Use the Cycling Gear Ratio Calculator to determine your gear ratio, rollout (gear development), and estimated speed based on your chainring, cog, tire size, and cadence. Optimizing your gear ratio is crucial for efficiency, climbing, and top-end speed.
Cycling Gear Ratio Calculator
Cycling Gear Ratio Calculator Formula
1. Gear Ratio ($\text{GR}$):
$$GR = \frac{\text{Chainring Teeth (C)}}{\text{Cog Teeth (G)}}$$
2. Rollout / Gear Development ($\text{D}$):
$$\text{D (m)} = GR \times \frac{\text{Tire Circumference (T) in mm}}{1000}$$
3. Speed ($\text{S}$):
$$\text{S (km/h)} = \text{D (m)} \times \text{Cadence (R) in RPM} \times 0.06$$
Variables
- Chainring Teeth (C): The number of teeth on the front sprocket (typically 34 to 53).
- Cog Teeth (G): The number of teeth on the rear sprocket (typically 11 to 32).
- Tire Circumference (T): The measured distance (in millimeters) the wheel travels in one rotation. This is highly dependent on tire size and pressure (e.g., 2096 mm for 700x25c).
- Cadence (R): The speed at which the cyclist pedals, measured in Revolutions Per Minute (RPM).
What is Cycling Gear Ratio?
The Cycling Gear Ratio is a fundamental concept in cycling, representing the mechanical advantage provided by the drivetrain. It is simply the ratio of the number of teeth on the front chainring to the number of teeth on the rear cog (C/G).
A higher ratio (e.g., 4.73) means the wheel turns more times per pedal revolution, which is suitable for flat ground and high-speed descents. A lower ratio (e.g., 1.00) means the wheel turns fewer times, making pedaling easier for climbing steep hills or accelerating from a stop.
The “Rollout” or “Gear Development” measurement further refines this, indicating the actual distance the bicycle travels forward for a single rotation of the pedals. This combines the Gear Ratio with the size (circumference) of the rear wheel, offering a complete picture of the gear’s mechanical output.
How to Calculate Gear Ratio (Example)
- Identify Inputs: A cyclist uses a 50-tooth Chainring (C=50), an 11-tooth Cog (G=11), a Tire Circumference of 2100 mm (T=2100), and a Cadence of 95 RPM (R=95).
- Calculate Gear Ratio: Divide the chainring teeth by the cog teeth. $\text{GR} = 50 / 11 \approx 4.545$.
- Calculate Rollout: Multiply the Gear Ratio by the circumference in meters. $\text{D} = 4.545 \times (2100 / 1000) \approx 9.545$ meters per revolution.
- Calculate Speed: Multiply the rollout by cadence and the conversion factor (0.06). $\text{S} = 9.545 \times 95 \times 0.06 \approx 54.4$ km/h.
Frequently Asked Questions (FAQ)
What is a good gear ratio for climbing?
A good climbing gear ratio is typically low, around 1:1 or less (e.g., a 34-tooth chainring with a 34-tooth cog gives a 1.0 ratio). This allows the cyclist to maintain a high, comfortable cadence on steep gradients.
Why is tire circumference important?
Tire circumference is critical because it converts the unitless Gear Ratio into Rollout (distance traveled). A larger tire will travel further for the same gear ratio, resulting in a higher speed for the same cadence.
What does a gear ratio of 3.0 mean?
A gear ratio of 3.0 means that for every full rotation of the pedals, the rear wheel rotates exactly 3 times. This is a common and versatile middle-range gear found on many road and hybrid bikes.
How do I find my tire circumference?
The most accurate method is to mark the tire and the ground, roll the bike one full wheel revolution, mark the ground again, and measure the distance between the two marks in millimeters.