FRC Flywheel Shooter Design: Compression, Speed, Backspin, and Hooding

A flywheel shooter looks simple: spin a wheel fast, feed a game piece into it, and watch it fly. But the gap between a shooter that scores from anywhere and one that sprays inconsistently comes down to a handful of physics decisions you make at the CAD stage — how much you compress the piece, how fast the wheel surface moves, how much rotational inertia you store, and how the hood shapes the exit angle and spin. This guide walks through each of those decisions with the real numbers behind them, so you can size a shooter on purpose instead of by trial and error.

Throughout, treat the physics models here as idealized starting points. Real game pieces slip, deform, and vary part-to-part, so every rule of thumb below is a place to begin tuning, not a final answer. The teams that shoot well are the ones who prototype early and measure relentlessly.

The projectile problem you're actually solving

Before touching a motor, understand what the shooter has to deliver. A launched game piece is a projectile, and once it leaves the shooter it obeys basic ballistics: it follows a parabola set by its exit velocity (speed) and launch angle, minus whatever air drag and spin effects modify the path.

For a projectile launched at speed v and angle θ (ignoring air resistance), the horizontal range is:

R = (v² · sin(2θ)) / g

where g = 9.81 m/s². Two things fall out of this immediately:

• Range scales with the square of exit velocity. Double the speed and you quadruple the distance. This is why exit velocity is the single most important output of your shooter, and why small speed inconsistencies turn into large landing-point errors downrange.
• 45° maximizes range for a given speed, but you almost never shoot at 45°. Real targets are elevated and have an opening you must drop into, so you usually launch steeper (often 50–70°) and accept that you need more speed than the pure range formula suggests. A steeper, higher arc also drops the piece into a target more vertically, which is more forgiving of small distance errors.

The practical takeaway: your design target is a specific exit velocity and exit angle, achieved repeatably. Everything downstream — wheel size, RPM, compression, motor choice — exists to hit that velocity consistently, shot after shot.

Exit velocity: wheel surface speed and the factor of two

A flywheel transfers energy to the game piece through friction at the contact patch. The speed the wheel's rim is moving is the surface speed:

v_surface = ω · r = (RPM · 2π / 60) · r

For a 4-inch wheel (radius 0.0508 m) spinning at 6000 RPM, ω ≈ 628 rad/s and v_surface ≈ 31.9 m/s. That is the theoretical ceiling on how fast the contact point moves — but the piece never leaves at the full surface speed. How much you actually get depends on the architecture.

Single flywheel against a hood

In a single-wheel shooter, the piece is squeezed between one spinning wheel and a stationary hood (or backplate). Model it as rolling without slipping: the wheel-side contact point moves at v_surface, the hood-side contact point is stationary at 0, so the piece's center travels at the average of the two — about half the wheel's surface speed — and it picks up heavy backspin in the process.

That factor of two is the defining trait of a single-flywheel shooter: to get a 16 m/s exit, you need roughly 32 m/s of surface speed. You "waste" half the rim speed spinning the ball, but you get backspin for free, which is often exactly what you want (more on that below).

Dual flywheel

In a dual-flywheel shooter, the piece passes between two counter-rotating wheels. Now both contact points move in the exit direction at v_surface, so the piece's center leaves at close to the full surface speed with little or no spin (if both wheels run at the same speed). You get roughly double the exit velocity of a single wheel at the same RPM, and you transfer energy from both sides, so recovery is faster.

Neither is universally "better." A single wheel with a well-shaped hood is compact, simple, and self-backspins — great when packaging is tight. A dual setup gives you independent control of speed and spin: run the wheels together for a flat, fast shot, or run the top wheel faster than the bottom to add backspin on demand. If you want a deeper look at how mechanisms trade off simplicity against control, the same tradeoffs show up in our intake design guide.

Compression: the knob nobody documents but everyone tunes

Compression is how much you squeeze the game piece as it passes through the shooter — the difference between the piece's free diameter and the gap you force it through. If a ball is 7 inches across and the gap between wheel and hood (or between two wheels) is 6 inches, you have 1 inch of compression.

