- Difficulty
- Advanced
- Build time
- 150-220 min
- Estimated cost
- $0-$30
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The foot traces a repeatable closed loop for ten crank turns, clears the floor during return, and avoids link collisions through the complete cycle.
Learning goals
- Identify how rotation of one crank pivot produces closed foot trajectory.
- Construct and explain a rotary-to-complex planar path system.
- Measure how the crank radius within a safe planned range changes performance.
- Diagnose losses caused by many pivot joints and link flex.
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 measured cardboard strips with color-coded holes for a large zero-load geometry model.
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.
- Operate as an unloaded tabletop module; do not build a ride-on machine or place weight on the leg.
Orient the build
Place the build so rotation of one crank pivot is on your left and closed foot trajectory 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
Choose and label dimensions
Prepare every link from one verified proportion set and measure center-to-center holes.
Group matching pairs before assembly.
Step 2
Build the ground frame
Reinforce the crank and fixed-pivot locations on two parallel walls.
Keep all pivot axes parallel.
Step 3
Install the crank link
Mount the measured crank radius and add a large slow handle.
Mark the zero-degree position.
Builder checkpoint: After install the crank link, the first subassembly should stay aligned when handled gently.
Step 4
Assemble the upper triangle
Join the first three links between fixed pivot, crank, and knee node.
Place each link in its planned depth layer.
Watch for: If this stage binds or drifts, inspect frame deflection before adding more parts.
Step 5
Assemble the lower chain
Connect knee, ankle, and foot links using the labeled hole order.
Check left-right orientation against the diagram.
Step 6
Join both subassemblies
Connect remaining cross-links without forcing any pivot.
Rotate through 30-degree increments and inspect clearance.
Builder checkpoint: After join both subassemblies, operate the build slowly and confirm that closed foot trajectory begins without binding.
Step 7
Add the tracing foot
Mount a blunt marker or pointer at the foot node over paper.
Keep the frame fixed while the marker barely touches.
Step 8
Trace and tune one cycle
Turn slowly through 360 degrees and close the path.
Fix collisions or wrong link planes before repeating ten cycles.
Builder checkpoint: At the final checkpoint, The foot traces a repeatable closed loop for ten crank turns, clears the floor during return, and avoids link collisions through the complete cycle.
See the engineering
Why it works
- Input
- rotation of one crank pivot
- Output
- closed foot trajectory
- Motion
- rotary-to-complex planar path
- Energy losses
- many pivot joints, link flex, plane interference, frame deflection
Why this works
Multi-link coupler path
Fixed link proportions and pivot locations constrain the endpoint to a repeating curve. Part of that curve approximates a level stance phase while the remainder lifts the foot for return.
Look for: Attach a marker to the foot and trace one full cycle without letting the leg support body weight.
Where the energy goes
Efficiency and losses
The ideal model leaves out many pivot joints, link flex, plane interference, frame deflection. 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 many pivot joints becomes visible or audible.
Math bite
Scale the linkage
Formula: new length = scale factor × reference length
- Reference link = 80 mm
- Scale factor = 1.25
Substitute: new length = 1.25 × 80 = 100 mm
Result: Every link and fixed-pivot spacing must scale by the same factor.
Uniform scaling preserves ideal path shape while changing path size.
Hole clearance and beam thickness do not scale perfectly.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Rotate the unloaded linkage one degree-controlled cycle by hand.
Success looks like: The foot closes a repeatable loop and every joint clears neighboring links.
Measure: Foot-path width, height, and stance-section flatness.
Change: the crank radius within a safe planned range
Keep constant: all other link lengths, pivot positions, frame, marker, and turn direction
- reference crank radius
- slightly shorter crank
- slightly longer crank after clearance check
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The linkage cannot complete a cycle | A link length, hole, or assembly branch is wrong | Stop at the lock and compare labels | Correct geometry rather than forcing pivots |
| Links collide | Moving members share the same depth plane | Turn through the collision slowly | Add spacers and assign separate planes |
| The foot path does not close | A pivot slips or the frame moves | Mark ground pivots and repeat | Brace frame and secure all collars |
| The foot drags on return | Link proportions or crank phase are wrong | Trace the full path without floor contact | Verify dimensions and zero orientation |
Choose your tradeoff
Preserve the chosen proportion set before experimenting. Small hole errors accumulate across many links, so improve pivot accuracy and frame stiffness before changing geometry.
Keep experimenting
Try another version
Paper path model
Build a flat cardboard linkage with no load.
Mirrored leg pair
Phase a second leg 180 degrees away for smoother support timing.
Path comparison
Digitize foot positions every 15 degrees and compare two proportions.
Build together
Classroom and access options
Classroom version
Teams can compare the crank radius within a safe planned range while keeping all other link lengths, pivot positions, frame, marker, and turn direction. 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.
- Color-code every link pair and use a numbered assembly map with pre-sorted pins.
Reflect on the design
- How did the crank radius within a safe planned range change the measured result?
- Where did many pivot joints affect the build most strongly?
- What evidence shows that multi-link coupler path explains the motion?
- Which change would improve closed foot trajectory without creating a new problem?
Glossary
- Multi-link coupler path
- Fixed link proportions and pivot locations constrain the endpoint to a repeating curve.
- Input
- The action or energy supplied to a system; here it is rotation of one crank pivot.
- Output
- The useful response produced by a system; here it is closed foot trajectory.
- 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 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.