Instead of coming up with a way to 'correctly' attach my accelerometer onto my electric bike (which has only delayed this project), today I just set the thing recording and threw it in my backpack.
The accelerometer on my phone collects data at about 203Hz, so the highest frequency it can deal with is about 100Hz.
Technical problems failed to record my outbound ride, but my inbound ride was recorded. Here's the data:
... and here's the spectrogram (using a 256 sampling window in a 4096 pad):
spectral range (left to right) goes from 0 to about 100 Hz. Temporal range is about 900 seconds, doing from top to bottom.
That bright white wiggly line is about 6 Hz. This corresponds to the rotational frequency of the tires if traveling about 28 mph -- which I was! The line is wiggly because I'm contantly slowing down and speeding up.
Check out the harmonics!
Pretty decent S/N -- I think I can work with this...
Here's a plot of the average spectrogram, which is just the sum of each row from the image above. The peak is about 5.84 Hz, which with 26 inch wheels works out to an average speed of 27.1 mph. I can't account for those other smaller peaks to the left at about 4 Hz and 1.6 Hz.
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