- Difficulty
- Beginner
- Build time
- 45-70 min
- Estimated cost
- $0-$12
- Age range
- 10-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The flywheel shaft runs without wobble, coasts measurably longer than the bare shaft, and reduces visible speed pulsing in a hand-cranked test.
Learning goals
- Identify how uneven hand-crank torque produces steadier shaft rotation.
- Construct and explain a rotary input-to-smoothed rotary output system.
- Measure how the radial position of equal masses changes performance.
- Diagnose losses caused by bearing friction and air drag.
Before you build
Materials, tools, and safety
Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Removable tape for motion marks
Low-cost swaps
- Use equivalent brick-compatible parts from any kit.
- Use cardboard beams and straw bearings for a larger demonstration model.
- Use two identical cardboard disks with equal coins taped symmetrically near the rim.
Project-specific safety
- Keep fingers, hair, and loose sleeves clear of moving parts.
- Turn the mechanism by hand; do not attach a high-speed motor.
- Use hand speed only, secure rim masses redundantly, and wear eye protection during coast tests.
Orient the build
Place the build so uneven hand-crank torque is on your left and steadier shaft rotation is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build a rigid bearing frame
Support one axle close to both sides of the planned flywheel.
Brace the frame against side motion.
Step 2
Balance the bare disk
Mount the disk at its center and let it settle from several positions.
Correct any heavy side before adding masses.
Step 3
Add symmetric rim masses
Place equal masses at opposite positions and secure each twice.
Recheck static balance.
Builder checkpoint: After add symmetric rim masses, the first subassembly should stay aligned when handled gently.
Step 4
Attach the crank and flag
Add a comfortable crank on one side and a short marker on the shaft.
Keep both clear of the wheel.
Watch for: If this stage binds or drifts, inspect frame vibration before adding more parts.
Step 5
Run the bare-shaft baseline
Remove the flywheel, give one measured quarter-turn push, and time the coast.
Repeat three times.
Step 6
Run the disk baseline
Install the balanced disk without rim masses and repeat the same push.
Record turns and time.
Builder checkpoint: After run the disk baseline, operate the build slowly and confirm that steadier shaft rotation begins without binding.
Step 7
Test rim mass
Install equal rim masses and repeat three trials.
Stop if wobble or frame vibration increases.
Step 8
Compare pulsed input
Turn with one gentle push per second and watch speed variation.
Compare the light and heavy configurations.
Builder checkpoint: At the final checkpoint, The flywheel shaft runs without wobble, coasts measurably longer than the bare shaft, and reduces visible speed pulsing in a hand-cranked test.
See the engineering
Why it works
- Input
- uneven hand-crank torque
- Output
- steadier shaft rotation
- Motion
- rotary input-to-smoothed rotary output
- Energy losses
- bearing friction, air drag, axle wobble, frame vibration
Why this works
Rotational energy storage
A rotating mass resists changes in speed. Mass placed farther from the axis raises rotational inertia strongly, so the flywheel absorbs and releases energy across each input pulse.
Look for: Give the same short crank push with and without the rim mass, then compare coast time and speed variation.
Where the energy goes
Efficiency and losses
The ideal model leaves out bearing friction, air drag, axle wobble, frame vibration. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where bearing friction becomes visible or audible.
Math bite
Compare rim-mass inertia
Formula: point-mass inertia I = m × r²
- Two masses total m = 0.04 kg
- Radius r = 0.06 m
Substitute: I = 0.04 × 0.06² = 0.000144 kg·m²
Result: Moving the same mass farther outward raises inertia by radius squared.
Higher inertia smooths speed but needs more torque to accelerate.
The disk's own distributed mass also adds inertia.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Give the bare shaft one marked quarter-turn push and release.
Success looks like: Every flywheel configuration remains balanced and the rim-mass version coasts longer on average.
Measure: Coast time and full rotations after the same input push.
Change: the radial position of equal masses
Keep constant: total mass, axle, frame, push angle, start mark, and timer
- no flywheel
- disk only
- equal masses near rim
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The wheel wobbles | Mass is off-center or axle bent | Let it settle and mark the low side | Rebalance and replace the axle |
| Coast time is shorter | Added bearing rubbing exceeds stored-energy benefit | Spin the shaft without the wheel | Restore collar clearance and alignment |
| A mass shifts | Attachment is not redundant | Mark each mass position before a trial | Stop and secure with two methods |
| Starting is unexpectedly hard | Inertia or bearing friction is high | Compare empty and loaded shaft starts | Reduce rim mass and inspect bearings |
Choose your tradeoff
Place equal masses symmetrically before moving them outward. More inertia improves smoothing and coast time but raises starting effort and the energy involved, so keep hand speed modest.
Keep experimenting
Try another version
Disk comparison
Compare a bare shaft with one cardboard disk.
Radius study
Keep mass constant and test three safe radii.
Energy estimate
Calculate rotational energy from estimated inertia and measured speed.
Build together
Classroom and access options
Classroom version
Teams can compare the radial position of equal masses while keeping total mass, axle, frame, push angle, start mark, and timer. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Use high-contrast tape to distinguish input and output parts.
- Replace a small crank with a wider handle for an easier grip.
- Add an audible paper clicker that counts rotations while the wheel coasts.
Reflect on the design
- How did the radial position of equal masses change the measured result?
- Where did bearing friction affect the build most strongly?
- What evidence shows that rotational energy storage explains the motion?
- Which change would improve steadier shaft rotation without creating a new problem?
Glossary
- Rotational energy storage
- A rotating mass resists changes in speed.
- Input
- The action or energy supplied to a system; here it is uneven hand-crank torque.
- Output
- The useful response produced by a system; here it is steadier shaft rotation.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
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Explore more guidesSources and build notes
An original BrickLabClips interpretation of a standard mechanical mechanism.
- Mechanism verification: Standard kinematics were checked for motion direction, constraint, clearance, and likely friction points.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.