A simple explanation of how skid patches are calculated by the
Skid Patch Calculator
Skid Patch Calculator
How does BikeCalc determine skid patches?
 Simplify the gear ratio to the smallest equivalent whole number ratio.
 For single legged skidders, the number of skid patches is the denominator.

For folks who can skid with either foot forward (ambidextrous skidders):
 if the numerator is even, the number of skid patches is still the denominator.
 if the numerator is odd, the number of skid patches is the denominator * 2.
48x20 => 24x10 => 12x5 results in 5 skid patches for both single and ambidextrous skidders.
43x18 results in 18 patches for single skidders but 36 for ambidextrous skidders.
Notice that the number of skid patches doubles not when the chainring is odd, but when the simplified gear numerator is odd, thus ...
42x16 => 21x8 results in 8 patches for single skidders but 16 for ambidextrous skidders.
What gear should I choose?
 Nonskidders should choose even toothed gears to maximize chain and sprocket life.
 Singlelegged skidders should choose odd toothed chainrings to maximize skid patches.
 Ambidextrous skidders can choose even toothed gears where the chainring simplifies to an odd number to get both extended drivetrain wear and doubled skid patches.
Where can I find out more?
fixed.org.au
and
Bike Forums
both have excellent discussions of the math behind skid patch calculations. I love
Sheldon Brown
but find the prior two explanations more convincing. The BikeCalc skid patch calculations are based on the
Bike Forums
thread.
See for yourself! Try the Skid Patch Calculator