Most robot motors are chosen by reading the stall torque on a listing and hoping. That number is the least useful one on the page. What decides whether a robot moves is the torque it needs at the wheel, the speed it needs to run at, and whether the motor can deliver both without cooking itself.
Start from the robot, not the motor
Four numbers describe the job: the mass to move, the top speed, the acceleration you want, and the worst slope or obstacle. Everything else follows from them.
| Input | Example | Why it matters |
|---|---|---|
| Total mass | 2.0 kg including battery | Sets the force needed to accelerate and climb |
| Top speed | 0.5 m/s | Sets the wheel RPM |
| Acceleration time | 0.5 s to top speed | Sets the peak force, usually the binding constraint |
| Worst slope | 10° ramp | Adds a constant force that never goes away |
| Wheel diameter | 65 mm | Converts force to torque and speed to RPM |
| Driven wheels | 2 of 4 | Divides the required torque per motor |
The three equations that do the work
wheel torque τ = F_total × r
wheel speed RPM = (v / (2πr)) × 60
motor power P = τ × ω (ω in rad/s)Work an example. A 2 kg robot on 65 mm wheels needs 0.5 m/s and should reach it in 0.5 s.
- Radius r = 0.0325 m. Acceleration a = 0.5 / 0.5 = 1.0 m/s².
- Acceleration force = m × a = 2.0 × 1.0 = 2.0 N.
- Rolling resistance on hard floor is roughly 0.03 × m × g = 0.6 N.
- Slope force at 10° = m × g × sin 10° = 2.0 × 9.81 × 0.174 = 3.4 N.
- Total force = 2.0 + 0.6 + 3.4 = 6.0 N.
- Torque at the wheel = 6.0 × 0.0325 = 0.195 N·m total.
- Split across two driven wheels: 0.098 N·m each, which is about 1.0 kg·cm.
- Wheel RPM = (0.5 / (2π × 0.0325)) × 60 = 147 RPM.
A yellow TT gearmotor is usually quoted around 0.8–1.2 kg·cm at 150–200 RPM at 6 V. On paper it just covers this robot. In practice you want at least twice the calculated torque, because the listing figure is measured near stall, the battery sags, and the floor is never ideal. Two TT motors at 1.2 kg·cm give about 2.4× margin over the 0.098 N·m requirement, which is why that combination works for a small indoor rover and struggles on carpet or a steep ramp.
Stall torque is not working torque
Motor listings quote several numbers that are easy to confuse:
| Figure | What it means | How to use it |
|---|---|---|
| Stall torque | Torque at zero speed, maximum current | Never design to it; the motor overheats and the driver may trip |
| Rated torque | Torque it can hold continuously | The number to design around, usually 20–50% of stall |
| No-load RPM | Speed with nothing attached | Overstates real speed; geared motors lose 10–30% under load |
| Rated current | Current at rated torque | Sets driver and battery sizing |
| Stall current | Current at zero speed | Can be 5–10× rated; the driver must survive a brief stall |
| Gear ratio | Motor turns per output turn | Multiplies torque, divides speed, adds friction losses |
A 12 V geared motor quoted at “30 kg·cm” is usually a stall figure at 12 V with a fresh supply. At 11 V through a driver that drops 0.3 V, with a gearbox at 70% efficiency, the usable continuous torque may be a third of that. The robot still moves; it just moves more slowly and heats up more than the spreadsheet predicted.
Match the motor family to the job
| Motor | Typical use | Strength | Weakness | Price |
|---|---|---|---|---|
| TT gearmotor (yellow) | Small 2WD rovers | Cheap, light, 3–6 V | Plastic gears, low torque, short life | ₹80–₹150 each |
| N20 gearmotor | Micro robots, 2–6 V | Compact, metal gearbox options | Very low torque, fragile shaft | ₹150–₹400 |
| 12 V geared DC (JGB37) | 4WD rovers, 3–10 kg | Good torque, encoder versions | Heavier, needs 12 V supply | ₹450–₹900 each |
| NEMA 17 stepper | 3D printers, arms, linear axes | Precise position without feedback | No speed, needs a driver, runs hot | ₹600–₹1,500 |
| Hobby servo | Steering, grippers, small arms | Built-in position control | Limited rotation, low continuous torque | ₹150–₹1,200 |
| BLDC with ESC | Drones, fast rovers, gimbals | High power density, efficient | Needs ESC and often feedback | ₹700–₹5,000 |
| Hub motor | E-bikes, large rovers | Torque in the wheel, simple drive | Heavy, expensive, hard to mount | ₹3,000–₹12,000 |
The servo vs DC vs stepper comparison goes deeper on control behaviour. For a first rover, the choice is almost always a geared DC motor with a encoder; for a joint that must hold position, a stepper or a servo.
