D · Interactive tool · Module 03
Motor Sizing Calculator
Torque and speed per motor from payload, wheel size, slope and acceleration, with an optional check against a candidate motor.
Preliminary engineering guidance. Results depend on the assumptions listed below and on simplified models. Verify with manufacturer datasheets and a test on your own hardware before committing to a design.
Robot
Steepest incline it must climb at speed.
Depends on wheel and floor; measure it by pulling the robot with a force gauge.
Drivetrain
Candidate motor (optional)
Result
Assumptions and limitations
- Force = rolling resistance + grade + acceleration:
F = m·g·Crr·cos θ + m·g·sin θ + m·awith g = 9.80665 m/s². Aerodynamic drag, turning scrub, wheel slip, bearing and cable drag are not modelled; they are small for a slow indoor robot and larger outdoors or at speed. - Torque per motor = F·r / (motors · gear ratio · gearbox efficiency). The load is shared equally between the driven wheels.
- Continuous torque uses rolling + grade only (steady speed); peak adds acceleration. Compare continuous with the motor’s rated torque and peak with its stall or peak rating, as stated in the datasheet.
- Speed check uses a straight speed-torque line (ideal DC motor). Real motors, brushless drives and current limits deviate from it. The mechanical power shown is at the wheels; electrical power is higher by the motor and driver losses.
- Gear efficiency, rolling resistance and margin are inputs you choose. The defaults are placeholders, not measurements of any product.
References
- Newton’s second law and the rolling-resistance model: any mechanics text; the formulas on this page are the complete model.
- Datasheet fields to read for a real motor: rated torque, stall (peak) torque, no-load speed, rated voltage, thermal resistance and maximum continuous current.