Build accurate, repeatable flywheel systems with better compression, spin-up, consistency, and velocity control. If it misses, there is a reason — and it is fixable.
// Section 01
How Flywheel Shooters Work
A flywheel shooter uses high-speed spinning wheels to accelerate a game piece and launch it. The flywheel stores rotational kinetic energy and transfers it to the piece in milliseconds.
When a game piece enters the flywheel gap, it contacts the spinning wheel and is accelerated by friction.
The wheel imparts its tangential velocity to the game piece over the contact arc
Longer contact arc = more energy transfer = higher exit velocity (up to a limit)
Too short an arc: inconsistent shots. Too long: the piece gets chewed up or jams
Double flywheel adds a second wheel on the opposite side — more consistent backspin for curved shots
🎯 Rule of Thumb
Consistency beats peak performance. A flywheel that puts 9 of 10 shots on target is worth more than one that sometimes makes incredible shots but misses 4 of 10. Design for repeatable energy transfer, not maximum power. Spin-up speed and shot consistency are the two numbers that determine your scoring rate.
The Three Variables That Determine Shot Outcome
Exit velocity: Set by flywheel RPM at the moment of contact. Varies if the wheel has not recovered from the previous shot (spin-down).
Exit angle: Set by your hood or backplate geometry. Fixed at build time — changes require physical adjustment.
Spin on the piece: Backspin from a single flywheel, topspin from a double flywheel. Affects trajectory arc and where the piece lands relative to where it hits.
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// Section 02
Single vs Double Flywheel
Two fundamentally different architectures. The choice affects spin-up time, motor budget, shot spin, and how easy the system is to tune.
Single Flywheel
+ Simpler build — one set of wheels, one motor
+ Lower motor budget (1–2 motors)
+ Easier to tune compression
− Puts backspin on the piece — affects trajectory
− Hood required to guide exit angle
− Spin-down between shots if flywheel is light
Double Flywheel
+ Two wheels cancel spin — piece exits without rotation
+ More energy transferred — faster exit velocity at same RPM
+ More consistent trajectory
− 2–4 motors (more motor budget required)
− Harder to tune — two wheels must be matched in speed
− Heavier, more complex mounting
💡
Which to choose? If your motor budget is tight or the game piece is forgiving (large target, close range), start with a single flywheel. If the game requires long-range accuracy or the piece needs to enter a narrow target without spin, a double flywheel is worth the complexity.
Flywheel Mass and Spin-Up
Flywheel inertia directly affects shot consistency — heavier = more stored energy = less spin-down per shot.
High inertia: less spin-down, more consistent shots in rapid fire — but longer spin-up time
Low inertia: fast spin-up, recovers quickly between shots — but more speed variation per shot
In practice: most VRC teams prefer high inertia for consistency over raw fire rate
Add mass at the wheel's outer radius to maximize inertia with minimum added weight
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// Section 03
Compression & Hood Design
Compression determines how much energy gets transferred to the piece. The hood controls the exit angle and backspin characteristics. Both must be tuned together.
Compression
Flywheel compression is the gap between the spinning wheel and the backplate or hood — measured at the tightest point.
If compression is too tight: the game piece slows the wheel excessively or jams
If compression is too loose: the piece barely contacts the wheel and exits slowly and inconsistently
Starting point: set compression to ~70–80% of the game piece diameter, then tune by shot
Make the hood adjustable on your first build — you will need to tune it
Start wide, measure, then tighten. Begin with a gap slightly larger than the game piece diameter. Reduce 1–2mm at a time and measure shot consistency at each setting.
Adjustable gap is worth building. Use a slotted hole or a shim stack at the backplate mount. Being able to change compression without disassembly saves hours of tuning time.
Measure exit velocity, not just accuracy. At different compression settings, the piece exits at different speeds. Use a measuring tape to spot the range change — it tells you exactly where energy transfer improved.
Hood / Backplate
The hood is the curved or angled surface that constrains the game piece as it exits the flywheel. It controls exit angle (launch angle), backspin, and how consistently the piece leaves the wheel.
Material: A smooth, low-friction hood lets pieces exit cleanly. High-friction surface on the hood adds more backspin — useful if you want the piece to “grip” a goal lip.
Angle: A steeper hood means a higher launch angle. A shallower hood means a flatter trajectory. Build in adjustability — even a 5° angle change significantly changes where the piece lands.
Adjustable vs fixed: Start adjustable. Once you find the correct angle for your game and robot position, lock it and apply Loctite.
⚠️ Stop Building If…
×
Shot variance is high Every shot landing in a different spot means inconsistent compression or the flywheel is not at target speed when you fire. Fix the mechanical issue first.
×
Piece deforms on exit Compression too high. Reduce gap immediately — deformed pieces score unpredictably and can jam.
×
Flywheel speed drops after every shot Flywheel mass too low for your firing rate. Add mass or reduce rate.
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// Section 04
Velocity Control & Programming
A flywheel running at inconsistent RPM produces inconsistent shots. Velocity control is the code that keeps the flywheel at its target speed across an entire match.
Why Raw Motor Power Is Not Enough
Running the motor at 100% does not mean 100% speed — battery voltage and motor temperature both affect RPM.
Lower battery = less voltage = lower motor speed = shorter shot distance
Hot motors after extended use also run slower
Solution: use a velocity controller (TBH or PID) that targets RPM, not motor power %
A "ready" indicator (Brain screen color, LED, or controller buzz) tells the driver when to fire
Velocity Controllers for Flywheels
Bang-Bang controller: Full power when below target RPM, zero power when above. Simple, works well for flywheels because the large rotational inertia smooths the on/off switching. See the Bang-Bang guide.
