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There's No Good Reason to Buy a Carbon Bike
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this post was submitted on 29 Jan 2024
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No. Carbon is more elastic but when it fails it fails completely. With metal frames they get a dent which you can easily repair.
Carbon's elasticity limit is far beyond what steel's plastic deformation point is though. That means a carbon frame will still be structurally sound as a bike frame after being through an impact that would bend a steel frame to be unusable. Steel is tough, carbon is strong.
Of course there is some impacts that will shatter it but a metal frame would've bent beyond any repairs from the same impact in 100% of the cases.
Moot point in reality, as unfortunately any deep gouges or signs of delamination are genuine cause for replacement of a CF part. The gouges are strong stress risers and cannot be ignored.
I agree that the damage can't be ignored when it happens but that's not my point at all. I'm just saying that the force needed to inflict this damage would have destroyed a metal frame to a greater extent rendering it immediately useless. That is also part of why carbon parts are so light. You need much less material to achieve similar strength.
Here's an example of the difference between a carbon and aluminium MTB frame of the same bike model. Again, I'm not saying these frames are undamaged, I'm just pointing at how much more repeated and specifically applied force is needed to damage them when talking about two parts used for the same application.
edit:fixed the link
I unfortunately can't agree with that sentiment. Composites fail in very complex ways, with part of a tested sample failing in tension, another portion in compression and a third portion of that same sample in torque (possibly other ways too, depending on layup etc).
To bolster my point, listen to the video you've linked. Long before the steel has started obviously yielding, you can hear some of the CF fibers failing. It'll take it, but your rated static strength is actually decreasing as those fibers in the downtube cross-section progressively fail. When failure in that video finally happens, it's indeed at a high value... But it was only tested to failure once. And therein lies the insidiousness.
You've taken a jump on your mtb several times, jump, jump, jump, not hearing the gentle pinging in your bike's headset area every time you land. One day you take the jump, as usual, but the headset and thus the forks & handlebars snap off. That's a problem.
I totally agree with everything you are saying. But you have to consider the application of said material. A bike frame that bends is a failed part and it does not matter how much more force it can resist, it is now useless. I also am speaking of catastrophic failure by the way, as in there is no bike anymore after this crash type of incident. In these cases, I believe the carbon bike will endure a greater amount of force than a steel bike. And that's also while being far lighter because at equal weight there is really no contest.
Well at this point we're talking fundamental properties, yeah, CF is stronger in tension than steel or aluminum, for less weight.
I guess the crux of my argument is that details like layup and such are critically important, mainly cuz the applied force cannot accidentally be permitted to focus on a small area and break the fibers there, repeat repeat repeat, progressively destroying that part.
At least metals are generally consistent in their properties in multiple directions. I'd need a lot of reassurance, technically, before I flew on anything with a fully carbon wing, particularly as the wing ages.
I understand that it does not do well beyond it's yield point. What I'm saying is that this yield point is higher when you are comparing specific applications like a bike frame for example. In the video, you can clearly see that the same force (in this case impact) just ruins the aluminium frame while the CF bounces back, repeatedly and while increasing the force applied. I am not saying that it's completely fine and safe, I'm saying it's still a usable bike frame even if unsafe if we are speaking peak strenght. This can make the difference between being able to ride back home and being stranded.