Monday, December 23, 2013

Final thoughts on sprocket use - pedal cadence and overall accuracy

From my two previous posts (#1 and #2) on my actual use of the 11-28 cassette, I had decided to not include data points where the recorded cadence went below about 50. My reasoning was that actually its quite hard to pedal that slowly and would likely only occur as I was rolling to a stop and changing gear ie soft pedalling.

Is this a sensible limit? Well I tried various values, and 48 seems about the best. Here's teeth vs speed for a 4.25 hour 125km ride. After I removed all the data points with either zero speed or zero cadence, I still had over 13,500 data points:

You'll see the fairly sharp cut-off on the lower side of the data cloud - thats the 48 rpm cut-off at work. When I adjust that value, the data either gets messier (ie noisier) or the line gets too sharp ie I'm removing signal.

So, 48 it is.

Next question, how does the actual data compare to modelled data? I created a matrix of teeth vs cadence, where the intersection is speed (in kph):

And a chart:

Now, if I scale and overlay them (I've drawn in lines to represent what my cassette offers):


Another thing you might notice is that that the lines of cadence are quite distinct, which we see zooming in:

What we're seeing here is that the GSC-10 only records integer values for cadence.

The cloud we see is thus where two imprecisions from sampling frequency and data recording surface:

  • cadence to 1 rpm only
  • speed and cadence recording at 1Hz intervals

If, in the example here, the number of teeth boils down to 17 ± .25 then that works out to be about 98% accuracy.

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