Wheels, Gears and Torque

One-sentence summary

How fast a robot moves and how much load it can pull are decided together by wheel diameter, gear ratio, torque, and how well the wheels grip the ground.

Why does it matter?

In the previous lesson we designed a chassis for our robot. But a good frame is not enough on its own. Once you connect the motor, the robot may crawl too slowly, get stuck on a ramp, or just spin its wheels in place.

The cause is usually not the code but the mechanical choices. With the same motor, the right wheel diameter, gear ratio, and a tyre that suits the surface give very different results. This lesson helps you understand the classic problem: "the motor is spinning but the robot is not moving."

You do not need heavy physics for these ideas. Everyday things you already know, like a bicycle and a door handle, use the very same principles engineers rely on in robots.

What is torque?

Torque is a turning force that tries to rotate something. A plain force pushes in a straight line; torque twists.

Think of a door handle. If you push the door close to its hinge, it is hard to open. Apply the same force far from the hinge, at the outer edge, and the door swings open easily. The force is the same, but the lever length changed.

We can think of torque simply like this:

Torque = Force × Lever length

For a motor, torque means "how strongly the shaft can turn." A high-torque motor can push a heavy robot up a slope. A low-torque motor may not move the same robot at all.

Everyday example: A wrench

To loosen a stuck bolt, a short wrench is hard work; you reach for a long-handled one. The longer the handle, the more torque you produce with the same arm. In robots, gears do exactly this job: they can increase torque without changing the force.

How wheel diameter affects speed

A motor shaft turns a fixed number of rotations per second. If that rotation rate stays the same and you make the wheel bigger, the robot travels farther per turn, so it goes faster. But there is a cost: a bigger wheel is harder to turn, so the pulling power (torque) drops.

The distance a wheel covers in one turn equals its circumference:

Circumference = 3.14 × diameter

Worked example

Suppose the motor turns the wheel 2 rotations per second.

• Small wheel (4 cm diameter): Circumference ≈ 3.14 × 4 = 12.56 cm. At 2 turns per second → about 25 cm/second.
• Large wheel (8 cm diameter): Circumference ≈ 3.14 × 8 = 25.12 cm. At 2 turns per second → about 50 cm/second.

Doubling the wheel doubled the speed. But now the same motor can pull less load and struggles on slopes. We are always balancing speed against pulling power.

Gear ratio: balancing speed and torque

In most robots the motor is not connected directly to the wheel. Gears sit in between. The ratio of the tooth counts of two gears is called the gear ratio, and it lets us trade speed for torque.

The basic rule is:

• Passing power from a small gear to a large gear increases torque and reduces speed.
• Passing it from a large gear to a small gear increases speed and reduces torque.

We gain one thing by giving up some of the other; gears do not create energy, they only redistribute it.

Example: a 3:1 gear ratio

Say the small gear on the motor has 10 teeth and the large gear on the wheel has 30 teeth. The ratio is 30/10 = 3.

• The wheel turns 3 times slower than the motor.
• But the torque at the wheel is roughly 3 times greater.

We turned a fast but weak motor into a slow but strong drive. For a heavy robot or a slope-climbing vehicle, that is exactly what we want.

Everyday example: bicycle gears

Bicycle gears are a gear-ratio system. On a steep hill you shift to a low gear: the pedals turn fast but feel light, the wheel moves slowly, and in exchange the force (torque) available for climbing goes up. On a flat or downhill road you shift to a high gear: each pedal stroke turns the wheel more, so speed rises but pedalling feels harder. Choosing gears on your robot follows the same logic.

Friction and traction

The torque a motor makes is only useful if the wheel can grip the ground. A wheel's grip on the surface is called traction, and it happens thanks to friction.

If there is too little friction, the wheels spin in place and the robot goes nowhere, like trying to walk on ice. If there is too much friction, the robot struggles to turn.

To improve traction:

• Use tyres or rubber-coated wheels; bare plastic slips.
• Balance the robot's weight evenly over the drive wheels.
• Match the wheel to the surface: a thin wheel on carpet, a wide tyre on smooth floor works well.

