Yesterday I did the same workout as 5.20, only yesterday I did it in my TT setup. If I compare the two, I see the power outputs for the work sections are almost exactly the same, but my speed in the TT setup is fully 2mph faster.
The iBike also lets me compare the aero benefit within yesterday's workout file by itself. I've calibrated my iBike to my PowerTap wheel that I use for training, so that the power readings for both meters (recording data on the same ride) are almost identical. The assumption the iBike is making for these readings, however, is that I'm in my road position (which is already pretty slippery), not my TT position. It measures how fast I'm going relative to wind speed and slope and my weight, and for the work periods in this workout it says I was putting out 348 watts for 20 minutes. The PowerTap tells me I was doing 295 watts for the same effort. The PT is correct, and the 50 watt difference is equal to the amount of aero benefit I received from my TT setup. And this setup was just with bibs and a jersey, road helmet and box aluminum clinchers, not skinsuit, pointy lid and deep wheels. According to this article, the helmet alone is worth another 9 watts.
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