AeroPod, PowerPod and Newton

I was in a discussion about the iBike on another forum recently when the issue of changing position and CdA came up... like usual. It suddenly occured to me that this isn't really a problem.

Body position is a factor in power production. If you've calibrated for a higher position, then the iBike will over-report actual *power* when in the drops. But my FTP is lower in the drops than on the hoods... so the higher power that the iBike reports, may be a more realistic measure (accidentally) of the stress level than an SRM (or PT or whatever), which doesn't account for body position.

For instance... I don't have a single FTP. It is ~10% higher on a climb than on my TT bike (or a low drops position). Riding for an hour at 270W in a low position is the same physical stress as riding at 300W with my hands on the tops. An SRM won't see this... but the iBike does.

You might think the iBike overcompensates for position on the flat, because the difference in aero drag will be greater than the difference in stress (that is why you went to the drops in the first place). But your body creates a "bow wave" of pressure that increases in effect the closer you are to the unit. In other words it partially compensates in the other direction... ie the power reading goes up by less than the change in drag.

So if you are interested in tracking training stress, then the iBike works fine as is... IMO. It probably even does a better job than other PMs!

I've noticed that with the Gen3 units much of the "resistance" against the iBike has disappeared. Even the drafting issue seems to have been greatly improved. So good job all you Velocomp people! Keep up the good work.

At first I dismissed your idea as accidental, but after I thought a bit more about it, if your position is optimized, then to first order improvements in CdA will be canceled by changes in FTP. So in this instance your point is valid, for minor deviations from the optimal position, keeping the unit at FTP should track your position-dependent FTP. But if your position is too relaxed to too aggressive, this cancellation will no longer apply.

The math: Consider z is a parameter describing position, and FTP is the FTP as a function of z, and CdA is also a function of z. Then:

Assume FTP / CdA predicts the iBike reported power, and predicts the speed on the flats.

d/dz (FTP/CdA) = 0
(d FTP/d z) / CdA - FTP * (d CdA /d z) / CdA² = 0
(1 / FTP) d FTP /d z = (1 / CdA) * d CdA /d z

So the fractional change in FTP with position = the fractional change in CdA with position if position is optimized.

I'm not sure about the issue with Crr, however.

"It was my understanding there would be no math." - Chevy Chase (playing Gerald Ford)

Indeed there is no mathematics involved because it is simply wrong.
FTP is weakly coupled to position, while CdA is much more strongly dependant on it. The proposed maths states that FTP and CdA are couipled in exactly the same way to position. There is absolute no supporting science for it.
Just use a profile "hands on top" and ride 30 minutes full out with position "hands on the drops". You will get an Intensity factor IF much above 1. No more must be said!

Charles wrote:Just use a profile "hands on top" and ride 30 minutes full out with position "hands on the drops". You will get an Intensity factor IF much above 1. No more must be said!

A more realistic comparison would a low-hoods position which I use most of the time (and where my iBike is calibrated), and drops. Have you actually tried this? And if you are so much faster in the drops then why don't you ride there all the time?