29 September 2015 ~ 0 Comments

Calibrating non-GPS Cyclocomputers

My Sigma 1606L at the end of 4 days on ”Last Chance.“ Thats a lot of miles for one ride!

Calibrate your bike computer (or cyclocomputer) so it is accurate—unless you have a GPS-based model. On my first brevet, my computer was so far off, that it was a major factor in my DNF (darkness, rain, sleet and a tiny bit of snow didn’t help either). To calibrate it you must know how to change the wheel size in the computer. Not all wheels are the exact same circumference. A 700 x 25c tire between brands can vary enough that during a 125-mile brevet you might be a mile or two off at the end of the day. Use a GPS to calibrate. Borrow one from a friend if you need to. I have a Garmin designed for hiking and generally don’t use it for cycling (too heavy & eats batteries like crazy). Don’t use an automobile odometer—they are all over the map.
Here’s how: Zero out the mileage on the computer & GPS. If necessary, put the GPS in your back pocket so it gets a clear shot of the sky for accuracy. Ride for 4 or 5 miles and compare the two. If the bike computer reads high, then you need to reduce the wheel size. Or visa-versa. Then ride several more miles and check it again. It may take several calibration attempts. It is simply trial and error. Finally, ride for 10–20 miles and check. I try to get mine to read within 0.1 mile accuracy for a 15 mile distance.
Don’t do this in mountainous terrain or a steep climb as some GPS models have trouble there. I have found that with my Sigma computer, I use 2110mm for Maxxis 700 x 25 ReFuse tires and 2113mm for Conti. 700 x 25 Gatorskins.

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