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Old 01-14-14, 02:27 PM
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wphamilton
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Originally Posted by gus6464
If anything I would think the smallest and thinnest bike frame would be most aerodynamic since it equates to a smaller area of wind resistance.
That is a very reasonable surmise and it's not clearly wrong. In most of the possible comparisons between frames, I'm guessing that it's accurate. Yet, I've seen enough data from different tests to convince me that, at least in some cases, the aero-shaped frame has an aerodynamic advantage over a same-sized thinner round-tube frame. Even so I have to question a lot of the assumptions about aero frame shape.

One, I don't believe that head tube and downtube cross sections are truly that aero in shape. They have the appearance, but they aren't actually long enough (front to back side of the tube) for the lower-drag shapes, in other words tapering too quickly. Second, if you draw the shape on paper and visualize different apparent wind angles, at some point not very far from dead on the aero tube shape loses all advantage, and is possibly worse than a simple cylinder. Third, it's not really a static system in that the streamlines (where there are streamlines) depend not only on the immediate surface and shape but also on what the air is doing in nearby areas. And finally, how much does it depend on how you're riding a particular bike, even if you have the exact same position on each? Rocking it back and forth or steady on the rivet, how often you gun it hard, how you attack the corners and what line, maybe these (and other differences I'm not thinking of) depend on how the bike responds and maybe that also affects the aerodynamics? I think that's probably true.

On the other side, there's something about the frame transitional points, like where the head tube meets the top tube, behind the seat tube, other places. If some frames test out more aero than others, I think - and this is just a surmise - that it's in large part if not entirely due to smoothing out the air flow at these places. So there may be something there.
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