Because I thought I could
Join Date: Dec 2004
Location: Wash DC Metro
Bikes: November, Trek OCLV, Bianchi Castro Valley commuter
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Search the Road Cycling forum, racing sub forum, and perhaps the Training & Nutrition forums. There have been numerous discussions of different power measuring systems. There have also been reviews in various online cycling magazines (Velonews, Pez). SRM is the "gold standard" and PowerTap seems to be the "good enough". The Polar power system measures the forces on the chain itself; I believe it requires precise measurement of chain weight and length and careful installation of the pickups. In side-by-side tests, the Polar system appears to be the least accurate (measured watts close to actual) and consistent (measured watts stay the same for the same conditions) of the 3. This doesn't make it a bad training tool, necessarily. Also, I believe most of the reviews were of the older system used with the Polar 720. I believe Polar's latest cyclecomputers with power option have a new version (same basic technology) of the power system that may work better than the older models.
I have a Polar 720 and really like it's HRM and bike functions; I've followed some of the discussions but have put off buying a power system for it as I don't really need it and couldn't justify the cost, even if someone is selling a used one.