Compression is what actually couples the wheel to the piece. Too little and the wheel slips on the surface — energy that should launch the piece just heats the contact patch, and your exit velocity collapses and varies shot-to-shot. Too much and you deform the piece so hard that you bog the wheel down, spike current, and again lose consistency (and you can damage the piece or the mechanism).

There is no universal number because it depends on the piece's material, size, and how squishy it is, plus your wheel durometer. But the method is universal:

1. Start with a modest compression — commonly in the ½ to 1 inch range for a large foam or rubber ball — set by your wheel-to-hood or wheel-to-wheel spacing.
2. Feed pieces through at a fixed RPM and measure exit velocity or landing point across many shots.
3. Increase compression until added squeeze stops buying you more speed and starts hurting consistency or spiking current draw. That plateau is your operating point.

Design the mechanism so compression is adjustable — slotted hood mounts, spacer shims behind the wheels, or an adjustable backplate. You will change it during prototyping, and a shooter you can't re-gap is a shooter you can't tune.

Wheel durometer and material

Compliant "squishy" wheels grip better and tolerate piece-to-piece variation, which is why they dominate shooters and intakes. AndyMark's compliant wheels, for example, come in four durometers identified by color — 35A (green, softest and grippiest), 40A (orange), 50A (blue), and 60A (black, firmest) — in 2, 2.25, 3, and 4 inch diameters, with the harder wheels rated for higher RPM (roughly 3500 up to 9000 RPM across the range, per AndyMark's product listing).

The general pattern teams use: softer wheels (35A–40A) for intakes and low-speed rollers where grip and forgiveness matter most, and firmer wheels (50A–60A) for high-RPM shooters where you need the wheel to hold its shape at speed and survive the RPM. A soft wheel at shooter speeds can balloon outward from centrifugal force and wear quickly. Always check the manufacturer's max RPM rating for the exact wheel before you spin it that fast.

Flywheel inertia and RPM recovery

Here's the part that separates a shooter that can fire rapidly from one that needs a long pause between shots. When the game piece hits the wheel, it steals energy from the flywheel and the wheel slows down — this is the "dip." How far it dips, and how fast it climbs back to setpoint (recovery time), governs your fire rate and your shot-to-shot consistency.

The energy stored in a spinning flywheel is:

E = ½ · I · ω²

where I is the moment of inertia and ω is angular velocity. For a solid disk, I = ½·m·r²; for a hoop with mass concentrated at the rim, I = m·r². The key insight: inertia scales with the square of the radius and directly with mass, so mass out at the rim matters far more than mass near the hub.

The design tension:

• More inertia (heavier, larger-radius wheel) means the wheel barely slows when it hits a piece — the RPM dip is small, so back-to-back shots stay consistent even before the motor fully recovers. The cost: it takes longer to spin up to speed initially, and longer for the motor to recover after a big dip, because the motor has more inertia to re-accelerate.
• Less inertia (light wheel) spins up fast and recovers quickly if the motor has torque to spare, but each shot causes a bigger dip, so if you fire before the motor catches up, your second shot is slower than your first.

Many strong shooters deliberately add a dedicated inertia mass — a machined aluminum or steel disk on the shooter shaft — to buy dip resistance without relying on the wheel itself. You size it so a single shot removes only a small, predictable fraction of the stored energy, keeping the RPM dip within a few percent. Because E scales with ω², a wheel spinning faster also stores disproportionately more energy, which is another reason high-RPM shooters tend to be more shot-tolerant.

The real recovery equation couples the flywheel inertia to the motor's torque curve, so this is exactly the kind of system worth simulating. WPILib provides a FlywheelSim model and a dedicated tuning-a-flywheel walkthrough so you can predict spin-up and recovery before you cut metal.

Backspin, hood geometry, and why your shots hook

Backspin — the piece rotating so its top surface moves backward relative to flight — does three useful things:

1. Flattens and stabilizes the trajectory via the Magnus effect: backspin generates aerodynamic lift, so the piece drops less than a pure parabola would predict, extending effective range and making the arc more repeatable.
2. Softens bounces. A backspun piece that hits a rim or backboard tends to kill its forward energy and drop, rather than bouncing away — much more forgiving of an imperfect shot.
3. Reduces sensitivity to small speed errors, because the lift partially compensates as speed varies.

