🌐 AWD with Central Diff: step-by-step build

All four wheels driven, with a differential in the MIDDLE of the car — like a rally car.

In a corner the front wheels drive round a bigger circle than the back wheels, so the front axle has to spin a little faster. If both axles were locked together, the tyres would fight each other. Rally cars solve this with a CENTRE differential between the front and rear shafts. Our centre differential lies long-ways in the middle. Two motors — the M and the L — push on its ring from both sides, and it shares their power between a shaft to the front axle and a shaft to the back axle. The whole front axle swings on a turntable, steered by the Servo standing on top, and a universal joint lets the front shaft bend round the corner.

• Level 5 of 5 (a big challenge)
• About 90 minutes
• 28 steps, 78 parts
• Powered by: Multi-motor
• Series: Gearbox & Drivetrain Cars

What you will learn

• Four-Wheel Drive: Power goes to both axles so all four wheels push.
• The Differential: Lets the two driven wheels spin at different speeds in a turn.
• Universal Joint: Passes turning power around a bend.
• Bevel Gears: Around the Corner: Slanted gears send spinning power around a corner.
• Servo Motor: A motor that turns to an angle and holds it.
• Turntables: A turntable lets a whole model spin round.
• Remote Control: Invisible light carries your commands to the model.
• 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.
• Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

• 🔒 Diff-Lock Mud Truck: Stuck in the mud? Slide the diff lock and both wheels are forced to turn together! (level 4 of 5, about 60 minutes, 33 steps)

Parts you need

• 2 × Beam 13
• 20 × Black pin (grippy)
• 2 × Open frame 5×11
• 6 × L-beam 2×4
• 1 × Axle 11
• 2 × Blue long pin
• 10 × Bush
• 2 × Corner gear (20 teeth)
• 4 × Half bush
• 1 × Double cross block
• 1 × Axle 6
• 1 × Axle 4
• 4 × Big tyre wheel
• 1 × Differential
• 3 × Axle 5
• 4 × Corner gear (12 teeth)
• 1 × M motor
• 2 × Axle 3
• 1 × L motor
• 1 × Universal joint
• 2 × Axle 2
• 1 × Servo motor
• 1 × Axle 12
• 1 × Axle joiner
• 1 × Turntable
• 1 × AA battery box
• 1 × IR receiver
• 1 × Speed remote

Steps

1. Stand a 13-hole beam on its edge — the right rail.
2. Servo frame: black pins in holes 2 and 4 of the rail (inside) and a tall 5×11 frame standing on them.
3. Rear bearing: a black pin in hole 8 of the rail (inside) and a black L-beam 2×4 on it, its short arm pointing up at hole 10.
4. Hold a second L-beam 2×4 inside the first, short arm pointing DOWN at hole 10 and long arm back to the last hole. Push an 11-long axle through the last hole of the rail and this L-beam — the rear axle. A blue long pin through hole 10 of the rail and both L-beams, and a bush on the rear axle outside the rail.
5. Slide a 20-tooth corner gear (teeth facing left), a bush and a half bush onto the rear axle from the left, so the gear sits just RIGHT of the middle — the mirror image of the front one.
6. Left bearing: slide a third L-beam onto the rear axle (short arm down) and a bush behind it. Hold the fourth L-beam (short arm up) outside the third and push a blue long pin through both at hole 10 — its end sticks out for the left rail.
7. Hold a double cross block standing up between the L-beams at hole 10; push a 6-long axle through the top holes of the upper L-beams and the block, and a 4-long axle through the bottom holes of the lower L-beams and the block.
8. Slide the left rail onto the rear axle and the long pin, and push a black pin through its hole 8 into the L-beam. Now the two rails are locked together.
9. Left Servo frame: black pins in holes 2 and 4 of the left rail (inside) and a tall 5×11 frame standing on them.
10. A bush on the left end of the rear axle and the two back wheels.
11. Centre drive: lay the differential long-ways between the rails and push a 5-long rear shaft from its back through the middle hole of the double cross block, with a bush behind the block and a corner gear on the 20.
12. Right motor: black pins in holes 6 and 8 of the right rail (outside) and the M motor pressed onto them, output pointing in through hole 7.
13. A 3-long axle through the rail and the L-beam into the M motor, with a tan corner gear on the differential's ring.
14. Left motor: black pins in holes 6 and 8 of the left rail (outside) and the L motor on them, output pointing in.
15. A 3-long axle through the rail and the L-beam into the L motor with a corner gear on the ring too.
16. Front shaft: push a 5-long axle into the front of the differential, then the universal joint, then a 2-long axle in front of it.
17. Slide the Servo down between the frames, orange output pointing straight DOWN. Push black pins through the 4th and 8th holes of the right frame's front edge into its side.
18. Two black pins through the left frame into the Servo the same way.
19. Bogie: push a 12-long front axle through the 2nd hole of a red L-beam, then slide on an axle joiner (as a spacer) and a 20-tooth corner gear, just left of the middle.
20. A half bush against the gear and the second red L-beam. Make the bogie stiff: a 5-long axle through the top holes of the two L-beams, with a bush on it in the middle so it can't fall out.
21. Two bushes on each end of the front axle.
22. A half bush on each end and the front wheels.
23. Lay the turntable on top of the L-beams.
24. Slide the bogie in from the front, under the Servo. Push a 2-long axle down through the Servo's orange output into the turntable, and slide a 12-tooth corner gear onto the front shaft so it meshes with the bogie's 20 gear.
25. Radio pack: the AA battery box…
26. …with 2 black pins in the low holes at its back end and the IR receiver pressed onto them.
27. Lay the pack over the centre drive and pin its front ear to the back edges of the Servo frames: black pins in their 6th and 7th holes.
28. Wiring: receiver cable onto the battery box. Both drive motors → RED, Servo → BLUE. Grab the remote.

Play time

Tape a paper flag on the front shaft and one on the rear shaft. Drive in a big circle and count the flag turns — the front shaft turns a bit MORE. That is the centre differential at work. Now hold the differential with a finger: the tyres scrub and hop in the turn!

Real-world inspiration

All-wheel drive with a centre differential (rally cars, Subaru); Sariel's Humvee. https://en.wikipedia.org/wiki/All-wheel_drive · https://sariel.pl/2011/04/humvee/