Week 2
Building a Robot That Moves
Turn a motor's spin into steady rolling, and design a chassis that moves without tipping.
- 65-80 minutes
- Mission: Build a stable rolling base and test how wheel, gear, and chassis choices change how it moves.
Now that students know what a robot is, they build one that actually moves. They learn how a motor spins an axle, how wheels and gears turn that spin into motion, and how gears trade speed for turning power (torque). They investigate why some rolling bases tip over and others stay steady - friction and traction for grip, and a low, wide center of mass for stability - and they meet differential drive, where driving two wheels at different speeds is what steers the robot. Then each student builds and tests a rolling base of their own.
By the end of this week you can
- Explain how a motor, axle, and wheels turn spinning into rolling
- Identify the wheels, axles, and gears on a rolling base
- Compare speed and torque, and how gears trade one for the other
- Use friction and traction to explain why wheels grip or slip
- Recognize stable and unstable chassis designs
- Explain how wheel spacing and center of mass affect balance
- Build or simulate a rolling base and steer it using differential drive
- Test a design and improve it based on the results
Learn
From a spinning motor to a rolling robot
A motor spins when it gets power, but a spinning motor by itself does not go anywhere. To move, the robot connects the motor to an axle, and the axle holds the wheels. When the motor turns the axle, the wheels turn, and the robot rolls.
So the chain is: motor spins, axle turns, wheels roll. If any link in that chain is loose or missing, the spin never reaches the ground and the robot stays put.
For example: A motor turning a bare axle with no wheel just spins in the air; A wheel pushed onto a turning axle rolls the robot forward
Gears trade speed for torque
Gears are toothed wheels that lock together so one can turn the other. When a small gear turns a big gear, the big gear spins slower but with more turning power. When a big gear turns a small gear, the small gear spins faster but with less power.
That turning power is called torque. Gears let you choose: a slow, strong robot that can climb or push, or a fast, weaker one that zips across a flat floor. You cannot get maximum speed and maximum torque at the same time - you trade one for the other.
For example: A bicycle in low gear: slow but easy to pedal uphill (high torque); A bicycle in high gear: fast on flat ground but hard to start (low torque)
Friction and traction: how wheels grip
Friction is the rubbing force between two surfaces that touch. Without friction, a spinning wheel would just slip in place and the robot would go nowhere - like tires on ice.
Traction is how well a wheel grips the ground, and it comes from friction. A rubber tire on carpet has lots of traction; a smooth plastic wheel on a slick floor has little. Good traction turns the wheel's spin into real forward motion.
For example: Rubber tires grip the road (high traction); A wheel spinning on a wet or smooth surface (low traction); Adding a rubber band around a slippery wheel to help it grip
Balance, stability, and center of mass
The center of mass is the average spot where a robot's weight is centered. A robot tips over when its center of mass leans out past its wheels.
To make a robot more stable, keep its center of mass low and its base wide. A low, wide base is hard to tip; a tall, narrow, top-heavy one tips easily. That is why race cars are low and wide, and why a stack of blocks falls when it gets too tall.
For example: A low, wide base stays upright on a turn; A tall tower of parts tips when the robot stops fast; Putting the heavy battery low in the chassis to lower the center of mass
Chassis design: the robot's frame
The chassis is the frame that holds the motors, axles, wheels, and battery together. A good chassis keeps the wheels lined up straight, holds the motors firmly, and puts the heavy parts low.
Design choices matter: wheels too far apart or too close, a loose motor, or a wobbly frame all change how the robot moves. The chassis is not just a box - it decides whether the robot rolls straight, turns cleanly, and stays upright.
For example: Wheels lined up straight so the robot rolls in a line; A firm frame so the motor does not wobble; Weight kept low and centered over the wheels
Differential drive: steering by speed
Most small robots steer with differential drive: two wheels, one on each side, each turned by its own motor. When both wheels spin at the same speed, the robot goes straight.
To turn, you drive the wheels at different speeds. The robot curves toward the slower wheel. If one wheel goes forward and the other goes backward, the robot spins in place. There is no steering wheel - the difference in wheel speed is the steering.
