One-sentence summary
The chassis is the skeleton that carries all of a robot's parts; a good chassis is balanced, sturdy and tidy, so the robot moves safely and smoothly.
Why does it matter?
In the previous lesson we saw that a robot works through the sense–decide–act loop. But all of that electronic brain, the motors and the sensors have to attach to something. That "something" is the chassis.
We can compare a chassis to a human skeleton. No matter how good the muscles, organs and skin are, if the skeleton is weak the body cannot stand. In the same way, even if your code is perfect, if the chassis bends, wobbles or lets parts shift, the robot will not work properly.
Think about these:
- If the motors are loosely attached, the robot shakes and drifts off course.
- If the weight is piled on one side, the robot can tip over while turning.
- If cables sit near a wheel, they can wind up and stop the robot.
So chassis design is a real engineering step to think about *before* you write any code.
What is a chassis and what does it do?
A chassis is the main body or supporting frame of a robot. The motors, wheels, battery, board and sensors all mount onto it. You can also think of it as the robot's "skeleton" or "frame."
A good chassis has three jobs:
- Hold all the parts firmly together.
- Carry the weight in a balanced way.
- Keep its shape while moving (not bend or flex).
Example: A simple two-motor vehicle chassis
The design we will use most in this module is a two-motor (differential-drive) vehicle. It looks like this:
front
[ ] sensor [ ]
+-----------------+
| O O | <- two drive wheels
| left motor right motor
| [battery pack] |
| [control board]|
+--------+--------+
o <- free-spinning support (caster)
back- There are two motors and two wheels at the front or sides. These push the robot forward and turn it.
- At the back, a single caster (a small free-spinning ball or wheel) keeps the robot balanced.
- The heavy parts (battery, motors) sit in the middle.
If the left and right wheels spin at different speeds, the robot turns. If both spin forward at the same speed, the robot goes straight. This simple idea is the basis of robot navigation.
Balance and weight distribution
For a robot to move without tipping or wobbling, its centre of gravity must be in the right place. The centre of gravity is the imaginary point where all of the robot's weight is gathered.
Principle 1: Put heavy things low and central
The heaviest part is usually the battery pack. If we place it as low as possible and between the wheels, the robot stands more steadily.
Everyday example: Think of a cargo truck. If you spread the load across the floor of the trailer, the truck stays firm on a bend. If you pile the whole load on the roof, it tips over. A robot is the same.
Principle 2: Spread the weight evenly
If we put all the heavy parts in one corner, that corner presses down, the opposite wheel does not fully touch the ground, and the robot pulls to one side. We try to spread parts evenly left-to-right and front-to-back.
Example: Comparing two designs
| Feature | Weak design | Good design |
|---|---|---|
| Battery position | On top | Low and central |
| Weight | Piled on the right | Balanced both sides |
| Cables | Hanging loose | Bundled and fixed |
| Result | Wobbles when turning | Goes straight and steady |
Sturdy and simple design principles
Good engineering does not mean "more parts"; it means "fewer, but the right parts." Here are useful starting principles:
Keep it simple
A complicated chassis has more places to break. For your first robot, a flat plate, two motors and one support wheel are enough. You can add more after it works.
Make the connections firm
Fix the motors with screws or tight brackets. Tape is a temporary fix; it loosens as the robot vibrates. A loose motor means a drifting direction.
Choose the material for the job (conceptually)
- Cardboard / foam: Cheap, easy to cut, great for a prototype. But it weakens quickly with moisture and weight.
- Wood / MDF: Stronger, easy to drill, good for school projects.
- Plastic sheet or acrylic: Light and durable; cutting it needs adult help.
- Metal / aluminium: Very strong but heavy and hard to work with.
Rule: Try a cheap, easy material first (cardboard), then move to a stronger material once the design works.
Tidy the cables
Bundle the cables together and keep them away from wheels and gears. Messy cables are both a safety risk and a source of failure.
Powering the chassis and motors: why a separate battery pack?
A control board (for example an Arduino) runs on a small current. But motors draw much more current. If we connect motors straight to the board's output pin, we can damage the board.
So we add two things to the chassis:
- A motor driver: a go-between circuit that turns the board's weak signal into a strong current for the motors.
- A separate battery pack: its own low-voltage batteries for the motors.
The board and the motor driver must share a common ground (GND) so they use the same "reference." But the motor power comes from the separate battery.
