Rennen Gear Calculator
Professional BMX Gearing, Rollout & Gain Ratio Precision Tool
Gearing Comparison Chart
Comparing Gear Inches for Cog Sizes 14 to 18
| Cog Size | Gear Inches | Rollout (In) | Gain Ratio |
|---|
Table showing variations based on current Chainring and Tire Diameter.
What is the Rennen Gear Calculator?
The rennen gear calculator is an essential precision tool designed specifically for BMX racers who require microscopic adjustments to their drivetrain setup. Unlike standard bicycle calculators, the rennen gear calculator accounts for the unique innovation of decimal chainrings (e.g., 44.1, 44.2), which allow riders to find “in-between” gears that traditional whole-number sprockets cannot provide.
Professional athletes use the rennen gear calculator to translate their physical power into maximum track speed. By understanding the relationship between the front chainring, the rear cog, and the wheel’s tire diameter, a racer can optimize their “holeshot” (initial acceleration) without sacrificing top-end velocity on the long straights of a BMX track. Common misconceptions suggest that only the chainring matters, but the rennen gear calculator proves that tire diameter and crank length are equally vital components of the performance equation.
Rennen Gear Calculator Formula and Mathematical Explanation
Calculating your gearing involves three core mathematical concepts: Gear Inches, Rollout, and Gain Ratio. The rennen gear calculator utilizes these formulas to provide a comprehensive view of your bike’s mechanical advantage.
Rollout = Gear Inches × π (3.14159)
Gain Ratio = (Tire Radius / Crank Length) × (Front Teeth / Rear Teeth)
To use the rennen gear calculator effectively, you must understand the variables involved:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Front Teeth | Chainring size (decimal supported) | Teeth (T) | 38T – 50T |
| Rear Teeth | Rear Cog size | Teeth (T) | 13T – 18T |
| Tire Diameter | Outer edge to outer edge | Inches | 18.5″ – 21.0″ |
| Crank Length | Center of BB to pedal spindle | Millimeters | 160mm – 185mm |
Practical Examples (Real-World Use Cases)
Example 1: The Holeshot Specialist
A racer feels their current 44/16 setup (53.63 gear inches) is slightly too heavy to get out of the gate quickly. Using the rennen gear calculator, they test a 43.8 Rennen decimal chainring. The rennen gear calculator shows a new gear inch of 53.38. This subtle 0.25 decrease allows for faster leg turnover in the first 30 feet of the track.
Example 2: Tire Change Adjustment
A rider switches from a 1.75″ rear tire to a 1.60″ tire. They notice the bike feels “easier” to pedal but they are spinning out too early. By inputting the new tire diameter into the rennen gear calculator, they realize their gear inches dropped from 54.0 to 52.8. To compensate, the rennen gear calculator helps them select a 45T chainring to bring their gearing back to the desired 54.0 range.
How to Use This Rennen Gear Calculator
- Enter Chainring Size: Type in your front sprocket tooth count. If you use Rennen decimal rings, include the decimal (e.g., 44.3).
- Select Rear Cog: Enter your rear cog size. Small changes here have a massive impact on the rennen gear calculator results.
- Input Tire Diameter: Measure the actual height of your tire or use standard industry averages (typically 19.5″ for 20-inch wheels).
- Add Crank Length: This is used by the rennen gear calculator to determine your Gain Ratio, which measures leverage.
- Analyze Results: Look at the “Gear Inches” for overall feel and “Rollout” to see exactly how far the bike moves per pedal stroke.
Key Factors That Affect Rennen Gear Calculator Results
- Tire Pressure: Higher PSI increases the effective tire diameter slightly, which the rennen gear calculator would reflect as a taller gear.
- Tire Wear: As knobs wear down, the diameter decreases, subtly changing your rollout.
- Crank Leverage: Longer cranks make a hard gear feel easier to turn, which is why the rennen gear calculator includes Gain Ratio.
- Decimal Precision: Standard rings only move in jumps of ~1.2 gear inches. The rennen gear calculator highlights how decimal rings bridge those gaps.
- Rider Fatigue: At the end of a long race day, a gear that felt perfect in practice might feel too heavy.
- Track Surface: Soft, muddy tracks require a lower gear inch than hard-packed, paved-turn tracks for optimal efficiency.
Related Tools and Internal Resources
- BMX Training Guide – Complement your gearing with the right physical training.
- Best BMX Tires – Learn how tire choice affects your rennen gear calculator inputs.
- Crank Length Calculator – Deep dive into leverage and biomechanics.
- Race Day Preparation – Ensuring your drivetrain is ready for the gate.
- BMX Frame Geometry – How chainstay length interacts with your gear choice.
- Drivetrain Maintenance – Keeping your chain and cogs efficient.
Frequently Asked Questions (FAQ)
1. Why does the rennen gear calculator use decimals?
Rennen specialized in producing chainrings with non-integer tooth counts to provide racers with ultra-fine tuning capabilities that standard gears cannot offer.
2. What is a “good” gear inch for BMX racing?
Most Pro-level racers stay between 53 and 55 gear inches, but this depends heavily on wheel size and rider strength.
3. How does rollout differ from gear inches?
Gear inches is a theoretical ratio, while rollout is the actual distance (usually in inches) the bike travels with one full rotation of the cranks.
4. Can I use this rennen gear calculator for 24″ cruisers?
Yes, simply change the Tire Diameter input to approximately 23.5″ or 24″ to get accurate cruiser results.
5. Does crank length change my gear inches?
No, crank length does not change gear inches or rollout, but it does change the “Gain Ratio” or how much effort it feels like you are putting in.
6. What tire diameter should I use for a 20 x 1.75 tire?
Most racers use 19.5″ or 19.6″ in the rennen gear calculator for a standard 1.75 inflated to high pressure.
7. Why is my gain ratio different from my gear ratio?
The gear ratio is just Front/Rear. The gain ratio is a more advanced metric that includes the leverage of the cranks and the radius of the wheel.
8. How often should I check my gearing with the rennen gear calculator?
You should re-calculate whenever you change tire brands, tire sizes, or if you feel you are struggling to maintain speed on specific track types.