🔒 Diff-Lock Mud Truck: step-by-step build

Stuck in the mud? Slide the diff lock and both wheels are forced to turn together!

A differential is great on the road: in a curve the outside wheel can spin faster than the inside wheel. But in mud it has a sneaky problem — if one wheel slips, ALL the power goes to the slipping wheel and the truck is stuck. Real off-road trucks have a differential LOCK. Ours is a shaft with two 24 gears right behind the back axle. Slide it in and it gears the left and right half-shafts together, so they must turn at the same speed. Slide it out and the differential is free again.

Level 4 of 5 (tricky)
About 60 minutes
33 steps, 97 parts
Powered by: Motor
Series: Gearbox & Drivetrain Cars

What you will learn

Differential Lock: Forces both wheels to turn together.
The Differential: Lets the two driven wheels spin at different speeds in a turn.
Servo Motor: A motor that turns to an angle and holds it.
Remote Control: Invisible light carries your commands to the model.
Bevel Gears: Around the Corner: Slanted gears send spinning power around a corner.
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.
Steering: Turning the front wheels makes a car follow a curve.

Build these first

🎮 First RC Car (Servo): Your first remote-control car: one dial drives, the other dial steers. (level 4 of 5, about 50 minutes, 27 steps)
🌀 Pull-back Diff Racer: A spring racer with a real differential, so it zooms round curves without skidding. (level 3 of 5, about 40 minutes, 23 steps)

Parts you need

2 × Beam 15
38 × Black pin (grippy)
2 × Long pin-axle block
2 × Beam 11
6 × Beam 7
1 × Beam 5
1 × Servo motor
1 × L motor
1 × Axle 2
2 × Half bush
4 × Beam 3
1 × Corner gear (12 teeth)
1 × Differential
2 × Medium gear (24 teeth)
2 × Axle 7
1 × Bush
2 × Dark medium gear (24 teeth)
1 × Axle 9
4 × Big tyre wheel
2 × Beam 9
4 × Grey pin (smooth)
2 × Steering arm
3 × Axle 3
1 × Gear rack
2 × Red axle-pin
1 × Tiny gear (8 teeth)
1 × LED lights
1 × AA battery box
1 × IR receiver
4 × Blue long pin
1 × Speed remote

Steps

Stand a 15-hole beam on its edge — the right chassis rail.
Push black pins into holes 1 and 3 of the rail (inside) and press a long pin-axle block onto them — a steering socket.
Make the rail longer: black pins in holes 13 and 15 (outside) and an 11-hole beam pressed on, sticking out at the back.
Front Servo post: a black pin in hole 5 (inside) and a 7-hole post standing on it. Black pins in its 4th and 6th holes, pointing inwards.
Back Servo post: a black pin in hole 9 and a 5-hole post, with a black pin in its 4th hole pointing inwards. Slide the Servo onto the three pins from the left, orange output at the front.
Drive motor: push a 2-long axle into the L motor's output and slide a half bush onto it, right against the motor.
Push black pins into the 4 side holes on the right of the motor (2 by the output, 2 at the back).
Motor posts: black pins in holes 8 and 14 of the rail (inside) and a 3-hole post on each, sticking up and down.
Slide the motor in from the left under the Servo, output facing backwards: its 4 pins click into the posts.
Rear axle: slide a tan corner gear onto the motor's axle. Hold the differential behind it so its ring meshes with the corner gear, hold a 24 gear next to it on the right, and push a 7-long half-shaft through the 5th hole of the rail extension, the gear and into the differential.
The left half-shaft: a 24 gear and a bush on a 7-long axle, pushed into the other side of the differential. The bush holds the gear against the differential.
Diff lock: hold two dark 24 gears next to the half-shaft 24s and push a 9-long axle through the 8th hole of the extension and both gears.
Push black pins into the Servo's 2 front side holes on the left and press a 7-hole post onto them.
Black pins into the 2 back side holes on the left of the motor and a 3-hole post onto them.
The same by the motor's output.
A black pin in the bottom hole of each of the three left posts, pointing out, and press the left rail onto them.
Slide an 11-hole beam onto the left ends of the half-shaft and the diff-lock shaft, and push black pins through its 1st and 3rd holes into the left rail.
Left steering socket: 2 black pins in holes 1 and 3 (inside) and a long pin-axle block.
Push on the two big back wheels.
Steering module: lay a 9-hole beam flat and push a grey pin up through each end hole.
Put a steering arm on each grey pin and slide a short axle into each arm.
Push on the front wheels.
Grey pins in the arm ends, the red tie rod on top, and the gear rack on the tie rod, teeth up.
Slide the steering module in under the front, then push 2 red axle-pins, axle end first, up through the 3rd hole from each end of its axle beam into the cross holes of the pin-axle blocks.
A 3-long axle into the Servo's output with the pinion on the rack, and a half bush on the front end so the pinion can't slide off.
Headlights: push the two LED lights into the round holes left and right on the Servo's face.
Radio pack: take the AA battery box.
Push 2 black pins into the low holes at the back end of the box and press the IR receiver onto them, dome up.
Black pins into the 2 lowest holes of the front ear, on both sides.
Blue long pins into the 2 lowest holes of the back ear, on both sides.
Press a 7-hole post onto each pair of pins, hanging down.
Lower the pack over the motor: the front posts go inside the rails at hole 12, the back posts inside the extensions at hole 10. Push a black pin through each rail and extension into the bottom of each post.
Wiring: receiver cable onto the battery box. L motor → RED (channel A), Servo → BLUE (channel B). Grab the remote.

Play time

Mud test: put one back wheel on a sheet of paper on a smooth floor and drive. With the lock OUT, the paper wheel just spins. Slide the lock IN — both wheels pull and the truck drives off the paper! Remember to unlock it before a tight turn.

Real-world inspiration

Sariel's compact axle with a differential lock; locking differentials on off-road vehicles. https://sariel.pl/2012/05/compact-axle-with-a-differential-lock/ · https://en.wikipedia.org/wiki/Locking_differential

Build next

🏆 Independent Rally Car: Every wheel on its own springy arm, a differential, universal joints and Ackermann steering. (level 5 of 5, about 90 minutes, 47 steps)
🌐 AWD with Central Diff: All four wheels driven, with a differential in the MIDDLE of the car — like a rally car. (level 5 of 5, about 90 minutes, 28 steps)