Originally Posted by
livedarklions
Aero generally isn't really that effective against headwinds for the simple reason that they usually are at least slightly perpendicular to the direction your heading--i.e., also a bit of a crosswind. We're streamlined for the air hitting us directly in the face, not the one hitting you in the cheek..
This is actually false. Ground-level air speed tends to be fairly low, to where the speed of a bike+rider causes the wind to nearly always be frontal-ish relative to the bicycle. And at typical moderate crosswind angles, the advantage of an aero posture is sometimes actually
bigger than for wind striking dead-on. (This is true of a lot of aero equipment, as well. A lot of people dislike deep-section wheels in strong crossinds due to the side forces, but they actually tend to see a smaller drag reduction over shallow wheels when the wind is hitting head on.)
For a true 90-degree wind angle, yeah, leaning down will do little to nothing to reduce wind side force. But that's a very uncommon phenomenon.