Ehow instructions for circular pitch...
Instructions
1
Measure the gear's diameter. For this example, imagine a diameter of 6 inches.
2
Multiply this answer by the constant pi, which is approximately 3.142: 6 × 3.142 = 18.852.
3
Divide this answer by the number of teeth around the gear. For example, if the gear has 28 teeth: 18.852 ÷ 28 = 0.673. This is the gear's circular pitch, measured in inches.
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http://www.ehow.com/how_8660734_calc...lar-pitch.html
You might start by wrapping the hub with the track and counting how many cogs you need.