Cervelo P2k (P2sl)
The P2K is Roadbikereview.com's winner of the "Consumer Choice Award" for 2002 & 2003, leaving industry observers to note "the best thing Cervélo can do to the P2K frame is nothing, it's perfect as is". So we raised the parts spec with the aero carbon seatpost, Velomax Vista wheels, Syntace F99 stem, Vittoria Rubino Pro tires and Selle Italia C2 saddle.
To optimize aerodynamics, the tubes need to have a very specific shape, a profile determined by the National Advisory Committee on Aeronautics (NACA). This shape can only be obtained by extruding the tubes, as forming round tubes imposes too many shape limitations. As an example, a round tube has about 5 times the drag of a NACA-profile, while a crushed tube is about 3 times as bad. A very important part of the shape is its thickness-to-chord ratio. If this ratio is high (meaning the tube is relatively wide compared to its depth), the air flow will detach, creating turbulence and high drag.
It is also important to note that the correct NACA profile is not round at the front but eliptical. Aerotubes with a round front and sharp trailing edge are becoming more and more popular (because it they are easy to make out of a round tube, and because they will easily take a round seatpost if used as a seattube). But unfortunately they offer hardly any aero benefit, as the bluntness of the round front already deflects the airflow before it reaches the trailing edge.
The aerodynamic downtube of the P2K is differentially butted, which means that the wall thicknesses around the perimeter are not constant. The reason we vary these wall thicknesses is that the material isn’t as useful at each spot. Both tubes are especially needed to provide lateral (sideway) stiffness. It also provides in-plane (vertical) stiffness, but that is not a critical factor, as the frame structure by itself already provides ample in-plane stiffness. It is actually a good idea to try to reduce the in-plane stiffness (and thus increase comfort), which can also be accomplished with the differential butting.
In order to decrease the drag of the rear wheel, the P2K seattube has a cut-out for the rear wheel. This feature can decrease the drag on a complete bike (complete with aerowheels) by as much as 30%, provided the rearwheel is positioned close behind the cutout. However, having the tire further behind the cutout (large gap between frame and tire) offered almost no improvement compared to having no cut-out at all. Short of a curved cut-out such as on the P3, no other refinement on the frame is going to have as big an improvement as a deep, close fitting rear wheel cut-out.
Which wheelsize is faster depends on how big you are and the position you ride in. It all comes down to putting the rider in a comfortable and aerodynamic position, and designing the bike around that.Rider Position: If the rider rides in an aero position, the handlebars and aerobars will usually be in a relatively low position, lower than on a road bike. In a small frame size, this low position of the bars can conflict with the size of a 700c front wheel, as the big wheel and the low bars wouldn't leave enough room for a headtube. Hence for small sizes we recommend 650c wheels. For larger sizes, a good position can be obtained over either a 650c or 700c front wheel, provided that in both cases the headtube is kept short enough (shorter than on a road bike of the same size).
Bike Aerodynamics: Since both wheel sizes will work on bigger frames, the optimal frame will be the one with the lowest drag. While a 650c wheel has slightly less drag and uses a slightly more aero fork (shorter fork legs), a 650c frame will have a 2" longer headtube, assuming an identical aerobar position. Our windtunnel testing shows that due to that shorter headtube, a 700c bike will actually be slightly more aero than a 650c bike. Therefore, for the larger sizes we recommend 700c wheels, but for mid sizes the customers' current equipment and other factors will also enter into the equation.
A steep seat angle (around 78 degrees) in combination with aerobars works well for most people, as it enables a more aerodynamic back position without compressing the hip angle. Triathletes especially benefit from this, as it will improve their running coming off the bike. Most time trialists (although there are many exceptions) prefer a more relaxed seat angle (74-76 degrees). But the bottom line is that one has to try both options to find out what works best. This is why both the P2K and the P3 come with a custom aero seatpost that can be adjusted to suit seat positions of 74-79 degrees with a simple flip of the head. Now riders have the flexibility to experiment with their position without risking that their bike will not be able to accommodate them.
In about 2005, the P2SL was released. The P2 SL is a SPECIAL EDITION version of the P2K, offering a lighter frame (100g) with the more scratch-resistant finish like the P3 SL.