Woven Carbon vs. Unidirectional

Woven fibers have long been the standard of composite structures, but are not necessarily correct for every application. We long recognized both the strength and the function of the woven fabric, along with it's inherent toughness and durability, however, recent advances in fiber technology, and construction technique have led Zipp to take an alternate design route. Previously, a woven fabric was the norm for the outermost covering of any carbon component. Along with offering excellent multi-axis properties, a weave lends great toughness and durability to a composite structure. Now, new construction methods and new fiber placement theory allow us to use woven fibers only in areas were it is needed. The result: Same Zipp toughness, same Zipp durability, VERY LOW new Zipp weight, and improved stiffness.

Uni-directional fiber

Uni-directional fiber, or UNI as it is commonly referred, comprises the majority of the Zipp rim. Since a carbon (graphite) filament will work most effectively if loaded along it's axis, fiber orientation becomes a critical aspect as the designer struggles to define load paths. The difficulty is determining the exact load paths in a rim, which has resulted in Zipp's exclusive radial fiber technology, and ultimately ICT. Placing continuous (unbroken) radial fibers running from the inner diameter to the outer diameter of the wheel creates a structure, which alone can handle over 85% of experienced wheel loads. Additional UNI fibers connect, stiffen, and transmit forces incurred on other axes within the rim along with providing critical vibration damping properties. Zipp spent months of R&D just defining load paths and force directions within the rim, allowing us to specify exact fiber orientation. The result is a new series of the lightest, stiffest, and strongest rims ever made.

Woven fiber

Woven fabric has long been a favorite of aerospace, auto racing, and bicycle enthusiasts due to it's 'Jack of all trades' nature. A woven fabric contains fibers oriented on at least two axes, in order to provide great all-around strength and stiffness. A sheet of woven fabric once cured can take flexural and tensile loads on multiple axes, and even exhibits good stiffness properties off axis. The real benefits of woven materials, however, come from their behavior in less than ideal circumstances, such as when punctured or exposed to bearing loads. 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, behave as if unbroken. Therefore, if a hole is punched in a weave (i.e.: 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. This is why we also use a woven fiber at the tire seat and outer diameter, where the wheel is most subject to impact and damage.

What does this mean for bicycle wheels?

Ultimately, the design incorporates uni-directional fibers to handle all of the tensile and compressive loading within the rim, giving the perfect mix of strength and stiffness without sacrificing the famous Zipp ride quality. The braking surface and tire mounting surface are even encapsulated with a woven silica/ceramic fabric along with woven carbon fabric to handle all compressive forces due to braking, and provide the ultimate in durability and toughness at the tire seating area. Overall, the combination of fiber types and orientation results in a spoke pull through strength of 950 lbf, more than 50% greater than the ultimate tensile strength of a 14 gauge spoke (straight pull), increased durability, and the highest stiffness/weight, and strength/weight ratios that we know of.