Originally Posted by
DrIsotope
Except that other that between the green and checkered flags, speed is a meaningless metric. I could draft a van all day and do a century at a 30mph average. Or I could just ride downhill. Or one day the wind is in my face and the other is at my back.
Speed is in not always indicative of effort. A power meter shows work done and ignores speed altogether. Anyone who has ever ridden up a hill knows it can take every bit as much work to climb a hill at 7mph as to cruise on flat ground at +20mph.
A power meter is one way to eliminate a lot of variables.
I agree with your statements that speed is not indicative of effort and it does eliminate variables. That is why I like time better for measuring improvement. Power is just one factor in going fast. I think there are too many variables, and too much variability in the measuring devices to make focusing on power very useful. Then what do you do with that number. Defining if power is for a sprint, or a TT, or up a hill will in theory tell you or the coach what to change, but more power generally comes at a cost someplace and the only measure I know of telling you if it was "worth it" is seeing the time go down.
Time over course may decrease by becoming more aero, taking better lines, changing cadence, and varying power application such as applying more power before cresting the hill, coasting downhill. Looking at the time for a "regular" ride is as useless as looking at your power. But when you want to measure, you pick the same undisturbed course and ride it for a time that you think is important. That time, or that power tells you how you are doing. If only one measurement device could be used on a repeatable course, I'd use the watch.
On that same course ridden for a test, the variance in PMs (hub, pedal, crank or different ones) will be much more than the variance in watches. Get on one bike with one PM and hold 300W and the time will be different on the same course than another bike, or another PM at the same 300W.
We use PMs when testing equipment and position as that is the only consistent thing (short of a wind tunnel) available. Once the equipment is dialed in, then time is the best indicator of what you are trying to achieve.