What exactly is carbon fibre?


New Member
Oct 24, 2001
What exactly is carbon fibre, i mean technically, how is it made and why is it so expensive?
Carbon fibre is made of one of life's base elements - carbon. Carbon fibres are fine filaments, made up of elemental carbon. Fibre types range from amorphous carbon to crystalline graphite. <br /><br />The main features of carbon fibre as a material for design and manufacture are:- <br /><br />1. Incredibly high tensile strength, approximately 15 times the strength of construction steel. <br />2. High rigidity, up to 3 times the stiffness of steel.<br />These two features mean that carbon fibre is a great material for making high performance products. It is also a great material for design. The high strength to weight ratio, in combination with high rigidity, offers superior design freedom. <br /><br />This has led to carbon fibre being used in a diverse range of products, from yachting to high performance motor vehicles, to sporting goods. <br /><br />Working with carbon fibre is a real challenge for the designer. The material is complex, with movable fibre orientation, and it's performance parameters are not yet well understood in the general design community. <br /><br />As designers begin to understand carbon fibre, as material costs fall, and as more efficient process technologies are developed, carbon fibre will become a major material used to produce a wide variety of products.<br /><br /> for good information on carbon fibr.
Well i would have tried to explain it but that website says pretty much all that need to be said.<br /><br />Carbon fibre is expensive because growing the crystals and making the moulds takes a long time as can curing the finished product. For example the brake discs on a F1 car take 150 days to make and most of that is curing time in an oven.<br /><br />Another thing to note is that although most carbon fibre products are finished with a clear resin and matt cross weave pattern for aesthetics quite often there are several layers underneath the skin where fibres have been laid in different orientations to resist forces in certain directions.
I have heard that carbon frames start sagging or deminishing in strength or whatever the right description would be. Is this correct? Does anyone know whether this is true and possible?
[quote author=e-male link=board=20;threadid=2762;start=0#23518 date=1039599913]<br />I have heard that carbon frames start sagging or deminishing in strength or whatever the right description would be. Is this correct? Does anyone know whether this is true and possible?<br />[/quote]<br /><br />That might have been the case in the old days! but its not now, carbon frames are pretty hard to beat IMO!
A few more comments. <br /><br />While I am not a cyclist yet (I have been threatening to buy my first bike soon) I am a mechanical engineer and have some more &quot;techie&quot; stuff on carbon fiber. I am quoting from memory so please shoot me down if you read something differently, it is most likely more accurate than my memory.<br /><br />As Vo2 mentioned, carbon fiber is incredibly strong in TENSION, but not as strong when in compression and much weaker than any other material used when placed in shear such as is common with lateral loads. The problem with bikes is that most loads are compressive not tensile, that is, the force is usually transmitted in a way that tries to squash (compressive) rather than stretch (tensile). If carbon fiber is designed properly it can effectively handle compressive loads well. While some strength is sacrificed by designing for compressive loads as opposed to tensile loads this sacrifice can easily be made up by &quot;reinforcing&quot; the design element. Reinforcing means one of two things bigger or more, in either case that means weight. Because of the high strength to weight ratio of carbon fiber, it is easy to add more material to handle compressive loads and still remain lighter. So, the only problem remains those shear loading effects. That problem is overcome by discussing the second part of your question, how is it made.<br /><br />To make carbon fiber, you start with a normal fiber, something similar to, say, seat belt material. Not actual seat belt material, but if you look closely enough at it you get a feel for what the grain structure of carbon fiber looks like. Through a rather complex mechanism, carbon fibers are impregnated into the lattice structure of the parent fiber in such a way that grain boundaries line up along their long axes, this is what gives it it's incredible uni-directional strength. What you wind up with is thin sheets of material that are flexible and incredibly strong in tension along one axis of stress/strain. To help overcome the weakness in the lateral loading or shear plane of force, sheets of material can be layered on top of each other and glued together at 90 degree angles to each other. This overlapping of alternate grain orientations allows for strength to be maintained in all directions but is also key to how carbon fiber can be used by a good designer. Case in point. Forks. Bear in mind, I know nothing practical about cycling yet, other than what I have read (I have read a lot) but a basic engineering analysis of forks shows that 90% of the load they bear are compressive loads. So if I layer carbon fiber sheets together and my main concern is to hold up to compression, I will layer 75% of the material longitudinally along the main axis of force transmitall and layer the other 25% perpendicular to that axis to reinforce. I am left with a member that will withstand more compressive loads than shear but that is okay because I do not expect major shear loads. This is why designers love carbon, all the equations they use to design with can now be applied with precision to design elements to the exact loads they will see, no wasted material. But there are a lot of challenges. <br /><br />As for the part about sagging of losing its strength, again, I am only guessing, but I know that carbon fiber parts are made by overlapping layers of material that are glued or bonded together in some other manner. I can envision a scenario in which an element that is subjected to repetitive loading/unloading along the same axis might cause layers of material to delaminate from on another. This might manifest itself as a &quot;flex&quot; or a &quot;spring&quot; in the material that was not originally there but this is all just speculation.<br /><br />I have rambled enough for now, hope I have answered more than I have confused.<br /><br />
I'll agree about carbon fiber composites being relatively weak in compression. But as for shear, all you've got to do is orient the fibers at a 45 degree angle to the shear plane. In other words the fibers run diagonally. So that they are oriented along the principal stress axes.

Unless you were talking about interlaminar shear in which case you are right they are **** poor, since interlaminar shear strength is governed by the strength of the "glue" holding the fibers together.

I'd like to see a frame made with only a carbon fiber down tube. Everything else could be metallic. Since the down tube is loaded primarily in torsion (twist) you could orient all the fibers at plus or minus 45 degrees and have an enormously stiff frame in the bottom bracket area.

Then you could make the rest out of metal to take advantage of metallic's superior bending and compressive properties.
Mr_Potatohead said:
I'd like to see a frame made with only a carbon fiber down tube.

Interesting -- that's basically the opposite of how Lemond's are constructed (w/metal down tube and chain stays, and CF top/seat tube and seat stays).