🔄 Reverse-to-Turn Car: step-by-step build

Only one motor — but it can still turn! Drive backwards and the wheels swing round.

Some toy cars have only one motor, yet they can turn round corners. How? The motor drives the back wheels, and its big gear also spins a clutch gear that turns the steering. That clutch gear SLIPS when it is pushed too hard. Driving forwards, the steering is pushed against a stop pin: the wheels stay straight and the clutch gear just slips. Drive backwards and the steering swings all the way round until it can go no further — then it slips again. Forward, back, forward: a three-point turn with one motor!

• Level 4 of 5 (tricky)
• About 50 minutes
• 24 steps, 69 parts
• Powered by: Motor
• Series: Gearbox & Drivetrain Cars

What you will learn

• Clutch & Driving Ring: A part that connects or disconnects a gear from its axle.
• Steering: Turning the front wheels makes a car follow a curve.
• Friction: Friction is the rubbing force that slows things down.
• Bevel Gears: Around the Corner: Slanted gears send spinning power around a corner.
• Rack & Pinion Steering: A gear rolling on a straight rack slides it sideways.
• 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.
• Switching Direction: Flip which way electricity flows and the motor spins backwards.
• Triangles Are Strong: A triangle cannot be squished — a square can.
• Bushes & Spacers: Bushes stop things sliding off an axle.

Build these first

• 🎛️ One Motor, Knob Steering: The motor drives, YOU steer — turn the knob as you walk beside it. (level 3 of 5, about 40 minutes, 21 steps)

Parts you need

• 2 × Beam 15
• 13 × Black pin (grippy)
• 2 × Long pin-axle block
• 3 × L-beam 3×5
• 1 × Axle 11
• 2 × Giant gear (40 teeth)
• 4 × Yellow-hub wheel
• 6 × Bush
• 1 × AAA battery box
• 1 × M motor
• 1 × Direction switch
• 2 × Axle 4
• 8 × Half bush
• 1 × Beam 7
• 1 × Double cross block
• 2 × Axle 5
• 2 × Beam 9
• 4 × Grey pin (smooth)
• 2 × Steering arm
• 2 × Axle 3
• 1 × Gear rack
• 2 × Red axle-pin
• 1 × Medium gear (24 teeth)
• 2 × Corner gear (12 teeth)
• 1 × Axle 6
• 1 × Clutch gear (24 teeth)
• 1 × Extension wire

Steps

1. Stand a 15-hole beam on its edge — the right chassis rail.
2. Push black pins into holes 1 and 3 of the rail (inside) and press a long pin-axle block onto them — a steering socket.
3. Right column post: black pins in holes 5 and 7 (inside) and an L-beam 3×5 on them, long arm standing up at hole 5.
4. Clutch post: the same in holes 8 and 10.
5. Motor post: the same in holes 12 and 14, and a black pin in its 2nd hole from the top, pointing inwards.
6. Push an 11-long axle through the last hole of the rail, put a giant 40 gear on it, outside, and push the right back wheel on after it so the gear can't slide off. A grey bush on the inside, against the rail.
7. Power pack: take the AAA battery box.
8. Press the M motor across the back end of the box (its underside clicks onto the studs), output pointing right.
9. Press the direction switch onto the front left corner of the box.
10. Slide the power pack in from the left: the pin goes into the hole under the motor's orange output. Push a 4-long axle through the top of the back post into the motor, then slide on a bush, a 40 gear — it drives the back wheels — and a half bush.
11. Slide the left rail onto the axle and a bush onto the axle outside it. A black pin in hole 5 (inside) and a 7-hole post on it — the left column post.
12. Hold a double cross block standing up between the column posts and push a 5-long axle through both posts' 4th holes and its bottom hole. A 4-long axle through the left post's 6th hole, a yellow half bush and into its top hole.
13. Push a grey bush onto the end of the short top axle, outside the left post. Left steering socket: 2 black pins in holes 1 and 3 (inside) and a long pin-axle block.
14. Push on the left back wheel.
15. Steering module: lay a 9-hole beam flat and push a grey pin up through each end hole.
16. Put a steering arm on each grey pin and slide a short axle into each arm.
17. Push on the front wheels.
18. Grey pins in the arm ends, the red tie rod on top, and the gear rack on the tie rod, teeth up.
19. 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. Push a black STOP pin up through the tie rod, 2 holes left of the middle.
20. Steering column: slide a 5-long axle through the round hole of the cross block. On the front end: a half bush against the block, a 24 gear that sits on the rack, and another half bush.
21. On the back end of the column: a bush, a half bush and a tan corner gear.
22. Clutch shaft: hold a half bush, a tan corner gear and another half bush in a row inside the middle post, and push a 6-long axle through the top of the post and all three. The corner gear meshes with the column's corner gear.
23. Outside the post: a bush right against it, so the axle can't fall out, then the 24 CLUTCH gear, locked on with a half bush. It meshes with the motor's 40 gear.
24. Wiring: plug the switch's cable onto the battery box and stack the motor's plug on the switch (use the extension wire if needed).

Play time

Switch forwards: the car drives straight. Flip to backwards: it swings round in a curve. Flip forwards again: straight off in a new direction! Can you get it out of a dead-end corridor made of books using only the switch?

Real-world inspiration

Single-channel "reverse-to-turn" toy RC cars and the back-and-turn trick of robot vacuums; slip clutches. https://en.wikipedia.org/wiki/Slip_clutch · https://www.technicopedia.com/fundamentals.html