Horizontal Spinners

A horizontal spinner is a robot whose weapon spins flat, parallel to the floor, on a vertical axis. The weapon usually sticks out past the front (or all round) of the robot, so it can hit opponents from a wide range of angles. Horizontals are popular in every insect class, partly thanks to famous robots like Tombstone. This page covers what's specific to horizontals; 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 horizontal throws broken parts sideways, which is where people stand. 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

Bar: a long bar with a tooth at one or both ends, usually extending well beyond the chassis. Long reach and simple to make, but a bar stores the least energy of the common shapes for its mass and diameter.[1] Single-tooth (asymmetric) bars give more bite.
Disc: a round plate with one or more teeth. Stores more energy than a bar of the same mass and size and is harder to bend, but has less reach for its weight.[2]
Shell and ring spinners spin the robot's whole outer body or a ring around it. They are full-body weapons with their own challenges; see Weapons – Shell spinner and Ring spinner.
Undercutters mount the weapon low, just above the floor, under or in front of the chassis. They attack wheels, wedge edges, forks and the underside of opponents.
Overhead / high-mounted horizontals sit above the chassis. They can reach over wedges to hit the opponent's body and top-mounted weapons, but leave the low front edge open to wedges and verts (see Weapons).

Some designs are invertible: the weapon sits mid-height, so the robot works either way up, acting as an undercutter one way and a high spinner the other.

Strengths and weaknesses

Strengths

Wide attack zone. A horizontal can hit from the front and sides, and it's easier to aim than a vert.
No gyroscopic wheel lift when turning. The weapon's axis points straight up and doesn't change direction when the robot turns, so a horizontal can turn quickly with the weapon at full speed.[3]
Long reach with a bar keeps opponents away from the chassis.
Good against exposed wheels and parts. Undercutters in particular can tear off wheels and break forks.

Weaknesses

Recoil. The hit pushes the horizontal sideways and backward across the floor. Against a heavy, stable opponent, a big hit can throw the horizontal into the wall or a hazard, and the shock can damage its own frame.[4]
Tipping and gyro tumble. If the robot gets tilted (by riding up a wedge, or being hit from below), the gyroscopic effect can make it tumble or "dance" uncontrollably.[5]
Low wedges and verts can get under the weapon.
Usually only one effective way up unless designed to be invertible.

Matchups

Weapon matchups are often compared to rock-paper-scissors: no design beats everything, and driving and build quality usually matter more than the weapon type.[6][7] Typical tendencies:

Opponent	How it usually goes for a horizontal
Vertical spinner or drum	Often difficult. The vert or drum can hit the horizontal's weapon from below and throw the whole robot, and its recoil goes into the floor while the horizontal's goes sideways. A horizontal can still win by hitting the vert's weapon, forks, wheels or side, especially with a strong, low weapon.
Wedge or control bot	Depends on the wedge. A low, hard, well-supported wedge can deflect hits and push the horizontal into walls. A horizontal can break hinged forks or wedgelets and attack wheels; high-mounted horizontals can reach over the wedge.
Flipper or lifter	Often favorable if the horizontal keeps the flipper away with its reach. If the lifter gets underneath, it can tip the horizontal and set off a gyro tumble.
Another horizontal	A weapon-on-weapon fight. Strong weapons, good bearings and good weapon height matter most; both robots can get thrown hard.
Hammer or axe	Usually favorable, since the horizontal's reach can hit before the hammer is in range.

Design considerations

Recoil and wall bounce

Every hit pushes the horizontal back as hard as it pushes the opponent. Since a horizontal can't push that force into the floor the way a vert can, it gets knocked sideways and often bounces off the arena wall, which can damage it or leave it facing the wrong way. Ways to cope:

A stiff, strong frame and well-supported weapon mount so the robot survives its own hits.
Good traction and a low center of gravity.
Drive control after the hit: expect to be knocked around and be ready to recover.
Bite matters: a weapon that grips the opponent transfers energy into it; one that glances off mostly sends both robots spinning. See Bite.

Horizontals can also reach the arena walls. Under SPARC rules, spinning weapons that can hit the outer arena walls in normal use need the event's approval in advance.[8]

Weapon height

Weapon height decides what you hit. A low weapon hits wheels, wedge edges and forks; a high one hits bodies, top armor and weapons. Undercutters shouldn't be too low: leave enough clearance that floor bumps, seams and jolts from hits don't make the blade touch the floor.[9] Some builders make weapon height adjustable with spacers, to suit each opponent.

Gyroscopic lifting

A horizontal turns freely, but its weapon still resists tilting. If the robot rides up a wedge or gets hit from below, the weapon can push it into a wobble or tumble, and spin-up torque can also tilt the chassis. A low, wide chassis and a weapon close to the chassis help. See Gyroscopic effects.

Shafts and mounting

The weapon shaft takes big sideways loads on every hit. Many insect horizontals use a dead shaft (a fixed bolt or shaft supported top and bottom by the chassis) with bearings in the weapon hub, rather than relying on the motor's own shaft. Undercutters often look like an upside-down full-body spinner mount, with the shaft ends rounded and sticking out below the blade and above the chassis.[10]

Powering a horizontal in the insect classes

As of 2026; parts change quickly.

Brushless outrunner motors are the standard weapon motor 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.[11]
Belt drive from a motor beside the weapon is common because the belt can slip a little and protect the motor from shock.[12] Outrunners aren't designed for heavy sideways loads on their cans, so get the belt tension right and support the pulley close to the motor.[13]
Direct drive / hub motors (the weapon bolted to the motor's can) are common in 150 g and plastic ant horizontals because they're compact, but every impact goes straight into the motor bearings.

See Spinner Design for more.

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 must not spin outside the arena or a testing area.[14] Check your event's own rules too.
A long horizontal bar is easy to lock with a strap or pin through the chassis; make sure the lock really stops the weapon and can't fall off.
Test the weapon only in a test box or arena. Horizontals throw debris sideways, so stand well clear of the weapon's plane even outside the box.
Check your failsafe every time you change radio settings, and see LiPo Battery Safety.

Famous examples

From BattleBots robot profiles, as of 2026.

Tombstone – horizontal bar spinner; BattleBots Season 2 (2016) champion.[15][16]
Valkyrie – horizontal disc spinner.[17]
Blade – horizontal bar spinner from South Korea.[18]

Open-source designs

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

Nemo (1 lb plastic ant) – a beginner-friendly spinner with only three printed parts.
Tombstone Tribute (1 lb).
Undercutter Battlebot (150 g) – undercutter with STEP files.
DNS (3 lb) – forked undercutter.
Endbots Vector Remix (3 lb) – a remix of the popular Endbots Vector.