Do carbon MTB frames crack easily?

Question

I have been looking to buy a new full sus mtb (Vitus sommet 29 Cr) and saw that the main frame is carbon fibre with the rear triangle being aluminium. I love the bikes specs and aesthetics but can't decide to spend the money with all the stories and videos I have seen of carbon frames cracking. I was wondering if carbon frames crack easily (where do they mostly crack) and if its overall worth buying this bike. Love the bike and really want it but don't want to risk having to replace it in a few years when I have just saved up enough money to buy it. I should also mention that their website says they hold a 5 yr warranty on frames but don't want to faff with replacements if aluminium is better (for a carbon fibre bike this is cheap £1.3k due to new season bikes coming). Love to know your thoughts as i don't want to miss this great bike and deal. https://vitusbikes.com/products/vitus-sommet-29-cr-mountain-bike-2022 <---(bike im looking at)

Answer 1

First of all, carbon is stronger than aluminium. It's mechanically on par with steel, while being much lighter. That's why it's the go-to material in aerospace and race cars, and it's also why it is excellent for bicycles.

Why, then, do we keep hearing so many scary stories about carbon failing? Three reasons:

1. Carbon is mostly reserved for applications where weight matters a lot – because when weight doesn't matter a lot, metals are usually the more economic alternative. And that means that engineers will typically not leave it at using this light material, but also make it as thin as possible, i.e. so thin that it's just enough for withstanding the expected forces.

   This tends to be not so much the case for metals, for different reasons:

   • Most applications of steel don't have weight as a top priority, so they are often overbuild just to be sure and to not require so much testing.
   • Aluminium has particularly poor fatigue properties. That means parts need to be designed not only so they withstand the expected forces, but also to do so with minimal flexing, because any flexing would lead to a quick deterioration of the mechanical properties. I.e., with aluminium manufacturers simply can't afford to go as close to the limits as with carbon – it would lead to failures that can't be excused as mishandling and must be covered by warranty.

   The latter point also is the reason for aluminium's reputation as making harsh and inflexible bikes.

2. Carbon is more complex than metals. In particular, metals are at least approximately isotropic. This, for one thing, makes it easier to get the design right, but it also means any abuse (forces in directions the part wasn't really designed to withstand at all) is more likely to be "accidentally" survived by metals.

   This is also where we could see the recent implosion of the OceanGate Titan submersible, which is thought to be a result of a particular delamination failure mode of the carbon fibre hull. This might have been avoidable with more suitable layup to catch the application-specific compressive loads, but that's very difficult to design. And even if you do get it right-according-to-spec then a crash is still more likely to result in catastrophic failure, if it involves force-direction that the spec simply doesn't consider possible.

3. Metals can deform plastically. That's when you actually do exceed the materials limits, strictly speaking, on e.g. a rock hit, but only end up with dents. Now, this is actually a bit of a contentious point, because a metal frame with a dent may already be unsafe to ride at full load. But will still generally hold up better than a carbon frame under the same conditions, which would have cracked, and such cracks are very likely to propagate even under moderate loading.

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None of this really takes away from the excellency of carbon fibre, but it does mean that carbon needs to be more deliberately designed to every situation it's supposed to handle. What exactly that means differs with application:

• For a small drone propeller, weight is quite important but even total failure is pretty inconsequential. Therefore the design will have very low safety margins.
  In these kind of applications, the answer to your question would be “yes, it does crack easily“.

• A road bike needs to handle a lot of forces that the top riders put through the pedals, and that for a long time without fatigue. But there's isn't too much expectation that if a crash does happen, the bike must survive intact. Similar perhaps for a satellite (if any unexpected forces appear, chances are the mission will be lost anyway).
  In these kind of applications, the answer to your question would be “you can be fairly sure it doesn't crack from normal use, but will crack easily from any abuse”.

• For a formula race car's driver cell, you have to handle an absolutely insane beating when the car crashes into a wall. The regulations stipulate that the cell needs to survive this, so the designers can't save too much weight here. And in that instance carbon does prove that it can be a great material even for hard impacts – though to be sure, you wouldn't re-use a crashed cell. But you wouldn't do that if it was made from aluminium, either.
  In these kind of applications, the answer to your question would be “it may crack from ‘intended use’, but will still do its duty”.

• For an airplane, there is extreme scrutiny and cracks won't be tolerated. Weight does still matter a lot (so you can't just go with lots of steel to be sure), but also planes are expensive and lots and lots of money will be spent on design and testing.
  In these kind of applications, the answer to your question would be pretty much “no, it won't crack, except in a crash that is anyway deadly”.

• An enduro bike is pretty much expected to crash somewhat regularly, or at least take severe landings, rock strikes etc.. So in this case, good design requires that it survives this and remains usable.
  Mind, that does not mean all enduro bikes actually cope very well with abuse. Especially for rock strikes, the design typically relies on rubber pads in addition to the carbon itself. If you don't use these, well, it can be a problem. But still: a good carbon enduro bike does not crack easily. Plus, the abuse margins mean that fatigue is hardly an issue at all, I would expect a carbon bike to typically last longer than its aluminium pendant.

tl;dr carbon fibre is a difficult subject. The question “do carbon mtb frames crack easy” can't be answered universally, but the question “does a good enduro MTB frame with suitable frame protection crack easily” can be answered quite confidently with “no”.

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To add a personal anecdote: I have by now had to replace and/or service almost all of the parts on my carbon enduro bike (including the aluminium wheels and cranks), because of both wear and crash damage. The actual carbon frame, however, is still perfectly ok.

Answer 2

Carbon frames are just fine. As with any other frame materials they are designed to take the loading and stress of normal riding for their intended purpose.

That isn't to say they can't be damaged if you take a spill and bike lands awkwardly, but that could be said about any bike. Being listed as an Enduro bike, safe to assume this one is built to take a knock or two.

Without knowing the ins and outs of the Vitus warranty, you might find that damage due to falls or crashes isn't covered anyway. You might want to consider a crash replacement policy too if available.

One other thing, just to confuse you more. Quite often the build kit on the top alloy version will be better than the lowest carbon variant. Means you can save yourself a little bit of coin and get a better level of bike for very little weight penalty. Worth thinking about in the enduro space where the real action is pointing downhill sometimes a little added extra weight is actually a good thing. Also note, suspension platforms and bike geo is getting so refined now, a few extra grams here and there make almost no difference in climbing ability.