- Difficulty
- Beginner
- Build time
- 35-55 min
- Estimated cost
- $0-$5
- Age range
- 10-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The sealed racer rolls at least 1 metre along a clear lane, keeps all internal parts contained, and repeats three runs without the band detaching.
Learning goals
- Identify how pre-rolling that twists an internal rubber band produces self-driven can rotation.
- Construct and explain a stored elastic twist-to-rolling translation system.
- Measure how the preload turn count changes performance.
- Diagnose losses caused by axle-hole friction and internal mass rubbing.
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
- Masking tape
Low-cost swaps
- Use clean recycled packaging whenever it has similar stiffness.
- Substitute paper clips, binder clips, or twist ties for specialty fasteners.
- Use a sturdy lidded food container with both ends fully closed and no sharp metal edges.
Project-specific safety
- Wear eye protection when stretched elastic, magnets, or spinning parts are present.
- Test at floor or tabletop height and keep the path clear of people.
- Use a non-glass container, fully seal both ends, wear eye protection, and keep the lane clear of people.
Orient the build
Place the build so pre-rolling that twists an internal rubber band is on your left and self-driven can 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
Inspect the container
Confirm both ends close securely and no metal edge is exposed.
Reject cracked or sharp containers.
Step 2
Mark axle centers
Find and mark the center of both circular ends.
An adult should make small reinforced holes.
Step 3
Prepare the internal mass
Tie two washers into a padded bundle below 40 grams.
Attach the bundle to the rubber-band midpoint so it hangs freely.
Builder checkpoint: After prepare the internal mass, the first subassembly should stay aligned when handled gently.
Step 4
Thread the band
Pass one band end through each center hole and anchor it to an outside dowel.
Keep the mass centered inside.
Watch for: If this stage binds or drifts, inspect band hysteresis before adding more parts.
Step 5
Seal both ends
Close lids, tape every seam, and cover external anchors.
Shake gently to confirm containment.
Step 6
Run a zero-preload roll
Place on a lane and confirm the shell rolls without severe wobble.
Listen for the mass hitting a wall.
Builder checkpoint: After run a zero-preload roll, operate the build slowly and confirm that self-driven can rotation begins without binding.
Step 7
Store the energy
Roll the can backward through three measured turns while holding the lane.
Set it down without adding a push.
Step 8
Compare three trials
Test three, five, and seven safe preload turns.
Record distance, direction, and internal sounds.
Builder checkpoint: At the final checkpoint, The sealed racer rolls at least 1 metre along a clear lane, keeps all internal parts contained, and repeats three runs without the band detaching.
See the engineering
Why it works
- Input
- pre-rolling that twists an internal rubber band
- Output
- self-driven can rotation
- Motion
- stored elastic twist-to-rolling translation
- Energy losses
- axle-hole friction, internal mass rubbing, shell slip, band hysteresis
Why this works
Internal elastic drive
Rolling the shell twists a rubber band around an off-axis internal mass. When released, the band applies torque to the container while the mass resists rotating with it.
Look for: Listen for internal rubbing and compare travel after different numbers of preload turns.
Where the energy goes
Efficiency and losses
The ideal model leaves out axle-hole friction, internal mass rubbing, shell slip, band hysteresis. 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-hole friction becomes visible or audible.
Math bite
Find rolling distance
Formula: distance = turns × π × diameter
- Can diameter = 0.10 m
- Can turns = 5
Substitute: distance = 5 × 3.14 × 0.10 = 1.57 m
Result: Five shell rotations correspond to about 1.57 metres without slip.
Stored energy may produce fewer or more visible rotations after release depending on preload.
Floor slip and internal rubbing change travel.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Roll the sealed can once by hand before adding stored twist.
Success looks like: It travels at least 1 metre and all parts remain securely contained.
Measure: Distance, shell rotations, path drift, and internal impacts.
Change: the preload turn count
Keep constant: can, band, mass, surface, lane, and release method
- 3 turns
- 5 turns
- 7 turns if the band remains safe
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The can does not move | Band is not twisting or mass rotates with shell | Open only after securing the can and inspect | Retie the mass off-axis and band midpoint |
| It wobbles badly | Holes are off-center or mass hits a wall | Roll with no preload | Recenter holes and shorten the mass hanger |
| Travel is short | Internal rubbing or shell slip is high | Listen during release | Center the mass and choose a grippier floor |
| A lid loosens | Tape or fit is inadequate | Inspect after every low-turn trial | Stop and reseal before any further use |
Choose your tradeoff
Center the axle holes and keep the internal mass clear before adding more preload. Extra turns can increase travel only until band friction, shell slip, or safety limits dominate.
Keep experimenting
Try another version
Observation window model
Use a transparent safe container to watch the mass.
Mass comparison
Test two contained masses under 40 grams.
Target race
Tune preload to stop nearest a marked line.
Build together
Classroom and access options
Classroom version
Teams can compare the preload turn count while keeping can, band, mass, surface, lane, and release method. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Offer pre-cut parts and tactile or high-contrast measurement marks.
- Split roles so one builder can hold, another assemble, and another measure.
- Use a large container, a start cradle, and tactile preload marks around the rim.
Reflect on the design
- How did the preload turn count change the measured result?
- Where did axle-hole friction affect the build most strongly?
- What evidence shows that internal elastic drive explains the motion?
- Which change would improve self-driven can rotation without creating a new problem?
Glossary
- Internal elastic drive
- Rolling the shell twists a rubber band around an off-axis internal mass.
- Input
- The action or energy supplied to a system; here it is pre-rolling that twists an internal rubber band.
- Output
- The useful response produced by a system; here it is self-driven can rotation.
- 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
A familiar household engineering activity implemented with original instructions and controlled tests.
- Classroom engineering basis: A common educational challenge implemented with original dimensions, tests, diagrams, and instructions.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.
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