Building a Car that Moves – Engineering for Kids

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In last year’s Car-themed class, our students decorated toy cars. This year, I wanted them to BUILD a model car that moved. (Here are the design options we considered, along with source citations.) At bare minimum, I wanted cars that would roll down a ramp. But, even better, I wanted to motorize the cars – build something that would move on its own.

After LOTS of research, tinkering, contemplating, and starting over, we came up with two very different designs that we’ll be using in this weekend’s class. Here’s a “how-to” tutorial on our retractable badge car. Check out the clothespin racer here.

These cars are designed to be done by children as young as 3 years old (with lots of adult support). A 6 – 7 year old should be able to assemble a car with minimal support.

Retractable Badge Car

Note: to see a larger version of any photo, just click on it.

Materials : corrugated plastic or cardboard for car body; plastic straws for axle bearings, 1/4″ dowels for axles, wooden wheels, glue gun and/or Tacky Glue, and Retractable Badge Holders. (Note: the badge holders I link to there are cheap… but 4 out of 12 broke the first time we pulled the string out…. So, if you order, order more than you need!)

Preparation (Parent or Teacher):

Step One: Use pliers to take the plastic badge holder with the snaps off of the retractable clip. Caution: Do NOT cut off the plastic doo-dad on the end of the string! If you do, the string will retract completely into the badge, never to be seen again.
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Step Two: Cut out the car body. It should be 5″ wide, and 7 1/2″ long. Cut out the tab as shown in illustration. (Or print this PDF to use as a template.)

car-body

Step Three: Cut dowel in half – making two 6″ segments.

Step Four: Cut straw – you’ll need two 1-inch segments, and one 4 or 5-inch segment.

Step Five: Use hot glue gun to drizzle glue on the edges of the wheels… this is to give them traction. Without it, the wooden wheels just slip and the car won’t move. Alternate: you can also wrap rubber bands around the rim of the tire. Or steal rubber tires off a toy.

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Assembly by the kids:

Step One: Decorate the car body if you choose to. If you’re using corrugated plastic, you’ll need to use Sharpies or Sharpie Paint Markers. These look good, but are, of course, not washable. I paid the penalty this week, because the one kid who got marker all over their clothes in my class was of course my own kid!

Step Two: Clip the badge holder to the front of the car, so the clip is on top, and the holder on the bottom.

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Step Three: On the bottom of the body, on the back end, tape two 1-inch segments of plastic straws to each side of the gap. It’s important these be straight / lined up with each other so the axle will run straight. Tape a 4 or 5-inch segment of straw straight across the front end of the car, just below the badge. (Adult should check for straightness.)

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Step Four: Slide the wooden dowels through the straws.

Step Five: Put the wheels on. If they fit tightly and don’t slip around on the dowel, it’s best not to glue, so you can take it apart later if you want / need to. But, if they slip around on the dowel, you have to glue them in place so that when the axle turns, the wheels turn – otherwise this car won’t work. Use the Tacky Glue for this.

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Step Six: Pull out the string for the badge holder and tie it around the axle. An adult will need to do this – it’s pretty hard to tie a string snugly, especially when it’s trying to retract into its holder. Then tape the plastic doodad at the end of the string onto the axle – this will help the string not just spin round and round when we want it to turn the axle.

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Step Seven: Test it! Turn the car over – while pressing down just a but, pull it back about 10 – 15 inches  to wind the cord. Release. It will travel forward! (A little less distance than the length of the cord.) Check out the video below.

Taking the Retractable Badge Car to the Next Level

If you put bigger wheels on the badge car, it will go MUCH further. Try gluing CD’s on top of the wooden wheels (make sure they’re centered!) and check out what happens!

CD’s aren’t the perfect wheel… they don’t have any traction, so you can’t “pull-back” the car to wind it up. You have to hand wind the string, then set it down and release. You might also try some other kind of large wheel – such as wheels cut from a cardboard box.

Obviously, if you’d rather just start with the large wheel, you might not need to purchase the little wooden wheels – you could start with figuring out how to affix the big wheel directly to the dowel.

Have fun playing with cars!

Note: All the activities described in my posts are from Family Inventor’s Lab, a parent-child cooperative class in Bellevue, WA. We are a play-based, STEM focused class for preschool through early elementary (kids age 3 – 7). We do a wide variety of fun, hands-on activities to learn about Science, Tools, Engineering, Nature, and Art. We also sing songs and read stories. Most of our activities are cheap, easy, and use everyday materials that most families would have in their homes (or their recycle bins!), so our activities are appropriate for classroom teachers, parents who homeschool, or after school programs.

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