it cure. all done.

  18. #18
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    question, do you see many carbon fiber bike park bikes? dirt jumpers, BMW freestyle?

  19. #19
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    Quote Originally Posted by Dichotomous View Post
    question, do you see many carbon fiber bike park bikes? dirt jumpers, BMW freestyle?
    Not yet...
    The simple mater of the fact is carbon is being targeted at much higher price point than that found on DJer and "park bikes" because at the moment, the process does cost quite a bit higher than welding and heat treating extruded Al tubes. Once the process of dealing w/ carbon becomes more affordable, you will. And, considering the application of carbon/resin which is indeed softer than aluminium oxide, in a mostly dirt environment a carbon DJer would be quite ideal since there isn't the sharp impact of large rocks to deal with.

    The UV caused brittleness has been addressed in the resin... I'm thinking your car hood must be either a few years old, or you went and cheaped out and bought one that didn't have the correct resin, but this is purely speculation on my part.
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    Quote Originally Posted by Dichotomous View Post
    yeah, at worst you can throw a few more layers on top to shore it up, epoxy and carbon fabric, wet it and lay it and squeeze it, let it cure. all done.
    Yeah right...
    Prob good enough for your car hood, but it sure as hell won't be enough to be taken back out on a WC DH course gain. Once the fiber is cut, the part has no more strength because the fiber/resin matrix sole purpose is to transform compression forces into tension forces. So, unless you have some top secret military nano-tech where you can rejoin the fibers on the molecular level, you're not going to repair anything structural "in your garage". BTW, do you realized that "few sheets" results in milometers of thickness, where as in the SC V10, the head tube region is damn near 1/2" thick of carbon/resin?
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    uv resistance CAN be fixed in the resin. often it is not. it NEVER needs to be addressed with aluminum.

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    Quote Originally Posted by Pau11y View Post
    Yeah right...
    Prob good enough for your car hood, but it sure as hell won't be enough to be taken back out on a WC DH course gain. Once the fiber is cut, the part has no more strength because the fiber/resin matrix sole purpose is to transform compression forces into tension forces. So, unless you have some top secret military nano-tech where you can rejoin the fibers on the molecular level, you're not going to repair anything structural "in your garage". BTW, do you realized that "few sheets" results in milometers of thickness, where as in the SC V10, the head tube region is damn near 1/2" thick of carbon/resin?
    do some searching please. and they DO have nanofibers that you can add into your epoxy. the specific fibers cut wont regain their strength. but a minor cut can but gone over with about that much carbon, roughed up along the frame and since those fibers were only pulling, the new fibers pulling is the same thing. if you cut through or break through 1/2" thick, that frame is done.

  23. #23
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    my frame is a carbon frame with a couple of aluminum pieces. I have broken the aluminum parts of the frame. but not the carbon. don't be afraid of carbon. and i am a bigger guy who is hard on stuff.

  24. #24
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    Quote Originally Posted by Dichotomous View Post
    question, do you see many carbon fiber bike park bikes? dirt jumpers, BMW freestyle?
    I notice that you did not include DH bikes in your question Why would that be?
    15mm is a second-best solution to a problem that was already solved.

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    Interview with Joe Graney of Santa Cruz bikes:

    T-HOFFonline – Joe Graney- Santa Cruz Bicycles engineer- Carbon fiber & the V-10

    Seems convincing to me. Yes, I realize he wants to sell bikes for his company, but what motivation would Santa Cruz have for selling people weak, dangerous products -- the ruin of their company's reputation (at the least) and the risk of losing millions in personal injury lawsuits? I'm pretty sure they're confident in the strength of this product.

    If it were me and money were no object, I'd give CF a try. I believe the product is just going to get better and cheaper in the future, too, just as aluminum frames are better and cheaper than they were 15 years ago. Maybe in time it will become the industry standard, as well. Who knows?

    Oh, and chromoly frames are not stiffer than aluminum or carbon, as was mentioned somewhere above. Every BMXer knows a chromoly frame is pretty flexy compared to aluminum.

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    Quote Originally Posted by The R View Post
    Oh, and chromoly frames are not stiffer than aluminum or carbon, as was mentioned somewhere above. Every BMXer knows a chromoly frame is pretty flexy compared to aluminum.
    1x1x6" solid beam, which is stiffer, carbon, chromo, or al, or heck, Ti?

