- Difficulty
- Intermediate
- Build time
- 90-140 min
- Estimated cost
- $0-$8
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The wheel completes ten turns without rubbing the frame, six gondolas stay upright, and the axle shows no visible permanent bend.
Learning goals
- Identify how hand-crank rotation produces rotating rim with gravity-oriented gondolas.
- Construct and explain a rotary-to-rotary with free pendulums system.
- Measure how the gondola pivot height changes performance.
- Diagnose losses caused by axle friction and rim wobble.
Before you build
Materials, tools, and safety
Reuse-material cost: $0-$3 with reused materials. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Pencil
- Scissors
- Low-temperature glue gun or tape
- Compass or string circle guide
- Adult-operated craft knife
Low-cost swaps
- Use clean shipping-box cardboard instead of buying sheets.
- Replace hot glue with strong tape and folded tabs.
- Use one solid disk per side when cutting ring centers is difficult.
Project-specific safety
- An adult should handle craft knives and make difficult starter cuts.
- Let hot glue cool before pressing a joint or testing moving parts.
- Turn slowly, keep gondolas empty or loaded only with paper, and keep fingers away from axle bearings.
Orient the build
Place the build so hand-crank rotation is on your left and rotating rim with gravity-oriented gondolas 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
Cut matched wheel sides
Trace two equal 35 cm circles and mark their centers before cutting.
Laminate weak areas across corrugation.
Step 2
Build straight spokes
Attach four crossed spokes to each side with equal angular spacing.
Keep hubs centered using a template.
Step 3
Join the wheel halves
Space both sides 8 cm apart with equal rim connectors.
Check the assembly against a flat table for wobble.
Builder checkpoint: After join the wheel halves, the first subassembly should stay aligned when handled gently.
Step 4
Build the support towers
Brace two A-frame walls around a straight axle at equal height.
Add a wide base and diagonal legs.
Watch for: If this stage binds or drifts, inspect frame flex before adding more parts.
Step 5
Install and center the wheel
Slide the axle through both hubs and bearings.
Use collars to keep equal side clearance.
Step 6
Make equal gondolas
Fold six same-size boxes and place pivot holes above their center of mass.
Check each hangs upright by hand.
Builder checkpoint: After make equal gondolas, operate the build slowly and confirm that rotating rim with gravity-oriented gondolas begins without binding.
Step 7
Space and attach gondolas
Mark six equal rim positions and install free pivots between wheel sides.
Confirm boxes clear spokes and supports.
Step 8
Balance and test
Turn one revolution slowly, correct heavy spots, then run ten hand turns.
Pause at three angles and inspect gondola orientation.
Builder checkpoint: At the final checkpoint, The wheel completes ten turns without rubbing the frame, six gondolas stay upright, and the axle shows no visible permanent bend.
See the engineering
Why it works
- Input
- hand-crank rotation
- Output
- rotating rim with gravity-oriented gondolas
- Motion
- rotary-to-rotary with free pendulums
- Energy losses
- axle friction, rim wobble, unequal gondola mass, frame flex
Why this works
Hanging center of mass
Each gondola pivots freely and its center of mass stays below the pivot, so gravity restores it toward upright even while the attachment point travels around the wheel.
Look for: Pause at several wheel angles and watch each gondola settle with its floor below the pivot.
Where the energy goes
Efficiency and losses
The ideal model leaves out axle friction, rim wobble, unequal gondola mass, frame flex. 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 axle friction becomes visible or audible.
Math bite
Space six gondolas
Formula: angular spacing = 360° / count
- Full circle = 360°
- Gondolas = 6
Substitute: spacing = 360°/6 = 60°
Result: Attach one gondola every 60 degrees.
Equal spacing supports balance when gondola masses also match.
Cardboard thickness and glue may shift real mass distribution.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Turn one unloaded wheel revolution over ten seconds.
Success looks like: All gondolas clear the frame, hang upright, and the wheel completes ten smooth turns.
Measure: Rim runout, crank force, and gondola settling angle.
Change: the gondola pivot height
Keep constant: wheel, masses, spacing, axle, frame, and crank rate
- low pivot
- pivot just above center of mass
- higher pivot
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The wheel stops at one position | Mass is unbalanced | Release from several angles and mark the low side | Trim or counterbalance the heavy area |
| Gondolas tip sideways | Pivot holes differ or axle is not level | Hang each box separately | Match holes and level the support |
| The rim wobbles | Wheel sides are warped or hubs off-center | Rotate and view edge-on | Rebrace rim and recenter hubs |
| The axle bends | Wheel is heavy or bearing span wide | Observe axle at rest | Lighten wheel and move supports closer |
Choose your tradeoff
A lighter, balanced wheel turns better than a heavily reinforced but uneven one. Add stiffness symmetrically and keep gondola pivots free rather than tightening away wobble.
Keep experimenting
Try another version
Four gondolas
Use a smaller wheel and four boxes at 90 degrees.
Loading station
Add a gate aligned with the lowest gondola.
Drive ratio
Add a small hand-crank pulley and belt to slow the wheel.
Build together
Classroom and access options
Classroom version
Teams can compare the gondola pivot height while keeping wheel, masses, spacing, axle, frame, and crank rate. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Pre-cut repeated pieces and mark fold lines with high-contrast ink.
- Use large tabs, binder clips, and tape for easier one-handed assembly.
- Use large numbered gondolas and a crank with a wide grip.
Reflect on the design
- How did the gondola pivot height change the measured result?
- Where did axle friction affect the build most strongly?
- What evidence shows that hanging center of mass explains the motion?
- Which change would improve rotating rim with gravity-oriented gondolas without creating a new problem?
Glossary
- Hanging center of mass
- Each gondola pivots freely and its center of mass stays below the pivot, so gravity restores it toward upright even while the attachment point travels around the wheel.
- Input
- The action or energy supplied to a system; here it is hand-crank rotation.
- Output
- The useful response produced by a system; here it is rotating rim with gravity-oriented gondolas.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
An original BrickLabClips cardboard machine with dimensionally specified construction.
- Cardboard design verification: Dimensions, fold allowances, repeated-motion joints, and likely load paths received an editorial geometry review.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.