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24 Setting Custom Wheel Size
Figure 8- Measuring rollout for
wheel size
Table 1 approximates the circumfer-
ence of each tire size. Instead of using
an approximation, you can measure the
actual circumference of your wheel. You
need a helper, your bike, and a smooth
floor with enough room to roll the bike
about 7 feet (245 cm) in a straight line.
1. With the valve stem of the front wheel
directly over the floor Figure 18), sit on
the bike.
2. Have your helper mark the floor at the
valve stem.
3. While you sit on the bike, have the helper roll the bike forward
one revolution of the wheel, so that the valve stem is again
directly over the floor.
4. Mark the new location of the valve stem.
5. Measure the distance between the marks, in millimeters.
If you can only measure in inches, convert to millimeters by multiplying
by 25.4.
6. Use the result to program your computer, following the instruc-
tions on page 22.