Mousetrap Vehicle

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Mousetrap Vehicle is a two person building event in which teams build a vehicle powered by one or two mousetraps. The current rules for Division B in the 2021 Season call for the vehicle to travel from a Start Point to an Undetermined target point. In past seasons, the vehicle was supposed to be capable of pushing a 3oz paper cup 8 meters down a track, reversing direction, and stopping as close to the vehicle target point as possible. It is a Division B event for the 2020 and 2021 seasons, and it has been an event in both Division B and Division C in the past. In the 2020 and 2021 season, a practice log with at least 10 runs measuring the parameters of target distance, distance from target and one additional parameter is required. In the 2019 season, a practice log with at least 10 runs was also required, in which an additional parameter of the vehicle besides time, cup distance and vehicle distance must be recorded. Class B Safety Glasses are also required for competition.

An example device, described here. This vehicle is built to the 2010 rules, and it does not comply with the current rules.

How It Works

One mousetrap is mounted on the vehicle. Most often, a long dowel or other type of rod is attached to the mousetrap(s), and a string is attached to the rod, which is then looped around the axle. When the mousetrap is triggered, it pulls the rod and the attached string, which unwinds from around the axle, which turns the wheels, thus propelling the vehicle. The vehicle cannot be touched or pushed in order to get it started.

The 2020 Event

The goal of the 2020 event was to get one vehicle powered by 1 mousetrap as its sole means of propulsion to reach an undetermined target point between 9-12 meters with increments of 0.5m for regionals, 0.25m for states, and 0.05m for nationals. The vehicle should also hold a dowel that will determine the point at which the vehicle distance from target point will be measured

The 2019 Event

The object of the event in 2019 was for the vehicle to push an 3 oz paper cup, which is at least 5 cm tall, 8 meters from the Start Point (SP) to the Cup Target Point (CTP), and then reverse direction and return to a Vehicle Target Point (VTP). In 2018, the cup pushed was a 16 oz plastic cup, and it only had to pass a cup target line, not stop on a specific spot. Then, the vehicle had to come to a stop at a point behind the SP. In 2019, the VTP is within a specific range based upon the competition level, and it will be announced by the event supervisor after the impound period is over. The track is a smooth, level and hard surface at least two meters wide, there is no maximum width, however. Avoid testing the vehicle on a rough surface, as this may affect a test run and provide inaccurate information as to how the vehicle will perform in competition.

The 2013 Event

The object of the event in 2013 was to travel along a 0.75 m wide track for at least 8.50 meters and come to a stop at a determined target point. The exact distance is determined by the event supervisor, and will be between 9.00 m and 12.00 m. However, the interval the vehicle has to stop at changes between tournaments. One or two unmodified mousetraps may be used to power the vehicle, and they must be the only means of powering the vehicle. The vehicle must be started with an unsharpened and unused #2 pencil vertical to the floor, and once started teams are not allowed to interact with the vehicle. The vehicle must have a paper clip attached to the front, which is used as a measuring point. The vehicle should also be able to hold a dowel perpendicular to the floor. Teams have 8 minutes of time to set up, attach the dowel and start their two runs. During each run the vehicle must travel at least 8.50 m, or it will be scored in Tier 2.

Building

There are many different kits and books available in hobby stores and online to help educate participants on how a mousetrap vehicle works. These kits are usually made of balsa or another type of lightweight wood, but some kits are available that are made of plastic for home experimentation. Careful examination should be made to ensure that these kits do not require or contain any illegal parts. It is also possible to 3D print a vehicle, but this can be expensive and time consuming. A good vehicle can be made from scratch, too. The building materials that can be used vary greatly depending on the builders' preference and skill level. Experienced builders might use lightweight balsa wood and carefully crafted plans, whereas novice competitors may glue together some scrap wood with holes drilled for the axles. Many methods can be used to build a mousetrap vehicle, and there is no one best design. It is suggested that anyone interested go to the Mousetrap Vehicle Forum for more tips and information.

Wheels & Axles

CDs wrapped with rubber bands or balloon pieces for traction are a cheap and easy option. To ensure that they are centered properly on the axle if the hole is too big, participants can make their own spacers, or they can purchase them online. 3D printing is also an option, but can require many tests to make sure they fit properly. Good wheels can be found at local hobby stores. Airplane wheels are often light, and come in a variety of sizes. However, they can be quite expensive. Robot wheels can provide excellent traction, but they tend to be small, and they can be quite heavy. Holes can be drilled in them to reduce weight, but this must be carefully done to prevent the wheels from becoming too weak. Laser cut wooden wheels can be very light, but they require either a laser cutter or someone to buy them from that has a laser cutter.

