Overview

This is a technical reference for building combat robots, focused on the 3 lb beetleweight class. It assumes you want to understand why a design choice is right, not just copy a parts list.

Two rules of thumb the whole sport agrees on: drive wins fights — build the drivetrain first and build it robust — and get under the opponent, because whoever is underneath controls the fight and disables the other’s weapon.¹

The five systems

Every combat robot — 150 g or 250 lb — is the same five subsystems. This wiki is organized around them.

System	Function	Key design variables
Drivetrain	Locomotion and turning	Motor Kv/torque, gear ratio, wheel diameter, traction, layout
Weapon	Deliver damage or displacement	Energy storage, tip speed, moment of inertia, spin-up time, bite
Power	Energy for the whole robot	Cell count (S), capacity (mAh), C-rating, bus current
Electronics	Control and safety	Radio link, ESCs, failsafe, removable link, wiring gauge
Structure	Hold together, absorb hits	Frame method, material properties, fastener thread engagement

Design order

A workable sequence for a new design:

1. Pick the class and archetype. Weight class sets the total mass budget; weapon archetype sets how that budget splits.
2. Weight budget. Allocate grams to each system before modeling anything. See Weight budgeting.
3. Drive package. Motor + gearbox + wheel that hits a target speed and has enough torque not to stall when pushed. See Power, torque & gearing.
4. Weapon package. Energy target, then motor and moment of inertia that reach it within an acceptable spin-up time.
5. Power system. Battery that supplies peak bus current and lasts a full 3-minute match with margin. See LiPo & LiHV batteries.
6. Structure. Package everything, then design the frame and armor around the load paths from weapon impacts.
7. Compliance pass. Check the design against the SPARC construction specifications and your event’s rules before cutting material.²

Units

SI throughout, with common shop units in parentheses. Energy in joules, mass in grams/kilograms, tip speed in m/s.

References

Footnotes

1. Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies: The Ultimate Guide to Building your Own 3lb Death Machine (Fall 2025 revision) — a builder-maintained beginner guide, originally for UTRA Combat and widely distributed through the NHRL Discord. Cited throughout this wiki for current North-American beetleweight practice. ↩

2. SPARC Robot Construction Specifications (Standardized Practices for the Advancement of Robot Combat), v1.4, 2023 — the widely adopted open ruleset in North America. Your event adopts a specific revision; follow that one. ↩