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Since the weather was generally crappy yesterday, I put the Kuota up on the trainer to do a trainer ride. I moved the speed sensors to the rear wheel (against consel – Polar manual says not to). I needed to move it there to allow me to do a comparision of the watts versus speed for both bikes / power systems using my Kurt Kinetic trainer as teh go between.

Before putting it up on the trainer, I moved the speed sensor and the wheel magnet and got them to talk to each other as it were. But when I started riding, it looked like it was skipping a beat since I was getting readings of 3-6 mph… So I stopped and angled the sensor toward the wheel, problem fixed.

I started out my ride as a warmup around 200W in the little ring… No problem. The numbers “felt” right… My plan was to do a number of different intensity intervals for the dual purpose of a workout and to get some data for comparision. I want to get the numbers about right to help me with TT pacing. If 300 W on the screen = 250 W, for example the power meter won’t help anyone.

So I put in a VHS tape of the 1994 Pink Floyd Pulse Tour concert video and cranked it up for my workout. Once I got past my warmup, I put it in the big ring and watched my power drop… I was pushing 22-23 mph which should have been good for high 200’s wattage, but the Polar was showing 150W and even lower. Crap. The early optimism for this device was fading fast… If I can’t get power numbers in the 53×15 to 53×11 that won’t help any time trial I have ever done.

Many zip ties (and an old tube) had to die that morning.

So I get off the bike and look at the chain and the sensor… time to make some changes. After several combination of the provided spacers, I ended up using cut up pieces of inner-tube to shim up the front end.

Photos of the current install. Not perfect, but seems to be better in the big ring.

At the end of the ride, I wanted to see if I could figure out what gears (and their chain lines) worked better than others. I haven’t had time to fully work out that aspect, but it is clear that the power numbers for the Polar seem to skew lower. This can be corrected by increasing the chain linear density (g/mm)-once I get some more data. I half wonder if moving the speed sensor made things worse or is it that my on the road rides have been lower intensity / small ring type rides (or is there some harmonic due to the trainer / wheel being out of round or the effects of uneven chain tension around the pedal cycle, etc.) I did put it in the big ring on the road and when it was reporting 300W, it felt about right, but it is difficult to make those sort of judgments on the road.

I have tons of data that I can get out of WKO+ for the Felt/SRM combo, but I had only this lower instensity workout from back in November already in the software. Between getting it out of WKO+ (I found using SRM format works best) and converting km/h to mph, etc it will take a little bit of time, and honestly, this is just a side side-project. Plenty else to do besides this! (Without going into the data deeply, the lower cluster of power numbers at ~20 mph and 150-175W were when in big ring combinations.)