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  1. #1
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    Stomping on a BMC downtube

    I'm not looking to start a flame war between metal vs carbon,
    it was not "intended use", but whoaah that was a little too easy!


  2. #2
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    I'm pretty sure if that was a steel bike the tubes might not have broken, but they sure would have bent and flattened.

    It would have been different damage, but damage none the less.
    Too old to ride plastic

  3. #3
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    That bike has a delaminated top tube as well.

    Pity we don't recycle them or something, they just go in the bin.
    use a torque wrench

  4. #4
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    Quote Originally Posted by velodog View Post
    I'm pretty sure if that was a steel bike the tubes might not have broken, but they sure would have bent and flattened.

    It would have been different damage, but damage none the less.
    I'm inclined to agree with you, and I'm a fan of steel.

    The problem is, the market has tried to keep steel competitive with carbon in the WEIGHT department, and in doing so, must make the walls so thin that denting is a greater possibility.

    Carbon frames are just not meant to take impacts. If they were designed so, the weight savings would likely be lost. I also think the trend of building ATB frames out of carbon is dumb. Abrade a carbon frame against a rock or a tree as too often happens, and the frame is history. Metals are much more durable in the mountain bike arena.

    As was pointed out, the top tube on the BMC already has damage. I have to wonder whether the video's author bothered to see whether it could be repaired. I give credit to the rising industry of carbon frame repair facilities as in many cases the repairs are stronger than new, cheaper than steel frame repairs, and look pretty good even without paint.

  5. #5
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    Quote Originally Posted by velodog View Post
    I'm pretty sure if that was a steel bike the tubes might not have broken, but they sure would have bent and flattened.

    It would have been different damage, but damage none the less.
    The video doesn't work for me, but generally speaking if we're talking lightweight steel I think you're definitely correct. And carbon would be a lot cheaper to fix.
    Probably not always the case though. I have a 19 pound cross bike made of steel and while I don't know anything for fact I sense the tubing is much stronger than the typical carbon cross bike on the market. Heavier too.
    High end steel road bike vs high end carbon race bike I tend to think you're right on. Different damage but still damage.

  6. #6
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    Quote Originally Posted by Peter P. View Post
    Carbon frames are just not meant to take impacts. If they were designed so, the weight savings would likely be lost. I also think the trend of building ATB frames out of carbon is dumb. Abrade a carbon frame against a rock or a tree as too often happens, and the frame is history. Metals are much more durable in the mountain bike arena.
    yeah, I'm not sure how much of that is fact though vs less than optimal carbon designs that have hit the market (as opposed to what's possible with carbon) and internet group think.
    My carbon frame has taken some pretty good hits and been fine. Hits I'm pretty sure would have dented Columbus spirit or similar.

    And I know bikes and other things are apples and oranges but I can't help think about carbon hockey sticks. yes they can and do break from impact but they can also take some really hard impact over a long period of time while at the same time being lighter than possible with any metal. Not that most of what is on the market for bike choice reflects this.

    I'm also a fan of steel and Ti and retired my carbon frame in favor for it. But durability didn't factor into that decision.

  7. #7
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    Quote Originally Posted by Peter P. View Post
    I have to wonder whether the video's author bothered to see whether it could be repaired. I give credit to the rising industry of carbon frame repair facilities as in many cases the repairs are stronger than new, cheaper than steel frame repairs, and look pretty good even without paint.

    The video author is most likely the most experienced carbon bike repair specialist in the world. His yellow marks on the top tube is how I know it's delaminated.

    If he won't repair it, it's for a damn good reason.

    He will and does repair forks, something hardly any repair outlet will do, he will repair and rebuild anything if it's safe to do so.

    If it's not safe to do so, he will utterly destroy the product so nobody else can half-ass repair it and put it on the road or market.
    use a torque wrench

  8. #8
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    I guess that's the way to test the durability of bicycle frame under intended use.

  9. #9
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    This seems wrong. No matter how you hate carbon bikes, smashing them to bits is so senseless. Couldn't he turn it into a doorstop or planter, or some other useful item?

