Some bikes are greener than others

To cycle or not to cycle is no longer the question. To cycle will usually be the answer, at least for anything less than a few miles.
It's healthier and more environment-friendly. And people in busy city centres with limited parking probably know that it might actually be faster than driving too.
Now, having said that, there is the question of exactly how environment-friendly you are when you get yourself a bike to go around.

And the answer is: it depends on the bike you choose.

I'm not talking about the choice between buying a mountain bike, a speed bike or a traditional design bike like our grandparents used to have. That will depend on what you want to do and where you live. Try a fancy mountain bike in Amsterdam and leave it on the street and you're asking to see it gone within 5 minutes. Try using a traditional bike without gears up and down Lisbon's or Porto's steep cobblestone gradients and you're setting yourself up to quit on cycling within the first half hour.

What I mean is rather that how green your bike is depends on the material its frame is made of.
Traditionally, bikes were made of steel. But you see more and more people going around with aluminium frames, which are much lighter than steel. And the space age has also given us the carbon fibre bike, weighing less than one kilogram.
So, what should an environmentally conscious cyclist choose?

Well, definitely not a carbon fibre frame.

Carbon fibre is basically a composite material made up of carbon fibres planted on a plastic bedding or epoxy to make a material which is lighter than aluminium, but three times harder.
The problem is they are almost very difficult to recycle because after making the composite the fibres cannot be separated.
As a result, composites must be either burnt or ground. But even grinding is not a particularly good solution because while the ground composite is not usable for high value applications, the process of grinding is very expensive because the high resistance of the composite quickly damages the shredders and these need the parts to be replaced.

Besides, carbon fibre frames break apart easily and are likely to last you less than an aluminium or steel frame.
“Pardon? I thought you said they were very hard?”
Yes. Carbon fibre is very hard. But also very rigid. It does not bend at all.
This means that if it is subjected to strong forces, it responds by cracking. Aluminium and steel, are metals, which means they are able to bend and therefore adjust to stress without cracking.
Also, carbon fibre frames are made out of layers, which means that cracks may occur in internal layers that will not be visible until the whole thing breaks apart.

Dirk Van den Berk from Belgian bicycle manufacturer Granville said recently in an interview that it is perfectly possible that one single fall will mean the end of your carbon fibre frame. In fact, that is the reason that Granville’s models have only a 3 year guarantee for carbon fibre frames and a 10 year guarantee for aluminium.

So, unless you are a competition cyclist, forget carbon fibre.

What about aluminium and steel?
Well, here things are less clear.

The positive thing is that both are infinitely recyclable. So, your bike will not be waste when it comes to the end of its life.

But then the two production methods split.

Aluminium is very polluting to manufacture. I’m not an engineer but as far as I know you have to dip bauxite into highly a pollutant cryolite (sodium hexafluoroaluminate) bath, which has to be heated to 950 degrees. Then it still needs a permanent current to produce electrolysis leading to 20% to 40% of the cost of producing aluminium being energy costs.

It isn’t for anything that you see aluminium smelters mostly in developing countries where environmental standards are lower.

On the positive side, it is easier to recycle than steel while using only 5% of the energy required for producing from ore. Unfortunately, a percentage (15% according to wikipedia) of the aluminium is lost.

______________________

Producing steel on the other hand, requires actual fire, burning coal at extremely high temperatures on a permanent basis. This generates large amounts of CO2 which might be avoided when producing aluminium if the source of electricity used in aluminium manufacturing is green.
The production methods for steel, however, have improved significantly during the twentieth century and a large proportion of the steel we use nowadays is also recycled, as it is cheaper to recycle than to produce from new ore.

Of course, steel is also heavier than aluminium.

So, all in all, steel is probably the cleanest of the three options, though aluminium does not do too bad and it will probably help reaching for the bike if you live in hilly areas.
Carbon fibre (and glass fibre too I suppose) are probably best avoided for durability and sustainability reasons.

All of this said, even if you do want a composite frame on your bike, it will still be a hell of lot more environmental than going around with a car.