Is Aluminium alloy good for nothing?

Discussion in 'Cycling Equipment' started by sogood, Sep 29, 2006.

  1. bobbyOCR

    bobbyOCR New Member

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    If you read abou tests from large companies like Zipp, it is obvious that they do carry out tests to determine an average fatigue limit, but the cost as mentioned is most likely too high to commission an engineer to properly analyse fatigue limits. Their excuse is 'carbon's fatigue behaviour is unpredictable'

    I buy aluminium ;)
     


  2. sogood

    sogood New Member

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    So what exactly happened in that case? I saw the video and it was certainly most dramatic and I wouldn't want to be in that position.

    I read that some people attributed to the Aheadset design while other talked about carbon etc. Does anyone know the details? I guess the terrible road didn't help and is not something most of us would encounter.
     
  3. bobbyOCR

    bobbyOCR New Member

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    He crashed earlier in the race and complained about a 'loose' feeling in the headset (hence what BD said). They did not, however, tell him to switch bikes and he did not want to either. There was a large discussion about this some time ago, and we attributed the loose feeling to fatigue, indicating a near failure.
     
  4. garage sale GT

    garage sale GT New Member

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    What do you do for forks?
     
  5. sogood

    sogood New Member

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    Good thing about forks is that they are easily replaced and not too much money, even for carbon. Whereas replacing a damaged frame is big bucks.
     
  6. RickF

    RickF New Member

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    True, but for those who are worried about catestrophic failure of CF parts, failure of a fork is about as serious as a failure can get.

    BTW, I am one who has an all carbon bike and does not believe that carbon fiber is any more (or less) likely to fail than aluminum or modern steel bikes, but most of the arguments against carbon fiber have centered around the concern that they will fail without notice.
     
  7. sogood

    sogood New Member

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    You are absolutely right, there are indeed contradictions.

    Catastrophic failure is certainly a worry, although remote. But from a bike's lifespan point of view, a failed fork can easily be replaced whereas a frame failure would mean a total loss.
     
  8. dhk2

    dhk2 Active Member

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    Bro Deal, great post. May not be what people trying to select a new bike want to hear, but believe you've stated some universal truths.

    Concerning long-distance comfort of al, the longest distance guy I know completed his 1200 KM events (Boston-Montreal-Boston, Paris-Brest-Paris) on a CAAD3 C'dale. He has a heavier steel touring bike, but told me thats not needed for these events since he doesn't have to carry much weight. The CAAD3 is still going strong; probably has at least 100K miles on it now.
     
  9. garage sale GT

    garage sale GT New Member

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    1. Any amount of cyclic loading will result in fatigue failure of aluminum eventually. Maybe not in your lifetime but it will. Relevance? see second to last paragraph below.

    2. How is bicycling "low-cycle fatigue"?:rolleyes:

    3. Hypothetically speaking, you might get burned buying a used steel bike which secretly had ten million miles and had almost reached the end of its fatigue life, but you wouldn't crash because steel has excellent failure properties and would show slowly propagating cracks when the end of its fatigue life was reached.

    CF has excellent fatigue properties under normal conditions and parts are surely overdesigned for longevity and product liability reasons.

    However, I was hoping for an opinion, not gleaned from Trek ad copy, about what happens when it fails: how much warning do you get?

    I would say a bike is exquisitely instrumented because you ride it every day and can feel minor differences. This "feel" does not exist for airplanes; plus when a part starts to go bad, you have to know far enough in advance to land the plane. What's the story with bikes? Can you really feel an impending CF failure before it drops you? Any experience, anyone?

    The fact that Al (and maybe carbon) does not have a fatigue limit like steel is in my opinion important to cyclists only due to how it may propagate unseen damage; under normal conditions, Al and CFRP parts are probabaly overdesigned enough to where normal-use fatigue failure just won't happen.

    BTW how do carbon fiber parts withstand an occasional exposure to road salt?
     
  10. allgoo19

    allgoo19 New Member

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    It's not my wording, dhk2. I suppose you haven't read the article.

    == The research is the work of Tsu-Wei Chou, Pierre S. du Pont Chair of Engineering, and Erik Thostenson, assistant professor of mechanical engineering, and will be featured in an article in the Oct. 2 issue of the influential journal Advanced Materials ==

    It's Mr. Chou's wording. He seems to have a good credential to back up what he's saying.

