Although the weather may not yet show it, summer is just around the corner and with that everyone is starting to think about digging out their fancy summer wheels. This is for those of us that are yet to join the club of cyclists riding on more affordably repaired/replaced winter wheels when the roads are wet and sandy, and beautiful fine-tuned carbon wonder wheels to match freshly shaven legs and long days circling the local parcours in the summer heat. The step to carbon is a big one in many ways. The price is often a large step, but with that comes a long list of positives from vain and petty, to real-world performance gains.
Although carbon has been around for longer than a lot of my readers have been alive, it's still a mystery to most. You're not going to come out of this article with the understanding of a Material Sciences PhD candidate, but you will be able to confidently use fancy sounding words like "micro-filament" "Toray", and of course "bilateral plane stress fracturing"... just kidding, I just made that one up.
As this is realistically one of many articles, and Parallel is but a wheel company, I'm going to trim things down and give a short explanation of commonly used types of carbon fiber. Keep your eyes open for later articles breaking down more details about individual types and applications of carbon.
3k Woven Carbon Fiber (Toray)
Woven fabric has long been a favourite of aerospace, auto racing, and bicycle enthusiasts due to it's 'Jack of all trades' nature. Woven carbon is made up of fibers oriented in at least two axes, 0º, 45°, or 90°. By having fibers traveling is opposing directions, woven carbon has great all-around strength and a respectable strength-to-weight ratio. A sheet of woven fabric once cured can take flexural and tensile loads in multiple directions, and even exhibits good stiffness properties off axis. This is great and all, but where woven carbon really shines, and also why we see it so often in wheels, is when things go wrong. Woven fibers show great resistance when punctured or exposed to strong bearing loads (think cobbles, potholes, etc.) A carbon strand is strongest when straight, and therefore loses some of its theoretical strength when it is bent around other fibers to produce a weave, but if a strand is broken or punctured in a weave, the surrounding fibers, because of this bend, will behave as if unbroken. Therefore, if a hole is punched in a weave (think, spoke holes), strands punctured in one place, will behave at full strength within a very short distance (typically one to two weave widths) from the damage. With a unidirectional fabric, a fiber puncture will compromise strength for the entire length of the fiber, relying on the resin strength (or sandwiched woven fibers) to distribute loading across the broken fibers. These same properties also result in better toughness, and impact strength than uni-directional material.
Unidirectional Carbon Fiber (UD)
Uni-directional fiber, or UD as it is commonly referred, is used in many ultra-high-end rims and because of it's easily identifiable appearance, you probably already know all the brands that sport UD carbon. Carbon works most effectively when loaded along its axis, think about what was just covered about woven fabric carbon. With this mind, designers dealing with unidirectional carbon fiber (literally "one direction") have to take layering of the fibers, stress cycles, and ride properties in mind to create not only a strong wheel, but also a durable wheel overcoming UD's inherent inability to compensate for mid-fiber failure (spoke holes, etc.). The difficulty is determining the exact load paths in a rim, which has resulted in many different patterns of UD layering on rims, including the more popular "radial" fiber direction. Placing continuous radial fibers running from the inner diameter to the outer diameter of the wheel creates a structure, which alone can handle over 85% of normally experienced wheel loads. Additional UD fibers connect, stiffen, and transmit forces incurred on other axes within the rim along with providing critical vibration damping properties in this arrangement.
Decades of research and development defining load paths and force directions within carbon components is what has brought the bike industry from the early days of single-use rims, delamination, and just general terror about using carbon fiber all the way to today where carbon fiber stands alone as a strong and reliable material with an ever decreasing list of negative aspects.