Mousetrap Vehicle
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| Forum Thread: 2009 2010 |
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| Best of: 2010 |
Contents |
Overview
Mousetrap Vehicle is a car that you must design to go forward x amount of meters and come back to its original starting point.You will be allowed two un-modified mousetraps which means that the mousetraps must have all the original parts.
How it Works
The mousetraps are mounted on the car. When the mousetraps are 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.
| Pros | Cons |
|---|---|
| Easy to build. | Lots of friction. |
| Hard to get to work right. |
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.
| Pros | Cons |
|---|---|
| More power on axle. | Lots of friction. |
| Easy to get to go the distance. | Hard to get to work right. |
| Hard to get to come back |
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.
| Pros | Cons |
|---|---|
| More power on axle. | Takes a bit to get to get it perfect. |
| Easy to get to go the distance. | Needs a brake. |
| Easy to get to come back. | Needs a really long bar. |
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.
A Design from 2009-2010
Feel free to copy this design, it's creators/designers have graduated from high school and feel like some teams deserve a good place to start for next year. At the national competition, this design placed 14th at the University of Illinois and was based off a design that won second place as a trial event at the National Competition in Augusta, GA. Based on the above classifications it is a category 3 type car (I believe that these are the most reliable and quickest).
The most important part of the whole competition is speed, especially when you get to higher level tournaments. Keep in mind, however that no matter how fast your car is, it still needs to be reasonably accurate. Even if you can shave off a second, will it still stay within 4cm? That is a question you should ask yourself if you find yourself competing at the national level.
When my partner and I were building the car we would break it up into separate parts: the mousetraps, the chassis, the wheels, the axles, the lever arm, and final assembly. This is how I will write this guide.
The Mousetraps
It took my partner and I a while to learn this but the mousetraps actually vary considerably. We built a testing apparatus in order to gauge the initial strength of every mousetrap that we considered to use for our car. Unfortunately, the mousetraps would become worse over time but we found that it was not hugely important. We used Victor mousetraps and purchased as many as we could at a time in order to compare them. Once you have all of your mousetraps tested, set the best two aside.
The Wheels
We used two different types of wheels on our car. The wheels on the front were our braking wheels and were actually plastic airplane wheels. They can be found here and here (click on Lite Flite). The other wheels that we used were the drive wheels and were actually specially designed by us and cut on our school's laser cutter. If your school is not as fortunate as to have a laser cutter, you can always use cd's but the rest of this guide is written with the intention that you are doing it EXACTLY as we did. Because the drive wheels were cut out of wood they did not have a lot of traction. What we did to compensate for this is that we cut a balloon into strips and put the strips around the wheels. I believe that a guide to do this can be found here.
The Axles
My partner and I used two types of axles. We used a 5/32" hollow brass rod for the drive axle (we used brass because of its combination of strength and weight, we tried aluminum but it bent way too easily) and a 8-32 aluminum threaded rod for the brake axle. The brake was just a standard wingnut braking system used for many of the car events such as scrambler and electric vehicle. The threaded rod needs to be cut just long enough to go through the chassis and the airplane wheels whereas the brass rod needs to be cut just so that can just fit through the chassis and maybe a few washers (optional). Then, you need to thread the inside of the brass rod so that you can screw the drive wheels in.
The Chassis
The chassis needs to be super-lightweight. For this reason, my partner and I decided to use 1/2"x1/4" balsa wood (don't worry it's strong enough). These were cut longer than the wheelbase by about 5cm. Then we drilled 1/4" holes for the axles to go through. Our secret is that the axles were not 1/4" but we would put plastic spacers (purchased at your local hardware store) into the holes and these acted as bearings. I believe that ball bearings are overkill. They are just too heavy for this event.
Then we had to make wooden spacers to keep the longer wooden pieces (we called them runners) apart. We built the car so that the width of the chassis was the same as the two mousetraps side by side. I believe that there is no reason for it to be any wider. Then we super-glued the pieces together to form a rectangle (it's easier if you have made a jig for this). Make sure that when you're gluing the pieces together that the axles can spin freely or else there will be too much friction in the car.
The Lever Arm
The lever arm is actually the easiest part of the car to make (we just made it out of the same material as the drive axle, the hollow brass rod). Just imagine where your mousetraps will be on your chassis and measure the distance from there to the drive axle. Cut the lever arm to that length and drill a hole in it. This brings up the topic of string. We used braided fishing line which does not stretch. It also was very strong and had a small diameter. I would advise using it if it is at all possible. You might have to ask the guy at your local sports/fishing store where it is because it is sometimes hard to find.
Final Assembly
You should have all the parts that you need to finally put the car together by this point. Start by super-gluing the mousetraps together. Wait until they dry and glue them onto the spot where you want them on the chassis. Make sure they are facing the right direction! When they are closed, the metal part of the trap should be closer to one of the sides of the chassis than the other side and should be near the braking axle. Once the mousetraps are glued in, you can zip-tie the lever arm to both of the metal moving parts of the trap. Then, insert the braking axle and assemble the wingnut braking system. Once that is complete, screw on the airplane wheels. Then put the drive axle into the chassis and screw in the wheels that you have made. If there is too much left/right wobble, you can add in some washers to get rid of the extra space. There is one final step after this which is adding a peg to the drive axle. We accomplished this by wrapping a thin piece of wire around and then twisting it together. Then we super-glued it and trimmed it. It should be less than 0.5cm long or else it may be difficult for the loop on the end of the string to come off, allowing the car to coast to the finish line.
Some Final Advice
1. Some people advocate the use of lubricants to speed your cars up. When I tried using liquid lubricant it just gummed up the axle and solid lubricants like graphite would just slow the car down.
2. Try your best to do your runs on a clean surface; we had to clean out the space between the chassis and the axle many times last year. We've found hair and other gross stuff in there but mostly it just makes your car go slower.
3. If I had more time to test, I would have tried making a deceleration sequence on the end of the run during the coast to the finish line. Our car would lock up and skid way past the line. Even though we consistently got super fast times (around 10-11 secs), the skidding killed our distance score (around 15-20cm) which was equivalent to around 4-5 extra seconds.
4. Don't be afraid to build many many cars. We always had at least one backup car and the building experience will help make your cars higher quality and run faster. We built at least 5-8 chassis's last year and rebuilt the wheels around them (yes, the chassis's are reusable).
5. Make sure you test a lot so you know how to get your best score. Also, have a backup plan in case your car doesn't make it to the first line (an added score of 1000 is a lot worse than making it all the way back).
6. And finally, make sure that when you start the car that the brake is not still locked up. It should be as close as possible to it's final location but it should not be locked up or else the car will not run. (In case you haven't figured it out yet, the wingnut starts near a spot where it cannot move but moves in the opposite direction, then turns around when the car turns around and theoretically should stop exactly at the starting line)
Tips
CD's or vinyl records are common wheel ideas. Rubber bands can be placed around CD's to add traction.
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.
