Drivetrain layouts
Drive wins fights — it’s the only thing keeping you off a countout, and it delivers the weapon. Build it first and build it robust.1
Skid steer (tank drive)
Section titled “Skid steer (tank drive)”Standard layout: left and right sides driven independently, robot turns by driving the sides at different speeds. Simple, robust, mixed in the transmitter.
- 2WD, 2 wheels total: both wheels driven, full weight on traction, needs a skid or the weapon housing as a third contact point.
- 2WD, 4 wheels: front or rear pair driven, others are casters/idlers.
- 4WD: all four driven. Better traction, stability, and redundancy (keeps moving after losing a wheel or motor). Most competitive beetles are 4WD.1
How the motor connects to the wheel
Section titled “How the motor connects to the wheel”| Setup | How | Notes |
|---|---|---|
| Direct drive | Wheel on a combat gearmotor’s output shaft | Default. Simple, compact, robust with a shock-absorbing wheel and a combat-rated gearmotor. |
| Indirect drive | Belt or gear from the gearmotor to a wheel on a dead axle | Isolates the motor from wheel hits; frees up packaging. 4mm S3M belts and the Repeat drive axle are common. |
| Direct 4WD | One wheel on the motor, a belt/gear to a second wheel per side | Keep the two wheels rigid to each other so the belt stays engaged. |
| Indirect 4WD | Motor drives a double-wide pulley feeding front and rear wheels | Full motor isolation; one wheel loss doesn’t kill the side (e.g. Repeat Peter Bar). |
| Multi-stage reduction | Gear/belt train off a plain drone motor instead of a gearmotor | Cheaper/lighter motor, more design freedom; more complex and fragile. |
| Tangent / friction drive | Drone-motor shaft pressed directly against the wheel | Most compact and powerful; needs constant shaft-to-wheel pressure, hard wheels, and wheel protection. Fast and twitchy. |
Combat gearmotors have improved enough that plain direct drive is fine for a first build; reach for belts when you want packaging freedom or shock isolation.1
Melty brain (full-body spinner)
Section titled “Melty brain (full-body spinner)”The entire robot spins; translation comes from pulsing drive power once per revolution, timed by an onboard accelerometer/gyro and firmware. Chaotic, fun, and a real coding project — not a first build.1
Walkers and shufflers
Section titled “Walkers and shufflers”Legged or leg-like drives, used partly to claim a weight bonus. The numbers vary by event but under SPARC/NHRL they run roughly:21
| Locomotion | Beetleweight limit |
|---|---|
| Wheels / normal | 3 lb |
| Shuffler (feet on continuously rotating cams) | 3.75 lb |
| Non-traditional locomotion (weapon-driven, etc., no rolling contact) | 4.5 lb |
| True walker (≥2 independent DOF per leg, no rolling contact) | 6 lb |
| Multibot (NHRL) | 4 lb ÷ number of segments, up to 4 |
Confirm the exact clause with the event before designing around it.
Invertible driving
Section titled “Invertible driving”Insect robots get flipped constantly. Design to drive upside down:
- Wheels tall enough to contact the floor in both orientations, or
- A weapon/body shape that self-rights (Self-righting).
- The transmitter should have an invert switch that flips drive (and sometimes steering) so “forward” stays forward when inverted.
References
Section titled “References”Footnotes
Section titled “Footnotes”-
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: Drive Systems; General Rules → Weight Bonuses. ↩ ↩2 ↩3 ↩4 ↩5
-
SPARC Robot Construction Specifications and Tournament Rules, v1.4, 2023 — walker/shuffler weight bonuses and their qualifying criteria. Many events modify or drop these; check the ruleset you are entering. ↩