What are the pros and cons between an aluminium and carbon frame?

Question

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?

Answer 1

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/

Answer 2

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.

Answer 3

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

Answer 4

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.

Answer 5

As for crashworthiness: Materials which yield before failure absorb more energy. Cars pass major crash tests because they are made of cold rolled steel. That property reduces the G load on the victims like nothing else. The material property most closely related to energy absorption ability is called elongation. Elongation is what happens before the material actually comes apart, but AFTER it starts to yield. Carbon fiber has very little elongation, while 6061-T6 has 10- 13%. Engineers know this, so all kinds of work is being done to improve CF energy absorption, e.g using PEEK as a resin. CF does absorb some energy, especially when loaded in shear, but that is not how a diamond frame tube is basically loaded. The members are loaded in tension and compression although there is some torsional flexing particularly near the head tube. Fork legs have almost no shear going on, hence the complaint that CF forks just snap. IMO CF forks are dangerous compared to most aluminum forks

Answer 6

One question is whether carbon fiber is the right material for let us say BMX frames. GT made a nice looking box beam frame called the UB2. The problem was that the factory riders would not ride it and went back to aluminum, at least for a time, because what they said UB2 was sluggish, especially out of the gate. Having recently learned about this complaint, I've been investigating whether the viscoelastic property of the resin in the carbon fiber was possibly absorbing rider energy and it seems likely that was. We have another clue because road frames made of carbon fiber are praised for their damping qualities, and so there may be an advantage for a road frame which is actually a disadvantage at a gate start. Some top pro BMX riders have stuck to aluminum as well, and we may have a clue why that is. Resin content of carbon fiber structures is 40 to 50% and it turns out that in the shear and torsional direction, carbon fiber reinforced polymers absorb up to 8 times as much energy as aluminum, around 3-4 percent. In fact plastics are used in conjunction with metals to damp vibration and energy. It has a name which is "constrained layer damping." You put viscoelastic material in the structural system and it damps the system and reduces stresses by dissipating energy