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Almost every question Bicycles.SE about damaged carbon frames seems to get the same answer: "the frame isn't safe anymore."

How strong of a hit is needed to break a carbon frame? There are little stones that get thrown up by the wheels and hit the bottom of the frame, from time to time the bike falls down, etc.

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Thanks for the edit. It's great. –  Alexander Jan 22 at 19:00
    
There are too many variables for a definitive answer, I consider the primary failure mode of carbon fibre to be a loss of element strength initiated by delamination, leading to tearing of individual fibres. –  Emyr Jan 29 at 15:49
    
Considering the availability of carbonfibre downhill rigs, I think impacts strong enough to structurally damage the frame would destroy a steel or aluminium frame. Impacts which might only dent a metal frame could equally dent a CF frame. Small dents are ok if there is no delamination. –  Emyr Jan 29 at 15:52
    
@Emyr thank you. –  Alexander Jan 29 at 18:57
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2 Answers

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The real problem with carbon is that it only has two modes -- not broken and broken. When stressed beyond its strength limits the carbon fibers break and the frame is seriously weakened. Plus, once this has happened, the "threshold" for further damage drops lower and lower. And, except for cracks that MAY OR MAY NOT be reasonably visible, there's no obvious warning sign of impending failure -- nothing is "bent".

Steel, on the other hand, will (unless it's super-hard stuff) "yield" ("plastic" deformation) before it breaks, and "yielding" (within limits) only slightly reduces the strength of the metal. Thus the frame will visibly bend before it breaks (unless, of course, it's run over by a truck or some such), and catastrophic failure is very rare.

(Aluminum is somewhat in-between the other two. It unfortunately has a tendency to "work harden", and thus can, over a long term (years of rough roads), develop cracks that lead to catastrophic failure without obvious bending before-hand. But it reacts more like steel in the face of short-term impact-type forces.)

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Thanks. I see this answer as almost what I looking for. But yet those bother me: 1) Is there any strength limit that can be said (few times I saw on the forum about chain drops, that can cause a failure - what the power of stress in it?) 2) Broken forks: as I see it, the most stress in fork is from braking, so carbon have to be most strength in the front-back direction. However there are many forks that broke in mid. What kind of hit have it to be, to cause it? –  Alexander Feb 22 at 17:53
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@Alexander - The three problems with carbon are 1) there is a lot of variation in durability between manufacturers and manufacturing technique, 2) even given a specific technology, no one has enough experience with it to know how far you can push it, and 3) there's no good way to tell when it's been pushed too far. –  Daniel R Hicks Feb 22 at 18:08
    
Thank you very much. Will you please share your own preference - carbon, aluminum or steel? –  Alexander Feb 22 at 18:45
    
I was going to add a 4th problem: 4) Carbon costs an arm. –  Daniel R Hicks Feb 22 at 18:46
    
My own preference is steel, but then I'm a bit of a Luddite. (I program smartphones for a living but don't own one.) And perhaps more to the point I've never been a racer or hot dogger but rather I tend toward cycle touring and distance commuting. And I've never been into flash or felt a need to have "the latest and greatest", but rather seek durability and reliability in a bike. –  Daniel R Hicks Feb 22 at 18:52
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I have a feeling that this has been covered many times! But here's a detailed paper about carbon fibre compared to other materials:

http://calfeedesign.com/tech-papers/technical-white-paper/

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I see, but my English is pretty bad to read all this. On this forum I didn't find an answer to this. If you could just to conspect in several words what about the strength of carbon, I will be greatfull. –  Alexander Jan 22 at 10:03
    
Well, it really depends on a lot of things - frame geometry, where the damage has happened, how well the bike was made, what sort of building process, what direction the force is coming from. I don't think there can be a precise answer! Carbon Fibre has a higher strength to weight ratio than any of the other available frame building materials. –  7thGalaxy Jan 22 at 10:08
    
You also have to remember that all other frame materials can, and do, break. Carbon isn't unique in that damage can write off the frame. And some seemingly trivial knocks can cause significant damage while bigger ones might not. –  user814425 Jan 23 at 13:01
    
I read/skimmed through parts of the above article and it seemed like he kept repeating the same list of platitudes again and again with little in the way of hard information. –  Daniel R Hicks Feb 23 at 20:09
    
@DanielRHicks I'm not sure that's entirely fair.. although I'll concede he's trying to sell carbon bikes, so there's a bias there. I don't want to sound like a carbon-prophet/obsessive, but I do think there's a lot of disinformation going around on all sides. The site linked does have some hard information on the strength/weight available, and I think the pros/cons table is informative and reasonably fair. (although to have a pro for carbon as strength, then qualify it with a "design dependant" in the cons is cheeky.) –  7thGalaxy Feb 23 at 22:11
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