- Difficulty
- Intermediate
- Build time
- 90-130 min
- Estimated cost
- $0-$25
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The rover rolls straight for 1 metre with limited drift, completes a controlled arc, and pivots in place without the chassis rubbing the floor.
Learning goals
- Identify how independent left and right wheel rotation produces forward, curved, or pivoting chassis motion.
- Construct and explain a two rotary inputs-to-planar vehicle motion system.
- Measure how the left-right wheel-turn ratio changes performance.
- Diagnose losses caused by wheel slip and axle friction.
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 bottle-cap wheels on two independent skewer axles in a cardboard 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.
- Test on a clear floor at walking speed and keep hair and fingers away from drive axles.
Orient the build
Place the build so independent left and right wheel rotation is on your left and forward, curved, or pivoting chassis motion 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
Square the chassis
Build a rigid rectangle with a marked centerline and equal left-right dimensions.
Check diagonal measurements for squareness.
Step 2
Install independent axles
Support one drive axle on each side without mechanically linking them.
Keep axle axes parallel.
Step 3
Match the drive wheels
Mount equal wheels at equal track width and mark one point on each rim.
Check side-to-side wobble.
Builder checkpoint: After match the drive wheels, the first subassembly should stay aligned when handled gently.
Step 4
Add the balance support
Place a caster or low-friction skid on the centerline away from drive wheels.
Set chassis level on the floor.
Watch for: If this stage binds or drifts, inspect caster drag before adding more parts.
Step 5
Measure wheel rollout
Roll each wheel one turn beside a ruler and compare distance.
Correct tire or diameter mismatch.
Step 6
Run a straight trial
Rotate both wheels equal turns over a 1-metre taped lane.
Measure sideways drift at the finish.
Builder checkpoint: After run a straight trial, operate the build slowly and confirm that forward, curved, or pivoting chassis motion begins without binding.
Step 7
Run an arc trial
Turn the outer wheel twice for every inner-wheel turn.
Trace the chassis center path.
Step 8
Run a pivot trial
Rotate wheels equal amounts in opposite directions at low speed.
Check that the center stays near one point.
Builder checkpoint: At the final checkpoint, The rover rolls straight for 1 metre with limited drift, completes a controlled arc, and pivots in place without the chassis rubbing the floor.
See the engineering
Why it works
- Input
- independent left and right wheel rotation
- Output
- forward, curved, or pivoting chassis motion
- Motion
- two rotary inputs-to-planar vehicle motion
- Energy losses
- wheel slip, axle friction, unequal wheel diameter, caster drag
Why this works
Differential-drive kinematics
The chassis forward speed follows the average wheel speed, while turn rate follows the difference between right and left speeds divided by wheel spacing.
Look for: Mark both wheels and compare their turns during straight, arc, and pivot trials.
Where the energy goes
Efficiency and losses
The ideal model leaves out wheel slip, axle friction, unequal wheel diameter, caster drag. 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 wheel slip becomes visible or audible.
Math bite
Predict pivot turn
Formula: heading change = (right distance - left distance) / track width
- Right distance = 0.314 m
- Left distance = -0.314 m
- Track = 0.20 m
Substitute: change = (0.314 - (-0.314))/0.20 = 3.14 rad
Result: The ideal heading change is about 180 degrees.
Opposite equal wheel travel pivots around the chassis center.
Floor slip and caster drag change the real angle.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Move both wheels through five equal forward rotations on a taped line.
Success looks like: The rover travels one metre with less than 10 cm lateral drift and wheels turn freely.
Measure: Wheel turns, travel distance, lateral drift, and pivot angle.
Change: the left-right wheel-turn ratio
Keep constant: wheels, track width, chassis, floor, support, and starting line
- 1:1 straight
- 2:1 arc
- 1:-1 pivot
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The rover veers during equal turns | Wheel diameters or friction differ | Measure one-turn rollout separately | Match wheels and free the tighter axle |
| Pivot center moves far away | Wheel travel magnitudes differ or one slips | Count rim marks during the turn | Match turns and improve equal traction |
| The chassis rocks | Caster height or frame twist is wrong | Place on a flat surface and press corners | Adjust support height and brace chassis |
| Wheels scrub heavily | Track is wide or support drag is high | Try the pivot on smoother flooring | Reduce track width or support friction |
Choose your tradeoff
Straight tracking requires matched effective wheel diameter and friction, not just matching labels. Calibrate wheel rollout first, then correct control turns rather than bending the chassis.
Keep experimenting
Try another version
Hand-pushed rover
Mark and turn wheels by hand without motors.
Straight-line calibration
Create a correction factor from measured rollout.
Motor control
Add encoders and closed-loop speed matching in a later robotics guide.
Build together
Classroom and access options
Classroom version
Teams can compare the left-right wheel-turn ratio while keeping wheels, track width, chassis, floor, support, and starting line. 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.
- Use large left and right control knobs with distinct tactile markers.
Reflect on the design
- How did the left-right wheel-turn ratio change the measured result?
- Where did wheel slip affect the build most strongly?
- What evidence shows that differential-drive kinematics explains the motion?
- Which change would improve forward, curved, or pivoting chassis motion without creating a new problem?
Glossary
- Differential-drive kinematics
- The chassis forward speed follows the average wheel speed, while turn rate follows the difference between right and left speeds divided by wheel spacing.
- Input
- The action or energy supplied to a system; here it is independent left and right wheel rotation.
- Output
- The useful response produced by a system; here it is forward, curved, or pivoting chassis motion.
- 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.