I have to make a self propelled car for a physics project. My criterial are as follows: 1)the car must be made of items found at home that normally don't fill the role that they have on a car; 2) the method of propelling the car can only be air, spring action or rubber band power -- no combustion of any kind; 3) the car is sturdy; 4) the car can run more than ten runs; 5)the car can move more than 20 m; 6) the car moves in a straight line.
I don't even know where to begin! Any suggestions
When I was in college, my Mechanical Design class had a problem similar to this. We were given a bag of junk and had to build a vehicle that could go over a hill and stop in a designated place. Our only power source was a rubber band, and catapult-things had to bring 90% of the mass to the finish line.
Here's some stuff I learned from that project that might help you.
1. The limit on power is how far you can stretch the rubber band without causing the wheels to "spin". This is a friction problem...more weight on the wheels means it can take more force, but you have more mass to move, so you need more force. There is a tradeoff.
2. How much power is transmitted to the wheels is a function of the size of the axle vs the size of the wheel (assuming you just wrap the rubber band around the axle).
3. Rubberbands don't have the same power as they relax at all stages for the same amount of "stretch". If you need extra power halfway through the expected path (as we did with the hill) or just want it to keep accellerating for longer, you can have it "change gears" by putting something wider around the axle and wrapping some of the rubber band around the narrow axle and some around the wider axle.
4. If you want it to go in a straight line, you need either machine the car very carefully, or use large wheels. The fewer turns the wheel has to take, the more likely it is to end up where you aimed it. I used large wheels and manufactured my car with hand tools. Some people made cars that would fit in a child's palm but had to place the vehicle on the track with micrometers to get it to end up where they aimed it.
5. To get it to stop in a single place, the usual "brake" we used was a string the same length as the desired travel distance. When the string runs out, one of the axles stops turning. Unless your wheels had a lot of friction on the axle, you needed a ratchet to keep it from rolling backward after stopping, because the string stretches during braking and kind of acts like a weak rubber band the other direction.
6. Most of us "wound up" the engine mostly by dragging the car backwards on the floor, the distance we wanted it to go.
7. Making the chasse of the car was the easy part. Most of the fiddly parts were getting the gear ratios right between the rubber band and wheel/axle assembly (less important in your case because you don't have a hill halfway down the course, when the rubberband power is starting to get weak)
The most useful material we used was plexiglass, since it is easily manipulated with things like bandsaws, drill press or even hand tools, and can be glued together with solvents. I'm not sure that's allowed in your case....given household materials, you could make a very light but strong for its weight vehicle mostly out of cardboard, but you'd need a frame strong enough to take the tension of the rubber band..likely made of wood.
You might want a way to add weight right over the axle, and think hard about how to make the axle, what to use as "Bearings" so the axle can spin (a steel rod through drilled hole in plexiglass worked ok for me, but your tools and materials seem a little cruder). Also how to make round wheels...a lot depends on what "Bag of junk" you find around the house.
So...look for wheels. Look for axles. Look for a way to build a frame sturdy enough to hold two axles with rubber band at full tension. Experiment with different sizes of wheels and tensions and axles to get it to meet your specs (20' distance, straight line). Build it as sturdy as possible - you'll need to make a LOT of test runs before you think it will perform reliably in the field.
Also begin soon - it will likely take a lot more time than you think. Many of us (including myself) were awake 2-3 days straight before the actual test, in order to get our normal college workload done and still find enough time to complete the testing. (Most of us had cars "in time" but testing showed many last second flaws....). Don't put yourself in that situation.