Continuing with the theme of creating a bike specifically for triathletes, Specialized ignored the UCI rules governing frame tube shape. Those rules state that a frame’s tubes cannot be more than three times deeper than their width. This limits aerodynamic performance and is the reason other brands, such as Trek, are using truncated airfoil tube shapes. Passing those rules aside gives Specialized the freedom to create the Shiv’s shockingly deep headtube and downtube. The headtube is 4.1 times deeper than it is wide (4.1:1 ratio). The downtube has a 3.9:1 ratio, the fork blades have a 3.4:1 ratio and even the seatpost violates UCI regulation.
Specialized road product manager and aerodynamicist Mark Cote explained that the aspect ratios were tuned for the specific wind conditions faced by different portions of the bike. He says that the rear of the bike experiences wind at a narrower yaw angle at a lower speed than the front of the bike because of interference created by the bike and rider, therefore these parts of the bike need different tube shapes. In addition to the radically deep tubes, Specialized added a fairing behind the steerer stack extending above the toptube. Many aerodynamicists say a gap behind the steer stack creates substantial drag and this fairing, called the Control Tower, helps minimize that drag. Cote says the difference is not massive, but is measurable in the wind tunnel.
Instead of using an integrated front brake, Specialized opted for an external caliper. Cote says the extra drag created by the exposed housing is tiny and he believes the ability to easily adjust the brake offsets that aerodynamic sacrifice.