  10. #10
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    Quote Originally Posted by Peter P. View Post
    I'm inclined to agree with you, and I'm a fan of steel.

    The problem is, the market has tried to keep steel competitive with carbon in the WEIGHT department, and in doing so, must make the walls so thin that denting is a greater possibility.

    Carbon frames are just not meant to take impacts. If they were designed so, the weight savings would likely be lost. I also think the trend of building ATB frames out of carbon is dumb. Abrade a carbon frame against a rock or a tree as too often happens, and the frame is history. Metals are much more durable in the mountain bike arena.



    As was pointed out, the top tube on the BMC already has damage. I have to wonder whether the video's author bothered to see whether it could be repaired. I give credit to the rising industry of carbon frame repair facilities as in many cases the repairs are stronger than new, cheaper than steel frame repairs, and look pretty good even without paint.
    Watch this and get back to me...

    https://www.youtube.com/watch?v=w5eMMf11uhM
    I work for some bike racers
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  11. #11
    .je
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    Quote Originally Posted by Peter P. View Post
    I have to wonder whether the video's author bothered to see whether it could be repaired.
    Here is a followup video where he explains how damaged that frame actually was, and explains how damaged the frames that he has are. In the background are dozens more frames he's scanned or examined. He also lists some industrial background he has in structures.

    Last edited by .je; 2 Weeks Ago at 11:50 AM.

  12. #12
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    For those not familiar about the author. A little background on him. His name is Raul (sp?), is probably one of the most experience carbon specialists in the cycling industry. He has done consulting to the who's who big names in the cycling industry. Prior to cycling, his background is some 20+ odd years working for Boeing doing carbon airplane parts, from rudders to fuelsage, etc. He sometimes speaks at conferences about material science and composites. Recently, he was one of the speakers a such conference at the University of California Irvine (near my place), where players like Boeing, Lockeed, would send their reps to attend. So the guy knows carbon.

    Interestingly, one popular question that he get asked in almost every single one of his video is:
    "Well Raul, seems like you're pretty critical on carbon parts, you seem to find faults in almost all carbon frame you cut up. Then what brand frame do you ride?"
    Raul: "I don't buy. I build my own carbon frame, using a different construction technique used by the bigs in the industry".

    Regarding this specific BMC frame, he mentioned there was damage at the junction where the top tube joins the seat tube and at the bottom bracket area. Damage was too extent to worth reparing. But there was no damage to the downtube. Hence, he too this opportunity to stomp the downtube for the purpose of demonstration on an otherwise unrepairable frame. Now we can sort of validate on what sort of hit this downtube can take.

    I wish he would have stand on and compress the rear triangle (like a sandwich) to see if it would break too. But I think if the triangle would to break, it could potentiall introduce big cracks into the downtube too, which would prevent him from doing his stomping experiment.

    now, we just need somebody to stomp on the downtube of a modern steel frame to see how it would hold up, mainly as a curiosity.

  13. #13
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    Quote Originally Posted by aclinjury View Post
    Now we can sort of validate on what sort of hit this downtube can take.

    I wish he would have stand on and compress the rear triangle (like a sandwich) to see if it would break too. But I think if the triangle would to break, it could potentiall introduce big cracks into the downtube too, which would prevent him from doing his stomping experiment.

    now, we just need somebody to stomp on the downtube of a modern steel frame to see how it would hold up, mainly as a curiosity.
    What would all that stomping prove about bicycle frame?

  14. #14
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    Quote Originally Posted by bvber View Post
    What would all that stomping prove about bicycle frame?
    Absolutely nothing, but its still fun to watch

  15. #15
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    Quote Originally Posted by cxwrench View Post
    Watch this and get back to me...

    https://www.youtube.com/watch?v=w5eMMf11uhM
    First of all, this is a really old video. I remember watching this probably 5-6 years ago, when mtb highend bikes were switching to carbon fiber. Not sure why they reposted this video in 2017.

