🏆 Independent Rally Car: step-by-step build

Every wheel on its own springy arm, a differential, universal joints and Ackermann steering.

Rally cars fly over bumps, rocks and jumps. The secret: every wheel has its OWN arm and its own spring, so when one wheel hits a bump the other three stay on the ground. The back wheels are driven by the L motor through a differential. Because the wheels move up and down, the drive shafts need universal joints — bendy couplings that pass on the spin at an angle. At the front, the Servo steers through a rack, and the steering arms are angled so the inside wheel turns more than the outside one. That is called Ackermann steering. This is a big build — take it corner by corner, like a real race team.

Level 5 of 5 (a big challenge)
About 90 minutes
47 steps, 137 parts
Powered by: Motor
Series: Gearbox & Drivetrain Cars

What you will learn

Suspension & Springs: Springs let wheels bounce over bumps while the body stays calm.
Ackermann Steering: The inside wheel turns more than the outside wheel.
The Differential: Lets the two driven wheels spin at different speeds in a turn.
Universal Joint: Passes turning power around a bend.
Servo Motor: A motor that turns to an angle and holds it.
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.
Triangles Are Strong: A triangle cannot be squished — a square can.
Bushes & Spacers: Bushes stop things sliding off an axle.
Steering: Turning the front wheels makes a car follow a curve.
Rack & Pinion Steering: A gear rolling on a straight rack slides it sideways.

Build these first

🛻 Bouncy Monster Truck: Four big wheels on four springy shock absorbers. Boing, boing! (level 4 of 5, about 45 minutes, 29 steps)
🔒 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 15
1 × L motor
3 × Axle 2
12 × Half bush
8 × Blue long pin
4 × Beam 3
48 × Black pin (grippy)
8 × Beam 7
3 × Beam 11
6 × Beam 5
1 × Servo motor
2 × Hard spring
4 × Suspension arm
7 × Axle 4
1 × Corner gear (12 teeth)
2 × Universal joint
2 × Axle 5
1 × Differential
3 × Bush
4 × Yellow-hub wheel
2 × Long pin-axle block
2 × Soft spring
2 × Axle 3
2 × Steering arm
2 × Cross block
1 × Gear rack
1 × Medium gear (24 teeth)
1 × AA battery box
1 × IR receiver
1 × Speed remote

Steps

Stand a 15-hole beam on its edge — the right main rail.
Drive motor: push a 2-long axle into the L motor's output and slide a half bush onto it, right against the motor.
Motor posts: blue long pins through holes 5 and 11 of the rail, sticking out 2 on the inside, and a 3-hole post hanging down on each.
Slide the L motor in from the left, output facing backwards: the blue pins go into its side holes. Push black pins through the bottom holes of the posts into the motor.
Rear pivot beam: hold a 7-hole beam under the rail and push black pins through its 1st and last holes into the middle of the motor posts.
Tall post: a black pin in hole 6 (inside) and an 11-hole post standing on it. Brace: a black pin in hole 3 (outside) and a 7-hole beam on it, leaning back towards the post.
Rear shock tower: hold a 7-hole beam level between the post and the brace, and push a blue long pin through the brace's 6th hole, the tower and the post's 5th hole.
A black pin in hole 8 of the rail and a 5-hole post on it, pinned at the top to the tower.
…and another 5-hole post at hole 10.
Black pins into the 2nd holes of the tall post and the first 5-hole post, pointing inwards, and slide the Servo onto them from the left, over the L motor.
Front post: a 5-hole post pinned to the Servo's two front side holes.
Front shock tower: a 7-hole beam pinned to the tops of the front post and the 6th hole of the tall post.
Left side: push pins into the 4 left side holes of the motor — blue long pins in the top two, black pins in the bottom two.
Press a 3-hole motor post onto each pair.
Left pivot beam, pinned into the middle of the motor posts.
Black pins into the Servo's 4 side holes on the left: 2 at the front, 2 at the back.
Press the posts onto them: the 11-hole tall post and a 5-hole post at the back, a 5-hole front post at the front.
Black pins in the bottom holes of the tall post and the 5-hole back post, pointing out, and press the left main rail onto all four pins.
Left brace and rear shock tower, held by a blue long pin like on the right.
Pin the tower to the back post, and add the last 5-hole post at hole 10.
Left front shock tower.
Right rear spring: hold a red (hard) shock absorber outside the tower and push a black pin through it into the tower's last hole.
Right rear arm: a suspension arm outside the pivot beam, on a 4-long axle through the arm's front hole and the beam's 6th hole. A half bush on each end so the axle can't fall out — the arm still swings freely.
Slide a tan corner gear onto the motor's axle. Hold a universal joint inside the arm's back hole and push a 5-long wheel axle through the arm into it; a 2-long half-shaft into its other end.
Slide the differential onto the half-shaft from the left, its ring on the tan corner gear. Push a 2-long half-shaft into its other side and a universal joint onto that.
A black pin through the arm's 3rd hole into the bottom of the spring. A bush and the right rear wheel on the wheel axle.
Left rear spring.
Left rear arm.
A 5-long wheel axle through the left arm into the universal joint, the pin into the spring…
…a bush and the left rear wheel.
Right front corner: a suspension arm with a black pin in its 1st hole, and a long pin-axle block pressed on beside it.
Push a blue long pin through the 3rd hole of the block and the arm so it sticks out 1 on the inside, and a yellow (soft) shock absorber onto it.
A steering arm under the block, pointing backwards, on a 3-long king-pin axle. A half bush on the bottom end so the king pin can't fall out.
Hold a cross block inside the steering arm, pin hole towards the back, and push a 4-long stub axle through the front wheel, the arm and the block.
Hang the right front corner on the rail: a 4-long axle through the rail's 2nd hole and the arm's back hole, with a half bush on each end. A black pin through the front shock tower into the top of the yellow spring.
Left front corner: a suspension arm with a black pin in its 1st hole, and a long pin-axle block pressed on beside it.
Push a blue long pin through the 3rd hole of the block and the arm so it sticks out 1 on the inside, and a yellow (soft) shock absorber onto it.
A steering arm under the block, pointing backwards, on a 3-long king-pin axle. A half bush on the bottom end so the king pin can't fall out.
Hold a cross block inside the steering arm, pin hole towards the back, and push a 4-long stub axle through the front wheel, the arm and the block.
Hang the left front corner on the same way, half bushes, spring pin and all.
Tie rod: lay the red 11-hole beam flat under the two Ackermann cross blocks and push a black pin up through each end hole into a cross block.
Lay the gear rack on the tie rod, teeth up.
A 4-long axle into the Servo's output with a bush, a 24 pinion on the rack and a half bush on the front end.
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 on the roof and pin its back ear to the tall posts: black pins in their 9th and 10th holes.
Wiring: receiver cable onto the battery box. L motor → RED, Servo → BLUE. Grab the remote!

Play time

Make a bump course with pencils and a folded towel. Drive over it slowly and watch each wheel move on its own. Then drive tight circles: look how the inside front wheel is turned more than the outside one.

Real-world inspiration

Sariel's narrow independent suspension with a differential lock; double-wishbone and trailing-arm suspension in rally cars. https://sariel.pl/2020/10/narrow-independent-suspension-with-a-differential-lock/ · https://en.wikipedia.org/wiki/Double_wishbone_suspension