The control logic is the same in either case:
// Drive a two-motor robot forward (logic, with a driver board)
int leftFwd = 5; // left motor direction pin
int rightFwd = 6; // right motor direction pin
int leftSpeed = 9; // left motor speed pin (PWM)
int rightSpeed = 10;// right motor speed pin (PWM)
void goForward() {
digitalWrite(leftFwd, HIGH);
digitalWrite(rightFwd, HIGH);
analogWrite(leftSpeed, 120); // test at low speed
analogWrite(rightSpeed, 120);
}Note: Here the motor power comes from the separate battery, through the driver board. The Arduino only sends the "how fast and which direction" signal.
Mini practice
In this activity we will plan a chassis on paper, before mounting any motors.
- Draw the top view of an A4 sheet (a rectangular plate).
- Mark the two drive wheels and the support wheel at the back.
- Place these parts on the paper and write their positions: battery pack, control board, motor driver, front distance sensor.
- Mark the centre of gravity with an "X". Is it between the wheels and low down?
- Show where the cables will run with a dashed line; keep them away from the wheels.
Check question: If the robot drifts to the right when going forward, could one reason be that the weight is piled on the right? Review your design with that in mind.
Common mistakes
Putting the heavy battery on top
Weight up high makes the robot prone to wobbling and tipping. Move the battery to the bottom and centre.
Holding motors with tape
Tape loosens with vibration, the motor shifts, and the robot loses direction. Prefer screws or firm mounting.
Connecting motors straight to the Arduino
Motors draw a lot of current and can burn out the board. Always use a motor driver and a separate battery, and connect the common ground.
Leaving cables loose
A hanging cable can wind around a wheel and stop the robot. Bundle and fix the cables.
Starting too complicated
Adding too many parts on the first try makes faults hard to find. Start simple, then improve once it works.
Safety note
A moving robot can pinch a finger, hair or a cable, fall off an edge, or run into things. To work safely:
- Clear a wide, empty test area before testing; make sure there are no drop-off points like a table edge nearby.
- Keep fingers, hair and cables away from wheels and gears.
- Test the robot at low speed the first time; start the speed pin with a small value.
- Use a motor driver and a separate, low-voltage battery pack for the motors, and connect the common ground to the board. Never use mains (wall) electricity.
- Get help from an adult for any work that needs cutting tools, drilling or hot surfaces.
- Disconnect the robot from its power source when testing is finished.
Lesson summary
- The chassis is the skeleton that carries all of a robot's parts; it should be balanced, sturdy and tidy.
- A two-motor, differential-drive vehicle is a simple and strong design for beginners.
- Placing the heaviest part (the battery) low and central improves balance.
- Keep it simple, make connections firm, choose the material for the job, and tidy the cables.
- Motors need a motor driver and a separate battery; they share a common ground with the board.
Check questions
- What can a chassis be compared to in a robot, and what are its three main jobs?
- How does a two-motor vehicle chassis make the robot turn?
- Why do we place the heavy battery low and central in the robot?
- Why don't we connect motors straight to the Arduino, but use a motor driver and a separate battery instead?
- What three safety steps would you take when testing a robot for the first time?
Answers
- A chassis can be compared to a human skeleton. Its three jobs: hold the parts firmly together, carry the weight in a balanced way, and keep its shape while moving.
- If the left and right wheels spin at different speeds the robot turns; if both spin forward at the same speed it goes straight.
- Because it lowers and centres the centre of gravity, so the robot stands more steadily and does not wobble or tip while turning.
- Motors draw far more current than the board can supply; connecting them directly can damage the board. The motor driver turns the signal into a strong current, the separate battery provides power, and the common ground shares a reference.
- Example: Clear a wide, empty area, keep fingers/hair/cables away from the wheels, and test at low speed the first time. (Adult help and using a separate battery are also valid.)
Source and verification note
For “Chassis and Mechanical Design”, verification focuses on whether the relationship between What is a chassis and what does it do? and Balance and weight distribution remains consistent across examples. Robot behaviour cannot be explained by code alone; mechanical structure, power system, sensor placement and surface conditions must be evaluated together. Test results should be recorded over several runs on the same course.
Next lesson
Wheels, Gears and Torque: We will learn how a motor's power reaches the wheel, and how gears change speed and force.