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hippy
The air trapped between the tyre and the seat tube cut out has to be sheared, because the boundary layer attached to the tyre is moving past the boundary layer attached to the cut out. The power absorbed in that shear layer goes up as the the layer gets thinner (the drag force is inversely proportion to the thickness of the shear layer for an idealised Couette Flow). At some point, it becomes worthwhile to increase the thickness of that shear layer to reduce that drag even if the resulting gap makes the translational drag of the complete system slightly greater. When Look analysed both the power needed to push the bike through the air and the power needed to spin the wheel, they discovered that a gap of 10-12mm was optimal on their (596 Tri) frame with a disc wheel. This effect is missed in wind tunnel tests which fail to measure the power needed to spin the wheels. Shortly after Look released the 596 Tri along with their explanation for the unfashionable gap, a lot of elite track teams suddenly found some longer chains in their parts bins :-)