TT bikes are reaching a level of design maturity in which gross functional performance features really are not all that different--there is a convergence of design, in other words. What's really interesting are these trends:
--Functional design elements are becoming very similar--low mounted, behind BB rear brakes, integrated stems, hidden front brakes, fared rear wheels are the norm.
--The also is a move toward integrated design by the major manufacturers--Trek, Specialized, etc. This integration offers the promise of additional incremental gains in aerodynamics, but often comes at the cost of limited ecologies (you have to buy frame and components from one manufacturer) and sometimes limited adjustability (e.g., Specialized's Shiv has the same standover height for all frame sizes; the only variation is reach).
--Increasingly, we are seeing bikes/systems optimized for specific conditions and/or specific rider speeds; it appears that a good number of bikes and wheels are being designed for yaws in the 10-15 degree range of yaw, which are angles normally seen by mid-pack competitors (or fast riders on exceptionally windy days). The new generation of superbikes, in other words, might not be optimized for the fastest riders (which comprise a small segment of the marketplace).
--Increasingly, manufacturers' are designing to consumer demand, and not necessarily for optimal performance. Note the trend toward massively wide wheels to accommodate the wide tires preferred by most riders. Soon enough, large manufacturers will design frames around specific wheel lines, or offer their wheels of their own optimized for their own frames (Trek?). Often accessories will be developed to accommodate rider need, whether real or not (e.g., Trek's bolt-on boxes, Cervelo's faux water bottle, etc.).
--Increasingly, we will see aesthetics as a primary difference among brands (see Look's kinked top tube, Argon's diamond shaped tubing, or Ceepo's heavily shaped tubes). I suspect that most of these aesthetic differentiators will prove to be aerodynamic detriments; think of them as the new century's version of the 90s Italian paint schemes.
--Given that UCI restrictions still will be the primary driver of frame and component design, it will be difficult to buy a good, mid- to top-tier bike that won't be aerodynamically efficient, provided that one is attentive to fit and basic details.
--Increasingly, frames, wheels, and components will be designed outside of a tunnel, based instead on computational fluid dynamics and proprietary software models. This, in turn, will lead to a number of conflicting and mostly unverifiable claims as to absolute and comparative performance.
--We now seem to be at the point where we were in the late 80s and early 90s, when everyone rode a round tube steel frame and simple clip-on bars. Most new developments will center on component and accessory development, such as electronic shifting (value is arguable for TTs), tubeless road tires (not so much arguable for TTs), and hydration systems (internal frame bladders, like CAT's Cheetah?).
Don: Cycling Tech >