As one who rides a 1998 vintage Proflex full sus mtb with original Easton carbon swingarms, and a 30 year old steel road bike, I don't think I really have an axe to grind in the carbon debate. However, I think it is important that newbies are informed of the strengths and weaknesses of carbon vs other materials.
I think everybody can guess that for the same volume carbon is lighter than steel. Actually for a certain volume carbon is roughly 1.6 times heavier than water, while steel is 8 times, titanium 4.5 times and alu 2.5 times.
These
tables, published by an ex composite manufacturer (alas, we are not the only one hit by the economy), is a comparison of modulus (think of it as stiffness, as in spring), ultimate strengths etc. of carbon (in various forms) and other materials (including steel, alu and ti). I think the figures are quite instructive.
In the first table, it shows carbon's amazing ultimate strength in resisting tension and compression in the direction of the fibre - it beats steel even for the same volume (at 1/5 th of the weight!). Its modulus however is lower, so it would flex more if of the same size.
Of course bike manufacturers don't use the same "amount" or volume of carbon composite to build a bike as they would with steel or alu, or else carbon frames/parts would have been 5 times lighter than their steel counterpart. I think for frames they are about half the weight of steel ones (1 kg vs 2 kg, very roughly speaking). That combined with their modulus characteristics probably explain why carbon bikes are often "comfortable" bikes, because a carbon tube is still quite flexible compared to a steel one even if it is 2.5 times thicker. Given steel is 3.2 times heavier than alu, and alu frames are not 1/3 the weight of steel one and yet it is only half as stiff as steel, that also partially explain why alu bikes are often stiff (although geometry, and especially tube diameter also contribute significantly to that too).
A weakness of carbon composite, however, it that it is not an isotropic material. That means its characteristic is not the same in all directions. It is because what is strong is the fibre, but only in the direction of the fibre. The effect of this is shown in table 2. As can be seen, when loading is exerted in a direction not in the direction of the fibre, it is actually a rather weak material. In fact at 45 degrees it is about 10 times weaker than steel, so doubling the volume used is not going to help a whole lot either.
Hence we have all these stories of carbon frames being damaged by clamping. Because bikes are slim machines, with tubes thin, at present there is just no way for manufacturers to align fibre in all directions so that bikes can take "odd" loads or knocks. This is a weakness that the above metals do not share - their structural characteristics are the same in all directions.
Although it is a characteristic that is not shown in the tables, carbon is also far more susceptible to damage by impact than steel, alu or ti. The best way to describe it is probably that it is a little glass like, and can shatter relatively easily.
Despite the weight and some incredible strength advantages, I think for the above reasons carbon as the technology stands today is not necessarily the best material for all bikes for all people, even if cost was no object. I guess this is why sometimes a good percentage of threads in e.g. Bikeradar's workshop forum is about cracked carbon chain stay, busted carbon seat tube/post, Trek shrugging off their carbon warranty etc.
Of course, if one goes into ownership with eyes wide open, fully aware of carbon's strengths as well as weaknesses, and can appreciate even if not able to take full advantage of the benefits, then that is great too! However I am not too sure how effective or keen manufacturers or vendors of carbon bikes/parts are in broadcasting do's and don'ts -I think that is not right.
Although modern aircrafts and racing cars are brim full of carbon, I personally wouldn't directly conclude that it must then be fine for bikes. I think bikes as they are built from carbon today have certain limitations that jets and cars don't share. I think these larger machines afford far greater opportunity to apply
omnidirectional carbon fibre weave, or effective monocoque construction e.g., that can mitigate carbon's weaknesses, that bike builders would find hard to duplicate.
Apologies for the rant. Hope it helps somebody, if only a little.
ps in the table:
UD is unidirectional (i.e. unlike fabric which is a weave)
HMCF is High Modulus Carbon Fibre, a treated version that is stiffer and less "springy"
