I have shown that there is no significant difference in accuracy between doing calibration on a 25-m rather than on a 300-m course when using rim readings in spoke intervals. The secret of using the 25-m course is to coast rather than ride along it, so as to avoid wobble and follow the line of the course very closely.
I create the 25-m course along a straight white line on the edge of the road by measuring 82 feet 0.25 inches plus zero offset with the loop at the beginning of the tape over an imbedded nail. (Temperature correction can usually be ignored, but measurement should be done while cloudy or in the shade.) Seated on the bicycle, I start rolling by pushing the right pedal down to the bottom of its travel and keep it there while coasting. Occasionally I touch the ground with my left foot as necessary to keep accurately along the white line.
Replicate determinations in each series of four were all within one or two tenths of a spoke interval from one another. Also the % difference of the calibration factor differed from that from a determination on a 300-m course done at the same time by 0.03%,0.05%, 0.04%, and neg 0.04%. (I am ignoring 0.06% found in the case where measurements on the 300-m course showed variability.) Raw data can be found in the Excel file at:
http://home.earthlink.net/~caverhall/calibration25m.xls
I still think it best to use only one calibration course at home with pressure monitoring. However, if one is set up away from home using the short version has the following advantages:
1. A good site is much easier to find.
2. The measurer can do the job very quickly by himself.
3. Measuring just one length of tape means there is less room for error.
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