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Mousetrap Cars and Rubber Band Cars

At our Oracle, we get a number of questions that are something like:

How do I make a car powered by rubberbands? What should the body be made of? What should the wheel and axle be made out of? How do I get the rubberbands to power the car?

and

I need to know how to design a mouse trap car so that it runs fast. And what kinda things need to be incorporated for it to run smooth.

 

Both of these questions are basically requests to use a limited source of stored power to propel a car of some sort for a distance. First, of all, this is a great chance to experiment. Try things out. Try to understand where the losses will be. Have fun.

Another option is to look on the Internet, like you're doing here. Look at our archive at SEPS, as well as go to a search engine like Google.com and search for mouse trap car and you can get your answers quickly without waiting for someone to answer a question to a problem that is regularly assigned to students to study motion.

Given that, let's look at a combination of past answers:

You have to decide whether you want to go fast or far. This determines the "gear ratio" you want, which in turn tells you whether you want really big wheels and a little drum on the axle to pull the string, or more normal wheels and a bigger drum. Check out the sites at the end of this page, and that will make sense. The answer will depend partly on what "road" you are running the car on. On a smooth surface (hardwood, or vinyl), you don't need a lot of power to get started. But on carpet or in grass, you need enough power to get moving, even if you don't need it really when you are underway. Like in a real car, you really would like to use "low gear" to get moving, then "high gear" to drive at speed.

Friction anywhere is BAD!! Everything should run as smoothly as possible. The "bearings" hold the axles to the car frame should be as smooth as possible: maybe make the axles out of plastic rod, and a drop of oil might be useful too if the materials are not too porous (e.g., plastic instead of wood). Anywhere else that anything rubs against anything else, try to make the parts as smooth as possible. Air resistance is also a factor, but really not so much with these model cars.

Energy lost to air resistance goes up as the square of velocity, so if you're being judged on distance, a low speed and very high gear ratio is desirable. The car should make the best use of its spring-stored energy. If maximum speed is what's important, then you optimize it differently.

Rubber band cars are powered by wrapping up a rubber band around one axle of the car, so that as the rubber band tries to unwind, it turns the axle and the wheels. What you want is the most power you can get for the longest time, and the car needs to be light but strong enough that it doesn't break as you wind up the band.

 

I have already built my car, it runs about 10 metres, and it is powered solely by an elastic band. I have 4 wheels, set up like a real car. You take one end of the elastic and hook it to the nail on the axle hooked to the front two wheels, and then hook the other end to the nail on the rear axle conecting the two wheels. Then you simply wind the wheels until the elastic is nice and tight.

 

Mousetrap powered cars use the release of the spring to pull a string from around a drum or axle rather than a rubber band for the stored energy. another thought is to consider how you might increase the energy stored in the mousetrap; for example, is there any reason why the whole mousetrap frame has to be used? If not, then you could cut off one side and over-rotate the mousetrap bail past the usual 'cocked' position to store more energy before release. You'll have to figure out if that's legal, and how to trigger the car.

Again, the main idea here is to experiment and have fun.

Last updated 18 May 2007