Effective rim diameter: This is the most important measurement! It is the diameter of the circle made by the ends of the spokes when the wheel is built. Note that a drop-centred rim, a dimpled rim, and an undimpled rim may have the same outside diameters, but that their effective diameters may vary. Try to estimate this measurement to an accuracy of 1 mm or so.
Cross Pattern: This is just the number of other spokes that a spoke crosses on its journey from the hub to the rim.Other than veteran Rudges ( 0 cross in a Multi back wheel, 2 cross in a veteran front wheel ), most wheels use 3 or 4 cross.
Number of spokes: Sure to be an even number! On a "typical" veteran motorcycle, you might find 36 at the front and 40 at the rear, but BSA owners will delight in 32 at the front, and NSU owners will be busy with 48 in the rear. The time Rudge Multi owners save by spoking 0 cross will be lost as they cut and thread the 80 spokes for the back wheel!
Hints for advanced uses: For asymmetric spoke flanges, you can use the calculator to do one half of the wheel at a time. Start with one flange, but instead of using the real flange spacing, use twice the distance from that flange to the centreline of the rim. The calculator will give the spoke length for that side. Repeat for the other. Remember that some asymmetric wheels have different cross patterns on each side. For any wheel where the rim is drilled far from the centreline, you will get a better estimate of the spoke length if you decrease the flange spacing by an amount equal to the spacing between the left and right spoke holes in the rim.
My wife's disclaimer: My wife has read this page over my shoulder, and would like everyone to know that my data sheet was eaten by vermin in the shed. Jazzy and Flippy (the house cats) would like everyone to know that they would gladly eat vermin daring to enter the house.