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Combat Robots Wiki

Weapon geometry & bite

Bite

Section titled “Bite”

Bite is how far into the opponent a tooth reaches before it makes contact — how much of the tooth actually engages:

bite ≈ (closing speed × 60) / (weapon rpm × n_teeth)

Bite roughly determines how much of the weapon’s stored energy transfers on a hit. Low bite grinds and pops; high bite gives the big violent hits. Stored energy means nothing if you can’t deliver it, so modern weapon design is largely about maximising bite:1

  • Fewer teeth — a tooth comes around less often, so the opponent moves deeper before contact. This is why most competitive weapons run one tooth.
  • Lower rpm — same effect. Builders often don’t run full speed: consistent bite beats raw energy, and tip speed above ~300 mph is rarely worth it.
  • Faster closing speed — charge the opponent (back up, then drive in hard).

The limits still apply: too much bite in one hit can stall the weapon or bounce both robots apart before full energy transfers.2 The current answer to that is a single big tooth run at moderate rpm, not many small teeth.

Tooth / impact point count

Section titled “Tooth / impact point count”
Teeth Behavior
1 The competitive default. Most bite, biggest hits. Must be asymmetric — balance it with the CAD center-of-mass tool.
2 Naturally balanced; used on very large-diameter weapons whose low rpm still gives enough bite.
3+ Low bite, smooth “grinding” damage, gentle on the weapon. Rare on competitive robots.

Odd or single-tooth weapons are balanced by an asymmetric body or a counterweight opposite the tooth, checked against the CAD center of mass.

Rotor diameter and engagement height

Section titled “Rotor diameter and engagement height”
  • Diameter sets tip speed for a given rpm (v = ω·R) and reach. Bigger rotor = more energy and range, more mass, more gyro. A larger diameter also buys bite: the tooth travels a longer arc per revolution.
  • Engagement height: where the rotor contacts the opponent.
    • Vertical spinner — set slightly above the floor so a front wedge or forks feed the opponent’s edge into the rotor.
    • Horizontal spinner — set to strike the opponent’s mid-height, above their wedge line but below their armor’s strong points.

Tooth shape

Section titled “Tooth shape”
  • Rake angle — the angle between the tooth’s impact face and the radius line to its tip. Neutral to about +10° is the usual choice: enough to dig into armor, not so much that the tooth rolls over and wears. High positive rake (>20°) bites hard but wears fast — situational, e.g. hammer-saw blades. Negative rake resists wear and won’t stick in wooden arena kickplates, but struggles to bite; seen on some large blunt-impact horizontals.1
  • Tooth support — back the impact face with as much material as possible (a “swept” profile behind the tooth) so it doesn’t roll backward on hits.
  • Tooth width — the tooth bends hardest at its base, not its tip. Make the base wide with generous fillets and taper toward a sharp point.1

Matching geometry to the motor

Section titled “Matching geometry to the motor”

The rotor’s moment of inertia must be small enough that the weapon motor spins it up in 1.5–4 s and recovers between hits. If spin-up is sluggish:

  • reduce rotor mass or move mass inboard (smaller I),
  • raise rpm target (taller belt/gear ratio into the rotor, or higher Kv motor),
  • accept lower per-hit energy in exchange for a weapon that’s actually turning when contact happens.

Verify the tip speed with the Bristol Bot Builders tip-speed calculator or the Ember weapon calculator — aim for roughly 250 mph (~110 m/s), and rarely design past 300 mph.31

First weapon

Section titled “First weapon”

A single-tooth vertical disc or a compact drum, moderate diameter, driven by a well-matched brushless outrunner on a belt (or an off-the-shelf hubmotor). It self-rights, it’s forgiving of driving, and the geometry is easy to get right.

References

Section titled “References”

Footnotes

Section titled “Footnotes”

  1. 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: Spinning Weapon Design (Weapon Geometry and Bite, Maximizing Bite, Designing for Durability). ↩ ↩2 ↩3 ↩4

  2. Meggiolaro, M. A. RioBotz Combat Robot Tutorial, v2.0, PUC-Rio, 2009 (CC BY-NC-ND) — the “bite” model, tooth-count trade-offs, and matching rotor inertia to the motor. ↩

  3. Bristol Bot Builders — Tip Speed & Drive RPM Calculator. ↩