🚧 Twin-Motor Dozer: step-by-step build

Wide caterpillar tracks, one motor per track, and a blade that cranks up with a worm gear.

A bulldozer pushes mountains of sand around. It needs huge grip, so it rolls on wide caterpillar tracks instead of wheels. Each track has its own motor: the M motor drives the left track and the L motor the right track. Both forwards — push! One forwards, one backwards — it spins on the spot. The blade hangs on two push arms. Turn the crank and a worm gear slowly lifts the arms. Worm gears lock, so the blade stays exactly where you leave it.

Level 5 of 5 (a big challenge)
About 100 minutes
33 steps, 124 parts
Powered by: Multi-motor
Series: Gearbox & Drivetrain Cars

What you will learn

Tracks vs Wheels: Tracks spread the weight and grip rough ground.
Remote Control: Invisible light carries your commands to the model.
Worm Gear: The Lock: A worm gear turns a gear super slowly — and never slips back.
Linkages: Beams joined by pins change one motion into another.
Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
Mounting a Motor: A motor must be pinned tight, or it spins itself instead of your wheels.
Plugging In: Power Functions: Battery box → (switch or receiver) → motor. Plugs stack like bricks.
Bushes & Spacers: Bushes stop things sliding off an axle.

Build these first

🪖 RC Tank: Two motors, two tracks, one remote. Spin on the spot and climb over anything! (level 4 of 5, about 60 minutes, 27 steps)
🚜 Skid-Steer Loader: No steering at all: left motor, right motor — spin it round on the spot! (level 4 of 5, about 65 minutes, 25 steps)

Parts you need

2 × Beam 13
12 × Black pin (grippy)
3 × Open frame 5×11
2 × Axle 7
2 × Giant gear (40 teeth)
6 × Blue long pin
2 × Beam 5
8 × Bush
2 × Axle 8
11 × Half bush
2 × Beam 9
3 × Axle 5
2 × Cross block
3 × Tiny gear (8 teeth)
1 × Double cross block
2 × Axle 6
4 × Track wheel
1 × M motor
2 × Axle 3
1 × L motor
48 × Wide track pad
1 × Worm gear
1 × Hand crank
1 × AA battery box
1 × IR receiver
1 × Speed remote

Steps

Stand a 13-hole beam on its edge — the left side of the hull.
Push black pins into holes 3 and 4 (outside) and press a tall 5×11 frame onto them, standing up at the front — the left tower.
Left front: a 7-long axle through the hull and the tower's corner, with a giant 40 gear on it. Push blue long pins into the tower's 4th and 5th bottom holes, pointing out.
Press a 5-hole beam onto the long pins and the axle — it holds the axle from the outside too — then a bush on the axle so it can't fall out.
Left back: an 8-long axle through the last hole, with a half bush on the inside end so it can't fall out and a bush on each side of the gap.
Blade: two 9-hole push arms with a 5-long axle through their front holes.
A cross block on each end of that axle, round hole at the bottom, and a black pin in each round hole pointing forwards.
Press the 5×11 frame onto the two pins, standing across — the dozer blade. It can tip a little on its axle, so it always sits flat on the ground.
Slide the blade in from the front until the back holes of the push arms line up with hole 6. Hold a tiny 8 gear between the arms and push a 5-long axle through the hull, both arms and the gear — the blade pivot.
Slide the right hull beam onto the pivot axle. Hold a double cross block standing up in the middle at hole 8 and push a 6-long axle from the left through a yellow half bush, both hull beams and its bottom hole — the worm stand.
Black pins in holes 9 and 11 (outside) and the right tower frame at the back. Push a 5-long axle through the tower's front edge (3rd hole) into the block's top hole.
Right back: an 8-long axle through the hull and the tower's corner, with a half bush on the inside end so it can't fall out. A bush on the stand's short axle, outside the tower.
On its outside end: a giant 40 gear and two bushes.
Right front: a 7-long axle through the 2nd hole with a bush on it, and blue long pins in holes 4 and 5, pointing out.
Press a 5-hole beam onto the pins and the axle, then a bush on the outside so the axle can't fall out.
Push the four big track wheels on.
A half bush on the end of each axle, so the track wheels can't slide off.
Push black pins into the 3rd and 5th holes of the left tower's front edge, pointing inwards.
Press the M motor onto them, lying across, output in the 4th hole pointing left. A 3-long axle through the tower into it with a tiny 8 gear on the 40 and a half bush on the end.
Right tower: black pins in the 3rd and 5th holes of its back edge, the L motor on them (output pointing right), and a 3-long axle through the tower into it.
The tiny 8 gear on the L motor's axle, meshing with the back 40, and a half bush on the end.
Left track: link 12 wide pads together along the bottom and round the back wheel.
Add the other 12 pads over the top and round the front to close the loop.
Wrap the left track round the left track wheels.
Right track: link 12 wide pads together along the bottom and round the back wheel.
Add the other 12 pads over the top and round the front to close the loop.
Wrap the right track round the right track wheels.
Blade lift: slide a 6-long axle through the middle hole of the double cross block, with a half bush and a worm gear right on top of the tiny 8 gear, and the crank on the back end.
A half bush on the front end of the worm axle, so the worm stays right over the 8 gear.
Radio pack: the AA battery box…
…with 2 black pins in the low holes at its back end and the IR receiver pressed onto them.
Lay the pack across the tops of the motors and pin its ears to the towers with blue long pins (8th hole: left tower at the front, right tower at the back).
Wiring: receiver cable onto the battery box. M motor → RED, L motor → BLUE. Grab the remote.

Play time

Make a sandpit out of rice or dried beans in a tray (or a pile of bricks). Lower the blade, push a heap to one side, spin on the spot and push it back. Can you build a ramp with the blade?

Real-world inspiration

Crawler bulldozers; LEGO Technic RC tracked models such as 42065. https://en.wikipedia.org/wiki/Bulldozer · https://en.wikipedia.org/wiki/Lego_Technic