- 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 rack moves freely, both wheels return near straight ahead, and the inner wheel angle exceeds the outer angle through left and right steering tests.
Learning goals
- Identify how rotation of a steering pinion produces unequal left and right wheel pivot angles.
- Construct and explain a rotary-to-linear-to-angular system.
- Measure how the steering-arm tie-rod hole changes performance.
- Diagnose losses caused by rack friction and tie-rod play.
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 a cardboard rack, bottle-cap front wheels, and paper-fastener kingpins on a flat chassis.
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 the model on a table or floor only; it is not a ride-on steering system.
Orient the build
Place the build so rotation of a steering pinion is on your left and unequal left and right wheel pivot angles 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 rectangle
Brace front and rear axle reference lines square to a centerline.
Mark the approximate rear-axle center.
Step 2
Mount the kingpins
Place left and right front pivots at equal distances from center.
Keep both kingpin axes vertical and parallel.
Step 3
Build steering knuckles
Attach free-turning wheels and inward-facing steering arms to each kingpin.
Match wheel and arm heights.
Builder checkpoint: After build steering knuckles, the first subassembly should stay aligned when handled gently.
Step 4
Angle the steering arms
Point imaginary arm lines toward the rear-axle center when wheels are straight.
Keep left and right geometry mirrored.
Watch for: If this stage binds or drifts, inspect tire scrub before adding more parts.
Step 5
Install the rack
Guide the rack across the chassis with a centered pinion and end stops.
Mark its straight-ahead position.
Step 6
Connect equal tie rods
Join rack ends to steering arms without preload at center.
Use free pivots in separate planes.
Builder checkpoint: After connect equal tie rods, operate the build slowly and confirm that unequal left and right wheel pivot angles begins without binding.
Step 7
Measure left steering
Move rack left in small steps and record both wheel angles.
Identify the inner wheel for that turn.
Step 8
Measure right steering
Repeat symmetrically to the right and roll the chassis through curves.
Tune tie-rod holes if behavior differs by side.
Builder checkpoint: At the final checkpoint, The rack moves freely, both wheels return near straight ahead, and the inner wheel angle exceeds the outer angle through left and right steering tests.
See the engineering
Why it works
- Input
- rotation of a steering pinion
- Output
- unequal left and right wheel pivot angles
- Motion
- rotary-to-linear-to-angular
- Energy losses
- rack friction, tie-rod play, kingpin friction, tire scrub
Why this works
Ackermann approximation
Steering arms angled toward the rear axle center make the inner wheel rotate farther than the outer wheel. Extended wheel axes then meet near one common turning center.
Look for: Place angle guides under both wheels and compare inner and outer steering angles at the same rack position.
Where the energy goes
Efficiency and losses
The ideal model leaves out rack friction, tie-rod play, kingpin friction, tire scrub. 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 rack friction becomes visible or audible.
Math bite
Compare ideal wheel angles
Formula: cot(outer) - cot(inner) = track / wheelbase
- Track = 120 mm
- Wheelbase = 200 mm
- Inner angle = 30°
Substitute: cot(outer) = 120/200 + cot(30°) = 0.6 + 1.732 = 2.332
Result: outer angle ≈ arctan(1/2.332) = 23.2°
The outer wheel should turn less than the 30-degree inner wheel.
This model uses an approximation and has joint clearance.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Center the rack and roll the chassis straight for 50 cm.
Success looks like: Both turn directions show a larger inner-wheel angle and the chassis rolls without severe tire scrub.
Measure: Left and right wheel angles at equal rack travel.
Change: the steering-arm tie-rod hole
Keep constant: track, wheelbase, rack travel, tie-rod length, and wheel size
- outer arm hole
- middle arm hole
- inner arm hole
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| Both wheels turn equally | Steering arms are parallel instead of angled | Extend arm lines toward the rear axle | Re-angle mirrored arms inward |
| One side will not center | Tie rods or rack starting position differ | Measure both rods and rack offset | Match lengths and reset center |
| The rack binds | Guides are tight or pinion mesh varies | Disconnect tie rods and test rack alone | Align guides and adjust mesh |
| Wheels scrub badly | Angles do not share a turning center | Draw extended wheel-axis lines | Move tie-rod pickup points and retest |
Choose your tradeoff
Steering-arm angle, tie-rod length, and pickup position interact. Change one mirrored dimension at a time and preserve a reliable straight-ahead center before chasing a perfect curve.
Keep experimenting
Try another version
Parallel steering
Start with equal wheel angles and observe scrub.
Geometry overlay
Draw extended wheel axes at several steering positions.
Bump-steer check
Add suspension travel and measure unintended steering change.
Build together
Classroom and access options
Classroom version
Teams can compare the steering-arm tie-rod hole while keeping track, wheelbase, rack travel, tie-rod length, and wheel size. 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 steering wheel and tactile center mark on the rack.
Reflect on the design
- How did the steering-arm tie-rod hole change the measured result?
- Where did rack friction affect the build most strongly?
- What evidence shows that ackermann approximation explains the motion?
- Which change would improve unequal left and right wheel pivot angles without creating a new problem?
Glossary
- Ackermann approximation
- Steering arms angled toward the rear axle center make the inner wheel rotate farther than the outer wheel.
- Input
- The action or energy supplied to a system; here it is rotation of a steering pinion.
- Output
- The useful response produced by a system; here it is unequal left and right wheel pivot angles.
- 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.