    But the video is majorly flawed, and I would argue intentionally misleading even. But during the time of the original release of this video, Santa Cruz had a major vested interest in releasing such video because they were selling high end carbon fiber frames.
    But I'm not here to dispute the theoretical characteristics of what carbon fiber could made to be (i.e, it could be made to be stronger than aluminum and steel, given the right construction parameters).

    Still, the test is flawed. For example, here's is a valid critique by a poster in the comments section:

    The "smash the frame against concrete"-test at the end is majorly misleading.

    A carbon frame that suffered from these impacts might feel safe in the hand and not show visible damage but internally those impacts lead to delamination which will result in cracks. As those delaminations are not visible it is a high risk for a rider to use a frame that suffered from such an impact yet from the outside you will not be able to tell if the frame is damaged. This is a reason why more and more companies offer carbon-bike inspection by using x-ray, ultrasound etc. to detect internal damage. This however was costly if performed after each (even minor) crash.

    For that reason aluminium-frames are much safer in that way, that you can easily detect damage from the outside.
    Furthermore quality-control is a major issue in the production of carbon frames. As carbon frames are assembled by hand even the tiniest imperfections (air bubbles, wrinkles, missalignment) can lead to major structural deficits.
    (More in-depth explanation here: https://www.youtube.com/watch?v=EZbg5hCRyvs)

    Lastly the first tests shown in this video are also not quite fair in that regard, that major damage to the carbon frame happens before the sudden cracking. If you listen to it carefully you can here the fibres bursting much sooner (at forces similarly to the aluminium frame). Those cracking sounds indicated damage to the bike that would already render it unsafe.

    In conclusion this video can lead to a quite wrong impression about carbon fiber strength. Ultimately carbon is more vulnerable to sharp impacts and are much more costly to inspect. Fatigue cant be assesed that easily either.
    This test would have been more valid if they had scanned each frame (aluminum and carbon) for internal damage after each iteration. That means they needed to use ultrasound and x-ray, if they wanted their experiment to be robust.

  16. #16
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    Quote Originally Posted by DaveG View Post
    Absolutely nothing, but its still fun to watch
    Just like this.



    Yeah, that was little too easy. If that truck frame was made out of titanium, it would've had a different result.

  17. #17
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    Quote Originally Posted by bvber View Post
    What would all that stomping prove about bicycle frame?
    the author literally explained why he did it in the video above posted by .je
    In this case it's not to prove a point, but to characterize and to validate.
    Furthermore, in science, you can't prove anything; you can only disprove things.
    Last edited by aclinjury; 2 Weeks Ago at 09:47 PM.

  18. #18
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    Quote Originally Posted by Peter P. View Post
    Carbon frames are just not meant to take impacts.
    That's not entirely correct.

    Carbon fiber structures -- like a bicycle frame -- are anisotropic: They exhibit different properties in different directions.

    Yes, it is rather trivial to damage a carbon bicycle frame by standing on the downtube, because the frame is not designed to resist impact/weight/deflection from that direction. Because in normal use [sic] a bicycle never receives impact/weight/deflection from that direction.

    otoh there are plenty of examples of carbon fiber resisting impact from the direction that it was designed to resist impact...which is usually fore/aft on a bicycle frame. That's because even if you never crash, your bicycle is still deflecting along its longitudinal axis (the direction of travel) in normal use. One can find plenty of examples of head-on collisions where the carbon fork was undamaged...even if the steel frame that fork was attached to was damaged.

    In short: Standing (or stomping) on the side of a bike frame tells you nothing about how that frame will perform when somebody is riding that bike.

  19. #19
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    Quote Originally Posted by Bob Ross View Post

    In short: Standing (or stomping) on the side of a bike frame tells you nothing about how that frame will perform when somebody is riding that bike.
    I totally disagree. Let's say you are just riding along and Chuck Norris jumps onto the road and delivers an explosive jumping front kick to your top tube. Wouldn't you want to be riding the bike best able to handle that?

  20. #20
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    Quote Originally Posted by Bob Ross View Post
    That's not entirely correct.