The Body

The body should be lightweight but sturdy. Some designs have a 3D printed centerpiece to hold the mousetraps, and basswood sticks as frames. Carbon fiber is also a good option, but may be expensive. Wood cars can be heavy, but they will withstand more punishment, which makes them better for first-time competitors to learn the basics of the vehicle. Keep in mind that if a vehicle breaks, it can be expensive, and it could even potentially cause competitors to not have a vehicle for a competition.

Mousetraps

Wooden mousetraps are most commonly used because they are cheap and have a large snap range. Commonly used brands are Victor and Tomcat wooden mousetraps. The rod should be firmly attached to the mousetraps to prevent power loss. Zipties are a good option, as they are easy to fasten and remove. It may be best to move the mousetraps as far from the driving wheels as possible, as this enables the participants to maximize the length of the lever arm and drive string. If the lever arm sticks beyond the axle, the string must extend back to the axle to wind around it, meaning that any additional string is wasted. Furthermore, the initial power from the trap will be wasted until the lever arm is vertically positioned over the axle.

It is recommended to mount the mousetraps in a way that they can be removed and replaced, as mousetraps lose some force after each use. Swap out for new ones before competition and after a few uses while practicing, as they can greatly alter the speed of the run. Four holes are permitted to be drilled in each mousetrap in order for them to be mounted to the base of the car with screws and nuts.

Number of Winds

To figure out the number of winds needed, all that must be done is get the circumference of the drive wheels and divide the distance by the circumference. However, due to inertia and other factors, many test runs must be performed to find out exactly how many winds the vehicle's drive string initially needs. Some vehicles require more or less than others, even with identical wheels and axles. Careful attention should be taken to ensure that the track is free of debris that could affect the vehicle. High-caliber teams should take every detail to attention to help ensure a winning score.

Reversing

The simplest method of making the vehicle go backwards is by back-winding the string, where string is wound in one direction, hooked around some sort of peg or hook, and then rewound in the other direction. An easy "hook" to make is a ziptie tightened and glued into place around the axle. While using this method it is important to remember you must wind the wheels forwards first (so the vehicle travel backward), wrap around the peg, and then wind the wheels backward to ensure that the vehicle travels forward upon activation.

Another method is to use two mousetraps with two drive arms, with one drive axle wound in one direction making the vehicle go forward, and then after trigger the second mousetrap with a different drive arm and string wound so the vehicle goes backward.

Adjusting the Vehicle

To reach the next level, the vehicle's design should be adjusted to better fit a variety of properties. There is no single strategy that will yield a perfect vehicle, and adjusting one element to fix one property of the vehicle may cause a negative change in another property. This is all part of the design process. All of the following tips require testing to reach the vehicle's full potential.

Making The Vehicle Go The Distance

If the vehicle cannot travel the minimum distance, try the tips below. If the vehicle does not move at all under its own power, go to the next section. It most likely will need more power.

  • Make the vehicle's drive axle thinner. This will allow more winds of the string around the axle, making the vehicle go farther.
  • Make the drive wheels larger. Wheels with a larger circumference will travel a greater distance with each revolution.
  • Make the drive string longer. This will also require a longer lever arm.
  • Make the vehicle lighter.

Warning: The first three suggestions will reduce the car's torque as a side effect.

Making The Vehicle Faster

Making the vehicle travel quickly is an important factor in the score. Follow these tips to make a vehicle go faster. Please note that the first two are the opposite of the distance tips, so the participants will need to find a balance between the distance the vehicle will travel and speed. They will generally want to just be able to make the distance, with a little wiggle room.

  • Thicken the axle: Wrapping some tape radially around the axle or adding a spool. It may be surprising as to how much faster this makes the vehicle. This also helps if the vehicle will not move at all.
  • Use smaller wheels. They will require less force to complete a rotation.
  • Make the vehicle lighter.

Making The Vehicle Accurate

Another factor in the score is the accuracy of the vehicle when stopping at the stopping point. It is common to include a braking system in mousetrap vehicles to ensure the vehicle's stopping on the right spot. Especially common are wingnut brakes. Please see the brakes used in the previous event, Scrambler, for an example, or the picture at the link in the next section.

