Make a square wheel car activity | Fizzics Education

# Square wheel car

### You will need:

A cardboard box

Scissors, a ruler, a pencil, paper and tape

A box of water bottles

Two wooden kebab sticks and a plastic straw

1

Line up the water bottles in a row and then tape them together.

2

Measure the diameter of the base of your water bottle.

Multiply that number by 1.2 (we’ll go through “why” in mathematics further down the page).

3

Using the scissors, cut out 4 squares with a side length of the number calculated above.

4

Draw 2 lines on each of your square wheels by connecting the opposite corners together (it should look like a cross on each wheel). This allows you to find the center of each wheel.

Poke a wooden kebab stick through one of the crosses on a wheel where those two lines intersect.

5

Test that you can roll one of the square wheels along the taped together water bottles.

Tape the wooden kebab stick onto the wheel to make the connection strong.

6

Measure the width of your water bottle row where the sides are straight (avoid the neck of the bottle). Whatever the width, subtract at least 30mm off that number so that your square wheel car has enough room to move along the water bottle track.

7

Cut out a cardboard rectangle for the base of your car. You need to use the width of the water bottles minus 30mm as calculated above for the width of your rectangle. For the length you need enough it would be worth making it the side length of the wheels multiplied by 3 so you have lots of clearance for both sets of wheels to spin next to each other.

8

Cut your plastic straw in half & tape each half of the straw on the base of car securely.

This straw needs to act as the housing of your kebab stick wheel axles. It’s best to make sure that this straw is aligned squarely with cardboard base (you can use a protractor if you’d like to check that these angles are 90 degrees!).

Slide your kebab stick with the cardboard square through the straw and then add another square on the other side. You’ve made the first axle with square wheels!

9

Repeat again for the other side of the cardboard base, making sure that the wheels can’t hit each other when they spinÂ and you are done!

10

You might want to push another kebab stick through the middle of your car base and tape it so that you have a handle to use when moving your square wheel car along the water bottle track.

11
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### What is going on?

There is a fair bit of mathematics hidden behind this experiment. You’ll also have seen that the car base didn’t move up and down but rather moved along the track at the same height the whole time. This is because the square wheels followed the dips and rises of the curved water bottles exactly, making the axles of the car not move up and down vertically at all. The line that each edge of the square wheels were following is called a catenary curve.

As your square wheel moves along the curved track you’ll notice that the sides of the square follow the curve of the water bottle without jamming into the bottles. This is the same issue engineers have to sort out when meshing gear teeth together.

The mathematics can be daunting for younger kids but great to follow through with high school math students. The Exploratorium has done a neat summary of the mathematics. Note that l is the side of the square wheels and d is the diameter of the water bottles.