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    I'm guessing since you're trying to make a point that I'll go with the chro-mo beam, But we're not talking solid beams, we're talking tubes made for bicycles welded together to make a frame. A chro-mo bike frame has a lot more flex than an aluminum one.

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    Quote Originally Posted by Dichotomous View Post
    ...the strings keep the resin from pulling itself apart.
    Your understanding of "carbon composite" is backwards.
    The resin gives those threads form, else they're just limp strings. When producing carbon parts, you're trying to get rid of as much of the resin as you possibly can...hence the vacuuming bagging process. If executed properly, it's the strings' tensile strength, thru orientation, that overcomes the compression forces, which is being held in place by the resin. The resin doesn't support any force as you've described.

    Anyway, I can pick apart you post bit by bit and embarrass you by pointing out your mistakes. No real point in doing this. Maybe you should read up more on the fundamentals of carbon composites, and what kind of tech is being applied today to overcome some of its drawbacks.
    The bottom line is Al has reached its peak of potential, carbon composites hasn't even come close to what it can potentially do. Is today's carbon composite tech perfect for bike application? No, there are compromises being made, but that can be said of ANY material...aluminum included. But are those compromises made by carbon composites acceptable...for the most part, yeah. The argument on this thread is which part of that compromise you can/can't live with. And, from your mis-understanding of the technology, you're drawing some pretty off conclusions.
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    Quote Originally Posted by The R View Post
    I'm guessing since you're trying to make a point that I'll go with the chro-mo beam, But we're not talking solid beams, we're talking tubes made for bicycles welded together to make a frame. A chro-mo bike frame has a lot more flex than an aluminum one.
    same tubing size, same thickness, same geometry entirely. now your answer? so why aluminum? its lighter. point made.

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    The aluminum frame is both stiffer and lighter. The chromoly frame is marginally more durable, but it is not as stiff as the aluminum.

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    Quote Originally Posted by Dichotomous View Post
    same tubing size, same thickness, same geometry entirely. now your answer? so why aluminum? its lighter. point made.
    I'm going to call this BS until you show me some lab results.
    I say carbon composite...a sample that's properly executed/designed to meet the requirement of your spec.
    Why? Because they're not putting that parameter of material inside a plane's wing made of anything else other than carbon, and those guys have done their homework (Boeing, Lockheed, Airbus, etc). But, keep in mind, these guys design these parts to have flex so as to not kill the plane's occupants.
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    Quote Originally Posted by Pau11y View Post
    Your understanding of "carbon composite" is backwards.
    The resin gives those threads form, else they're just limp strings. When producing carbon parts, you're trying to get rid of as much of the resin as you possibly can...hence the vacuuming bagging process. If executed properly, it's the strings' tensile strength, thru orientation, that overcomes the compression forces, which is being held in place by the resin. The resin doesn't support any force as you've described.

    Anyway, I can pick apart you post bit by bit and embarrass you by pointing out your mistakes. No real point in doing this. Maybe you should read up more on the fundamentals of carbon composites, and what kind of tech is being applied today to overcome some of its drawbacks.
    The bottom line is Al has reached its peak of potential, carbon composites hasn't even come close to what it can potentially do. Is today's carbon composite tech perfect for bike application? No, there are compromises being made, but that can be said of ANY material...aluminum included. But are those compromises made by carbon composites acceptable...for the most part, yeah. The argument on this thread is which part of that compromise you can/can't live with. And, from your mis-understanding of the technology, you're drawing some pretty off conclusions.
    lets go there then! with no "force" or compression, how do those strings stay where they are? in a structure facing more than simply tension, you need compression, thats where the resin comes in. you dont need much, but you need whats there, otherwise wouldn't it be better and considerably lighter to JUST use the carbon cloth? coll your shirt in a tube shape and tell me how well it does in compression from anywhwere. twist it, push it, bend it. you NEED compression resistance.

    tell me again how the resin/plastic does nothing? carbon is ONLY the tensile reinforcement of a plastic or resin shape. again, if the carbon fabric or unidirection strings (think hair here, try all of the above with hair too) can not do the job alone, they NEED the resin. and if its needed, then its a structural component and has properties that must be measured and CAN be lacking.

    now, where is my understanding lacking? I'm always eager to soak up info and have mistakes corrected by those better than me.

