Rennen Gear Calculator

Rennen Gear Calculator for BMX & Track Cycling .rennen-calc-container { max-width: 800px; margin: 0 auto; padding: 20px; font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif; color: #333; background-color: #f9f9f9; border: 1px solid #e0e0e0; border-radius: 8px; } .rennen-calc-header { text-align: center; margin-bottom: 30px; } .rennen-calc-header h2 { color: #d32f2f; margin-bottom: 10px; } .rennen-calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 20px; } @media (max-width: 600px) { .rennen-calc-grid { grid-template-columns: 1fr; } } .rennen-input-group { margin-bottom: 15px; } .rennen-input-group label { display: block; margin-bottom: 5px; font-weight: 600; font-size: 0.9em; } .rennen-input-group input, .rennen-input-group select { width: 100%; padding: 10px; border: 1px solid #ccc; border-radius: 4px; font-size: 1em; } .rennen-btn { width: 100%; padding: 12px; background-color: #d32f2f; color: white; border: none; border-radius: 4px; font-size: 1.1em; cursor: pointer; font-weight: bold; margin-top: 10px; transition: background 0.3s; } .rennen-btn:hover { background-color: #b71c1c; } .rennen-results { margin-top: 30px; background-color: #fff; padding: 20px; border-radius: 4px; border-left: 5px solid #d32f2f; box-shadow: 0 2px 5px rgba(0,0,0,0.05); display: none; } .result-row { display: flex; justify-content: space-between; margin-bottom: 10px; padding-bottom: 10px; border-bottom: 1px solid #eee; } .result-row:last-child { border-bottom: none; margin-bottom: 0; padding-bottom: 0; } .result-label { font-weight: 600; color: #555; } .result-value { font-weight: 700; color: #d32f2f; font-size: 1.1em; } .rennen-article { margin-top: 40px; line-height: 1.6; color: #444; } .rennen-article h3 { margin-top: 25px; color: #222; } .rennen-article ul { margin-bottom: 20px; padding-left: 20px; }

Calculate Gear Inches, Rollout, and Decimal Gearing for BMX Racing

Front Chainring (Teeth) Supports decimal inputs for Rennen threaded gears.

Rear Cog (Teeth)

Tire Diameter (Inches) 20 x 1-1/8 (Junior/Expert) 20 x 1-3/8 (Expert XL) 20 x 1.50 (Pro Section) 20 x 1.60 20 x 1.75 (Pro Standard) 20 x 1.85 20 x 1.95 20 x 2.10 24 x 1.75 (Cruiser) Custom (Enter below)

Custom Tire Diameter (Inches)

Gear Ratio: 0.00 : 1

Gear Inches: 0.00″

Rollout (Distance per Revolution): 0.00″

Development Meters (Metric): 0.00 m

Mastering Your BMX Gearing with the Rennen Calculator

In the world of BMX racing, the difference between a holeshot and second place is often measured in fractions of an inch. The Rennen Gear Calculator is designed for serious racers who need to fine-tune their bike setup. Unlike standard cycling calculators, this tool accommodates the precision of decimal gearing—a concept popularized by Rennen Design Group to allow for micro-adjustments in gear ratios.

What is Rollout?

While "Gear Ratio" tells you the mechanical advantage of the drivetrain, Rollout is the most critical metric for a racer. It measures exactly how far your bike travels with one complete revolution of the pedals.

Low Rollout (e.g., 52-53 inches): Provides explosive acceleration out of the gate but may cause you to "spin out" on long straights.
High Rollout (e.g., 55+ inches): Offers higher top-end speed for long tracks but requires more power to get moving initially.

Understanding Decimal Gearing

Standard gearing usually involves integers (e.g., 44/16). However, the jump between a 44/16 and a 45/16 gear is massive in racing terms. Rennen introduced threaded rings and cogs that allow for "decimal" teeth equivalents (e.g., 44.2 teeth). This calculator allows you to input these decimal values to find the "magic gear" that sits perfectly between standard sizes, optimizing your power output for specific track conditions.

How to Use This Calculator

1. Front Chainring: Enter the number of teeth. If you use Rennen decimal gears, you can enter values like 44.1 or 44.2.
2. Rear Cog: Enter the rear cog size (usually 13-18).
3. Tire Diameter: Select your tire size. Since actual diameter varies by tire pressure and brand, we use standard racing approximations. You can also enter a custom diameter if you have measured your tire's rollout height.

// Handle custom tire input visibility document.getElementById('rennen-tire').onchange = function() { var val = this.value; var customGroup = document.getElementById('custom-tire-group'); if(val === '0') { customGroup.style.display = 'block'; } else { customGroup.style.display = 'none'; } }; function calculateRennenGearing() { // 1. Get Inputs var frontStr = document.getElementById('rennen-front').value; var rearStr = document.getElementById('rennen-rear').value; var tireSelect = document.getElementById('rennen-tire').value; var customTireStr = document.getElementById('rennen-custom-tire').value; // 2. Parse Numbers var front = parseFloat(frontStr); var rear = parseFloat(rearStr); var tireDiameter = parseFloat(tireSelect); // If custom tire is selected if(tireSelect === '0') { tireDiameter = parseFloat(customTireStr); } // 3. Validation if(isNaN(front) || front <= 0) { alert("Please enter a valid Front Chainring size."); return; } if(isNaN(rear) || rear <= 0) { alert("Please enter a valid Rear Cog size."); return; } if(isNaN(tireDiameter) || tireDiameter <= 0) { alert("Please select or enter a valid Tire Diameter."); return; } // 4. Calculations // Gear Ratio = Front / Rear var ratio = front / rear; // Gear Inches = Ratio * Wheel Diameter (This is the "Old School" number) var gearInches = ratio * tireDiameter; // Rollout = Gear Ratio * Tire Circumference (Circumference = Diameter * PI) // This assumes rollout is Distance Per Revolution var rollout = gearInches * Math.PI; // Metric Development (Meters) // Rollout is in inches, convert to meters (1 inch = 0.0254 meters) var developmentMeters = rollout * 0.0254; // 5. Update UI document.getElementById('res-ratio').innerText = ratio.toFixed(3) + " : 1"; document.getElementById('res-inches').innerText = gearInches.toFixed(2) + '"'; document.getElementById('res-rollout').innerText = rollout.toFixed(2) + '"'; document.getElementById('res-meters').innerText = developmentMeters.toFixed(2) + " m"; // Show results container document.getElementById('rennen-result').style.display = 'block'; }