There are UD,3k and 12K option when you check our carbon bike,rim.so what is the guideline？Actually,the inside layer of bike rim is always UD, only the top layer when naked can be specified to these different finish types.There is very little difference in performance between each of the different patterns of carbon fiber. The choices for the handlebars and saddles change only the look , not the strength as it affects only the last visible fiber layer.The performance of the rim is more influenced by how the carbon is cut and laid in the mold. The appearance of the rim can be changed by adding a cosmetic layer of either UD, 3K or 12K weave.
UD or 3k or 12k specify the carbon weave pattern. UD carbon(Uni-Directional) has fibers that run the same way rather than being woven together. 3K means there are 3,000 filaments per 'tow'. 12K means there are 12,000 filaments per 'tow'.The carbon fiber grade and orientation will affect the strength of carbon rim(eg. 0 degree, 45 degree, 90 degree). The layup method is a crucial factor for every company. UD weave(in certain direction) creates a very strong structure. BUT, if you drill a large hole in the middle of the rim, it has compromised the strength of the entire structure because you've cut/drilled almost all the fibers. For 3k, it’s OK to drill a hole in the middle which would not weaken the rim structure as much because your cut would have only severed a small portion of the total fibers and the drilled area is able to get support from it's neighboring (non-cut) carbon fibers. 3k weave is better because it is not as brittle as the UD. It displaces some of the pressure because it is not as hard as UD. However, this doesn't mean 3K is always better, because UD generally gives rim the most strength and allows the engineer more control in the layup patterns to optimize the rim structure. This is the reason we use 3K fabric to support all the drill holes in our rims while the majority of the rim itself is made of UD fiber. From the UD and 3k outlook rims, they are only last carbon layer difference and others are the same.
Also there is usually an option to choose between matte or glossy. Matte is more stealthy look, glossy will make the surface more shiny.After rim sanding, we do matte or glossy finish to cover the rim. For the matte finish, the bicycle rim surface scratches(micro marring) may be not easy to see and it will help to hide imperfections in carbon layers. But it requires more maintenance to look clean because oils, sweat, and smudges may show more easily.