    What credential you have to go against his opinion, dhk2?
     
  11. benkoostra

    benkoostra New Member

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    Take a look at the new line of Giant frames. Carbon/alu hybrids. Very interesting stuff.

    I have an aluminum bike. It had a carbon post, but it broke, so now it's all metal!

    I think it rides very smoothly and well. I thought I might notice a difference in ride quality when I switched posts, but I didn't.
     
  12. garage sale GT

    garage sale GT New Member

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    What happened when it broke though!?!? Please describe the failure of your post from start to finish, if you would.
     
  13. dhk2

    dhk2 Active Member

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    Hey, I totally agree with the statement of Mr Chou, just really do like the choice of words..seriously. How did you read that I disagreed with the statement? The quote describes a typical failure mode of CF beams like wings and rotor blades; certainly not a controversial finding, IMO.

    Credentials are easy to quote, but they really don't mean much. Have always thought that if you have to resort to citing degrees and positions held, the point is already lost.
     
  14. dhk2

    dhk2 Active Member

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    1. You're talking about the concept of "fatigue limit", the point of stress loading on the s-n curve below which no fatigue occurs. But this is "high-cycle" concern, out at 10 million, 100 million cycles and beyond. To me, that makes the issue basically irrelevant for bicycles, since everything I've read says they fail due to "low-cycle" fatigue.

    2. "Low cycle" refers to high loads that cause fatigue failures fairly quickly, like in 10,000 to 100,000 cycles. For bikes, this would be major bumps and potholes, as well as heavy climbing out of the saddle. I'm not an expert on fatigue theory, and can't explain the different mechanisms that distinguish "low-cycle" from "high-cycle" fatigue, but what I've read says that "low-cycle" loading is what kills bike frames and forks, not any very small high-cycle loads, eg like road buzz.

    3. Your concern about crack propagation rates is a good one. In aerospace applications, having adequate time to detect impending failures in CF is an important design and certification consideration. Obviously, no one wants an aircraft which has parts that can fail suddenly, either CF or aluminum, steel or ti.

    Just an anecdote, but the only guy I know that has fatigued two Trek CF frames hasn't crashed either time. The cracks gave him plenty of warning and time to get home and take the frame to the dealer. Agree if a frame or fork doesn't give an indication and just snaps, that would be a poor and dangerous design. But, with the millions of CF forks now being ridden, abused and crashed everyday, believe they must be pretty well designed and durable. If not, riders would be thrown to the ground regularly by CF forks, and I just don't think that's happening in the real world. After all, the CF itself has high tensile strength, so even if the resin is cracked, the fibers should serve to hold everything together for a good while.

    I continue to bring up CF forks because that's the most critical application, IMO. If you're willing to trust your life to a CF fork, the frame material shouldn't matter much.
     
  15. garage sale GT

    garage sale GT New Member

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    According to Mechanical Engineering Design, by Shigley and Mischke, McGraw-Hill, 1989, low cycle fatigue is considered 1-1000 cycles.

    Pedaling can induce significant cyclic stresses to the frame, seatpost, handlebars, etc.
     
  16. bobbyOCR

    bobbyOCR New Member

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    The thing is, Giant need to sell. MAny people believe carbon is better than aluminium in every possible way because of marketing. Carbon post don't do anything, either do carbon bars, because most of them are aluminium wrapped with carbon.
     
  17. garage sale GT

    garage sale GT New Member

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    :rolleyes: oh, boy.
     
  18. Bigbananabike

    Bigbananabike Member

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    Steel is real?!!
    Aluminium is real:) My two aluminium framed bikes are as comfortable as any steel bike(Team Raleigh, Pinarello etc) I've had and they're lighter and stiffer too.
     
  19. Bigbananabike

    Bigbananabike Member

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    ==========================================================
    Recently a Carbon fork(steerer too) broke(flat piece of road, no real bumps) in a race I was in. A quality bike too. The guy who it unfortunately happened to was in a bunch in front of mine. We went past him(thankfully no other riders with him) with his head in hands being ministered to by others and ambulance on the way.
    So - it does happen but obviously not often thankfully.
     
  20. Bigbananabike

    Bigbananabike Member

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    Carbon expensive? There are carbon forks(not the sort of quality I'd like to ride) coming out of China/Taiwan for $9 US each.
    We can expect carbon bikes or bikes with carbon bits to filter down in the near future...
     
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