🔢 Odometer Cart: step-by-step build

Five turns of the wheels make the flag go round just once — a cart that counts!

How far did you push your cart? This one can tell you! A tiny 8-tooth gear on the front axle turns a giant 40-tooth gear standing up above it. The giant gear is so big that the wheels must turn five whole times before the red flag goes round once. Real cars have a counter like this, called an odometer — and the Romans had one too, 2000 years ago!

  • Level 1 of 5 (super easy)
  • About 15 minutes
  • 9 steps, 27 parts
  • Powered by: Push
  • Series: Gearbox & Drivetrain Cars

What you will learn

  • Gears: Gears are wheels with teeth that push each other round.
  • Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
  • Axles: An axle is a cross-shaped rod — anything on it turns together.

Build these first

  • ⚙️ Gear Friends: Three gears holding hands — spin one and they all dance. (level 1 of 5, about 8 minutes, 6 steps)

Parts you need

  • 2 × Beam 11
  • 5 × Black pin (grippy)
  • 2 × L-beam 3×5
  • 2 × Axle 10
  • 6 × Half bush
  • 2 × Bush
  • 1 × Tiny gear (8 teeth)
  • 1 × Giant gear (40 teeth)
  • 1 × Thin beam 3
  • 1 × Axle 6
  • 4 × Big tyre wheel

Steps

  1. Stand an 11-hole beam on its edge — the left side.
  2. Push two black pins into the 3rd and 4th holes of the beam, on the inside. Hang a red L-beam on them with its long leg standing straight up — its corner hole lines up with the 2nd hole.
  3. Push two 10-long axles through the 2nd hole (and the red corner hole) and the 10th hole. Put a yellow half bush on each axle end outside the beam.
  4. Slide a grey bush, the tiny 8-tooth gear and a second grey bush onto the front axle. The gear sits in the middle, a bush on each side of it.
  5. Slide the second red L-beam onto the front axle, long leg up, and push two black pins into its outside face.
  6. Slide the second 11-hole beam onto both axles and the two pins, then lock the axle ends with two more half bushes.
  7. Counter time! Hold the giant 40-tooth gear between the towers with a thin red 3-hole beam (the flag) on its face. Push a black pin through the flag's 2nd hole into the gear, then a 6-long axle through the towers, the gear and the flag's 1st hole. Rotate the whole counter shaft slightly so the gear teeth fit neighbouring gaps, not tooth against tooth. The flag turns round with the gear — that is how it counts!
  8. Lock the counter axle with a half bush on each end.
  9. Push on the four big wheels.

Play time

Point the flag forward, then push the cart slowly until the flag points forward again. Measure how far it went — then use your cart to measure the hallway in "flag turns"!

Real-world inspiration

The Roman hodometer and the odometer in every car — https://en.wikipedia.org/wiki/Odometer