mim’s blog

My parents got me a bike trainer for Hanukkah this year and I’ve been riding inside while the weather is crappy. On Graham’s recommendation, I got the Kurt Kinetic Road Machine. It’s a fluid trainer, so the resistance comes from an impeller stirring a bunch of silicone fluid, which provides resistance that’s supposed to mimic that of the road and wind. The back wheel of the bike rolls on a little metal cylinder that’s connected to the impeller.

When I started riding on it, one of the first things I noticed was that it makes a pitched sound and the pitch goes up as I go faster. I decided that it would be cool to record the sound at different speeds to see if I noticed any regularities. After finding a good placement for the mic (the crappy computer mic again), I managed to get some decent recordings, the spectrogram of which you can see below. The x-axis is minutes and the y-axis is Hz. I’m riding at three or four different speeds, which are pretty accurately represented by the rising and falling of the lines in the spectrogram.

After seeing the spectrogram, I hypothesized that the sound might be the same frequency as the roller and impeller. The roller is 2 1/8 inches in diameter, so going one revolution on it is like traveling 2.125*pi inches on the ground. Using some dimensional analysis, that means that one revolution per second is the same as going 0.379 mph (2.125 * pi inches/s * 3600 s/hr * 1/5280 miles/foot). That means that if I were going 23 mph outside, the roller would be spinning at 61 rotations per second, or 61 Hz and that’s about what I saw in the spectrogram, along with its harmonics.

Unfortunately, My bike computer is hooked up to my front wheel, so it’s useless when I’m riding the trainer. I’d like to see how accurate this calculation is and then make a calibrated audio speedometer for the trainer. Even without that, it could at least track the frequency the trainer is humming and tell me how fast I’m going relative to the rest of my ride.

This entry was posted on Tuesday, March 11th, 2008 at 7:31 pm and is filed under ideas, research. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.