The frame is UCI-legal. Cervélo offers a triathlon-specific fork that is not. The frame conforms to the regulations stating that tubes must be no more than three times deeper than their width, but Cervélo used a loophole to stretch the seat tube beyond the typical interpretation of the rule. Cervélo senior advanced R&D engineer Damon Rinard says the UCI allows “gussets” that support and connect the frame tubes as long as they are no deeper than the original tube dimension. The P5’s seat tube is 27mm wide, which means it must be 81mm or shorter in the longest direction, and the seat tube is almost exactly that length. The gusset connecting the seat tube and the top tube, however, is another 81mm. These connected elements create a surface that is 162mm at its longest point. A second gusset is used to connect the seat stays to the seat tube that extends the segment of the tube deeper than the UCI’s 3:1 ratio lower on the seat tube. At its widest point, the P5 actually has a 6:1 ratio, yet it still abides by the UCI’s 3:1 rule. Go figure.
Cervélo contends that building a bike with a single tube shape doesn’t make sense. The rider’s body and wheel influence the air flowing around the bike in different ways at different sections of the bike, so Cervélo broke the bike into discrete segments to design shapes optimized for each condition. As a result, the P5 has different tube shapes from the aerobar to the back of the frame. They’re sticking with a teardrop-shaped airfoil instead of a truncated airfoil design for the downtube and aerobar, but some portions including the seat tube have a chopped, flat tail.
The same P5 frame is used for both the triathlon versions and road time trial versions of the bike. As a result, Cervélo did not use outrageously deep profiles for the downtube or head tube such as those on the Quintana Roo Illicito and Specialized Shiv. The triathlon fork, however, is extremely deep. The fork blades and the extension off the front of the bike both create airfoil shapes much deeper than the road version. We tested both bikes in very windy conditions and noticed a slightly greater influence from the wind on the triathlon version.
Cervélo elected to stick with horizontal dropouts, which make wheel removal and reinsertion more difficult than vertical dropouts. These dropouts also allow the gap between the seat tube and the tire to be finely adjusted. Rinard says a gap of 2-6mm is aerodynamically ideal and there is no performance difference within that range, taking into account the rotational aerodynamic resistance against the wheel in its direction of motion created when the air moving with the tire collides with the frame. Hexagonal head screws are sunken into the dropouts. They can be backed out to space the wheel away from the frame to widen the gap or accommodate a 25c tire. The bike is optimized for 23mm tires and when the screws are all the way in the bike, tires of this width fit precisely with the frame. The 27mm-wide seat tube is another change Cervélo made from the P4 to the P5 because of the trend to wider wheels and tires. The P4’s seat tube is 25mm wide, a shape that is optimal for narrower tires, but the P5’s is 27mm at its widest point. The broader tube helps smooth airflow from the frame to the tire and should also improve lateral stiffness.