The diameter of your fork steerer has a noticeable effect on how your bike performs. Evolved Steerer Design applies the best steerer diameters and taper design to achieve the goals of that specific model.
A larger steerer diameter adds torsional stiffness with no weight penalty. A smaller steerer diameter is more compliant and can also be used to reduce head tube frontal area, and therefore, overall aerodynamic drag.
How do we select which one to use? By defining performance goals – weight savings, aerodynamics, stiffness, comfort, and usability -- at the start of the frame design process, Cervélo engineers can choose the best steerer diameters/tapers to help achieve those objectives.