- Difficulty
- Advanced
- Build time
- 120-180 min
- Estimated cost
- $0-$25
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The wheel moves through at least 30 mm of controlled travel, returns after compression, and changes the opposite side by less than 5 mm.
Learning goals
- Identify how vertical force at one wheel produces guided wheel travel and spring deflection.
- Construct and explain a linear force-to-constrained arc motion system.
- Measure how the upper-arm length changes performance.
- Diagnose losses caused by pivot friction and spring hysteresis.
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 craft-stick arms, paper-fastener pivots, and a rubber-band spring on a cardboard chassis wall.
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 a lightly loaded wheel and shield stretched elastic from faces during compression tests.
Orient the build
Place the build so vertical force at one wheel is on your left and guided wheel travel and spring deflection 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 the chassis wall
Brace a vertical panel on a wide base and mark four pivot locations.
Check the wall stays perpendicular.
Step 2
Install lower arms
Mount a matched arm pair in separate planes around the wheel location.
Keep inner pivots coaxial front to back.
Step 3
Install upper arms
Add a shorter or equal matched pair above the lowers.
Preserve free pivots and planned spacing.
Builder checkpoint: After install upper arms, the first subassembly should stay aligned when handled gently.
Step 4
Build the wheel carrier
Join upper and lower outer pivots with a stiff upright.
Mount a free wheel centered on the carrier.
Watch for: If this stage binds or drifts, inspect tire rubbing before adding more parts.
Step 5
Add the spring element
Connect a light elastic or spring between lower arm and chassis.
Set moderate preload at ride height.
Step 6
Add travel stops
Limit droop and compression before arms or wheel hit the chassis.
Mark the ride-height position.
Builder checkpoint: After add travel stops, operate the build slowly and confirm that guided wheel travel and spring deflection begins without binding.
Step 7
Measure the path
Move wheel in 10 mm steps and record vertical position and camber line angle.
Release after each point.
Step 8
Check independence
Build or hold a reference wheel on the other side and compress only this module.
Measure unintended opposite movement.
Builder checkpoint: At the final checkpoint, The wheel moves through at least 30 mm of controlled travel, returns after compression, and changes the opposite side by less than 5 mm.
See the engineering
Why it works
- Input
- vertical force at one wheel
- Output
- guided wheel travel and spring deflection
- Motion
- linear force-to-constrained arc motion
- Energy losses
- pivot friction, spring hysteresis, arm flex, tire rubbing
Why this works
Double-control-arm geometry
Upper and lower arms locate the wheel carrier through two arcs. Their lengths and pivot spacing shape vertical travel and camber while a spring stores energy and supports the chassis.
Look for: Place a vertical line on the wheel and measure its tilt at full extension, ride height, and compression.
Where the energy goes
Efficiency and losses
The ideal model leaves out pivot friction, spring hysteresis, arm flex, tire rubbing. 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 pivot friction becomes visible or audible.
Math bite
Estimate spring rate
Formula: spring rate k = force / deflection
- Added force = 2 N
- Spring deflection = 20 mm = 0.02 m
Substitute: k = 2/0.02 = 100 N/m
Result: The spring rate is about 100 newtons per metre in this range.
A stiffer spring deflects less for the same load.
Elastic elements may be nonlinear and show hysteresis.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Compress the unloaded wheel 10 mm and release.
Success looks like: The carrier moves without binding, returns near ride height, and the chassis wall remains fixed.
Measure: Wheel travel, return error, and camber angle.
Change: the upper-arm length
Keep constant: lower arms, spring, chassis, wheel, pivot spacing, and test force
- equal arm length
- slightly shorter upper arm
- same geometry with different spring anchor
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The carrier binds | Front and rear arm pivots are not coaxial | Disconnect spring and move by hand | Realign pivot axes and spacer planes |
| The wheel tilts excessively | Arm lengths or inner spacing create large camber gain | Measure at three positions | Adjust upper-arm length or pivot height |
| The wheel does not return | Spring force is low or pivots rub | Lift by hand and test each joint | Reduce friction and adjust preload |
| The chassis wall bends | Spring load lacks bracing | Watch wall during compression | Triangulate wall to base and reduce load |
Choose your tradeoff
Change geometry separately from spring settings. Arm proportions control the wheel path; spring anchor and preload control support. Tight pivots may hide useful motion under friction.
Keep experimenting
Try another version
No-spring geometry
Move the carrier by hand and plot its path.
Adjustable spring anchor
Compare wheel rate at two leverage positions.
Roll-center sketch
Extend arm lines and estimate the instant center.
Build together
Classroom and access options
Classroom version
Teams can compare the upper-arm length while keeping lower arms, spring, chassis, wheel, pivot spacing, and test force. 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 a large wheel handle and high-contrast extension, ride-height, and compression marks.
Reflect on the design
- How did the upper-arm length change the measured result?
- Where did pivot friction affect the build most strongly?
- What evidence shows that double-control-arm geometry explains the motion?
- Which change would improve guided wheel travel and spring deflection without creating a new problem?
Glossary
- Double-control-arm geometry
- Upper and lower arms locate the wheel carrier through two arcs.
- Input
- The action or energy supplied to a system; here it is vertical force at one wheel.
- Output
- The useful response produced by a system; here it is guided wheel travel and spring deflection.
- 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.