Skid Patch Calculator

Skid patches are the specific points on your rear tire that make contact with the road surface when you lock your legs to slow down on a fixed gear bicycle. Because the drivetrain on a fixie has a direct 1:1 connection between the pedals and the rear wheel, the tire always contacts the ground at predictable, repeating positions determined by your gear ratio.

The number of skid patches is a function of the chainring and cog tooth counts - specifically, the cog teeth divided by the greatest common divisor (GCD) of both values. A higher number of skid patches means wear is distributed across more points on the tire, significantly extending its lifespan.

On a fixed gear bike without a front brake (or for riders who primarily use leg braking), skidding is the main method of deceleration. Every time you skid, the same points on the tire wear down. With a low skid patch count - say, just 1 or 2 patches - you'll develop flat spots and blow through tires in weeks. With 15+ patches, wear spreads so evenly that tires last many times longer.

Choosing the right gear ratio isn't just about cadence and speed - it's also about practical tire economics and safety. A flat-spotted tire has less grip in wet conditions and is more prone to blowouts. Understanding skid patches helps you make an informed decision when building or modifying your fixed gear setup.

The formula for skid patches is straightforward: Single-leg patches = Cog teeth ÷ GCD(Chainring, Cog). The GCD (Greatest Common Divisor) is the largest number that divides evenly into both the chainring and cog tooth counts.

For ambidextrous skid patches, if the single-leg count is odd, you double it (because skidding with the opposite foot forward hits entirely different points on the tire). If the single-leg count is even, the ambidextrous count stays the same, since the alternate foot lands on existing patch locations.

For example: a 49/17 setup has GCD(49, 17) = 1, so you get 17 ÷ 1 = 17 single-leg patches. Since 17 is odd, ambidextrous gives you 17 × 2 = 34 patches - excellent tire longevity.