Spinner types

Kinetic weapons are categorized by spin axis orientation and rotor shape. See Weapon energy & gyro effects for the underlying physics.¹

Axis orientation

	Vertical spinner (horizontal axis)	Horizontal spinner (vertical axis)
Hit direction	Launches opponent (and self) upward	Sweeps sideways, launches rotationally
Self-righting	Weapon often self-rights the robot	Usually needs separate srimech
Gyro when turning	Nose pitches up/down	Robot tips onto a corner
Wedge interaction	Works with a front wedge to lift into the rotor	Must catch the opponent’s edge
Typical feel	Controllable, consistent	Huge energy, drives itself around on hits

Vertical spinners (horizontal axis)

The most common and generally most competitive weapon type — they brace against the floor on a hit, so they stay planted and dump maximum energy into the opponent.²

Drum: a wide, small-diameter drum across the front. Generous attack area, doubles as front armour, easy to package compactly, fast spin-up.
Eggbeater / beater bar: a rectangular beater — the middle ground between a drum and a bar. Good balance of weight, area, energy, and bite. Forgiving; an excellent first vertical weapon.
Bar: a stiff bar with mass at the tips. Most energy per gram, low part count. Bends if under-built.
Large-diameter vert (tower / reach vert): a big, thin, light disc or bar. Reach advantage and lots of bite from the large arc, for relatively little weapon weight.
Disc: a rigid machined/waterjet disc, usually a single large tooth. Predictable, controllable bite; heavier than a bar for the same energy.

Horizontal spinners (vertical axis)

Bigger weapons and more reach, but they can’t brace against the floor, so they fly off after a hit.

Undercutter — blade below the chassis, near the floor. Pokes under opponents, hard for wedges or vertical spinners to reach. Generally the most competitive horizontal.
Midcutter — blade at mid-chassis height, on arms off the top and bottom plates. Most reach and best at hitting chassis and wheels directly.
Overhead — blade above the chassis. Runs a larger weapon and drives well, but the high blade is vulnerable to vertical spinners and it’s unstable inverted.

Full-body spinners

360° attack area and very heavy weapons; hard to build and often unreliable.

Shell: spinning shell over the top and outside — also full-coverage armour. Heaviest weapons, long spin-up, can’t drive inverted.
Ring: spinning ring around the robot only, so it can drive inverted, at the cost of a complex, friction-prone bearing system.
Meltybrain: the whole robot spins on one or two wheels; drive is pulsed once per rotation from an onboard gyro. Chaotic and a real coding project.

Overhead attack (hammer-saw)

A weapon on the end of an overhead arm, swung down onto the opponent’s top armour — usually a small vertical spinner (“hammer-saw”), sometimes a saw or drill. High odds of hitting internals, since few robots armour their top.

Choosing (beetleweight)

Priority	Pick
First weapon, want reliability	Drum or eggbeater (vertical)
Max damage, accept build difficulty	Bar or large-diameter vert
Reach and control on a judged win	Undercutter, or a disc vert + forks
Understand the sport first	Control bot — no spinner

See Which spinner suits a first build and Weapon motors & ESCs to spec the drive.

References

Meggiolaro, M. A. RioBotz Combat Robot Tutorial, v2.0, PUC-Rio, 2009 (CC BY-NC-ND) — weapon taxonomy and the trade-offs between rotor shapes. ↩
Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Section: General Robot Archetypes. ↩