Building a self-propelled toy car is actually a very easy thing to do and is also a project a father and son can do together when the son is still pretty young. Also, it’s a project that has a virtually unlimited number of ways in which it can be done, though means of propelling the car remain the same.
In essence what you do is build or obtain a very light box that is anywhere from a couple of inches to about a foot in length, add wheels and a hook on the front inside part of the box, and rubber band for providing the power and an axle for connecting the wheels and for wrapping the rubber band around.
The box doesn’t have to actually be a whole box, you can use just a piece of corrugated cardboard if you choose, just so long as it is strong enough to withstand the pressure of the rubber band when you’ve got it pulled taut. Also, it’s best if you have one of those wide band rubber bands, because they work the best with little toy cars.
At any rate, you connect the pieces and parts together in such a way as to have a box with wheels on it, and a rubber band running from the front inside of the box or piece of cardboard, to the back axle. This way, when you press down on the car and push it backwards, the rubber band is forced to wrap around the back axle. By holding the back wheels firm with your hand while holding the car and repeatedly pushing it backwards on the floor or table, you build up more and more tension in the rubber band. Then, when you stop, set the car on the table or floor and let go, the car lurches ahead as the rubber band unwinds itself on the back axle.
To make such a car, you can use an old cigar or tissue box for the main body, wood wheels from a tinker toy set, and or an old broken toy car and the very round and smooth kind of chop-sticks for the axles, though you will likely want to cut them so be sure you won’t need them anymore. For the hook you can use a picture hanger type or if you don’t have a hook you can always just staple one end of the rubber band to the inside front end of the box. As for the rubber band on the other end, where it attaches you’ll need to glue it with something strong so it doesn’t slip when the tension first starts, rubber glue works okay, but super glue is best. Also, so that your car doesn’t spin out when you first let go, you might try putting smaller rubber bands on them so that you’ll be able to get some traction on slippery surfaces.