Rc Gear Ratio Calculator

Calculate Final Drive Ratio (FDR), Rollout, and Theoretical Speed

Table 1: Pinion Gear Matrix (FDR for current Spur & Internal Ratio)

Pinion Teeth	FDR	Rollout (mm)	Speed Impact

What is an RC Gear Ratio Calculator?

An RC Gear Ratio Calculator is an essential tool for remote control car enthusiasts, racers, and hobbyists. It helps you determine the mechanical advantage of your vehicle’s drivetrain by calculating the Final Drive Ratio (FDR) and Rollout based on your spur gear, pinion gear, and internal transmission ratio.

Whether you are tuning a touring car for a tight technical track or setting up a drag racer for maximum top speed, understanding your gear ratio is critical. Incorrect gearing can lead to overheated motors, swollen batteries, or sluggish acceleration. This calculator allows you to experiment with different gear combinations virtually before buying parts or wrenching on your car.

By using an RC gear ratio calculator, you can optimize your setup for specific track conditions, balancing torque (acceleration) against top speed to achieve the fastest lap times while keeping your electronics safe.

RC Gear Ratio Formula and Explanation

To understand how your RC car performs, you need to grasp three main concepts: The Raw Gear Ratio, the Final Drive Ratio (FDR), and the Rollout. Here is the breakdown of the math used in this tool.

1. Raw Gear Ratio

This is the relationship between the spur gear and the pinion gear.

Formula: Spur Teeth ÷ Pinion Teeth

2. Final Drive Ratio (FDR)

The FDR accounts for the internal transmission of the car (belt or shaft drive reduction). This is the most common metric used in spec racing.

Formula: (Spur Teeth ÷ Pinion Teeth) × Internal Ratio

3. Rollout

Rollout measures how far the car moves forward for one single revolution of the motor shaft. It is crucial for cars with foam tires where tire diameter changes as they wear.

Formula: (Tire Diameter × 3.14159) ÷ FDR

Table 2: Key Variables in RC Gearing

Variable	Meaning	Unit	Typical Range
Spur Gear	Main gear on the central shaft	Teeth	60 – 120
Pinion Gear	Small gear on the motor	Teeth	15 – 50
Internal Ratio	Drivetrain reduction (Fixed)	Ratio	1.7 – 3.0
Tire Diameter	Height of the tire	Millimeters (mm)	55 – 110

Practical Examples of Gearing

Example 1: Touring Car (Stock Class)

Scenario: You are racing a 17.5T Stock Touring car. The track is medium-sized with flowing corners.

• Spur: 100T
• Pinion: 45T
• Internal Ratio: 1.9 (e.g., Xray T4)
• Calculation: (100 / 45) × 1.9 = 4.22 FDR

Interpretation: An FDR of 4.22 is geared towards speed. If the motor comes off the track too hot (over 160°F), you might need to increase the FDR (smaller pinion) to reduce load.

Example 2: Off-Road Buggy (Modified)

Scenario: You have a 2WD Buggy with a powerful 7.5T motor for a dirt track.

• Spur: 78T
• Pinion: 22T
• Internal Ratio: 2.6
• Calculation: (78 / 22) × 2.6 = 9.21 FDR

Interpretation: A higher FDR number (9.21) means more torque and less top speed. This is ideal for off-road tracks where acceleration out of jumps and corners is more important than straight-line speed.

How to Use This RC Gear Ratio Calculator

1. Identify your Spur and Pinion: Count the teeth on your gears or read the markings. Enter these into the “Spur Gear” and “Pinion Gear” fields.
2. Find your Internal Ratio: Consult your car’s manual. Common ratios are 1.9 or 2.0 for touring cars, and 2.4 to 2.6 for buggies.
3. Measure Tire Diameter: Use calipers to measure your tire height in millimeters. This is vital for calculating Rollout accurately.
4. Optional Electrical Specs: If you want to estimate speed, enter your Motor KV rating and Battery Voltage.
5. Analyze the Results: Look at the FDR. Lower numbers mean higher top speed (but more heat). Higher numbers mean more acceleration (and cooler running).

Key Factors That Affect RC Gearing Results

Simply calculating the ratio is the first step. Several factors affect how that ratio translates to on-track performance:

• Motor Temperature: This is the limit of gearing. If you gear for too much speed (low FDR), the motor pulls excessive amps and overheats. Always check temps after 2 minutes.
• Track Size: Large tracks with long straights favor a lower FDR (Speed). Small, technical tracks favor a higher FDR (Torque).
• Tire Wear (Foam Tires): As foam tires wear down, their diameter decreases. This reduces your rollout and top speed. You must increase your pinion size to compensate as tires get smaller.
• Motor Timing: Increasing mechanical timing on the motor increases RPM but reduces torque. You usually need to increase the FDR (gear down) when adding timing.
• Vehicle Weight: Heavier cars put more load on the motor. A heavy short-course truck needs a higher numerical gear ratio than a lightweight pan car to avoid overheating.
• Surface Grip: High-grip carpet imposes more drag on the drivetrain than loose dirt, often requiring a conservative gear ratio to prevent motor fade.

Frequently Asked Questions (FAQ)

What is a good FDR for 17.5T touring car?

For most 17.5T “Blinky” (zero timing) classes, a starting FDR is usually between 3.8 and 4.5, depending on the motor brand and track size. Always check motor temperature.

Does a larger pinion mean more speed?

Yes. Increasing the size of the pinion gear (more teeth) lowers the FDR number, which increases top speed but reduces acceleration and increases motor heat.

What is the difference between FDR and Rollout?

FDR is a ratio of gears only. Rollout takes the tire size into account, telling you the actual distance traveled per revolution. Rollout is more accurate for comparing setups between cars with different tire sizes (e.g., F1 vs Touring Car).

Why is my motor getting too hot?

Overheating usually means you are “over-geared” (pinion too big / FDR too low). The motor is struggling to push the car. Try a smaller pinion gear.

How do I calculate Internal Gear Ratio?

If you don’t know it, rotate the spur gear exactly once and count how many times the tires rotate. Divide spur rotations by tire rotations. Alternatively, divide the differential gear teeth by the drive pulley/gear teeth.

What is “Gearing Down”?

Gearing down means changing gears to get more torque and less speed. This usually involves using a smaller pinion or a larger spur gear (Higher FDR).

Does battery voltage affect gear ratio?

No, the mechanical ratio is physical. However, higher voltage (e.g., 3S vs 2S) increases RPM, so you often need to gear down to manage the extra heat and speed.

Can I use this for rock crawlers?

Yes. Rock crawlers typically aim for very high FDRs (e.g., 40:1 or 80:1) for extreme low-speed control.