Most cheap carbon bike frames come in both 3K or 12K versions. Many people ask which is best.

Firstly, what is the difference between the two types?

Carbon fiber is made of thin carbon “threads”, each of which is made from even smaller carbon filaments. 3K or 12K refers to the number of filaments per thread – ie, 3ooo or 12ooo filaments per-thread.

Originally, most carbon fiber was woven using 3K fiber. During the early to mid 2000′s, though raw carbon fiber increased in price as more and more of it was consumed in military and aerospace applications.

New carbon fiber manufactures developed processes to use 12K fiber instead of 3K. This new fiber could be manufactured cheaper, but had the disadvantage of being more difficult to handle and mold, especially on tight circumferences as well as being not as strong as 3K filaments. For example, cstsales.com lists the strength of 3K fiber at 7400 ft/lb while 12K is listed at 1800 ft/lb. (Edit – it has been pointed out that these measurements are actually for weight per length, not strength at all. Doh!)

Now, most handling problems have been overcome and for bicycle frames the molding problems are not an issue. The finish on the frames looks different though, because 12K filaments cannot be woven as tightly as 3K fiber.

3K carbon fiber weave. It is impossible to weave 12K fiber as tightly as this because it will not flex into the tight circumferences required.

12K carbon eave close up

3K Carbon Weave close up

Unfortunately, for most bike frames it can be difficult to know how much of the frame is manufactured from each type of carbon fiber. Carbon fiber frames are manufactured by laying down layers of fiber weave with resin, and we can only see the outer, cosmetic layer. Without destructive testing and/or a forensic investigation of a cut up frame it’s impossible to know what (if any) effect weave has on a frame’s riding characteristics.

In conclusion, you are probably best off buying the type you like the look of, or whatever is available.

Update: I found this document from hexcel which lists the strength of various fiber type. It gives some useful information, but really shows how the exact grade of fiber used has a much bigger effect than if it is 3K or 12K.