Polar Power & Resonance

So the program my coach has me on is power based. This meant I had to get out that damn Polar power meter and install it on the bike I'd use for most of my training. It only took a couple of hours and things seemed to work great, until I noticed that in one, and only one, gear combination the power went through the roof. So my wonderful wife suggested I try using her chain (wipperman connex, rather than my cheapie SRAM). Lo and behold the problem went away!

So we took out one of her spare chains to put on my bike. In life there are these moments when something tugs at the back of your brain and asks for your attention. I had one. As we're prepping the chain my wife asks "Should we leave it the same length as mine?" The obvious answer was "Yes!", but I said "Gee, it should probably be the appropriate (shorter) length for my bike. How could the overall length possibly matter?" Bzzzzzzzzzzzt, wrong answer.

What happened was that the resonance (that's what must be going on, because clearly the pickup is reading the wrong, higher frequency) moved from one gear combo to another (less bothersome) one. Crap! By the way, Polar will absolutely disavow any knowledge of this phenomenon. I'm having trouble getting my head around it, since the source must be in the static chain tension. I don't know if a longer chain length throws the resonance onto a gear combo you'd never use (say, 50x26) or just in between two existing ones. It doesn't matter, when my new chains come in I'll set one up as long as I can and test it. If the resonance is still there, then I'll cut it down and hope it falls into limbo between two gears. I'll let you know.