Weapon motors & ESCs

Nearly all modern insect-class kinetic weapons use a brushless outrunner driven by a drone-market ESC, connected to the rotor by a belt or by a purpose-built hubmotor.

Motor selection

Trade-offs for a weapon motor:

Size — the 4-digit number is diameter × length in mm, of either the whole can or the stator depending on brand (check the listed dimensions). Bigger stator = more torque and thermal mass = faster, cooler spin-up. The weapon motor is usually the largest motor on the robot.
Kv and voltage — the point of higher voltage is lower current, not more power. A given motor size has a fixed maximum power regardless of Kv. A high-Kv motor reaches it on fewer cells but draws more current and runs hotter; a low-Kv motor on more cells makes the same power at lower current and temperature. Experienced builders raise voltage and drop Kv to cut heat, then match rotor I and belt ratio to hit the target rpm.¹
Overvolting — 3-minute matches let most brushless motors run well past their rated voltage (roughly +50%) without trouble.¹
Rough starting points:¹
- Vertical spinners — ~2836-size outrunners, or 2838/2845 inrunners.
- Horizontal spinners — 3530/3536 outrunners, or thin 4006/5008 stator-size “pancake” motors for large low-rpm blades.
- Cheap AliExpress outrunners work; BadAss and SunnySky are the known-good quality brands.

Belt drive vs hubmotor

Belt (or gear) reduction is the traditional setup: a small high-Kv motor spins a high-I rotor with reasonable spin-up, and the belt’s give isolates the motor from impact shock. Common tooth profiles are S3M, HTD3M, HTD5M, and XL; some builders run a toothless pulley or a belt straight off the motor can so it can slip under shock. See Shafts, bearings & mounting.
Hubmotor — a motor built specifically to survive a rotor bolted straight to its spinning can, with oversized bearings. Off-the-shelf beetleweight hubmotors (e.g. Repeat Robotics 2812, Just ’Cuz RDY-5536) have made direct drive practical: compact, robust, and beginner-friendly.¹
Plain direct drive — a rotor on a stock motor shaft — does not survive in beetleweight. Use a belt or a hubmotor.¹

ESC selection

Don’t trust the printed current rating. Drone ESC ratings assume propeller airflow; sealed in a robot they sustain far less. Pick a proven ESC for your weapon size rather than sizing to the label.¹
Prefer AM32 firmware; BLHeli_S is fine for small weapons (ESC firmware).²³
Cell-count rating matched to the battery — and note a “6S” ESC may not survive 6S LiHV.¹
The weapon ESC needs no reverse; it needs a controlled ramp and no active braking.
On 5S–6S, fit a capacitor across the ESC power pads (ESC firmware).

Spin-up current and the battery

Spin-up briefly pulls the largest current the robot ever draws. This sizes:

battery C-rating and capacity (LiPo batteries),
main wiring gauge and the removable link rating,
ESC peak rating.

A sagging battery or thin wire shows up as a weapon that spins up slowly and an ESC that cuts out or desyncs.

Firmware setup (weapon)

Motor timing: medium/high for outrunners; raise if the motor stutters or desyncs under load.
Startup power / ramp: low enough to avoid desync, high enough for acceptable spin-up.
Motor direction: set so the rotor throws opponents the intended way.
Low-RPM power protection: off or low — it can prevent spin-up under weapon load.
Signal-loss behavior: motor must stop on lost signal — verify on the bench with the rotor in a test box.

References

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: Brushless Motors; Brushless ESCs; Hubmotors; Power Transmission. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷
AM32 ESC firmware and the AM32 wiki — settings reference for timing, startup power, and low-RPM protection. ↩
BLHeli / BLHeli_S firmware (see the manual PDFs in the repo); browser config via ESC-Configurator. ↩