🌀 Pull-back Diff Racer: step-by-step build

A spring racer with a real differential, so it zooms round curves without skidding.

Race cars have their engine lying long-ways, and so does this one! The pull-back motor turns a little corner gear, and the corner gear turns a differential on the back axle. The differential lets the two back wheels spin at different speeds, so in a curve the outside wheel can go faster than the inside wheel. Set the front wheels with the steering knob, pull the car back and watch it swoosh round a bend!

Level 3 of 5 (medium)
About 40 minutes
23 steps, 70 parts
Powered by: Pull-back
Series: Gearbox & Drivetrain Cars

What you will learn

The Differential: Lets the two driven wheels spin at different speeds in a turn.
Bevel Gears: Around the Corner: Slanted gears send spinning power around a corner.
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.
Steering: Turning the front wheels makes a car follow a curve.
Rack & Pinion Steering: A gear rolling on a straight rack slides it sideways.
Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

🏁 Pull-back Racer: Pull it back, let it go — ZOOM! A racer with a spring for an engine. (level 2 of 5, about 12 minutes, 10 steps)
🏎️ Steering Go-Kart: Turn the steering wheel and the front wheels really turn — just like a real car. (level 3 of 5, about 30 minutes, 25 steps)

Parts you need

1 × Pull-back motor
1 × Axle 5
16 × Black pin (grippy)
1 × Open frame 5×7
4 × Half bush
2 × Beam 15
4 × L-beam 3×5
8 × Red axle-pin
6 × Double cross block
3 × Axle 7
1 × Corner gear (12 teeth)
1 × Differential
4 × Yellow-hub wheel
2 × Beam 9
4 × Grey pin (smooth)
2 × Steering arm
2 × Axle 3
1 × Gear rack
2 × Cross block
1 × Axle 9
1 × Beam 5
1 × Axle 8
1 × Knob wheel
1 × Tiny gear (8 teeth)

Steps

Engine: stand the pull-back motor up so its cross hole points front-to-back, and push a 5-long axle through it, sticking out at the back.
Find the row of 5 holes along one edge of the motor. Push a black pin half-way into the first and the last of them, from the back, and press the red 5×7 frame onto them, standing across. Slide a half bush onto the axle, right behind the frame.
Push a black pin into the bottom row of the frame, 2 holes from each side, pointing backwards.
Stand a 15-hole beam on its edge — the left side of the racer. Push black pins into holes 11 and 13 (inside) and press an L-beam 3×5 onto them, short arm standing up at hole 10.
A red axle-pin into the L-beam's corner hole, axle end inwards. Push a double cross block onto it, standing up, and a 7-long axle through the top of the L-beam and the block's top hole.
Slide a second double cross block onto the other end of the axle and push a red axle-pin into its bottom hole, pin end outwards.
Press the second L-beam onto the axle and the axle-pin, push 2 black pins through its long arm, and press the right side beam onto them.
Slide the spring engine in from the front: the 2 pins behind the frame click into the middle holes of the double cross blocks.
Slide the tan corner gear onto the back end of the engine axle, slanted teeth pointing backwards. Hold the differential right behind it, so the gear meshes with its big slanted ring, and push a 7-long axle through each side into it — the two half-shafts.
Push on the two big back wheels.
Steering module: lay a 9-hole beam flat and push a smooth grey pin up through each end hole.
Put a steering arm on each pin, pointing forwards, and slide a short axle into each arm.
Grey pins in the arm tips and the red tie rod on them.
Lay the rack on the tie rod, teeth up, and push a black pin up through the 2nd and 8th holes of the flat beam.
Dashboard posts: black pins in holes 2 and 4 of the left side beam (inside) and an L-beam 3×5 on them, long arm standing up at hole 4. A red axle-pin in its middle foot hole, axle end inwards.
The same on the right side beam.
Put a cross block on each black pin of the steering module, its cross hole pointing sideways, towards the back.
Slide the steering module under the front of the racer. Push a double cross block onto each red axle-pin, then push a 9-long axle from the side through the first hole of the side beam, both cross blocks, both double cross blocks and out through the other side beam.
Push on the two front wheels.
Red axle-pins into the 3rd and 5th holes of both posts, axle ends inwards.
Press a double cross block onto each pair of axle-pins, standing up, and push a black pin into each block's middle hole, pointing forwards.
Press a 5-hole dashboard beam onto both pins and slide an 8-long axle through its middle. Push a knob wheel onto the back end, snug against the beam, and a half bush behind it.
On the front end: a half bush, the tiny pinion right on the rack, and another half bush to hold it there.

Play time

Set the steering knob a little to the left, pull the racer back and let go — it drives a smooth curve. Now lift one back wheel off the floor and pull back: that wheel spins and the spring runs out! That is the one weakness of a differential.

Real-world inspiration

Rear-wheel-drive race cars with a long-ways (longitudinal) engine drive the rear differential through a bevel pinion. https://en.wikipedia.org/wiki/Differential_(mechanical_device)

Build next

🔒 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)