Originally Posted by
alan s
You don't really need to run tests to determine which is more aerodynamic. Just look at how much is exposed to the wind, and you can easily tell. If you sit upright, a backpack will add nothing to windage, but it you ride in a lower position, it will increase your windage. This difference is measurable at any speed, but the differences are magnified at higher speeds due to the exponential relationship between windspeed and drag.
Sorry but I disagree! The reason is, the shape and texture can matter as much as the reference size, and additionally what the air is doing at that point. Is it attached to the surface in a smooth flow, a momentary separation and reattachment, turbulent, low pressure? What about parasitic drag? And then we have real world considerations such as what happens when the wind shifts even a few degrees. It can become complicated very quickly and the situation we see on a bicycle is more dynamic than we'd think at first grasp.