Example

A robot with a very strong motor can still spin its bare plastic wheels on start-up, even on a flat floor. Fit rubber tyres to the same robot and the torque reaches the ground, so it accelerates smoothly. Sometimes the fix is not a stronger motor but a better tyre.

Mini practice

Make a small design decision on paper. No code needed; the goal is to think about the mechanical balance.

Your task: A robot has to climb a gentle ramp, and you only have one type of motor.

Answer these questions:

1. To make the robot stronger, would you use a bigger or a smaller wheel? Why?
2. If you raise the gear ratio from 1:1 to 3:1, what happens to torque and speed?
3. If the wheels slip on the ramp, which two things would you try without changing the motor?

Justify each decision in a single sentence. Example format:

Decision: I will use a smaller wheel.
Reason: A smaller wheel gives less speed but more torque,
so climbing the ramp becomes easier.

If you have a robot, test it at a low speed with the current wheel first, then with a different wheel, and observe what changes on the ramp.

Common mistakes

Trying to fix every problem with a bigger wheel

A bigger wheel adds speed but lowers torque. For a slope or a heavy load, a smaller wheel or a higher gear ratio is usually the better choice.

Thinking a gear creates energy

A gear ratio converts speed into torque (or the reverse); it does not produce free power. When you raise torque, speed falls, so plan for that from the start.

Forgetting about traction

Even with perfect code and a perfect motor, a slippery wheel will not move the robot. The wheel's grip on the ground matters just as much as the motor.

Connecting the motor straight to the wheel and running out of torque

Small hobby motors on their own often cannot supply enough torque. Adding a gearbox (reduction gear) in between can be the difference that actually gets the robot moving.

Safety note

A moving robot can pinch a finger, hair, or a cable, fall off a table, or run into something.

• Clear a safe, open area for testing; try the robot on the floor or a wide tray, not at the edge of a table.
• Keep your fingers, hair, and cables away from spinning wheels and gears; gears can pinch badly.
• Always start at low speed; increase it only after you see how the robot behaves.
• Drive the motors through a motor driver with a separate low-voltage battery pack, not straight from the microcontroller, and connect the grounds (GND) of both sources together. This both protects the controller and supplies the current the motor needs.
• Never use mains (wall socket) electricity; use suitable battery packs only.
• Ask an adult for help when fitting motors, tools, and gears.

Lesson summary

• Torque is a turning force and decides how much a robot can pull and how well it climbs.
• A bigger wheel adds speed but lowers torque; a smaller wheel does the opposite.
• A gear ratio trades speed for torque; it does not create energy, only redistributes it.
• Bicycle gears are an everyday example of a gear ratio.
• A motor's torque only reaches the ground and moves the robot when there is enough traction (friction).

Check questions

1. What is torque, and how does the door-handle example explain it?
2. If the motor's rotation rate stays the same and you double the wheel diameter, what happens to speed?
3. How does a 3:1 gear ratio change speed and torque?
4. Why does a cyclist shift to a low gear on a steep hill?
5. Why might even a robot with a strong motor fail to move?

Answers

1. Torque is a turning force that tries to rotate something (Torque = Force × Lever length). Pushing a door handle far from the hinge increases the lever length, so the same force makes more torque and the door opens easily.
2. The robot goes roughly twice as fast, because the wheel covers twice the distance per turn. However, the pulling power (torque) drops, so the motor can pull less load.
3. The wheel turns 3 times slower than the motor (speed goes down) but the torque at the wheel roughly triples. We give up speed to gain power.
4. In a low gear the pedals turn lightly and the wheel slows down; in exchange, the torque available for climbing goes up. This lets the cyclist climb a steep hill with less effort.
5. If traction is too low (the wheels are slippery), the motor's torque cannot reach the ground, the wheels spin in place, and the robot cannot move. The problem may be friction, not the motor.

Source and verification note

For “Wheels, Gears and Torque”, verification focuses on whether the relationship between What is torque? and How wheel diameter affects speed 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

Choosing a Power Source: Picking the right battery pack to safely supply the voltage and current that the motors and controller need.