🔀 Differential Demo Cart: step-by-step build

Push it round a corner and watch: the outside wheel spins faster than the inside one!

When a car goes round a corner, the outside wheels have a longer way to go than the inside wheels. So they must spin faster — but how, if they are on the same axle? The answer is a clever gearbox in the middle of the axle called a differential. This cart has a see-through job: tan hubcaps on the back wheels and a speedo knob in the middle, so you can watch the differential share out the turning.

• Level 2 of 5 (easy)
• About 22 minutes
• 10 steps, 34 parts
• Powered by: Push
• 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.
• Drive Train: The chain of parts that carries power from the motor to the wheels.

Build these first

• 🌬️ Bevel-Gear Windmill: Turn the handwheel at the bottom — the sails spin at the top! (level 3 of 5, about 25 minutes, 16 steps)
• 🛒 Push Buggy: A sturdy buggy with a push bar held up by super-strong triangles. (level 2 of 5, about 15 minutes, 12 steps)

Parts you need

• 2 × Beam 11
• 4 × Black pin (grippy)
• 2 × L-beam 2×4
• 1 × Double cross block
• 1 × Axle 4
• 1 × Axle 3
• 8 × Half bush
• 1 × Cross block
• 1 × Bush
• 1 × Corner gear (12 teeth)
• 1 × Axle 5
• 1 × Differential
• 2 × Axle 7
• 1 × Axle 8
• 1 × Knob wheel
• 4 × Yellow-hub wheel
• 2 × Tan knob wheel

Steps

1. Stand an 11-hole beam on its edge — the left side.
2. Push black pins into holes 6 and 7 of the beam, on the inside, and snap a small L-beam onto them, long leg up.
3. Stand a double cross block on the outside of the post, over the beam. Push a 4-long axle through it and the post's 2nd hole, and a 3-long axle through its top hole and the post's 4th hole. A half bush on the inside end of the short axle.
4. Now build the right side on the table: push black pins into holes 6 and 7 of a second 11-hole beam and snap the second small L-beam onto them, long leg up.
5. Slide a cross block upside down onto the lower axle. Slide a grey bush and a corner gear onto a 5-long axle and push it through the cross block, so the corner gear sits over the 10th hole.
6. Hold the differential against the beam, its ring of teeth pressed on the corner gear, and push a 7-long axle through the 10th hole into it. It is only HALF an axle! Lock it with a half bush outside the beam.
7. Slide the right side onto the car: its post goes over the end of the lower axle and its beam traps the differential. Push an 8-long axle through both 2nd holes and lock it with a half bush on each side.
8. Push the other half axle into the differential from this side and lock it with a half bush. Then push a red knob wheel onto the front end of the speedo shaft, right against the cross block, and lock it with a half bush.
9. Push on the four wheels.
10. Push a tan knob wheel onto each back half axle as a hubcap, and lock each one with a half bush.

Play time

Push the cart in a tight circle and watch the hubcaps: the outside one whizzes, the inside one crawls. Now lift the cart and turn one back wheel forwards and the other backwards — what does the speedo do?

Real-world inspiration

"Around the Corner" (Chevrolet, 1937) — the classic differential explainer — https://en.wikipedia.org/wiki/Differential_(mechanical_device)