---
title: "Choosing Gear Ratios for FRC Mechanisms"
tags:
  - gear ratio
  - mechanism
  - design
  - motor
  - torque
  - speed
  - frc
  - 2890
type: training-module
owner: 2890
status: active
sources:
  - "https://docs.wcproducts.com/frc-build-system/belts-chain-and-gears/gears"
  - "https://www.frcdesign.org/learning-course/stage1/1B/torque-speed/"
  - "https://blog.thebluealliance.com/2013/06/24/behind-the-design-understanding-motor-and-gearbox-design/"
growth: tree
---

# Choosing Gear Ratios for FRC Mechanisms

## Core Principle

**Speed and torque are inversely proportional.** A 4:1 gear reduction gives you 4× the torque but 1/4 the speed.

```
Gear Ratio = Driven Gear Teeth / Driver Gear Teeth

If ratio = 4:1 (input:output = 1:4):
  Torque → multiplied by 4
  Speed → divided by 4
```

## The Design Process

### Step 1: Define Your Goal

Ask these questions:
- What does this mechanism need to **do**?
- How **fast** does it need to move?
- How **heavy** is the load?
- What's the **time constraint** (game-specific)?

### Step 2: Calculate Required Output

For each mechanism:

| Mechanism | Key Metric | Typical Range |
|-----------|------------|---------------|
| Drivetrain | Top speed | 10-15 ft/s |
| Elevator | Lift speed | 3-6 ft/s |
| Arm | Rotation speed | 90-180°/sec |
| Intake | Roll speed | Fast (1-2 sec) |
| Shooter | RPM | 3000-6000 RPM |

### Step 3: Match Motor to Load

**NEO Vortex specs (Team 2890's standard):**
- Free speed: 6784 RPM
- Stall torque: 3.6 Nm (0.36 kg·m)
- Peak output: 640W
- Continuous (40A): ~375W

**Work backward from desired output speed:**
```
Required output speed = X RPM
↓ 
Account for mechanism reduction (belts, chains, gears)
↓ 
Calculate motor speed needed
↓ 
Find gear ratio
```

## Drivetrain Gear Ratio Selection (Team 2890)

Team 2890 uses **L1 and L3** on MK4i modules with NEO Vortex:

| Ratio | Speed | Torque | Use When |
|-------|-------|--------|----------|
| **L1** (8.14:1) | ~14.4 ft/s | Lower | High speed needed, less pushing |
| **L3** (6.12:1) | ~12.8 ft/s | Higher | More pushing power, climbing |

**L1 = faster, less torque. L3 = slower, more torque.**

Swap based on game demands. Field-swappable.

## Mechanism Design Guidelines

### Elevators / Vertical Motion

```
Motor → Belt reduction → Sprocket → Chain → Carriage
```

- Target: 3-6 ft/s vertical
- Common ratios: 4:1 to 10:1 per stage
- Watch for staging — two-stage elevators need compounding reduction

### Arms / Swing Motion

```
Motor → Gearbox → Arm
```

- Target: 90-180°/sec rotation
- Consider start-up torque (gravity fight at low angle)
- PID control essential for repeatable positioning

### Shooters / Flywheels

```
Motor → Gearbox → Wheel
```

- Target: 3000-6000 RPM wheel speed
- High reduction = more torque at wheel, faster spin-up
- Common ratio: 3:1 to 5:1

### Intake Rollers

```
Motor → Direct or light reduction → Roller
```

- Target: Fast roll-in (under 2 seconds)
- Low reduction or direct drive works
- Rubber roller = good grip, no gear reduction needed

## Motor Sizing Rules of Thumb

1. **Stall torque > Load torque at worst angle**
2. **Free speed > Desired output speed by 2× minimum**
3. **Current draw at stall < 80% of breaker rating**
4. **Continuous torque > Average load torque**

## Common FRC Motor/Gearbox Pairings

| Motor | Typical Gearbox | Output | Use |
|-------|----------------|--------|-----|
| NEO Vortex | Built-in 4:1 | 1696 RPM | Drivetrain, mechanisms |
| NEO Vortex + planetary | External reduction | Variable | Elevators, arms |
| Falcon 500 | Integrated | 1680 RPM | Drivetrain, high torque |
| Kraken X60 | Integrated | ~1680 RPM | Newer alternative to Falcon |

## The Calculation

```
Desired output RPM = X
Motor free RPM = 6784 (NEO Vortex)
Total reduction needed = 6784 / X

Example: Want 600 RPM output
  6784 / 600 = 11.3:1 reduction needed

Split across stages:
  Stage 1 (belt): 3:1
  Stage 2 (gearbox): 4:1
  Total = 12:1 — close to target
```

## Signs You've Got It Wrong

- **Too fast / not enough torque:** Robot stalls under load, wheels slip
- **Too slow / too much torque:** Mechanism moves too slowly to be useful
- **Motor overheating:** Too much load for continuous operation — need more reduction or bigger motor
- **Brownouts:** Current spikes from stalling — check breaker sizing

## For Team 2890 Students

When designing a mechanism:
1. **Define the task** — what does it need to do in the game?
2. **Pick the motor** — NEO Vortex is standard on 2890
3. **Calculate the speed you need** — game constraints (time limits, field size)
4. **Work backward** — gear ratio = motor speed / desired output speed
5. **Split across stages** — belt + gearbox is easier than single-stage high ratio
6. **Test and tune** — PID tuning can fix some speed issues, but wrong gear ratio can't be fixed with software

## Related

- [[swere-modules]] — MK4i gear options for drivetrain
- [[neo-vortex-motor]] — motor specs
- [[spark-flex]] — controller configuration
- [[motor-basics]] — understanding motor curves

---
*Research from web search — gear ratio design for FRC mechanisms*
*Queue: research complete — stored in wiki*