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I'm after a mountain bike and I've narrowed it down to the Cannondale F29 (based on reviews, plus I like the look of the bike).

My question is, should I go for the carbon bike or the alloy bike? I'm 6'4" and weigh about 125kg; I know carbon is very strong but I've heard when it goes, it just snaps (rather than alloy that bends).

Do you think this is a valid concern?

What are the pros and cons of each?

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If you're concerned about failure, go with steel. –  Daniel R Hicks Jan 21 at 23:09
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The engineering in a lot of cases is more important than the particulars of the material - there are a lot of very strong aluminum bikes and carbon bikes have been used for many hard races and what not. A well engineered bike should be fine provided its not abused regardless of if its made from good steel, aluminum or carbon fiber. In terms of repairability though, steel is probably the only one you can do reasonable repairs on. –  Batman Jan 22 at 0:13
    
possible duplicate of Carbon bikes make me nervous. Can you ease the tension? –  Tom77 Jan 22 at 8:20
    
What are your special needs that only can be satisfy with a carbon bike? Going carbon means you have to take extra care for your frame. Factor that into the cost. –  Look Alterno Feb 13 at 20:36
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4 Answers 4

up vote 3 down vote accepted

I've been looking into this a lot, and asking around at local bike shops, as I'm looking into getting a modern road bike. In terms of failure - both aluminium and carbon are quite brittle, compared to steel, and from what I've heard (all non substantiated) they are both about as failure prone as each other nowadays. Steel can also fail remember! I'd be interested in a well reserached article into failure rates of different frame materials! Well looked after, both types can last for a long time too.

One bike shop told me that cracks Carbon Fibre can be repaired to a certian degree, apparantly by injecting resin into the cracks then baking it back together, Aluminium can, apparantly, be harder to weld back together. However, he was trying to sell me a carbon fibre bike!

In theory, carbon fibre can be constructed so that the frame has different properties - stiff where it needs to be stiff, and more flexible where that would be required. It can give a smoother ride than aluminium.

When it comes down to it though, I suspect you can get badly made frames out of either material! Can you try riding both models and see which you prefer?

Edit: Here's a video of two frames being stress tested. Not the most scientific ever, but this particlar carbon frame takes more pubishment than that particular aluminium frame. Make of that what you will! http://www.pinkbike.com/video/243228/

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Carbon is seen as expensive and light while Aluminum is heavier and cheaper. Both are, to all practical purposes, more than robust enough for the job. At the price point you are looking at (for a hard tail), Carbon is a no brainier and superior in every way.

If you are worried about failure mode, both are as likely to fail catastrophically as each other, although for different reasons. Aluminum will fatigue and stress crack then fail, while carbon tends to fail from impact damage.

Do not worry about the frame failing, it happens, but to be honest, I have seen plenty of guys in bandages, casts and hospital with fully ride-able bikes, and have never personally met someone who got seriously injured breaking a frame.

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Some people do prefer the riding feel of the Aluminum frame though. Feels "springier" than Carbon (which feels much more like steel). –  Brian Knoblauch Jan 21 at 22:48
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@BrianKnoblauch - Most folks who've tried them say that aluminum is stiffer than steel. –  Daniel R Hicks Jan 21 at 23:08
    
@BrianKnoblauch I have to agree: Al is stiffer than steel. IMO Carbon can be stiffer too, depending on the design. –  andy256 Jan 22 at 2:56
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The frame design is the most important factor in the "springiness" of the bike, not the material. It's not a coincidence that ultralight springs are made from carbon fibre instead of metal. –  arne Jan 22 at 8:03
    
Just want to chip in that some people I know who want the best of both worlds have chosen a Titanium frame for the lightness and flex. –  Aaron Jan 22 at 15:24
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I have seen videos of an aluminum frame snapping at the weld where the front vertical shaft (where the fork is mounted to) shears off the two horizontal/crossbars. Though these people were doing lots of big jumps on their mountain bikes.

I think you will have issues with snapping in either case. A number of factors come into play here. Material quality (mixtures, purities, impurties, etc), how the material was formed (die-cast, poured, laser metal sintering, etc), the quality of the welds or glues that hold it together (I am assuming they use some form of gluing for carbon fiber), material thickness, construction design, and a few other factors.

The biggest benefit you get from either material over steel is low weight, with carbon fiber being the lightest by far. The next benefit is corrosion/rusting. Aluminum doesn't rust. But it can be corroded. Carbon fiber isn't a metal, so rusting has no possibility. But I would think that it would be more sensitive to acids and bases when it comes to corrosion.

Steels biggest advantage is flexibility, it will bend a lot more before it breaks.

I have been using aluminum bikes as of recently and they are plenty light for me for my mountain biking. I don't do any big jumps (at this point). So I don't know how it would hold up for that. But I can see it holding up pretty well.

And if I were doing a bunch of jumps I would want something with enough heft that I could more easily keep it under me. I would be worried that carbon fiber would be so light weight that I could lose it to a stiff wind while in mid air if I loosen my grip too much.

I will say this. Those carbon fiber road bikes are freaking sweet. I never thought I would be able to pickup a bicycle with one finger until I saw one of those.

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I'd say Carbon's also got the ability to be as flexible as steel - they can make sold forks with enough 'give' in them to smooth over the ride (cannondale call it "SPEED SAVE micro-suspension") But I guess that's more applicable to road bikes. –  7thGalaxy Feb 13 at 16:19
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I happen to own two almost identical mountain bikes other than frame. One aluminum (Orbea) with Niner carbon font fork. And the other an all carbon Niner with Niner carbon front fork. I was looking for a Niner carbon frame and found a whole bike at such a deal I bought it. Both are single speed and tubeless. The all carbon has 2.1 tires compared 2.25 on the aluminum.

The carbon just plain rides and performs nicer

  • Softer ride
  • More efficient pedal action
  • Lighter - easier to hop the front end and rear end
  • Take the bumps nicer
    Same section of roots/rocks I get bounced around less on the carbon and it has the smaller tires

Cost

  • Advantage aluminum

Ride

  • Advantage carbon.
    It has more flex and up and down.

Pedal efficiency

  • Advantage carbon.
    It has less flex side to side. Carbon can have an asymmetric flex pattern.

Longevity

  • Advantage carbon.
    Aluminum fatigues. Carbon does not fatigue. You ride an aluminum bike long enough and it will fail.

Catastrophic failure

  • ????
    This is where I think aluminum is given what I consider a false advantage. If I crush an aluminum can so what if it did not snap - it failed. If you stress aluminum past the yield point it may not snap but it fails in what I call a catastrophic manner relative to the rider. Stress aluminum past the yield point and it will deform significantly and you are going down. Front tire touches frame and you are going down. Frame / fork must be replaced. From a rider perspective that is a catastrophic failure. By strict definition a material scientist would say no that is not catastrophic failure as aluminum plastically deformed - that was not a brittle failure. So to be fair judge aluminum versus carbon on yield point. And you need to measure that bike to bike but for the most part the carbon bike is going to have a higher yield point. I give the advantage to carbon.

Ding

  • Advantage aluminum.
    If you take a hammer to a bike you could kill a carbon first. Don't take a hammer to your bike.

When carbon forks came out there was a lot of scare they are not as strong. A fork takes a lot of stress and you you see a lot of carbon forks today.

Aluminum Vs. Carbon Bikes

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