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Cyclecomputer Calibration Group B

If the model name is highlighted, clicking on it will take you to detailed setting instructions.

Avocet 20, 30 & 31: The calibration numbers based on tire size designations below are only approximate. Add 1 to these numbers for kilometers -- or for greater accuracy, do a rollout test and see John Allen's detailed explanation below.

Circumference in inches X 2.727

700 X 44 236
700 X 38 231
700 X 35 230
700 X 32 227
700 X 28 225
700 X 25 223
700 X 23 222
700 X 20 221
27 X 1 3/8 232
27 X 1 1/4 230
27 X 1 1/8 228
27 X 1 226
26 X 2.125 225
26 X 1.9 220
26 X 1.5 212
26 X 1.25 206
26 X 1.0 (559 mm) 205
26 x 1 (650C) 206
Wide Tubular 224
Narrow Tubular 223
26 X 1 3/8 222
24 205
24 x 1 188
20 X 1.75 (406) 158
20 X 1 1/4 (451) 173
17 x 1 1/4 142
16 x 1 3/8 137

Spoke Divider

John Allen's detailed math on Avocet inductive-pickup cyclometer calibration values:

If calibrating these Avocet computers using a rollout test:

The numbers are from an original Avocet 20 manual. The two results are slightly different. Use the appropriate result for greatest accuracy.

Wh are the numbers different? Warning: math ahead!

Avocet explains that 2.7273 is 30 / 11 (actually the repating decimal, 2.727272727...)

30/11 is 1/4 the ratio of miles per hour to feet per second, 15/22.

These Avocet computers are unusual, then, in being calibrated directly to miles rather than to miles by reference to kilometers.

But for kilometers, the same computers ask for the circumference in millimeters x 0.108.

Now let's see how the mile and kilometer calibrations compare:

25.4 is the number of millimeters in an inch.

25.4 * 0.108 is 2.7432 -- slightly but significantly different from 2.7272....

2.7432 *11/30 is exactly 1.00584

There are exactly 1.609344 kilometers in a mile, and 1.609344 / 1.600000 is also exactly 1.00584.

Therefore, the calibration numbers for kilometers will be slightly different, because the computer uses the rounded factor of 1.6 when converting miles to kilometers. The error if not adjusting for this is over 1/2 of 1 percent, which is significant, as bicycle computers can be calibrated by rollout to greater accuracy. The Avocet manual says to add 1 to the calibration number for kilometers, but this is only an approximation.

John Allen

Spoke Divider

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