Drum Spinners

A drum spinner (or "drum") is a robot with a short, wide spinning weapon mounted low across its front, spinning on a horizontal axis so the teeth strike upward. Drums are a close cousin of the vertical spinner: the difference is mostly shape. A drum is about as wide as the robot and has a small diameter, while a vert's weapon is tall and narrow. Drums are one of the most common and successful designs in the insect classes, because they're compact, tough and easy to aim. This page covers what's specific to drums; for the shared engineering (stored energy, tip speed, bite, spin-up, materials, shafts and bearings) see Spinner Design. For other weapon types see Weapons.

Safety: a drum stores a lot of energy for its size. Only spin it up in an arena or a test box, and always fit a weapon lock outside the arena. See Safety, Failsafes and the Safety section below.

Variants

Drum: a solid or hollow cylinder with teeth, either machined in or bolted on. For a given mass and diameter, a hollow drum stores the most energy of the common weapon shapes, because its mass is all far from the axis.[1] Solid drums are tougher but store less energy per gram.
Eggbeater: an open loop (often a square or rounded frame) instead of a solid cylinder. It can be larger in diameter for the same weight, giving more bite, and stores more energy than a disc of the same mass and diameter.[2] Eggbeaters create a lot of air drag, which grows rapidly with speed, so weapon motors and ESCs can overheat or hit current limits if you just add RPM.[3] See also Vertical Spinners.
Beater bar: a flat or shaped bar, often with one or two teeth, spinning across the front. Simpler to make than a drum and gives big bite for its weight, but stores less energy. The name comes from the spinning brush bar in an upright vacuum cleaner (see Weapons).
Full-body drum: the whole robot is a drum, with a chassis inside and wheels at the ends. The drum usually rides on bearings around the chassis and is driven from inside, and an arm or tail rests on the floor so the chassis doesn't spin with the drum.[4] They're impressive but hard to build, and few are seen at events.

Strengths and weaknesses

Strengths

Easy to aim. A drum covers the full width of the robot's front, so the driver just has to point and drive.
Compact and tough. The weapon sits low and close to the chassis, supported on both sides, so drums handle big hits well.
Recoil goes into the floor, as with verts, so a drum usually stays in place after a hit.
Upward throws flip opponents and cause fall damage.
Often invertible or self-righting, since many drums are symmetrical top-to-bottom or can gyro-right (see below).

Weaknesses

Small diameter means less energy and bite than a big vert of the same weight.
Gyroscopic effect lifts one side of the robot in fast turns, as with verts.
Low ground clearance and a weapon close to the floor make drums sensitive to arena seams and debris.
Weapon-to-weapon hits with other drums and verts are common and hard on belts, bearings and teeth.
Only dangerous from the front; a fast opponent that gets to the side can push the drum around.

Matchups

Weapon matchups are often compared to rock-paper-scissors: no design beats everything, and if your opponent brings a drum, another drum gives you no special advantage.[5] Driving and build quality usually matter more than weapon type.[6] Typical tendencies:

Opponent	How it usually goes for a drum
Another drum or vert	Weapon-to-weapon hits decide many of these fights. When two drums or eggbeaters meet head-on, the one with the higher tip speed tends to win, so builders who expect a lot of drums keep extra RPM in reserve.[7] A big vert has more reach and energy; a drum has a more stable chassis.
Horizontal spinner	Often favorable. The drum can hit the horizontal's weapon from below and send it flying while its own recoil goes into the floor.
Wedge or control bot	Depends on who gets lower. A low, hard wedge can deflect the drum and push it around. Many wedge builders swap in hinged drop forks to get under drums and eggbeaters.[8]
Flipper or lifter	Can be risky. A drum can damage the flipper's front, but if the lifter gets under it, the drum needs to be invertible or able to self-right.
Hammer or axe	Usually favorable, though a hammer landing on top of the drum's chassis can do damage.

Design considerations

Ground clearance

A drum works best when its teeth pass just above the floor, so they can catch the opponent's lowest edge. But very low clearance causes its own problems: arena seams, uneven floors and debris can catch the chassis or the weapon.[9] Many drums use replaceable forks or wedgelets in front of the drum to lift the opponent into the teeth; some instead slope the front back underneath the weapon so the large weapon can bite deeply.[10] Check both upright and inverted clearance if the robot is invertible.

Bite vs. tooth count

Drums are small and spin fast, so bite is often their weak point. Each extra tooth reduces bite (see Bite), so many drums use only one or two rows of teeth. The useful tooth height depends on your attack speed, weapon RPM and number of teeth:[11]

Useful tooth height = attack speed (per second) × 60 ÷ (RPM × number of teeth)

There's no single best balance: against hard, smooth opponents you want all the bite you can get, while drum-vs-drum fights favor speed over bite.[12]

Belt vs. direct drive

Belt drive is the most common arrangement. The motor sits behind or beside the drum and drives it with a belt, which can slip slightly and protect the motor from impacts.[13] Belts can be cut, thrown or worn, so carry spares, and get the tension right: brushless outrunners aren't designed for heavy sideways loads on their cans.[14]
Direct drive puts the motor inside the drum, with the drum acting as (or bolted to) the motor's rotating can. It's very compact and common in 150 g and 1 lb drums, but the motor's bearings take every hit. Ask Aaron notes that hollow motor-in-drum weapons become generally unsuitable for large robots, which store much more energy.[15]

See Spinner Design for more.

Gyroscopic effect, wide chassis and self-righting

A drum resists turning just like a vert, lifting one side in fast turns. A wider wheelbase (or 4WD) and smooth turning reduce the problem; the gyroscopic effect calculator helps you check a design.[16] The same effect lets many drums self-right by spinning up and turning sharply while upside down – see Self-Righting.

Mounting and balance

Support the drum on both ends with strong, well-braced mounts and easily replaceable bearings. In most cases a metal or very stiff frame is needed.
Balance the drum carefully; an unbalanced drum vibrates, wastes energy and wrecks bearings.

Powering a drum in the insect classes

As of 2026; parts change quickly.

Brushless outrunner motors are standard at every insect weight (see Motors, Insect Class Motors and ESCs). For beetleweights, Ask Aaron suggests about 150 W per pound of robot and about 6% of the robot's weight, typically a 28 mm or 36 mm outrunner.[17]
Both belt drive and motor-in-drum layouts are used in insect drums; weigh compactness against motor protection (see above).

Safety

SPARC rules require a clearly visible weapon lock whenever the robot is out of the arena, and it must be in place when you power on. Spinners must stop within 60 seconds of power being removed, using their own braking, and flywheels must not spin outside the arena or a testing area.[18] Check your event's own rules too.
A drum's lock usually goes through the chassis and between the teeth so the drum can't turn.
Test the weapon only in a test box or arena, and check your failsafe every time you change radio settings. See also LiPo Battery Safety.

Famous examples

From BattleBots robot profiles, as of 2026.

Minotaur – drum spinner from Brazil's RioBotz.[19]
Copperhead – drum spinner.[20]
Black Dragon – eggbeater spinner.[21]

Open-source designs

A few printable drum spinners from the list on 3D Printing (more there):

Terminator (1 lb) – well documented drum spinner.
CORROSION (1 lb plastic ant) – 4WD drum with a design write-up.
Break Check (1 lb plastic ant) – modular drum spinner.
Beater Bar Battlebot (150 g) – beater bar.
MicroNova (3 lb) – 4WD eggbeater with a STEP assembly.