Wheels and drivetrains

Learn

What you’ll learn

• How wheel size affects speed and distance
• How a two-wheeled robot steers (differential drive)
• Other drivetrains: casters, tracks, omni and mecanum wheels

Wheel size

Each turn of a wheel moves the robot one circumference:

Distance per turn = π × diameter

A 65 mm wheel (common in kits) moves about 3.14 × 65 ≈ 204 mm per turn. Bigger wheels go further per turn (faster), but need more torque to start and climb.

Differential drive

Most beginner robots have two driven wheels and a free caster wheel or ball for balance. They steer by changing the speeds of the two wheels:

Left wheel	Right wheel	Robot
Forward	Forward (same speed)	Straight
Forward fast	Forward slow	Curves right
Forward	Stopped	Pivots around the right wheel
Forward	Backward	Spins on the spot
Backward	Backward	Reverses

This is exactly what motors(left, right) does in the robot simulator.

Other drivetrains

Drivetrain	Pros	Cons
2 wheels + caster	Simple, cheap, turns on the spot	Caster can catch on bumps
4-wheel skid steer	More grip and pushing power	Wheels scrub when turning
Tracks	Grip on rough ground and sand	Slow, lose energy to friction
Omni / mecanum	Can drive sideways	Expensive, less grip
Car-style steering	Efficient at speed	Can’t turn on the spot

Examples

Example: how fast is my robot?

A gear motor turns at 200 rpm with 65 mm wheels. Speed = 200 × 0.204 m per minute = 40.8 m/min ≈ 0.68 m/s.

Example: driving a set distance

To drive 1 m with 65 mm wheels: 1000 ÷ 204 ≈ 4.9 wheel turns. With wheel encoders counting 20 slots per turn, that’s about 98 counts.

Example: steering in code

motors(60, 60);   // straight
motors(60, 30);   // gentle right curve
motors(60, -60);  // spin right on the spot
motors(-40, -40); // reverse

Practice

Try it

Write different motors(left, right) values and watch the path in the obstacle room.

Differential drive

▶ Run↺ Reset

Your code (JavaScript)// Try different pairs of numbers and press Run. function loop() { motors(60, 30); // left faster than right: curves right }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

CirclesEasy

Make the robot drive in a big circle, then a small circle. Big circle: motors(60, 45). Small circle: motors(60, 10). The bigger the speed difference, the tighter the turn.

Square danceMedium

Drive forward for 1 s, spin about 90° to the right, and repeat. Use millis() % 1600 to split time into a “drive” part and a “turn” part. Adjust the turn time until it’s close to 90°.

Show an answer

function loop() {
  const t = millis() % 1600;
  if (t < 1000) motors(50, 50);   // drive for 1 s
  else motors(50, -50);           // turn for 0.6 s
}

Open-loop turns drift, which is why real robots use encoders or a gyro to measure the angle.

Wheel mathsHard

Your robot has 80 mm wheels on 150 rpm motors. How long does it take to drive 3 m? Circumference = π × 80 ≈ 251 mm. Speed = 150 × 0.251 ≈ 37.7 m/min ≈ 0.63 m/s. 3 ÷ 0.63 ≈ 4.8 s.

Quick check

1. A 70 mm wheel moves about how far per turn?

70 mm140 mm220 mm490 mm

2. How does a differential-drive robot spin on the spot?

Both wheels forwardOne wheel forward, one backwardBoth wheels stoppedIt can’t

3. Which drivetrain can move sideways?

TracksMecanum wheelsCar-style steering2 wheels + caster

Check my answers