Gear: June 2011 Archives


June 25, 2011

I recently started biking again and in the interest of being able to more accurately measure my workouts, I moved my SigmaSport BC1100 bike computer from my race bike onto my training bike. Like basically all bicycle computers from the pre-GPS era, the BC1100 is of the wheel magnet/sensor loop variety: you mount a magnet to one of the spokes and and a sensor to the fork. Every time the magnet passes by it induces a current which is transmitted to the computer.1 Of course, this mechanism just measures rotational velocity (rotations per second). In order to measure road speed you need to know the circumference of the wheel and as, the battery had run out so whatever calibration I used to have was long gone.

If you read the manual for a typical bike computer you'll discover not just one but many calibration techniques arranged in a hierarchy of both accuracy in inconvenience that goes something like this:

  • Look up your wheel size in a table.
  • Measure the diameter and multiply by 3.14.
  • Roll the bike one wheel rotation and measure the distance traveled.
  • Roll the bike one wheel while sitting on it (to compress the front tire the way it would be if you were riding it) and measure the distance.
  • Roll the bike N rotations (plus sitting in it, etc.), measure the distance and divide by N.

Regardless of the technique, the basic principle is that perform the above procedure, get the circumference, and enter it into the computer. In the specific case of the SigmaSport, you want the circumference in millimeters, so for a typical 700C-sized road wheel, you want something around 2100mm. Anyway, I dutifully performed the procedure as specified (see instructions here) and entered the desired number (2037) into the computer. So far so good, except that once I actually got on the bike, it reported that I was going about 25 miles an hour on average and 30 mph on the flat. Seeing as typical time trial pace for amateur athletes is around 25 mph and I wasn't even breathing hard, either I was ready to sign up for the Tour de France or something was screwed up with the calibration. The second of these seemed more likely.

A little searching around the InterWebs quickly revealed the problem: this model doesn't have an internal adjustment for English versus Metric, so you need to divide by 1.6ish to convert to miles/hour. I guess it was cheaper to just have the units setting change the labels than to actually include a circuit that divided by 1.6. Turns out that this actually is on the SigmaSport web site, though not in the owner's manual. Unfortunately, it's labelled "Attention BASELINE 400, BASELINE 700, BASELINE 1200 & BASELINE 1200+ owners!". which doesn't really help, since I have a BC1100. Outstanding!

1. The really cool Jobst Brandt-designed Avocet cyclometers instead used a ring of alternating polarity magnets mounted around the hub, allegedly for better precision. They don't seem to be available any more.