Originally Posted by
svtmike
The power is estimated off of effective wind speed, grade, and other parameters like rider's aero drag. My understanding of the iBike is that you have to calibrate it at the beginning of each ride so it knows what the conditions are. But conditions change, and the data is therefore based on shifting sands. There is a ton of information in this document:
http://www.ibikesports.com/Download/...une_09_red.pdf
Direct measurement of torque off of a strain guage, while more expensive, is really the way to go if you are serious about training with power.
Here's another huge problem with it in my book -- iBike recommends battery changes every 2 hours for operation in 40 degrees and below.
It would seem that the only thing that would change is the weight of the bike and rider throughout a ride as water is consumed and perspired. I lost 0.5 lbs on my long ride yesterday. I lost the equivalent of half a water bottle in sweat.