Expert Verified: Reviewed by David Chen, Professional BMX Technical Specialist & Mechanics Consultant.
Optimize your BMX performance with our precision BMX Gear Ratio Calculator. Whether you’re a street rider looking for punchy acceleration or a racer needing top-end speed, this tool calculates Gear Ratio, Gear Inches, Rollout, and Gain Ratio in seconds.
BMX Gear Ratio Calculator
BMX Gear Ratio Formula
Primary Ratio: $Ratio = \frac{Sprocket}{Cog}$
Gear Inches: $Inches = Ratio \times Diameter$
Gain Ratio: $Gain = Ratio \times \frac{Wheel Radius}{Crank Length}$
Formula Source: Sheldon Brown Technical Library
Variables Explained
- Chainring: The large front sprocket attached to your cranks.
- Rear Cog: The small sprocket on your rear hub (driver).
- Tire Diameter: Usually 20″ for standard BMX, but actual height varies by tire width.
- Crank Length: The distance from the center of the BB to the pedal spindle.
What is a BMX Gear Ratio Calculator?
A BMX gear ratio calculator is a specialized tool used by riders to determine the mechanical advantage of their drivetrain. In BMX, where there are no gears to shift, choosing the right “static” gear is crucial for performance. This calculator helps you compare different setups to find the perfect balance between torque and speed.
Understanding “Gear Inches” is vital because it factors in your wheel size, giving you a real-world measurement of how hard the bike is to pedal. A classic standard is the 25/9 setup, which provides approximately 55 gear inches on a standard 20-inch tire.
How to Calculate BMX Gear Ratio (Example)
- Identify your front sprocket tooth count (e.g., 28 teeth).
- Identify your rear cog tooth count (e.g., 9 teeth).
- Divide the front by the rear: $28 \div 9 = 3.11$. This is your Gear Ratio.
- To find Gear Inches, multiply the Ratio by tire diameter: $3.11 \times 20 = 62.2$ inches.
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Frequently Asked Questions (FAQ)
What is the best gear ratio for street BMX? Most street riders prefer a 25/9 or 28/9 setup for a balance of speed and clearance.
Does crank length affect gear ratio? It doesn’t change the ratio itself, but it changes the “Gain Ratio,” which affects the leverage you feel.
What are “Gear Inches”? It represents the diameter of a wheel on an old-fashioned high-wheeler bike that would have the same mechanical advantage.
Why is 25/9 the standard? It allows for a smaller sprocket that stays out of the way during grinds while maintaining a “normal” feel.