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    Quote Originally Posted by The R View Post
    The aluminum frame is both stiffer and lighter. The chromoly frame is marginally more durable, but it is not as stiff as the aluminum.
    if the solid peice is stiffer, so is the frame, its just heavier. same is same. if shaped differently, then you have to factor in the shape. an I beam or tube made up of the same amount of material as a solid, is stiffer than that solid.

  34. #34
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    I think we need to read the 'whole' post that someone has made and not quote just a sentence or part of it as it may not be sufficient to explain the individual's point of view ... but when a thread like this or Shimano vs Sram etc comes up ... it makes an 'interesting' reading ...

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    Quote Originally Posted by Pau11y View Post
    I'm going to call this BS until you show me some lab results.
    I say carbon composite...a sample that's properly executed/designed to meet the requirement of your spec.
    Why? Because they're not putting that parameter of material inside a plane's wing made of anything else other than carbon, and those guys have done their homework (Boeing, Lockheed, Airbus, etc). But, keep in mind, these guys design these parts to have flex so as to not kill the plane's occupants.
    and every part of airplanes before this was aluminum, before that wood. before that wood and cloth. hey look, full reversal!

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    Quote Originally Posted by Dichotomous View Post
    lets go there then! with no "force" or compression, how do those strings stay where they are? in a structure facing more than simply tension, you need compression, thats where the resin comes in. you dont need much, but you need whats there, otherwise wouldn't it be better and considerably lighter to JUST use the carbon cloth? coll your shirt in a tube shape and tell me how well it does in compression from anywhwere. twist it, push it, bend it. you NEED compression resistance.

    tell me again how the resin/plastic does nothing? carbon is ONLY the tensile reinforcement of a plastic or resin shape. again, if the carbon fabric or unidirection strings (think hair here, try all of the above with hair too) can not do the job alone, they NEED the resin. and if its needed, then its a structural component and has properties that must be measured and CAN be lacking.

    now, where is my understanding lacking? I'm always eager to soak up info and have mistakes corrected by those better than me.
    This will be a longish reply...I'll edit this post in an hour or so as I need to get on the road right now. But think about how a wheel is laced...compression resistance is achieved thru tension.

    Edit: here we go...lemme address the wheel first
    The compression forces being supported by any spoke (the CF analog in this response) as a part of the wheel is basically negligible. After all, the nipple is not anchored in such a way that it can support the spoke being compressed. The circular shape of the rim is kept circular by spoke tension...the downward force of the rider's weight is felt at the axle, and responded to by tension to the top of the rim. The squashing effect experienced by the rim is countered by the 9 and 3 o'clock spokes. Across the bottom of the rim, the tension of the spokes keeps the rim straight to transmit the bike's load to the ground. The rim experiences compression from both the spokes tension and the rider weight from the hub. The circular shape distributes the any radial compression force on its plane evenly around its circumference. On side loading...say a right turn...a rider's weight is felt on the lower (rt) side of the wheel's spokes as tension. The load is transferred out to the rim, then the same process to the ground contact point. Therefore, a wheel stays together thru tension. When rolling along, it's primarily the tension across the top of the rim that supports the bike. The rim experience compression, but is countered by spoke tension.

    Now, the resin...
    If you ever played w/ a block of the most ideal (epoxy) resin, it's really plastic! It cannot support much loading at all before the pressure heats it up and turns it into a liquid. This isn't to say it experiences no compression at all, but it serves mostly to anchor fiber threads for tension. It does this by increasing the coeff of friction (bonding) between each carbon fiber thread, and sheets of carbon fiber cloth. Think the resin as the the tie and solder of wheels if you've ever done that. And, you know what happens to a tied and soldered wheel in terms of stiffness...

    The carbon fiber...
    Interesting stuff in how they make it (take notice of the tensile strength in the "Synthesis" section w/ respect to heating: Carbon (fiber) - Wikipedia, the free encyclopedia)
    Here's another link to a material data table (take notice of the ultimate tensile strength - SO high, density - SO low, hardness - from what I gathered...not hard(?), and Young's modulus - pretty much like that of steel)

    So, it's this ultimate tensile strength and low density/weight that engineers take advantage of. When you weave these strands (the simplest being 90 degree, and over and under) you're creating anchoring points at every intersection for a carbon fiber strand. Now, since they have no rigidity, the only way they can support any weight is thru tension, implying these fibers cannot be completely horizontal. When you do a over/under weave with say a horizontal distance of 1.5 times the carbon fiber strands diameter, you get an angle of:
    arctan(1/1.5) = 33.7 degrees, and there's your vertical component.

