Calibrating the Bicycle

The pre-measurement calibration is the initial step that must be performed in the measurement of a road course. The post-measurement calibration guards against systematic sources of error such as a slow leak. At least four pre-measurement and four post-measurement calibration rides are required.

1. The bicycle tires should be inflated hard, to the pressure indicated on the side of the tire.
2. Warm the tires by riding the bicycle for several minutes immediately prior to the calibration rides. This will reduce the variance in counts for the pre-measurement calibration and ensure a better measurement.
3. At one end point of the calibration course, slowly roll the front wheel forward, just through the next count. Lock the front brake and place the front wheel axle directly over the line. Record the count.
4. Ride the bicycle over the calibration course in as straight a line as possible and with the same weight and equipment on the bicycle as will be used during the actual race course measurement. A calibration ride should be one non-stop ride.
5. Stop the bicycle just before reaching the end of the calibration course and roll it slowly forward until the axle of the front wheel is directly over the line. Lock the front brake and record the count.
6. With the front wheel brake locked, turn the bicycle around and place the front wheel axle directly over the line for the next ride. Repeat steps 4 and 5.
7. Repeat the above procedure for a total of four rides, recording start and finish counts each time. Alternate directions on the calibration course. This will give you two rides in one direction and two rides in the opposite direction.
8. The spread among your calibration rides cannot exceed 2 or 3 counts for riding each direction of the calibration course. If your variation is greater, do more rides until your counts stabilize.
9. Add the results of each ride and divide by the number of rides. This gives the “average pre-measurement count.”
10. Divide this count by the length of the calibration course in kilometers (or in miles) to obtain the number of counts per kilometer (or per mile).
11. Multiply this by 1.001 to obtain the working constant. The “short course prevention factor” of 1.001 is intended to result in a course which is at least the stated distance, within the limits of measurement precision. It also helps ensure that (very) slight variations in the course layout on race day won't invalidate your measurement. This lengthens the course by one meter per kilometer or 5.28 feet per mile.

Now go measure the race course. When finished, return to the calibration course.

12. The post-measurement calibration must be performed as soon after the course measurement as possible. Repeat steps 3 through 11. Four post-measurement calibration rides are required.
13. Determine the average post-measurement count by adding all the post-measurement counts and dividing by the number of rides.
14. Determine the finish constant by dividing the average post-measurement count by the length of the calibration course in kilometers (or in miles) and multiply this by 1.001.
15. The constant for the day is either the working constant or the finish constant, whichever is larger. Although measurements using the average of the working and finish constants will be accepted, it is strongly recommended to use the larger constant.

Remember: Each day’s measurement must be preceded and followed by calibration runs. You may measure as much as you want in a day, just as long as calibration closely precedes and follows measuring (within a few hours). This is done to minimize error due to changes in tire pressure from thermal expansion and slow leakage. Frequent recalibration "protects" the previous measurement. A smart measurer will recalibrate frequently--you never know when a flat tire is coming!

When a course is measured by more than one cyclist, every cyclist who rides the race course must do his or her own pre-measurement and post-measurement calibration rides. A separate copy of the Bicycle Calibration Data Sheet must be completed for each rider, calculating individual riding constants for each rider. This procedure must be followed even when cyclists share the same bicycle, because riding constants will vary for different cyclists, depending on riders’ weights and riding styles.