The Cervélo S3 is pretty much where Cervélo began their aero road bike category. The S is for speed. But the new S3 is a result of Cervélo applying lessons learned all over their product range and seeing them put together here for an aero road bike that's does everything the old S3 does, only better.
The traditional sign of an aero road bike is a shaped, oversized down tube. Indeed, the S3's down tube has gotten larger. It has also gotten more complicated. Cervélo engineers learned that the air flows over frame tubes differently at different points. For example, the top tube does a pretty good job of hiding behind the head tube. But the down tube starts hidden behind the head tube and then becomes more exposed—and as its becoming more exposed, water bottles are added behind them, further changing the air flow. To better work with this reality, the down tube starts fairly narrow behind the shrouded head tube and then widens and flattens on the trailing edge as it reaches the BBright bottom bracket.
And that bottom bracket standard is wider and larger than most everything out there, which is better both for crank stiffness and efficiency, but also for Cervélo to build more around it for better frame stiffness and aerodynamics. Cervélo changed the seat tube cutout to better fit 25mm tires as well as better shroud the rear wheel. The slender ultra-thin seat stays are employed here because they both improve vertical compliance for greater comfort at a cost of almost nothing in terms of aerodynamics. The fork, which has undergone a change in shape, is 16% stiffer in the lateral plane and is shaped to match the curves of the front brake, thus minimizing any aero penalty.
There's also an interesting trade-off between the S3 and the S5. The S3 is stiffer, but less aero. It has a four-watt increase in drag over the S5. But the shaping and lay-up are such that the S3 is 23% torsionally stiffer at the head tube thanks to the tapered 1 1/8" to 1 3/8" steerer and nineteen percent stiffer at the bottom bracket. Perhaps this is why some of the Garmin-Sharp team choose this for their racing weapon.
The S3 picks up several other design initiatives from Cervélo's other bikes. It has Future Proof internal cable routing. Electronic, hydraulic, mechanical; it all works, just change the fittings. There are mounts under the down tube for a battery. And possibly the most important overlooked feature of Cervélo bikes is the sizing. Looking at seat tube lengths proved pointless with the advent of compact frames. Top tube lengths are better, but vary the seat angle of head tube length and the top tube suddenly is not such a fixed number. While Cervélo uses the top tube length to name the frame size, they really focused on making sure there was an even progression in terms of stack and reach numbers. With the sloping top tube they run, the biggest issue for you is to figure out what reach you want.
The S3 Frameset is the platform for fast riders and privateers. It comes with Cervélo's FK42 fork, a Cervélo Aero seat post, and an internal FSA headset.