Skid Patch Calculator

Fixed Gear Skid Patch Calculator

Optimize your gear ratio to maximize tire longevity.

Skid Patches

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Gear Ratio

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function calculateSkidPatches() { // Get input values var chainring = parseInt(document.getElementById('chainring').value); var cog = parseInt(document.getElementById('cog').value); var isAmbi = document.getElementById('ambi').checked; // Validation if (isNaN(chainring) || isNaN(cog) || chainring <= 0 || cog <= 0) { alert("Please enter valid positive numbers for chainring and cog teeth."); return; } // Function to find Greatest Common Divisor (GCD) var getGCD = function(a, b) { return !b ? a : getGCD(b, a % b); }; // Calculate simplest form of the fraction var gcdVal = getGCD(chainring, cog); var simplifiedNumerator = chainring / gcdVal; var simplifiedDenominator = cog / gcdVal; // Base skid patches logic var patches = simplifiedDenominator; // Ambidextrous logic: // If the numerator of the simplified ratio is odd, ambidextrous skidding doubles the patches. // If the numerator is even, the patches remain the same even if skidding with both feet. if (isAmbi && (simplifiedNumerator % 2 !== 0)) { patches = patches * 2; } // Calculate Gear Ratio for display (e.g., 3.0 or 2.8) var ratio = (chainring / cog).toFixed(2); // Analysis Message var analysisText = ""; if (patches === 1) { analysisText = "WARNING: 1 Skid Patch. Your tire will wear out extremely fast in a single spot. Change your gear ratio immediately."; } else if (patches < 5) { analysisText = "Low number of patches. Expect uneven tire wear. Rotate your tire frequently."; } else if (patches = 15) { patchBox.style.background = "#e8f5e9"; document.getElementById('skidPatchesResult').style.color = "#1b5e20"; patchBox.querySelector('div:first-child').style.color = "#2e7d32"; } else { patchBox.style.background = "#fff8e1"; document.getElementById('skidPatchesResult').style.color = "#f57f17"; patchBox.querySelector('div:first-child').style.color = "#f9a825"; } }

Understanding Skid Patches and Tire Wear

For fixed-gear cyclists ("fixie" riders), understanding skid patches is crucial for saving money on tires. Because the rear wheel is locked to the pedals, stopping often involves "skidding"—locking your legs to stop the rear wheel from turning while the bike slides to a halt.

If your gear ratio creates a low number of skid patches, the rear wheel will always lock up at the exact same point(s) on the tire rotation every time you skid. This causes the tire to wear down to the threads in just one or two spots, ruining the tire while the rest of the rubber remains fresh.

How the Math Works

The number of skid patches is determined by the fraction of your gear ratio (Chainring Teeth / Cog Teeth) reduced to its simplest form. The denominator of this simplified fraction represents your base number of skid patches.

Example: A 48t chainring and a 16t cog.

• Ratio: 48 / 16
• Simplifies to: 3 / 1
• Denominator: 1
• Result: 1 Skid Patch. This is the worst possible ratio. You will skid on the same spot every time.

Example: A 47t chainring and a 17t cog.

• Ratio: 47 / 17 (Cannot be simplified further as 17 is prime)
• Denominator: 17
• Result: 17 Skid Patches. This is excellent. The wear is distributed across 17 different spots on the tire.

Ambidextrous Skidding

Most riders have a dominant leg and tend to skid with the same foot forward every time. However, if you are an "ambidextrous skidder"—meaning you can initiate a skid with either your left or right foot forward—you can potentially double your skid patches.

There is a mathematical caveat: Doubling only occurs if the numerator of your simplified gear ratio is an ODD number. If the simplified numerator is even, the skid spots for your left foot overlap perfectly with the spots for your right foot, resulting in no gain in patches.

Recommended Gear Ratios

To maximize tire life, look for prime numbers in your chainring or cog size to prevent the fraction from simplifying down too much.

• Good: 47/17, 49/17, 51/17 (High patch counts)
• Bad: 48/16, 44/16, 51/17 (Low patch counts)
• The "Magic" Cog: A 17-tooth rear cog is a favorite among fixie riders because 17 is a prime number. It almost guarantees a high number of skid patches regardless of what chainring you use (unless you use a 34t or 51t chainring).

Use the calculator above to experiment with different tooth counts before buying your next chainring or cog.