A single flywheel produces backspin automatically. A dual flywheel produces backspin only if you make the two wheels spin at different surface speeds — typically the wheel on the "top" of the piece runs faster, so the top surface gets dragged backward. This is a genuine advantage of the dual architecture: spin becomes an independent, tunable output.

Hood geometry sets the launch angle and how long the piece stays in contact with the wheel (the wrap angle). A longer, curved hood keeps the piece pressed against the wheel through more of the wheel's rotation, giving more time to transfer energy and spin — but too much wrap adds friction and can bog the shot. The exit angle where the hood ends determines your launch angle. Two ways to handle range:

• Fixed hood: simplest and most robust. You shoot from known distances and vary RPM to change range. Great for a shooter that fires from a small number of set positions.
• Adjustable/variable hood: a servo or motor pivots the hood to change launch angle on the fly, letting you shoot accurately across a continuous range of distances. More mechanism and more to tune, but far more flexible. Combined with vision-based distance measurement, an adjustable hood plus variable RPM is how teams shoot accurately from many positions.

Choosing the motor, gearing, and current limit

A shooter is a speed machine, not a torque machine — you spend energy getting the wheel up to a high RPM and keeping it there against the periodic load of each shot. That shapes every motor and gearing choice.

Verified motor specs

These are the brushless motors you'll actually pick between, with figures from the manufacturers' own data:

NEO figures are REV's empirical values measured with a SPARK MAX (from the REV NEO datasheet); REV also publishes higher theoretical numbers (150 A stall, 3.75 N·m), but the empirical set is what to design around. Kraken figures are from WestCoast Products' Kraken X60 documentation, which lists both trapezoidal and field-oriented-control (FOC) commutation modes — FOC extracts more torque at the cost of more current. Falcon 500 figures are from VEX/CTR's published spec. For a fuller comparison of these three, see our NEO vs. Kraken vs. Falcon breakdown.

For a shooter, the free speed and the shape of the torque curve matter most. A single NEO or Kraken directly geared to a 4-inch wheel already gives you tens of meters per second of surface speed — often you'll gear down modestly or run direct, not gear up.

Sizing the gear ratio

Work backward from your target surface speed:

1. Convert target exit velocity to required surface speed (×2 for a single flywheel, ×1 for dual).
2. Convert surface speed to wheel RPM: RPM = v_surface / (2π·r) · 60.
3. Set the gear ratio so the motor's free speed, reduced by real-world load (you never hit free speed under load — plan for the wheel to run meaningfully slower than the free-speed math), lands your wheel above the target RPM with margin.

Because you want the wheel spinning fast, shooter reductions are gentle — often close to 1:1, sometimes a slight reduction to trade a bit of top speed for quicker spin-up and stiffer speed-holding. If any of the ratio math is unfamiliar, our gear ratios explainer covers reductions, tooth counts, and how free speed maps through a gearbox.

Current limiting: protect the wheel and the battery

Spinning up a flywheel from rest pulls a large current surge because the motor is near stall at zero speed. A stalled Kraken can theoretically draw hundreds of amps — far more than a single 40 A breaker allows continuously — so you must set a current limit in the motor controller (SPARK MAX/Flex, Talon FX). Current limiting:

• Keeps you from tripping the 40 A main breakers or browning out the robot on spin-up.
• Protects the motor from thermal damage during repeated spin-ups.
• Trades a slightly slower spin-up for a robot that doesn't sag its bus voltage every time the shooter winds up.

A common approach is a higher stator current limit for the brief spin-up and a lower supply limit to protect the breaker, tuned so the flywheel reaches speed quickly without dragging battery voltage down. Because a sagging battery directly lowers your achievable RPM (and therefore your shot distance), shooter consistency is tied to battery health — a tired pack shoots short. Our battery guide explains internal resistance and voltage sag, and you can sanity-check whether your shooter plus drivetrain fit inside the breaker budget with the current draw calculator.

