II. Typical Set-up
1. Marking the wheel rim
Use a black Sanford Sharpie Permanent Marker to create very permanent marks on the rim. Red can be used to designate smaller divisions. Assign increasing values in an anticlockwise direction as viewed from the right-hand side of the wheel and clockwise from the left-hand side. Designate a zero spoke on the same side of the wheel as is desired for the computer sensor.
Marking the rim requires estimation of 0.6 and 0.8 of a spoke interval for a 32-spoke wheel and 0.5, 0.6, 0.7, 0.8, and 0.9 for a 36-spoke. These can be measured accurately on a card and the card used repeatedly or simply eye-balled.
Time to mark a rim in fractions of a revolution is less than 25 min. Time to mark in spoke intervals is 5 min.
Note that if the above instructions have been followed correctly, fractional revolutions should increase with forward rotation of the wheel in the forks. Also, after completing installation, there should be excellent synchronization of the rim zero point with that of the meter. If a mistake is detected, simply turn the wheel 180 degrees in the forks.
(i) Wheel with 32 spokes in fractions of a revolution
Mark the first divisions using the following precise correlations:
Spoke 0 4 8 12 16 20 24 28
Rev 0 0.125 0.250 0.375 0.500 0.625 0.750 0.875
Finally, using the fact that 0.8 spoke intervals equals 0.025 rev and 1.6 spoke intervals equals 0.050 rev, work on either side of the first marks until the whole rim is marked in divisions of 0.025 rev.
Picture on left shows deep rim marked in fractions of a revolution where each digit indicates 0.1 rev. Picture on right shows portion of rim from 0 to 0.1 rev.
(ii) Wheel with 36 spokes in fractions of a revolution
Mark the first divisions using the following precise correlations:
Spoke 0 9 18 27
Rev 0.00 0.25 0.50 0.75
Finally, work on either side of the first marks until the whole rim is marked in divisions of 0.025 rev using the following correlations:
Spoke interval 4.5 3.6 2.7 1.8 0.9
Rev interval 0.125 0.100 0.075 0.050 0.025
An ordinary 36-spoke wheel marked in fractions of a revolution. Portion shown is from 0.85 to 0.15 rev. Red marks represent divisions of 0.025 and 0.075.
(iii) Wheel in spoke intervals
Marking a rim in spoke intervals is very fast and subsequent readings are very fast and accurate. Rotation can be read down to 0.003 rev equivalent to 0.06 of a count on the Jones, but the downside is that in order to correlate with meter readings in revolutions, the measurer has to divide rim readings by the number of spokes. Obviously though, this would be the method of choice when using a rented bicycle.
Wheel with 32 spokes marked in spoke intervals. Note the magnets on spokes 2, 10, 18, and 26.
2. Installation of magnets
By far the best design of wheel magnets is that by Sigma Sport for their cyclocomputers. (As of May 04, these magnets are available from Nashbar for $0.99.) They are snapped instantly into place onto the spokes without tools and twisted around the spokes for extremely accurate alignment. The only downside to the design is that by rough manhandling of the wheel the magnets can be knocked out of alignment. Use of metal sleeves provided with the magnets protects the alignment slightly, but I usually do not use them. The sleeves slightly alter the response of the sensor.
Install magnets immediately adjacent to the spoke nipples on spokes 2, 10, 18, and 26 for 32-spoke wheels and spokes 2, 10, 20, and 28 for 36-spoke wheels. (The zero spoke is that designated during the marking of the rim and is on the same side of the wheel as that desired for the sensor.) Pictures of the installed magnets can be seen in the sections above and below.
3. Installation of the sensor and computer
I describe here the installation of the Sigma Sport BC 600 as typical. (The BC 600 has been discontinued but as of May 04 is available from Nashbar for $12.95.) Like all Sigma Sport models, components are very rapidly installed without tools through the novel use of rubber O-rings. Installation time can be as little as 10 min.
Install the sensor on the upper inside surface of the fork so as to align with the magnets as shown on the left. Twist the magnets until there is a minimal clearance between them and the sensor. This adjustment is not critical as they will still function with as much as 20 mm from center to center. Note the white O-ring making the attachment. Rubber bands or electrical tape can be used instead. It might even be glued. I usually do not use the foam adhesive pad provided for the base of the sensor.
Sensors differ in their response, but with this sensor a position slightly forward of the center of the fork should result in synchronization of the zero point of meter with the point at which the zero spoke is closest to the ground within a mm of bicycle movement. However, operation procedure is such that the preciseness of synchronization will have no affect on accuracy of readings.
The computer mount is at the end of the cable leading from the sensor, Attach this instantly to the handlebars using a black O-ring.
Activate set-up of the computer by pressing the set button on the back for 5 sec. Select KMH and a wheel circumference of 2500 mm using the yellow and red buttons on the front and press the set button for 1 sec. Slide the computer onto the mount.
Test the installation as follows. Rotate the wheel until the lowest point is about 0.05 rev. Select " trip dist " with the red button and then zero the display with yellow button. Rotate the wheel forward slowly. The broken circle in the upper right-hand corner of the display should rotate every time a magnet passes the sensor and trip distance should increment by one the very instant that the zero spoke is closest to the ground.
Computer mount installed on the handlebars with black O-ring. Computer in place and showing a reading of 39 rev.
