VII. Single-Magnet Method
In this variation of the method only one magnet is used on the 20th spoke of a 36-spoke wheel or the 18th spoke of a 32-spoke wheel. (Spokes are numbered following the zero spoke in forward rotation) The obvious advantage is that the measurer does not have to acquire additional magnets. Also, when switching the computer back to regular bicycling use there is no need to twist magnets away from the sensor or to remove them. Even when not rezeroing the computer at each split, there is virtually no chance of generating a spurious impulse.
On the downside there is less chance of detecting a spurious impulse since the computer will always remain synchronized with the rim zero spoke. Each increment of the display represents 4 rev, and it may be necessary to monitor the turning of the wheel through a whole revolution without the benefit of the computer.
For instance, to measure 768.42 rev, stop immediately after the display registers 192 and without going through the zero point again move the wheel backward or forward to get 0.42 on the rim. However, to measure 769.42, stop again at a display of 192 but move the wheel forward through the zero point again and on to 0.42 indicated on the rim. To measure 771.42 with plans to rezero afterwards, stop immediately after a display of 193 and turn the wheel back through the zero point to 0.42 on the rim.
To try out the method, I measured a 3-km course that I had certified a few years before using a Jones. This course is the basic unit for longer courses and has been validated because a 12-km national age group record was set on it. Two individual measurements differed by only 0.03% and the new measurement differed from the old by only 0.08%.
Temp: 73 degrees throughout.
Length of calibration course: 400 meters.
Meter reading at end of calibration runs: 47 = 188 rev.
Pre-measurement calibration additional rev: 1.115, 1.11, 1.10, 1.125; ave, 1.1125
Post-measurement calibration additional rev: 1.09, 1.10, 1.09, 1.11; ave, 1.0975
Average additional rev for the day =(1.1125 + 1.0975)/2 = 1.105
Average constant for the day = (188 + 1.105) x 1.001 x 1000/400 = 473.235 rev/km
Rev/ 3 km = 473.235 x 3 = 1419.71
Ride 1: 1.5-km mark = 4 x 177 + 1.58 = 709.58 rev
finish mark = 4 x 354 + 2.93 = 1418.93 rev
Ride 2 ( return ): 1.5-km mark = 4 x 177 + 1.29 = 709.29 rev
start mark = 4 x 354 + 2.55 = 1418.55 rev
Difference between ride 1 and 2 for 3 km = (1418.93 - 1418.55) x 100 / 1418.55 = 0.027%
Difference between new and old measurements for 3 km = (1419.71 - 1418.55) x 100 / 1419.71 = 0.082%
VIII. Two-Magnet Method
In this variation of the method magnets are used only on the 2nd and 20th spokes of a 36-spoke wheel or the 2nd and 18th spokes of a 32-spoke wheel. (Spokes are numbered following the zero spoke in forward rotation) The obvious advantage is that the measurer has to acquire only one additional magnet. Also, when switching the computer back to regular bicycling use there is only one magnet to twist away from the sensor or to remove. Even when not rezeroing the computer at each split, there is little chance of generating a spurious impulse. If such an impulse occurred, it would be detected because synchronization of the zero spoke with the counter would be lost.
Of course calculations are a little more complicated because each display increment represents 2 rev. However there is little need to have to monitor wheel movement through whole revolutions without the counter. For instance, to measure 768.42 rev, stop immediately after the display registers 384 and without going through the zero point again move the wheel backward or forward to get 0.42 on the rim. To measure 769.42, stop again at a display of 384 but move the wheel forward through the zero point again and on to 0.42 indicated on the rim. However there is a slightly simpler way to make the latter measurement if rezeroing is planned afterwards. Stop immediately after a display of 385 and turn the wheel back through the zero point to 0.42 on the rim.