    When you push down vertically on a sheet of carbon fiber composite, all these over/under points would try to give just a tiny bit as you tension and un-tension the region's fibers. But since those intersections are all anchored at such a short length by the resin, the hardened cloth doesn't move because the fiber doesn't stretch. The resin does experience a bit of compression here. However, that's why in this config, the sheet is pretty weak. But if only a single layer is hardened (X-Y axis only), you can flex the sheet on the Z axis because these anchoring points do give a tiny bit and the over/under cannot resist all of the vertical force since it's only at a fraction of the 90 degrees. Effectively, it's like a spring kinda like when you coil up a length of steel. But if you laminated two sheets together, you now have a Z component to counter flex thru tension on the outside of the flex. Think welding two springs together along its side. The more layers you add, the more you counter the flex w/ higher and higher tension on the outside of the flex. It's kinda like when you side load your wheel...the outside of the flex is tensioned.

    If the resin can support a lot of compression forces as something stand alone, then it would imply the stuff is quite inflexible, because it would necessarily need to be very cohesive to keep itself together under pressure. If this is the case, you wouldn't be able to get the carbon fiber leaf spring used in some of those prosthesis. You can also tweak the weave and thus the amount of tension the fibers would experience per a given amount of deflection...the resin moves and then you get the flex.

    Re-reading your post, I think we're basically arguing the same point. But your idea of the resin capable of a lot of compression forces is what might be off.
    Last edited by Pau11y; 08-05-2011 at 05:50 PM.
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    Quote Originally Posted by chinaman View Post
    I think we need to read the 'whole' post that someone has made and not quote just a sentence or part of it as it may not be sufficient to explain the individual's point of view ... but when a thread like this or Shimano vs Sram etc comes up ... it makes an 'interesting' reading ...
    if it were named "mojo HD or Pivot firebird, things would have gone far different. Fun though. I love debate.

    I do enjoy discussions such as these, in case there is confusion as to my posts. I also really like carbon and intend to make many things of it when I stop remodeling my house.

    I'll adjust my original post to reflect that posters are stating that the MOJOHD is indeed built for strength over lightness. which is good.
    Last edited by Dichotomous; 08-05-2011 at 01:59 PM.

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    Quote Originally Posted by Pau11y View Post
    This will be a longish reply...I'll edit this post in an hour or so as I need to get on the road right now. But think about how a wheel is laced...compression resistance is achieved thru tension.
    well ok, but make sure you eliminate any compression resistance in the rim, use a string..... also remove the compression resistance between the spoke flanges in the rim, they must be able to move twoards each other.

    I will save you the trouble though. any structure which must any force other than tension, MUST include compression members or compression resistance.

  39. #39
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    why not steel or titanium?

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    Somehow I didn't see this coming. It sounds like a debate in Washington, everyone has a strong opinion whith their own facts to back them up. The truth is in there somewhere when you combine everything. Anyhew, I'm going to demo a Mojo HD next week. Unfortunately I have to drive to NY to find one to try. Still trying to find a Pivot to demo. I rode one briefly in a lbs parking lot the last ime I was shopping for a bike. Ultimately I found a good deal on the Nomad and that was the end of it.

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    Quote Originally Posted by The R View Post
    The aluminum frame is both stiffer and lighter. The chromoly frame is marginally more durable, but it is not as stiff as the aluminum.
    BMXs and steel xc bikes are only flexy because they insist on using such narrow and thin walled tubes. BMXs are always made out of horrible cheap steel too.

    I have hardtail with nice fat 1.75inch supertherm tubes, thick-ish walled stays and a 44mm head tube, noticably stiffer when hauling the bars than any alloy hardtail I've had (except the Scirocco I briefly owned, that was vicious).

    Talk to the builders in frame building, use decent quality metal and proper sized tubes, you'll get a steel bike at least as stiff as most alloy frames. Maybe not some of the ones with massive oversized hydroformed head tube junctions, but stiff enough.

    PS, I really hate people referring to steel frames as flexy, they ain't unless designed to be so!

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    My only reservation with C is it doesn't handle impacts all that well. C passes far tougher strength tests than AL, but it's still in the early days as far as impact resistance is concerned. Strength doesn't equal durability always. I still ride C bars in spite of this, however, I don't know how I'll feel when I put scratches and gouges in those bars.

