Mousetrap Vehicle

Mousetrap Vehicle for 2012"This is a building event. Prior to the competition teams will build a vehicle powered only by one mousetraps that is capable of traveling down a straight track as quickly as possible, and stopping as close to the finish line as possible. The exact track distance for Regionals is 10.00 m; for State between 8.00 and 12.00 m in 1.00 m intervals" --From NC state site.

How it Works
The mousetrap is mounted on the car. When the mousetrap is triggered, they pull their killbar, which is attached to a string, attached to the axle, attached to wheels. The pulling of the killbar pulls the string. So, if you wind the string around the axle, when the string is pulled, it will spin the axle, turning the wheels, propelling the car.

How to Make it Work
There are three easy designs to get it to come back:

Method 1

One mousetrap on each axle: you wind one mousetrap up and as it goes down it winds the other mousetrap up to pull it back. Normally to do this model you need to add a bar to your mousetrap to make it have more leverage to make it go the x amount of meters.

Also you need to have the string come off of the axle or you will start to head back towards the x amount of meters mark. However, you then are stuck with the problem of not having to wind up a mousetrap on the way back, which means less resistance, which needs a brake, or you'll overshoot the starting point.

Method 2

Two mousetraps on one axle with the other axle having a piece of rubber that gets stretched as you go the x amount of meters and will send it back to the beginning. Also normally you need to add a bar to your mousetrap to make it have more leverage to make it go the x amount of meters.

Method 3

The last model, thought to be the easiest, is to have two mousetraps connected to the axle and you wind it some amount of times and than wrap it around a pin on the axle and continue wrapping it the opposite way that you started. The number of times you wrap it depends on the circumference of the wheels. Standard 12 cm CD's need to have the string wound around 19 times each way. Normally you need to add a bar to your mousetrap to make it have more leverage to make it go the x amount of meters. In this case, you need double the length of the bar of the first model (on the first model, there are two bars).

An alternative to a brake is to attach two nuts to your axle and squeeze them really really tightly around your string. (You need tools for this; you can't get it by hand.) You need to make sure that when the mousetrap is fully unwound, the string has no slack at all. This way, when the mousetrap finishes unwinding after a run, the string will stop the axle, stopping the car.

Number of Turns
To figure out the number of winds needed, all you must do is get the circumference of your wheels that are being driven and divide the distance by circumference. Round up if you don't want a 1000 point penalty! However, due to inertia, testing is HIGHLY RECOMMENDED to find the number of winds. Some vehicles need more or less than others, even with wheels of the same size.

Standard 12 cm CD's need to be wound 19 times to hit 7.16 meters. However, this is MATHEMATICAL and is no substitute for REAL TESTING. When testing however, the mathematical number of winds is useful to use as a starting point.

Making Your Car Go the Distance
If for some reason, your car can't travel the full ten meters (2011), try these tips (note:if your car isn't moving at all, look at the next section, as you most likely need more power).

-Make your drive axle thinner, this will allow you to wind your string around the axle more times, and thus your car will go farther.

-Make your drive wheels larger; wheels with a larger circumference will travel more with each rotation.

-Make your drive string longer (note that this also entails that you make your lever arm longer, and you may also have to increase the distance between the two axles so you don't have string just hanging).

-Make your car lighter

Making Your Car Faster
Alright, so your car can now travel 10 meters. What's next? Make it faster! Follow these tips (note that they are opposite of the distance tips, so you will need to find a balance between the distance you can travel and speed; you will generally want to 'just'be able to make the distance with a little wiggle room).

-Thicken your axle: wrap some tape radially around the axle, you will be surprised as to how much faster this makes your car. This also helps if your car isn't moving at all.

-Use smaller wheels, they need less force to complete a rotation.

-Make your car lighter

Making Your Car Accurate
You've got a speedy car, and now you just have to keep testing 'till your car is accurate! I highly recommend the employment of a braking system, especially a wingnut system. Look up the scrambler braking systems for some ideas. Also, when testing, test with the 16 oz. cup, you will be surprised as to what a difference it can make. If your car is struggling in pushing the cup, look in the speed tip section.

Making Your Car Go Straight
Okay, you're almost done! Some years, straightness has been enforced more than others. If you cannot build the axles to be exactly parallel, then there are some other options. One commonly used solution is to build the car in two halves. The front axle will be part of the front half, and the rear axle will be attached to the back half. The two halves of the vehicle are connected together with a bolt that runs to the middle. Here is a modified example of this method. To adjust the path of the vehicle, simply loosen the bolt, turn the front half, and lock it in place by tightening the bolt again. Now you can adjust how straight or arched the path of your vehicle is!

General Tips
CD's or vinyl records are common wheel ideas. Rubber bands can be placed around CD's to add traction. Also, good wheels can be found at a local hobby store. Airplane wheels are often light and come in a variety of sizes. The only problem is they can sometimes be reasonably expensive.

If possible, try to test on a suface similar to the one that you will encounter at the competition. Testing on tile and then competing on hardwood will affect your vehicle's performance. Best way is to test on multiple surfaces, and keep data for each one.

While it is true that the distance you're aiming for (in 2010) is between 7 and 8 meters, you should NOT aim for 7.5 meters unless your vehicle's accuracy is off frequently. If you have an accurate vehicle, you should aim for somewhere in the 7.2 or 7.3 range to minimize distance, and thus minimize time, while still allowing room for a bit of error. After all, if every vehicle performs as well as yours, but all of the others go out 7.5 meters, and yours goes out 7.2, you have won the event on time score. However, if your vehicle is highly inaccurate, then it is best to aim for around 8, as a 50 point penalty is much better than an 1000 point penalty.

For string, fishing wire works quite well. Sewing thread breaks fairly easily. It is best to have thin string so that the thickness of the string doesn't interfere with winding the string.

When using the third method, it is best to move the mousetraps as far from the driving wheels as possible. This enables you to maximize the length of the string being wound. It is not practical to use a bar more than 34 cm (in 2010) as that is how large the distance between axles can be. The reason is that if the end of the bar ends up outside the axle (relative to the center of the car), the string must extend back to the axle to wind around it, so any additional string is wasted. Therefore, unless placing your mousetraps outside the non-driving axle, the maximum string length that can actually be would around the driving axle is around 70 cm.

When you wind the string, lock the mousetrap, hold the string tightly (you don't want any slack on the string around the axle), and turn the axle in the opposite direction that you want it to go. If you are using the third method, then you want to wind the trip back first, so you would spin the axle forward, wind around the pin, and spin the axle backward.