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

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