Sweep wrote:Forgive me if I'm being thick (quite possible) but am somewhat puzzled.
As I thought some cycling folks on rides had told me that carbon was good for soaking up road buzz/some sort of suspension.
How does that square with being stiffer?
Be easy on me - I may have just revealed again my total lack of knowledge of materials technology.
like the supposedly 'infinite fatigue life' claim this is -in a nutshell- little more than irrelevant nonsense when it comes to real structures.
The things to bear in mind are that
a) strength (and stiffness) is normally measured in uniaxial loading and is expressed in terms of the loaded cross section area, or in terms of the unit weight.
b) fibres are, er, fibres so are capable of withstanding loads in one direction only.
So real structures may see primary loads in one or two directions but between more complex real-world loading conditions and the fact that (in real structures) there are
always shear stresses generated within the structure, you need fibres going in many different directions in almost any real structure, if you want it to work and last. [ Imagine trying to build a house using string; not easy is it...? And you certainly wouldn't expect the house to be as stiff as any one piece of the string, would you...?] This means that the strength and the stiffness (per unit area or unit weight) in a real structure are nothing like the 'headline figures' might lead you to expect.
This fairly simple fact is nothing like as well understood as it should be, and it means that by the time you have a real structure with real loads on it, there ought to be more CF in the structure than you might expect, and it may be seeing loads in ways you wouldn't expect too.
The fibres also don't (in fact
can't) be packed to 100% density, any more than a bundle of sticks won't have gaps in it. There
has to be a percentage of resin in the structure and the resin is responsible for transferring the load between fibres and between layers; without the resin a CF object would just be a floppy mess. If the resin matrix starts to deteriorate in any way, this can initiate a failure; fibres are then exposed to stresses they wouldn't normally see which then causes them to start breaking. Often this process is near-silent, faster than you might expect and potentially deadly. Any time you see a woven layer in CF, this comes with a small penalty in terms of packing density.
It won't have escaped your notice that any laminated structure resists through-thickness stresses about as well as the glue (resin) between the fibres/layers/joints does, i.e. not very well. [Joints? Yes, most CF objects have joints in them, because this simplifies production. You can mould an entire frame in one go but this is rarely done because it is more expensive to do it this way.] In real structures the through-thickness stresses are never zero and if they are not allowed for in the right way then they can become the source of failures, pretty much in the same way as something made of wood will usually break in such a way as it will include some splitting along the grain.
So headlines such as 'infinite fatigue life' sound great but real structures contain stress concentrations and complex loading conditions which make this a nonsense. Talk of relative strengths of materials is also meaningless unless the tests represent the loading conditions seen in the real object, and they don't; simple as that.
CF is a wonderful material,
but there are a lot of
buts concerned with its use.
I have refrained from commenting in this thread thus far; the only type of reply which could possibly make the OP happy is one whereby it would be well known that
a) CF parts are always uniformly well designed and manufactured
b) it is easy to inspect CF structures and detect damage/ incipient failures well before they are likely to cause trouble
c) the failures were always easy to detect before the part actually fails catastrophically and
d) failures are in bike parts which have built in redundancy, where a failure can be tolerated at least for long enough for you to bring the bike to a halt.
Unfortunately none of the things on the list above is true, not for a CF fork, handlebar or stem, not even from a single manufacturer. The 'real worry quotient' is (or should be) something to do with risk x consequence, (where a), b) and c) are mostly to do with risk and d) is mostly to do with consequence) and for some uses the numbers never stack up to something tolerable. By the time you factor in age and the possibility of deterioration/damage the numbers (for parts like forks) rarely stack up to anything that will give you any comfort whatsoever.
"The light that shines twice as bright, burns for half as long"....?
Something like that....?
cheers
