Originally Posted by
Looigi
IMO, it's a sound concept but: Basically, it determines power from airspeed, road speed, and barometric pressure (rate of climb). To do that, it needs to know the total weight, aerodynamic coefficient of drag, the rolling resistance, and drivetrain efficiency. There is a calibration you do by riding out and back, but it's unclear to me how it uses that information. One obvious shortcoming is it can't tell if your position changes from the drops, hoods or tops, or if there are other changes to your Cd, like changing wheels, wearing a jacket, etc.. Smaller, but still possibly a factor is accounting for changes in total weight. I would also question how it handles or accounts for crosswinds.
IMO, it could be a useful training tool if those limitations and caveats are understood and accounted for.
Obviously the unit depends on a sophisticated algorithm to measure power, and I see from watching a video on Youtube that good readings depend on user input for things like riding position and rider weight, and bike weight. From there, it must estimate Cd, using input from the calibration ride. I suppose, then, that if you were going to change wheels, riding position, or clothes, you'd want to do another calibration ride to get the most accurate power measurements.
I'd also guess that the Newton+, which does all sorts of fancy aerodynamic calculations, is able to switch between algorithms for riding on hoods, or in the drops on the fly, without having to do another cal ride. It would probably be horrible having to push a button everytime you change hand positions, so maybe it does it some other way; I dunno.
Having to do at least some calibration before every ride (wind?) is something to consider when weighing systems against each other. I've never used a Powertap or other direct measurement system, but I presume that because it's actually measuring leg force (at crank, pedals, or hub), that you can just get on and go.