For example: Both wheels same speed: straight ahead; Left wheel slower than right: the robot curves left; Left wheel forward, right wheel backward: the robot spins in place
Words to know
- Motor:
- A part that spins when it gets power, giving the robot the movement it needs to roll.
- Axle:
- A rod that a wheel is attached to, so when the axle turns, the wheel turns with it.
- Wheel:
- A round part that rolls the robot along the ground when its axle spins.
- Gear:
- A toothed wheel that locks into another gear to pass along spinning motion and change its speed or power.
- Speed:
- How fast the robot moves. A small gear driving a big gear makes the wheels spin slower.
- Torque:
- The turning power a motor or gear has. More torque means more force to move a heavy robot or climb.
- Friction:
- The rubbing force between two surfaces that touch. It slows sliding and lets wheels grip instead of spinning in place.
- Traction:
- How well a wheel grips the ground. Good traction means the wheel pushes the robot forward instead of slipping.
- Balance:
- Keeping the robot's weight spread so it stays upright and does not tip over.
- Stability:
- How hard it is to tip a robot over. A low, wide base is more stable than a tall, narrow one.
- Center of mass:
- The average spot where a robot's weight is centered. A lower center of mass makes the robot harder to tip.
- Chassis:
- The frame or base of the robot that holds the motors, wheels, and everything else together.
- Differential drive:
- A way to steer by driving the two side wheels at different speeds: the robot turns toward the slower wheel.
Stay safe
Read these before you build or run a robot this week.
- Caution:Keep fingers, hair, and loose clothing away from spinning wheels and gears, and turn the motor off before swapping parts.
- Caution:Use child-safe scissors with an adult to cut cardboard and axle holes, and keep skewer points blunted.
- Caution:Test rolling bases in a clear area away from table edges, stairs, and drops.
- Note:Save your work often so a browser refresh does not lose your chassis and test log.
Do it
Choose your path
Do this course with a robot kit, the browser simulator, or unplugged with household materials. Pick one - you can switch anytime without losing your work.
Virtual chassis investigation
Robot kitExplore how wheel size, gearing, and base shape change how a rolling base moves, grips, and stays upright.
- You will change one thing at a time - wheel size, gears, base width, or weight - and watch how the base moves. Changing one thing at a time is how you tell which change did what.
- For each change, note what happened to speed, grip, and stability. Write down which setup rolled straight and fast, and which one stayed the steadiest.
Swap real wheels and gears on a kit base
You need: A programmable robot kit with two motors, Assorted wheels and gears from the kit, Chassis Test Log worksheet
Steps
- 1Build a simple two-motor base and drive it straight across the floor at a set speed.
- 2Swap the wheels for a bigger or smaller pair and drive the same distance again. Note the change in speed.
- 3Change the gearing (or the motor power) so the wheels turn slower, and feel how much harder the base is to stop - that is more torque.
- 4Add a tall part on top, then move the weight low, and see which version is easier to tip.
What success looks like: Bigger wheels roll faster but need more push to start; lower gearing gives more torque; a low, wide base is hardest to tip.
- At least three setups tested and logged
- Each change describes its effect on speed, grip, or stability
- One setup is named the most stable, with a reason
Safety: Keep fingers, hair, and loose clothing away from spinning gears and wheels. Turn the motor off before swapping wheels or gears.
If it doesn't work
- The base curves instead of going straight
- - Check both wheels are on tight and the motors run at the same speed; a loose wheel or uneven motors cause curving.
- A wheel spins but the base does not move
- - The wheel is slipping - low traction. Try a grippier wheel or a rougher surface.
Go further: Find the wheel-and-gear combination that climbs your ramp without stalling.
Virtual Chassis Lab
Design a rolling robot, then watch how it does on four tests. Change one thing at a time and read the tips to figure out why a design works or fails.
Your design
Test surface
Surface: Smooth floor
Live results
Test outcomes
- Straight lineGood
Drove straight and gripped the surface.
- TurningGood
Turned cleanly using the two wheels.
- Small rampGood
Climbed the ramp with power to spare.
- Slippery surfaceOkay
It moved slowly but kept some grip.
Why this happened
- Solid, stable design - low and wide with good grip.
What are you testing?
