Fixed gear skid patch calculator.

Worked example

A 46-tooth chainring with a 16-tooth cog reduces to 23/8. The rear wheel turns 23/8 times for each crank turn, so the tire lands on 8 distinct spots: 8 skid patches if you always skid with the same foot forward. The reduced numerator, 23, is odd, so skidding with either foot doubles that to 16. On a 700x25C wheel (2105 mm round, 26.4 in across) the gear is 2.875 × 26.4 = 75.8 gear inches, and 2.875 × 2.105 m = 6.05 m of development per crank turn.

How it works

Skid patches = the denominator of chainring ÷ cog in lowest terms. Divide both tooth counts by their greatest common divisor, and the denominator is the number of places the tire can land. For ambidextrous skidding the count doubles when the reduced numerator is odd and is unchanged when it is even. The tests confirm the rule against a rolling simulation for every chainring from 1 to 60 teeth and cog from 1 to 30.

Gear inches are the wheel diameter times the front sprocket divided by the rear sprocket (Sheldon Brown). Development is the distance travelled per crank revolution: the ratio times the wheel circumference.

Sources

Retrieved 2026-10-16. The skid patch rule is from Bikecalc and is verified by simulation in the tests; gear inches and development use Sheldon Brown’s definitions. Wheel circumferences are from Sheldon Brown’s tire-size table and vary with tire and pressure.

• Bikecalc, Skid Patch Math (reduced ratio, ambidextrous rule)
• Sheldon Brown, Glossary: Gear Inches
• Sheldon Brown, Glossary: Development
• Sheldon Brown, Cyclecomputer/GPS Calibration (tire-size circumference table)

FAQ

How do I calculate skid patches?

Divide chainring teeth by cog teeth and reduce the fraction to lowest terms. The denominator is the number of skid patches when you skid with one foot. 44/16 reduces to 11/4, so 4 patches.

What are ambidextrous skid patches?

If you can skid with either foot forward, the pedal can stop at two crank positions half a turn apart. The count doubles when the reduced numerator is odd: 44/16 is 11/4, odd numerator, so 8 patches instead of 4.

What gear gives the most skid patches?

Ratios whose tooth counts share no common factor, such as 47/17 or 44/15, give as many patches as the cog has teeth (17 and 15). Skidding with either foot doubles the count only when the chainring is odd, as with 47.

What are gear inches?

The equivalent diameter of the drive wheel on a high-wheel bicycle: wheel diameter times chainring teeth divided by cog teeth. It lets you compare gears across wheel sizes.

What is development?

The distance the bike travels for one turn of the cranks, usually in metres: the gear ratio times the wheel circumference. It is also called rollout.