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The number of skid patches is determined by the given gear ratio and is easy to calculate. Assuming you skid with the same foot forward each time, it is the denominator of the reduced fraction of the gear ratio, starting with the number of chainring teeth as the numerator. In other words, a 48×18 ratio would be 48/18, reduced to 8/3, yielding 3 skid patches. 48×17 doesn’t reduce further, and has 17 skid spots. The rare rider who skids equally often with either foot forward can double their skid patches with ratios that reduce to fractions with odd numbers in both slots.
Read the whole article from issue #9, and more bike repair and maintenance articles at www.urbanvelo.org/tech.
Missing the magical 47t chainring.
http://rideyourbike.com/images/skidspots.jpg
The chart was pared down to commonly available sizes. The math as explained works for the 47t ring.
It doesn’t matter what the ratio is if you just rotate the wheel about once every week.
Don’t bother referencing Sheldon Brown.