Spinner Design

Spinning weapons are the most common weapon type in robot combat, because they store a lot of energy and release it in a single hit. This page explains the basic physics behind spinner design, and the trade-offs builders make. For the different kinds of spinners (drums, vertical and horizontal spinners, full-body spinners and more), see Weapons.

Safety: a spinning weapon stores a lot of energy even in small classes. Only spin it up inside an arena or a test box, always use a weapon lock outside the arena, and follow the SPARC Robot Construction Specifications (for example, spinners must stop within 60 seconds of losing power).

Stored energy

A spinner works like a flywheel: the motor slowly builds up energy, and the weapon dumps much of it into the opponent when it hits. The energy stored is:

KE = ½ × I × ω²

where I is the weapon's moment of inertia (MOI) and ω is its speed in radians per second (RPM × 2π ÷ 60).

Two things follow from this formula:

Speed counts twice. Doubling the speed gives four times the energy. But speed is limited by how fast the motor can spin the weapon up, by the weapon's strength, and by bite (see below).
Mass far from the center counts most. MOI depends on how far the mass is from the axis, squared. A ring or bar with heavy ends stores much more energy than a solid disc of the same weight and size.

Approximate MOI for common shapes (mass m, radius r, length L):

Shape	MOI
Solid disc or drum (spinning around its axis)	½ m r²
Thin ring or hollow drum	m r²
Bar (spinning around its middle)	¹⁄₁₂ m L²

Real weapons are a mix of shapes. CAD software can calculate MOI exactly, and online calculators such as Run Amok's SpinCalc and the Bristol Bot Builders tip speed and KE calculator estimate energy and spin-up time. See OnlineTools for more.

Tip speed

Tip speed is how fast the outermost point of the weapon is moving:

Tip speed (ft/s) ≈ RPM × diameter (inches) × 0.00436

Typical full-combat spinners run at tip speeds of a few hundred feet per second. Some classes limit tip speed: for example, the SPARC Sportsman rules allow only 20 ft/s (see SPARC Restricted Class Regulations), and some events limit tip speed or weapon energy for their arena.

Bite

Bite is how deep a tooth can reach into the opponent before it hits. With more bite, the tooth lands squarely and transfers more energy; with too little, it skips off the surface and the hit is a glancing blow.

Bite depends on how far the robots close in during the time between one tooth and the next:

Bite = closing speed ÷ (RPM ÷ 60 × number of teeth)

Example: a closing speed of 10 ft/s (120 in/s), a weapon at 6,000 RPM (100 revolutions per second), and 2 teeth means 200 tooth passes per second, so each tooth gets about 120 ÷ 200 = 0.6 in of bite.

This is why many spinners use only one or two teeth, and why running a weapon faster isn't always better: past a certain speed you gain energy but lose bite.

Spin-up time

A heavier or faster weapon takes longer to reach full speed. During that time it's vulnerable, and after every big hit it has to spin up again. A weapon that reaches most of its speed in a second or two is often more useful than a bigger one that needs much longer. Spin-up time depends on the motor's power and torque, the gearing or belt ratio, the battery and ESC's ability to deliver current, and the weapon's MOI.

Gyroscopic effects

A fast-spinning weapon resists being tilted, like a gyroscope. When the robot turns, the weapon pushes back by trying to tip the robot sideways or lift one side:

Vertical spinners and drums are affected most. When they turn quickly, the robot can lift onto one wheel, which makes driving harder. Some robots use this effect deliberately to self-right, sometimes called a "gyro dance."
Horizontal spinners are mostly affected when the robot is tipped up, for example by riding up an opponent's wedge, which can make them tumble.
Stiffer frames and well-supported weapon shafts and bearings handle the forces better. Drivers learn to turn more smoothly with the weapon at speed.

Driving the weapon

Direct drive: the motor spins the weapon directly, or the motor sits inside the weapon (common in drums and some insect spinners). Compact, but the motor takes every impact.
Belt drive: a belt connects motor and weapon. The belt acts as a clutch and shock absorber, protecting the motor, and lets you change the ratio. Belts can slip or be cut.
Gear or chain drive: stronger, less slip, but no shock absorption.

Brushless motors are the standard choice for spinning weapons because of their power for their weight (see Motors and ESCs).

Weapon construction

Materials: weapons need to be tough, not just hard. Shock-resistant tool steels such as S7, hardened to about 54 HRC, are a popular choice for teeth and blades, while 4130/4140 chromoly and hardened AR steels are used too.[1] Plastic and printed weapons are common in 150 g and plastic classes.
Balance: an unbalanced weapon vibrates, wastes energy, and loads the bearings. Balance it carefully, and re-check it after any damage.
Bearings and shafts: the weapon's shaft and bearings take the full force of every hit. Supporting the weapon on both sides is much stronger than mounting it on one side only.
Teeth: fewer teeth give more bite; replaceable teeth make repairs easier.

Spinner Design

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