AeroPod, PowerPod and Newton

If I have a DFPM (powertap) and an iAero can I block (or otherwise ignore/disable) the wind port?
I really like knowing what part of my power goes to gravity/brakes/acceleration but I'm content to simply calculate the wind resistance from the DFPM measured power. I know that I won't be able to get wind speed out of it (because a higher wind speed has the same effect as a higher drag) but I will get the total power going to aerodynamic effects.

Simplistically you have wind+gravity+acceleration=power. You only need three out of the four, so I know it's possible. But it might mess up how the software operates (for example in giving summaries)...

Basically I'm thinking about using the ibike as a head unit for the powertap with the benefit of getting the power breakdown by wind/gravity/acceleration. It would free me of the constraints of a clear wind port/RWS.

Anybody tried something like this?

This is an interesting question.

There are actually two variables (or, if you're really picky, three):

1) Wind speed
2) drag coefficient
3) rolling resistance

All three of these variables, in addition to gravity and acceleration, can vary.

In the iAero we assume that rolling resistance is a constant, generally a good working assumption.

The iBike measures wind speed and, in combination with the DFPM data and other iBike data, computes the corresponding CdA.

If you do not measure wind speed, you won't know if your DFPM power change is a result of wind speed changes, rider position changes, or both.

Most certainly, if you assume that CdA is fixed then you could infer wind speed from the DFPM and other iBike data. However, our experience suggests that a fixed CdA assumption is not valid. Furthermore, because aerodynamic resistance increases as the square of velocity, any errors in the fixed CdA assumption could result in significant errors in calculated wind speed.

You left out drivetrain losses. These are modeled by iBike as a fixed fraction of power, but reality is more complicated. For example, a small chainring with the small cog will have substantially higher losses than the big chainring with a larger cog in the same gear ratio. Additionally, chain condition (how clean it is, how worn it is) has a substantial affect. For example Kraig Willett has estimated the difference between an old and new chain is around 3 watts.

For comparisons to PowerTap data, it doesn't matter. But for Quarq or SRM or Ergomo or Brim Brothers or anything else which works upstream of the drivetrain this will be an additional source of uncertainty.