- Straight line:
- Can it roll straight without slipping or wobbling?
- Turning:
- Can it turn cleanly without spinning out or being too slow?
- Small ramp:
- Does it have enough power and grip to climb a slope or push through resistance?
- Slippery surface:
- Can it still move when the floor has almost no grip?
Rolling-base challenge
Robot kitBuild a rolling base that drives straight, stays upright, and can steer with differential drive.
- Build a base with two driven wheels, one on each side, plus a support at the front or back so it does not tip. Keep the heavy parts low and the wheels lined up straight.
- Test three things: does it roll straight, can it steer by driving the wheels at different speeds, and does it stay upright when it stops and turns? Fix one thing at a time until all three work.
Physical-kit rolling base
You need: A programmable robot kit with two motors, wheels, and axles, A ramp or books for the tip test, Chassis Test Log worksheet
Steps
- 1Build a chassis that holds two motors firmly, one driving each side wheel, with a caster or skid to balance the third point.
- 2Keep the battery and heavy parts low and centered so the center of mass stays low.
- 3Drive it forward and adjust until it rolls straight, then drive the two wheels at different speeds to make it turn.
- 4Test its stability by stopping fast and by driving across a slight slope.
What success looks like: A kit base that rolls straight, turns by differential drive, and stays upright when it stops and on a mild slope.
- Rolls straight for at least one meter
- Turns left and right by changing wheel speeds
- Stays upright when stopping quickly
- Motors and wheels are held firmly
Safety: Keep clear of spinning wheels and gears while driving. Test in a clear area away from edges and stairs.
If it doesn't work
- It always drifts to one side
- - One motor may be faster or a wheel loose; match the motor speeds and tighten both wheels.
- It tips forward when it stops
- - The center of mass is too high or too far forward; lower the weight and move it back over the wheels.
Go further: Program it to drive a straight line, then a square, using differential-drive turns.
Predict
Commit to a guess before you test - then see how close you were. Your predictions save automatically.
How to check: Tilt each base on the ramp (or run the sim slope) and see which one tips first; compare to your guess.
How to check: Change the gearing so the wheels turn slower, drive up the ramp, and check whether it climbs better but moves slower.
Test & improve
Rolling-base speed and straightness test
Mark a start line and a finish line one meter apart. Drive the base from the start and record how many seconds it takes and how far off the line it ends up. Run it three times.
Measure: The time to cover the distance and how far the base drifted from a straight line
| Run | Time to finish (seconds) | How far off the straight line (cm) | Notes |
|---|---|---|---|
Tip-over stability test
Place the base on the ramp and slowly raise the ramp until the base tips over. Record the tipping angle (or the number of books) for a low, wide setup and a tall, narrow setup.
Measure: The angle or ramp height at which each setup tips over
| Setup | Tips at how many books / what angle | Stayed upright? (Y/N) |
|---|---|---|
Knowledge check
Answer these to check that you understand how a robot moves and stays steady.
1. Diagnose the design problem.
A student's robot is tall and narrow with the battery mounted on top. It drives straight fine, but it tips over every time it makes a fast turn.
2. How does a spinning motor make a robot roll across the floor?
3. You gear a robot down so its wheels turn slower. What do you gain, and what do you give up?
4. A robot's wheels spin fast but it barely moves on a smooth, slippery floor. What is the problem?
5. Which robot base is the hardest to tip over?
6. In differential drive, how does a two-wheeled robot turn left?
0 of 6 answered
Reflect
Your reflections save automatically.
Coming up next week
Next week your rolling base gets a brain: you'll write your first program so the robot drives a planned path on its own.
- Keep your rolling base built and working so you can program it next week.
- Charge your kit or bookmark the simulator and its block editor.
- Think about how you would tell someone the exact steps to walk from the door to a chair without watching them.
Finish Week 2
Complete these to mark the week done and unlock the next one:
- Complete the Virtual chassis investigation and log three setups (not done yet)
- Build a rolling base that rolls straight, steers, and stays upright (not done yet)
- Record the speed-and-straightness and tip-over tests (not done yet)
- Score at least 4 of 5 on the knowledge check (not done yet)
- Write your reflection (not done yet)