Originally Posted by
skinnyone
And given the angle of incidence of wind onto your wheels, it would have to be in a very small range to contribute any real gain.
I have no idea of your definition of "small" much less "very" so I can't really say if this statement is correct or not. I do know the test conditions and range over which Zipp observed negative drag and the typical yaw angles over which people ride. The overlap of the two is large enough that negative drag is possible over the majority of conditions likely to be seen during a time trial.
While it is true that all disks exhibit negative drag under some conditions (hold a disk wheel perpendicular to the wind and you'll feel it), most do not show it under typical riding conditions (the above example, for instance).