    Carbon fiber structures -- like a bicycle frame -- are anisotropic: They exhibit different properties in different directions.

    Yes, it is rather trivial to damage a carbon bicycle frame by standing on the downtube, because the frame is not designed to resist impact/weight/deflection from that direction. Because in normal use [sic] a bicycle never receives impact/weight/deflection from that direction.

    otoh there are plenty of examples of carbon fiber resisting impact from the direction that it was designed to resist impact...which is usually fore/aft on a bicycle frame. That's because even if you never crash, your bicycle is still deflecting along its longitudinal axis (the direction of travel) in normal use. One can find plenty of examples of head-on collisions where the carbon fork was undamaged...even if the steel frame that fork was attached to was damaged.

    In short: Standing (or stomping) on the side of a bike frame tells you nothing about how that frame will perform when somebody is riding that bike.
    That begs the question: Why wouldn't crashing a race bike be considered "normal use"? Not exactly far fetched.

    If it can be made to resist impact why isn't it? I always figured it was for weight weenie reasons and they thinned out the tubes to look good on the scales. But from what I gather of what you're saying not making them impact resistant is due to stupidity (well, I consider it stupid to not think crashing is 'normal' for a racing bicycle)

    As per my example of hockey sticks I think you definitely right that it could be designed to resist impact, so why not?
    Last edited by Jay Strongbow; 2 Weeks Ago at 07:30 AM.

  21. #21
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    Quote Originally Posted by Jay Strongbow View Post
    As per my example of hockey sticks I think you definitely right that it could be designed to resist impact, so why not?
    Helmets which are safety item, are designed to deal with impact from all sides. Bicycle frames are also designed to deal with certain impact but not the same category as helmets.

  22. #22
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    Quote Originally Posted by DaveG View Post
    I totally disagree. Let's say you are just riding along and Chuck Norris jumps onto the road and delivers an explosive jumping front kick to your top tube. Wouldn't you want to be riding the bike best able to handle that?
    Couple of major flaws with that hypothesis, one, Chuck Norris doesn't jump onto the road, the road bends and comes to him, two, front kick would hit the headtube, not top tube.

  23. #23
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    Quote Originally Posted by bvber View Post
    Couple of major flaws with that hypothesis, one, Chuck Norris doesn't jump onto the road, the road bends and comes to him, two, front kick would hit the headtube, not top tube.
    Yea, you may be right about that. What about a Total Gym randomly falling from the sky on your top tube? That could definitely happen

  24. #24
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    Quote Originally Posted by Jay Strongbow View Post
    That begs the question: Why wouldn't crashing a race bike be considered "normal use"? Not exactly far fetched.

    If it can be made to resist impact why isn't it? I always figured it was for weight weenie reasons and they thinned out the tubes to look good on the scales. But from what I gather of what you're saying not making them impact resistant is due to stupidity (well, I consider it stupid to not think crashing is 'normal' for a racing bicycle)

    As per my example of hockey sticks I think you definitely right that it could be designed to resist impact, so why not?
    Why would manufacturers want to risk adding 200 grams to a frame and make it much more impact resistant in case of a crash? This would defeat the purpose of selling frames AND at the same time get a bad rap for producing a "heavier frame". Like getting a double whammy! Not gonna sit well with the marketing boys upstairs. The racers don't care for impact resistant. And the randoneurs have already made up their minds to with steel/ti. But you right, I'll gladly take a 200g hit for a tougher frame.

  25. #25
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    Quote Originally Posted by aclinjury View Post
    Why would manufacturers want to risk adding 200 grams to a frame and make it much more impact resistant in case of a crash? This would defeat the purpose of selling frames AND at the same time get a bad rap for producing a "heavier frame". Like getting a double whammy! Not gonna sit well with the marketing boys upstairs. The racers don't care for impact resistant. And the randoneurs have already made up their minds to with steel/ti. But you right, I'll gladly take a 200g hit for a tougher frame.
    Probably a combination of that, cost and it may impact the way it bends (you know, laterally stiff and vertically compliant and that jazz)

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