The driver is part of the motor choice
An L298N drops 2–3 V across its output transistors. On a 6 V supply that leaves 3–4 V for the motor, so the robot is slower and the driver gets hot. A TB6612FNG or DRV8871 drops far less and is the better choice for anything battery-powered.
| Driver | Continuous current | Voltage drop | Best for |
|---|---|---|---|
| L298N | 2 A per channel with heatsink | 2–3 V | Bench tests on 12 V, not battery robots |
| TB6612FNG | 1.2 A per channel | About 0.5 V | Small rovers, 6–12 V |
| DRV8871 | 3.6 A peak, 2 A continuous | Low | Single motor per board, robust |
| BTS7960 | 43 A peak | Low | Large rovers, high-current motors |
| A4988 / TMC2209 | 1–2 A per phase | n/a (stepper) | NEMA 17 positioning |
Size the driver for stall current, not rated current, and add a fuse. A stalled geared motor can pull several amps through a driver rated for one, and the failure is usually silent until the board smells.
Gearboxes: torque is not free
A gearbox multiplies torque by the ratio and divides speed by the same ratio, minus losses. A 1:50 gearbox does not give 50× the torque; at 70% efficiency it gives 35×, and the output shaft and gears must be rated for that torque. Plastic gears strip, metal gears whine, and both add backlash that matters for positioning but not for driving.
Two more effects matter on a robot. First, the gearbox reflects the motor’s inertia, so a high ratio makes the output feel heavier and less responsive. Second, a geared motor back-drives poorly: pushing the robot by hand may turn the wheels but not the motor, which is good for holding a slope and bad for detecting a stall by current alone.
Check the supply before buying
- Voltage: a 6 V motor on 12 V runs fast and dies young. Match the nominal voltage, then allow for a fresh battery being 15–20% above nominal.
- Current: total stall current of all motors plus logic should stay under the battery’s continuous rating. A 2S 18650 pack with a 5 A BMS cannot run four motors that stall at 3 A each.
- Buck converter: if the motors run at 12 V and logic at 5 V, the buck converter powers only the logic, never the motors.
- Wiring: 20 AWG for small motors, 18 AWG for 12 V geared motors, and short runs. Thin jumper wires add resistance that looks exactly like a weak motor.
- Encoders: if you plan odometry, buy motors with encoders now. Retrofitting them is possible but rarely aligned well.
A selection checklist
- Calculate wheel torque at the worst case: acceleration plus slope plus rolling resistance.
- Double it for margin, then divide by the number of driven wheels.
- Calculate wheel RPM at top speed, then pick a motor whose rated RPM under load is at or above it.
- Check rated current and stall current against the driver and battery.
- Confirm the shaft, mounting pattern and wheel hub fit the chassis before ordering.
- Buy one spare motor and one spare driver. The spare is cheaper than a stalled project.
The servo torque calculator handles the joint case, and the chassis guide covers wheel size and mounting. If the robot must hold position against gravity, read the encoder guide before choosing a plain DC motor.
Common questions
How much torque does a robot motor need?
Calculate the force needed to accelerate the robot, climb the worst slope and overcome rolling resistance, multiply by the wheel radius, then double it for margin and divide by the number of driven wheels. A 2 kg rover on 65 mm wheels typically needs about 0.1 N·m per driven wheel.
Is stall torque the same as running torque?
No. Stall torque is measured at zero speed with maximum current, and a motor cannot sustain it. Design around 20–50% of stall torque for anything that runs continuously.
Should I use a stepper or a DC motor for a robot?
Use a stepper when you need position without feedback and can accept lower speed and higher heat. Use a geared DC motor with an encoder for driving, where efficiency and torque matter more than holding position.
Why does my robot slow down on carpet?
Rolling resistance and wheel slip rise sharply on soft surfaces, and the motors are probably already near their rated torque. Increase wheel diameter, add driven wheels, or choose a higher-torque motor.
Can I run a 6 V motor at 12 V?
It will spin much faster and wear out quickly. Use the rated voltage, or limit the PWM duty cycle so the average voltage stays near nominal.
Related: servo vs DC vs stepper, robot chassis guide, TB6612FNG driver, robot power guide.
The same torque and duty-cycle reasoning applies when sizing solar panel cleaning robots.
Once you know the torque and speed you need, BLDC motors and ESCs explains how to turn those numbers into a brushless setup.
If the load moves linearly rather than rotating, start with the linear actuator guide and size the screw or belt first.
If the load is driven by a cylinder rather than a motor, start from choosing between hydraulic, pneumatic and electric actuators.
Once the motor is chosen, the gearbox comparison decides whether that torque reaches the joint.
Frame size is not torque; read NEMA 17 vs NEMA 23 before ordering.
The gearbox choice matters as much: see harmonic vs cycloidal drives.
Steppers are open loop, so allow margin for lost steps rather than sizing the motor to the exact load.
Once the motor is chosen, the servo drive selection guide covers matching current, feedback and interface.
Choosing the motor is only half the job; the PID tuning guide covers making it behave.
Motor mounts and gearbox clearance are chassis decisions; the beginner chassis guide covers layout and stiffness.
Sources and further reading
- Pololu brushed DC motors — datasheet conventions for stall torque, current and gearmotors.
- NEMA motor standards — NEMA frame sizes used by steppers.
- Gear train — torque multiplication and efficiency losses.
Last reviewed: October 2026. Calculations use ideal rolling assumptions; measure current and temperature on the real robot before finalising a design. Prices are Indian retail ranges. Not bench-tested by this site.
