Shimano's Dura-Ace 9100 C40 Carbon Road Clincher Wheelset brings an aesthetic update to pair perfectly with the new groupset and carries over the speedy stability we've come to expect from a Dura-Ace wheels. As with its C35 predecessor, the C40's weight is centered at the hubs to reduce rim and rotational weight so it kicks up to speed fast. Combine that snappiness with a 35mm rim depth and you get the ideal balance of lightweight responsiveness and aerodynamic speed to manage a wide variety of terrain. The rim's construction, which is ultimately responsible for the wheelset's devoted following, remains the same. It involves an alloy body that's extruded and machined to thin the walls down to an almost two-dimensional 0. 6mm wall thickness--about half of the typical alloy rim wall thickness of 1 to 1. 3mm. This obviously reduces weight to a minimum, but it also means the overall structure is less sound, so Shimano buttresses the rim with a jealously guarded carbon laminate process. The only information that the brand is willing to divulge about this process is that phosphoric acid is used to anodize the alloy, and then variable carbon layers are fused to it in order to boost its structural integrity without undoing the weight gains made during the milling process. The result is a lightweight rim that's as durable and radially comfortable as alloy but still manages to approach the weight and drive stiffness of carbon. It also features an alloy brake track, which means it doesn't sacrifice stopping power in pursuit of weight reduction and also doesn't carry the risk of being distorted by internal pressure from the tube when it heats up during long, hot descents. Altogether, each component of the wheel has been designed as a symbiotic system for the ultimate in aerodynamics, efficiency, and pure speed.