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Combat Robots Wiki

Weight budgeting

Weight is the hardest constraint in combat robotics. Every class is defined by a maximum mass, and the design problem is dividing that mass between systems that all want more of it.

Start from the class limit

Section titled “Start from the class limit”
Class Limit Typical usable budget after fasteners/wire
Fairyweight 150 g ~140 g
Antweight (US) 1 lb / 454 g ~430 g
Beetleweight (US) 3 lb / 1361 g ~1300 g
Beetleweight (UK) 1500 g ~1440 g

Beetleweights can claim extra allowance for non-wheeled locomotion — roughly 3.75 lb (shuffler), 4.5 lb (non-traditional), or 6 lb (true walker) under SPARC/NHRL, plus NHRL’s multibot split. See Drivetrain layouts, and confirm the exact clause with your event.12

Allow 3–5% for hardware you never model precisely: fasteners, wire, heat-shrink, adhesive, zip ties, connectors. A useful CAD-stage number is a flat ~50 g allowance for screws and wiring on a beetle.2

Typical splits by archetype (beetleweight)

Section titled “Typical splits by archetype (beetleweight)”
System Vertical spinner Horizontal spinner Control / wedge
Weapon (disc/bar + motor + mount) 30–40% 35–45% 0%
Drivetrain (motors, gearboxes, wheels) 18–25% 15–22% 25–35%
Battery 8–12% 8–12% 8–12%
Electronics (RX, ESCs, wiring) 6–10% 6–10% 6–10%
Structure & armor 20–30% 15–25% 45–60%

A control bot spends its whole non-drive budget on structure and wedge geometry. A spinner robs structure to feed the weapon, and relies on the weapon to end fights before the thin armor is tested.

Method

Section titled “Method”
  1. Build the budget as a spreadsheet with a row per component and a running total column.
  2. Enter measured masses wherever possible — vendor spec sheets, or a 0.1 g scale for parts you own.
  3. For printed parts, estimate from slicer output (grams of filament) early, replace with measured mass after the first print.
  4. Keep a contingency row of 5% of the class limit. If you spend it before the robot is done, something has to be redesigned, not shaved.
  5. Re-weigh the assembled robot on the same scale you will see at tech inspection. Build in 5–15 g of margin for paint, tape, and scale variance.

Where the grams hide

Section titled “Where the grams hide”
  • Fasteners. A beetleweight can carry 40–80 g of screws. Use the shortest screw that gives full thread engagement; drop from steel to aluminium or titanium only where the load allows.
  • Wire. Copper is dense. Size it to current (Wiring & current) — 16 AWG main/weapon, 20–22 AWG drive on a beetle — and keep runs short.
  • Over-thick printed walls. Going from 3 to 4 perimeters on every part adds up. Thicken locally at load points instead, and keep infill low (5–10%).
  • Connectors. Every connector pair is 2–6 g of pure overhead. Use XT30 for the main battery lead in insect classes, not XT60, and solder the joints you don’t need to separate.

If you come in overweight

Section titled “If you come in overweight”

In rough order of least to most disruptive:2

  1. Re-slice 3D-printed parts lighter — cheap to reprint.
  2. Trim or drop side and back armour (keep the front pointed at the opponent).
  3. Shorten wiring.
  4. Remove non-critical hardware.
  5. Only then start drilling or pocketing solid parts — heaviest parts first, material removed slowly.

References

Section titled “References”

Footnotes

Section titled “Footnotes”

  1. SPARC Robot Construction Specifications, v1.4, 2023 — weight classes and weight bonuses. NHRL publishes the class limits used at its events. ↩

  2. Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Sections: What is a Beetleweight?; Weight Bonuses; Appendix VII (Weight Management). ↩ ↩2 ↩3