TBH (Take Back Half) controller: One gain, converges smoothly to target velocity, less oscillation than bang-bang for continuous-fire applications. See the TBH guide.
PID: More tuning parameters but most precise. Useful when you need the flywheel to hold exact velocity across different battery states. Start with bang-bang and move to PID only if consistency is not sufficient.
Ready-to-Fire Detection
The driver should never guess whether the flywheel is ready — build in a "ready" signal.
Brain screen color change: green = at speed, red = spinning up
Controller rumble when the flywheel reaches target RPM
Add a minimum inter-shot delay: even at speed, firing too fast gives the wheel no recovery time
Document your optimal inter-shot delay in the notebook — it changes with game piece weight variations
✅
Log flywheel RPM data to SD card during practice sessions. You will see exactly how much the speed drops after each shot, how long recovery takes, and whether your battery state affects performance. See the Data Logging guide for implementation.
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// Section 05
Accuracy Tuning
Accuracy comes from isolating one variable at a time. If you change hood angle, compression, and flywheel speed in the same session, you cannot know what improved the shot.
Systematic Tuning Protocol
Fix a shooting position. Mark tape on the floor. All accuracy tests happen from the same spot. If you move, you are not comparing the same thing.
Fix flywheel speed first. Choose a target RPM and verify the velocity controller holds it consistently. Do not tune hood or compression until speed is stable.
Tune compression. At fixed speed and position, adjust compression until the average shot lands closest to target. Record: setting, average landing, variance.
Tune hood angle. At fixed speed and compression, adjust hood in 2° increments. Shoot 5 times per setting. Record landing position average.
Test at different distances. Once tuned at one distance, test at match-relevant distances. Only change flywheel speed to adjust range — do not retune hood and compression for each distance if avoidable.
Sources of Variance (Troubleshooting)
Shot-to-shot speed variance: Velocity controller not converged, or firing before spin-up is complete
Shots drifting left/right: Flywheel not perpendicular to shot direction; piece entering asymmetrically; sidespin from misaligned feed
Shots drifting long/short over a match: Battery voltage dropping — use velocity control, not open-loop power
High variance on first shot, consistent after: Flywheel taking too long to reach target speed — spin up faster or wait for ready signal
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// Section 06
Testing Checklist & Notebook Evidence
Flywheel accuracy is meaningless without documented testing. This data also goes directly in your notebook — it is engineering evidence, not just practice.
🔬 Flywheel Testing Checklist
Spin-up time measured from 0 to target RPM
Time with stopwatch from motor enable to ready signal. Record seconds.
RPM recovery time after each shot
How long from shot to back-at-target. Should be <1s for most games.
10 consecutive shots from fixed position — record landing
Mark each landing point. Measure average distance from target and variance.
Compression setting documented with measurement
Gap in mm, measured with calipers. Record the final tuned setting explicitly.
Hood angle documented
Angle in degrees from horizontal. Photograph the final setting.
Accuracy at multiple match distances tested
Test from every position your robot will shoot from in a real match
Low-battery performance tested
Discharge battery to 50%, retest accuracy. Velocity control should hold.
Notebook Evidence
Tuning table: compression setting vs. average shot landing — shows systematic iteration
RPM data log showing velocity held across a simulated match (from SD card log)
Accuracy scatter plot — 10 shot landing positions plotted on a field diagram
Spin-up time measurement before and after adding flywheel mass — shows tradeoff analysis
You increase your flywheel RPM by 15% and accuracy at close range improves, but accuracy at medium range drops. Why?
Higher RPM causes motor temperature to increase, reducing torque mid-match
The encoder drift increases proportionally with flywheel speed
Higher surface speed changes the contact dynamics — more spin is applied to the game element, altering its flight path at longer distances
The compression gap widens at high RPM due to motor vibration
Correct. Flywheel speed affects how much spin the wheel imparts on the game element. More spin changes the flight arc — what works at one distance won't work at another. This is why testing at every scoring distance matters, and why your data table needs distance as a variable.
📝
Notebook color guide for this mechanism: ■ Purple — Decision matrix comparing this mechanism against alternatives. Include weighted criteria and selection conclusion. ■ Orange — CAD screenshot, build notes, and any design changes made during assembly. ■ Cyan — Test protocol with hypothesis, data table (n≥5), and conclusions. Include the failure mode you found and how you addressed it.
⚙ STEM Highlight
Physics: Rotational Energy & Compression Dynamics
A flywheel stores rotational kinetic energy — KE = ½Iω², where I is moment of inertia and ω is angular velocity. When a game element contacts the wheel, energy transfers through friction: the wheel decelerates slightly, the element accelerates. Compression (gap between wheel and hood) controls how much contact surface area is engaged. Too little compression = low energy transfer. Too much = the element deforms or jams. The sweet spot is where energy transfer is maximized without compromising exit consistency.
Recovery time after each shot (how fast the flywheel returns to target RPM) is determined by motor torque and the moment of inertia of the flywheel assembly — heavier flywheels store more energy but recover more slowly.
🎤 Interview line: "We tuned our flywheel by treating compression and RPM as two independent variables and testing them separately with a data table. Judges can see our test results — we found that optimal compression was more impactful than RPM at our primary scoring distance."