A friction car keeps rolling after you let go because your push spins a heavy flywheel inside it. The wheels drive the flywheel through gears that make it spin much faster than the wheels, and a fast, heavy spinning wheel holds energy. When you let go, the flywheel drives the wheels back through the same gears until friction uses the energy up. The name comes from the tyres’ grip on the floor, which is what lets your push reach the flywheel.
This page answers the questions people ask about friction cars, with a lab built on our Flywheel Friction Car (level 2, 15 parts, five steps, about 25 minutes). It is the moving cousin of why a hand-spun wheel slows down, which looks at one wheel coasting on a stand.
What is inside the Flywheel Friction Car?
Two 5×11 frames hold three axles. The back axle carries the 56 mm wheels and a 40-tooth gear. Three holes above it, a 9-long axle carries a tiny 8-tooth gear between two yellow-hub wheels. Those two yellow wheels are the flywheel. The build notes that they hug both gears, which keeps the gears in mesh. There is no spring, no motor and no clutch: just gears and wheels.
Why is the flywheel geared up?
Because speed counts for more than weight. The energy in a spinning wheel grows with the square of its spin speed. Double the weight of a flywheel the same size and it holds twice the energy; spin the same wheel twice as fast and it holds four times the energy. In the car, 40 teeth drive 8, so the flywheel turns 5 times for each wheel turn. At the same car speed, the yellow wheels hold 5 × 5 = 25 times the energy they would if they were simply fixed on the back axle. The gear ratio idea page explains counting teeth, and the gear ratio worksheet gives practice.
How many times does the flywheel turn when I push it?
Each wheel turn rolls the car π × 56 mm, about 176 mm. Push the car 1 metre and the wheels turn about 5.7 times, so the flywheel turns about 28 times. Try it in the lab: set the gearing, the push and the speed, predict, then check.
Interactive: Flywheel car lab. The Flywheel Friction Car’s 40 → 8 gear-up: push it a set distance or roll it at a set speed and predict how many times the flywheel turns, which way, how fast, and how much more spin energy the gearing buys. Five challenges.
Why does a friction car whirr?
Because the flywheel spins fast. At 1 metre per second, a slow walk, the 56 mm wheels make about 340 turns a minute and the flywheel about 1,700. The fast-turning gears and wheels are what you hear. That is also why the drawing in the lab shows the flywheel as a blur at higher speeds.
Why does the flywheel spin backwards?
Two gears that mesh always turn opposite ways. The car has one mesh between the back axle and the flywheel shaft, so the yellow wheels spin backwards while the car rolls forwards. Add a second mesh and the direction flips back, which is what the lab’s 25 : 1 what-if shows. The build’s own animation spins the flywheel the opposite way to the wheels at five times their speed.
Why do you push it several times before letting go?
A spring can only wind so far, but a flywheel keeps spinning between pushes, so each push adds to what is already there. That is why friction toys can be “pumped up” with a few strokes along the floor. A pull-back car works the other way: it winds a spring, which fills up after a short pull. Some shop-bought friction cars have a one-way clutch so pushing backwards does nothing. Ours has plain gears, so rolling it backwards spins the flywheel too.
Would more gearing always make it roll further?
No, and this is the most common wrong guess. A second 40 → 8 stage (25 : 1) would hold 25 times more energy than the built car at the same speed, but the flywheel would also feel 25 times heavier when you push the car to get it going, so you must put that energy in. Every extra gear also adds friction, which takes energy out the whole time. Whether it rolls further is a question for a test, not the maths. The lab compares gearings only: we haven’t weighed the parts, so it can’t predict distance.
Does it matter where the flywheel sits?
Yes. The Twin-Piston Engine (level 3, about 40 minutes) uses a yellow-hub wheel as a flywheel on the crank, the slow side of a 5 : 1 reduction from its motor. Its build notes that the smoothing effect hasn’t been measured. The Ripcord Top Launcher (level 2, about 20 minutes) does the opposite of the car: a pull on a toothed rack is geared up 5 : 1 to spin a top. The flywheel idea page sums up the idea.
Do real vehicles use flywheels?
Some have. The Swiss gyrobus, a flywheel bus, began regular service in 1953 between Yverdon-les-Bains and Grandson. At stops, roof booms connected to charging points and an electric motor spun the flywheel up to 3,000 rpm in between 30 seconds and 3 minutes. A full charge typically lasted about 6 km on a level route.
Try a fair test
The build suggests one: roll the car with the flywheel, then take the yellow wheels off and roll it again. Mark a start line, give the same number of pushes over the same distance, and measure how far it rolls after you let go. Repeat each three times.
| Set-up | Roll 1 (cm) | Roll 2 (cm) | Roll 3 (cm) |
|---|---|---|---|
| With flywheel | |||
| Without flywheel |
The blanks are for your results: we haven’t run this test. Hand pushes vary, so compare the spread before you decide. The steeper ramp test shows a way to give every run the same start.
The Flywheel Friction Car is in the Mechanical Wonders booklet pack, and its booklet is free on the build page.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- Where does a friction car store the energy from your push?
- In the Flywheel Friction Car, a 40-tooth gear drives an 8-tooth gear. How many flywheel turns for each wheel turn?
- The same flywheel spins twice as fast. How much more spin energy does it hold?
- Why does the car’s flywheel spin the opposite way to its wheels?
- A friend adds a second 5 : 1 stage. What is certain to happen?
