Tool 01 · Drive System
Answer the first question of every wheeled-robot build: what motor do I actually need? Enter your robot's numbers and get the required torque, RPM and power per motor — before you spend a rupee.
Robot + battery + payload
= 3.6 km/h
Lower = snappier = more torque
Please enter positive values in every field.
Required motor spec — per motor
Your motors must overcome three forces at once: gravity pulling the robot back on a slope, rolling resistance from the wheels deforming against the surface, and the inertia of the robot while accelerating. The total driving force is:
F = m·g·sin(θ) + Crr·m·g·cos(θ) + m·a
where m is mass, g = 9.81 m/s², θ is the incline angle, Crr is the rolling-resistance coefficient of your surface, and a is acceleration (top speed ÷ time to reach it). Torque and speed then follow from the wheel size:
Torque per motor = (F × wheel radius × safety factor) ÷ number of motors
Wheel RPM = (speed × 60) ÷ (π × wheel diameter)
The safety factor covers everything the ideal math ignores: gearbox friction, voltage sag, bumps, and the extra payload you will inevitably bolt on later.
| Surface | Crr | Notes |
|---|---|---|
| Hard floor / tile | 0.010 | Rubber wheels on smooth hard surface |
| Wood / smooth concrete | 0.015 | Typical indoor workshop floor |
| Carpet | 0.030 | Doubles or triples floor resistance |
| Short grass / dirt | 0.055 | Outdoor rovers, garden robots |
| Sand / rough terrain | 0.120 | Consider bigger wheels or tracks |
Values are practical engineering estimates for small rubber-tyred robots; real values vary with tyre hardness and load.
Once you know power and torque per motor, hobby motors fall into rough classes. Under ~5 W, micro metal gearmotors (N20 size) are cheap and tiny. From 5–25 W, the 25 mm and 37 mm DC gearmotors are the workhorses of desktop robotics. From 25–100 W you're into RS-550/775 class brushed motors or small brushless motors with external gearing. Above that, look at hub motors, e-bike motors, or wheelchair motors. The verdict box above places your build automatically.
It's torque: how many kilograms the motor can hold on a 1 cm arm. 1 N·m ≈ 10.2 kg·cm ≈ 141.6 oz·in. The nameplate above shows all three so you can compare listings directly.
Not for the mechanical requirement — torque and RPM are what the wheel needs regardless of voltage. Voltage matters when you pick the specific motor and its driver: a 12 V motor at 6 V gives roughly half its rated speed and much less torque.
Skid-steer turning scrubs the wheels sideways and can demand 2–4× the straight-line torque on high-grip surfaces. If you're building 4WD skid steer, use the 2.0 safety factor and grippy surfaces cautiously.
These are the part categories you'll shop for next. Links may be affiliate links (see footer disclosure).
N20 micro to 37 mm workhorse gearmotors with the RPM your result specifies.
Browse gearmotors →Pick a driver rated above your motor's stall current — L298N for small, BTS7960 for big.
Browse motor drivers →The wheel diameter you entered above, with hubs that fit your motor shaft.
Browse robot wheels →Size the pack after the motors — use our battery runtime calculator next.
Open the Battery Runtime Calculator →