Thanks for covering me 53x12. I never did get a chance to take my bike apart and weigh the frame. I think the S2 would come in pretty close to the S3. According to Damon Rinard, there's really no difference in weight between the two framesets.
From a post on slowtwitch.com:
Post #13 of 37 (2602 views)
Re: S2 vs. S3....any aero differences? [pyrahna] [In reply to] Quote | Reply
Performance-wise, just the component differences.
Our mission is to make Cervelo riders faster. This philosophy helps explain the engineering of the two forks.
We developed the S3 fork first, using the same advanced engineering tools we've honed in engineering the Rca and P5: CFD, FEA, laminate analysis, etc. The somewhat large surface area (due to the long chord) means we've chosen higher modulus fibres in some areas, to help reduce weight.
Afterwards, to make the S2 fork, we chopped a centimeter or so off the trailing edge of the S3 fork's CAD model. That reduction in surface area let us save some weight, which we put back into the fork in the form of thicker walls (but not as high modulus fibres). Those two changes combine in a cancelling effect: same weight and stiffness. In the end, the main difference is the S2 fork costs less.
Only afterwards did we find there was no measurable difference in the wind tunnel!
Cervelo S2 fork: weight neutral, stiffness neutral, aero neutral.
Lucky for S2 riders, eh!
Cannondale Road Engineering Department