Tuning for consistency

Once the hardware is built, consistency is a control problem: hold the flywheel at a commanded velocity and recover fast after each shot. The proven approach in WPILib:

1. Feedforward first. Characterize the shooter to find your feedforward gains — kS (voltage to overcome static friction), kV (voltage per unit velocity), and kA (voltage per unit acceleration) — and use SimpleMotorFeedforward to command most of the voltage directly from the target speed. Feedforward alone gets a well-built flywheel most of the way to setpoint, per the WPILib feedforward docs.
2. Add feedback for the dip. Layer a feedback controller on top to close the gap and recover after each shot. WPILib specifically recommends bang-bang control for flywheels: because a flywheel stops accelerating the instant you stop applying voltage, overshoot is rare, so an aggressive asymmetric controller (full power when below setpoint, nothing when at or above it) drives the fastest recovery without the destructive oscillation a hot P-gain would cause. See the bang-bang controller docs. A modest P term combined with feedforward is a common alternative.
3. Wait for "at speed" before feeding. Gate the feeder so the game piece only enters when the flywheel velocity is within a tight tolerance of setpoint. Firing mid-dip is the number-one cause of a "good" shooter that randomly misses.

General PID intuition — proportional, integral, derivative, and how to tune without inducing oscillation — carries over directly; our PID tuning guide is a good companion if the feedback side is new to you.

Beyond the control loop, the biggest consistency wins are mechanical: keep compression constant (rigid, repeatable hood mounting), keep wheels from wearing unevenly, keep battery voltage up, and measure landing points, not just RPM. A shooter that reads the right RPM but lands in a different spot every time usually has a mechanical variable — inconsistent piece feeding, a flexing hood, or slipping compression — hiding behind a good-looking number.

Frequently asked questions

Single flywheel or dual flywheel — which should a rookie team build?

Start with whichever matches your packaging and control goals. A single flywheel with a fixed hood is simpler, more compact, and gives you backspin automatically — an excellent first shooter if you fire from a few set distances. Go dual flywheel when you need higher exit speed at a given RPM, faster recovery from firing both sides, or independent control of backspin. Whichever you pick, make compression and hood angle adjustable and prototype early; the architecture matters less than your ability to tune it.

How much compression should I use?

There is no universal number because it depends on the game piece's material and size and your wheel durometer. Start modest (often ½ to 1 inch for a large foam or rubber ball), then increase compression while measuring exit velocity and consistency until more squeeze stops adding speed and starts spiking current or hurting shot-to-shot repeatability. That plateau is your operating point. Build the mechanism so you can re-gap it easily — you will change this value many times.

Why does my shooter's second shot go shorter than the first?

Your flywheel is dipping in RPM when the piece hits it and hasn't recovered before you fire again. Fix it three ways: add rotational inertia (a heavier or larger-radius flywheel, or a dedicated inertia disk) so each shot removes a smaller fraction of stored energy; improve recovery with feedforward plus a bang-bang or P controller; and gate the feeder so a piece only enters when the wheel is back within tolerance of setpoint. Also confirm your battery isn't sagging — a weak pack lowers achievable RPM and shoots short.

What RPM and wheel size do I need?

Work backward from your target exit velocity. Convert it to required surface speed (multiply by ~2 for a single flywheel, ~1 for dual), then to wheel RPM with RPM = v_surface / (2π·r) · 60. A larger wheel needs fewer RPM for the same surface speed and stores more energy, but takes up more room and is heavier to spin up. Most shooters land on 3–4 inch compliant wheels running a few thousand RPM, geared close to 1:1 off a NEO, Falcon, or Kraken. Always verify your chosen wheel's max-RPM rating before spinning it that fast.

Do I really need backspin?

For most arcing shots into an elevated target, yes — backspin generates lift (the Magnus effect) that flattens and stabilizes the trajectory, extends effective range, and makes bounces off a rim or backboard much more forgiving. A single flywheel gives it to you for free; a dual flywheel gives it only when you run the two wheels at different speeds, which is also how you tune it. The main case for minimal spin is a very flat, direct shot where you want maximum speed and a straight line rather than a lofted, forgiving arc.