    Also, check the Pivot forum, the carbon rocker on the FB had issues and people were breaking them. There's some good pictures on mtb of these broken rockers. My nomad's c upper link has been perfect for 2.5 years. All C is not equal: there are far more variables than with metals. This is a good thing because the material can be customized to a far greater extent than metals, and like mentioned above, we are nowhere near the apex of C technology. Who knows where composite technology with be in 5 years?

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    Quote Originally Posted by slimat99 View Post
    My only reservation with C is it doesn't handle impacts all that well. C passes far tougher strength tests than AL, but it's still in the early days as far as impact resistance is concerned. Strength doesn't equal durability always. I still ride C bars in spite of this, however, I don't know how I'll feel when I put scratches and gouges in those bars.

    Also, check the Pivot forum, the carbon rocker on the FB had issues and people were breaking them. There's some good pictures on mtb of these broken rockers. My nomad's c upper link has been perfect for 2.5 years. All C is not equal: there are far more variables than with metals. This is a good thing because the material can be customized to a far greater extent than metals, and like mentioned above, we are nowhere near the apex of C technology. Who knows where composite technology with be in 5 years?
    You're right on C being a bit weak on impact. But the key word here is "composite". SC added a stainless plate on the chain stay...which def falls under the "composite" ideal. I don't believe anyone's set a hard/fast rule to where you can't integrate other materials into carbon composites to get the ideal properties you need. Hell, Kevlar, Spectra, and if you want to go nutz, braided Ti or hastelloy mesh can be put on the surface...
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    I'm pretty sure several brands use Kevlar in the downtube to protect against rock strikes..

    OP - Demo the bikes you're interested in & make your own decision. I think the bike scene is at the point where a CFRP frame from a reputable mfgr is going to be perfectly fine. Use common sense & use it for it's indended purpose (don't expect a composite XC race frame to hold up to DH abuse), but that holds true for any bike/component.

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    This post got me thinking. I've never seen any hydroformed steel
    frames, why not? Also Trek claims their new carbon frames are
    made to have more impact resistance than aluminum. I don't know
    if any of this is true, but it makes me think.

    Best, John

  46. #46
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    Quote Originally Posted by Dichotomous View Post
    question, do you see many carbon fiber bike park bikes? dirt jumpers, BMW freestyle?
    because $1000 dj hardtail frames dont sell.

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    Quote Originally Posted by John Kuhl View Post
    This post got me thinking. I've never seen any hydroformed steel
    frames, why not? Also Trek claims their new carbon frames are
    made to have more impact resistance than aluminum. I don't know
    if any of this is true, but it makes me think.

    Best, John
    I hope trek has figured something out that others haven't. They and Specialized have the money for R&D so I would assume they will progress C tech better than anyone else.

  48. #48
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    I have an old cf trek 9800 hardtail that's probably close to 9 years old now, I rode it hard for 3 years then I went full suspension. That same bike has been through several transformations including singlespeed and then snowbike, now it's got plans to be transformed into a 1x9 (or 1x10) winter road bike and around town errand spinner to save the roadie from road salt. My point is even though it was built to be one of the lightest frames of its day, even though it doesn't see abuse like it used to it's still going strong. cf can be just as strong or stronger than other materials, usually lighter and stiffer depending how its designed. Its weakness may be hard direct impacts but that's why the layup is built thicker around vulnerable areas, alum will fail also from a dent, fatigue and stress crack. Am I going to ditch my steel SS, heavy alum downhill and jumper, or scandium xc bike just to go all CF? No. I have plans to sell the freeride and xc bike and buy an AM bike, very possibly cf, final choice to be decided yet, but having bikes made of everything except ti, I have no reservations about riding cf hard and am not the least bit worried as long as it is built for the type of riding I plan to do on it. If cf could not handle it why would companies like trek and santa cruz make lighter downhill bikes out of the stuff and bomb it down world cup courses for all to see it fail?
    Last edited by masterofnone; 08-06-2011 at 04:03 PM.

  49. #49
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    Quote Originally Posted by b-kul View Post
    because $1000 dj hardtail frames dont sell.
    Actually the price would probably reflect something more along the lines of $1400-$1600!

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    Quote Originally Posted by 2clue View Post
    Actually the price would probably reflect something more along the lines of $1400-$1600!
    true, but in a few years it should (hopefully) come down.

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