🎚️ Two-Speed Pull-back: step-by-step build

Choose your gear before the race: CLIMB or SPRINT! Slide the back axle to swap gears.

Racing drivers choose their gear before the start. Now you can too! The spring engine has two gears on its axle: a tiny 8 on one side and a 16 on the other. The back axle carries a 24 and a 16, and it can slide sideways by one hole. Slide it one way and the 8 meets the 24: CLIMB gear, slow and strong. Slide it the other way and the 16 meets the 16: SPRINT gear! You will build the engine block on its own first, then drop it into the frame — just like a car factory.

  • Level 3 of 5 (medium)
  • About 35 minutes
  • 13 steps, 35 parts
  • Powered by: Pull-back
  • Series: Gearbox & Drivetrain Cars

What you will learn

  • Gearbox: A box of gear pairs where you choose which pair works.
  • Stored Energy: Energy can be saved up in a spring and let out later.
  • Mounting a Motor: A motor must be pinned tight, or it spins itself instead of your wheels.
  • Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.

Build these first

  • Sliding-Gear 2-Speed Cart🕹️ Sliding-Gear 2-Speed Cart: Your first real gearbox: slide the crank axle to switch between strong gear and fast gear! (level 2 of 5, about 22 minutes, 8 steps)
  • 🐢 Pull-back Tortoise: Slow but super-strong: a spring car that can climb up a book! (level 2 of 5, about 30 minutes, 12 steps)

Parts you need

  • 1 × Pull-back motor
  • 14 × Black pin (grippy)
  • 2 × Open frame 5×7
  • 2 × Open frame 5×11
  • 1 × Axle 12
  • 1 × Medium gear (24 teeth)
  • 2 × Small gear (16 teeth)
  • 4 × Yellow-hub wheel
  • 1 × Tiny gear (8 teeth)
  • 1 × Axle 9
  • 1 × Axle 10
  • 2 × Long pin-axle block
  • 2 × Red axle-pin
  • 1 × H frame 7×11

Steps

  1. Engine block: take the pull-back motor, long body pointing forward. Push a black pin half-way into the front one of its two top holes and into the first and last hole of its bottom row, on this side.
  2. Press a red 5×7 frame onto the three pins. Turn the motor over and push three black pins half-way into the same holes on the other side.
  3. Press the second red frame on. Push two black pins into the outside of the first frame: the 2nd hole up at the back and at the front.
  4. Two more black pins in the same holes of the second frame.
  5. Stand a 5×11 frame up on its long side — the left wall.
  6. Slide the engine block sideways onto the wall: its two pins go straight into the 2nd and 8th holes of the top bar.
  7. Press the second 5×11 wall onto the pins on the other side.
  8. The sliding back axle: a 12-long axle through the bottom back corners, with a 24 on the left and a 16 on the right, one hole further out. Push the right back wheel on.
  9. Hold the tiny 8 at the left and a 16 at the right of the middle back holes, and push a 9-long axle through them, the walls and the motor. Then push on the left back wheel.
  10. Push a 10-long front axle through the front corners and put the two front wheels on it.
  11. Roof supports: push black pins into the outside of both red frames' top bars, in the 5th and 7th holes.
  12. Press a long pin-axle block onto each pair of pins, and push a red axle-pin up into the middle hole of each block, pin end on top.
  13. Press the H frame onto the two red pins — the roof.

Play time

Set up a ramp and a flat race track. Try both gears on both tracks. Which gear climbs? Which gear goes furthest on the flat? Why?

Real-world inspiration

Pre-selector gearboxes (Wilson) — https://en.wikipedia.org/wiki/Preselector_gearbox