For the 2019 competition the vehicle target point was between the vehicle start point and cup target point, and thus the typical wingnut breaking system which could have been utilized for the 2018 competition needed to be modified a little. The wingnut must be able to travel along the threaded rod forward, but then be stopped before it reaches where it started. Some methods to achieve this include some sort of one-way door, or using a wingnut with one wing cut off and an adjustable peg on the side of the chassis so that the wing will go freely forward, but then be stopped by the peg during reversal.

Making The Vehicle Go Straight

In the 2019 competition, the track must be at least 2 meters wide. Therefore, traveling straight is important. If the axels cannot be built to be exactly straight and parallel, then there are some other options. One commonly used solution is to build the vehicle 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. 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 it is possible to adjust the curve or lack thereof without taking the vehicle apart.

General Tips

  1. If possible, test the vehicle on a surface similar to the one that will be encountered at the next competition. Testing on tile and then competing on hardwood will affect the vehicle's performance. The best way if the competition surface is unknown is to test on multiple surfaces and keep data for each one. Also, the importance of clean floors cannot be emphasized enough: the results will be varied if the floors are dusty or have small pieces of trash on them.
  2. Possible places to practice can be:
    1. a school gym
    2. a school cafeteria
    3. school hallways
  3. For string, Kevlar fishing line works quite well. Sewing thread breaks fairly easily, and regular monofilament fishing line will stretch under stress. It is best to have a thin string, so that the thickness of the string does not interfere with the winding of the string.
  4. When building, it may be best to move the mousetraps as far from the driving wheels as possible. This enables the participants to maximize the length of the lever arm and drive string. If the lever arm sticks beyond the axle, the string must extend back to the axle to wind around it, meaning that any additional string is wasted. Furthermore, the initial power from the trap will be wasted until the lever arm is vertically positioned over the axle.
  5. It is recommended that the drive string be wound around the axel, and then lock the trap(s), eliminating the requirement to pull the string tight as it is wound.

Scoring

The goal is to have the lowest final score. Each team is given a total of 8 minutes to attempt 2 runs. The lower score of the 2 runs will be the team's Final Score. If a team cannot successfully complete a run in the 8-minute block, they are given participation points and scored in a lower tier, but they are still scored above no-shows.

The run score is calculated as follows: Vehicle Distance in cm + 2*(Cup Distance in cm) + Run Time (in sec) + Penalties

Vehicle Distance

Vehicle Distance is calculated as the distance from the VTP to the front bottom edge of the dowel, to nearest 0.1 centimeter. If the dowel covers the VTP, the Vehicle Distance is 0.

Cup Distance Score

The Cup Distance Score is the distance in cm to the nearest 0.1 cm from the CTP to the closest part of the cup when it has reached the final resting position. A score of 0 is given if the entire cup covers the CTP.

If the cup does not pass the line, the score is the distance to the nearest 0.1 cm from the cup furthest from the CTP to the CTP + 300. If the cup tips over during the run, then measurement is made from where the cup comes to rest.

Run Time

The run time begins as soon as the mousetrap is actuated, and is measured until the Mousetrap Vehicle stops completely for 3 seconds.

Penalties

  • A penalty of 250 points is given for teams with incomplete practice logs.
  • A penalty of 500 points is given for teams that do not impound their practice logs.
  • A penalty of 2,000 points is given for every competition violation, with a max of 8,000 points.
  • A penalty of 5,000 points is given for every construction violation, with a max of 15,000 points.
  • A penalty of 10,000 points is given for every run with a vehicle not impounded during the impounded period.

Ties

Ties are broken in the following sequence:

  1. Lower Cup Distance Score
  2. Lower Vehicle Distance
  3. Lower Run Time
  4. Lower Run Score of the other run

2013 Scoring

The lowest score wins. A team's final score is the one out of the two runs that gives them the higher rank. The score is calculated as follows:

Run Time = Distance Score + Time Score

The Distance Score is the distance from the measurement point to the target point. This is measured in centimeters to the closest 0.1 cm.

The Time Score is the time taken to complete the run multiplied by 5.

Tiers

  1. Tier 1: A run with no violations.
  2. Tier 2: A run with competition violations. This consists of any run where the vehicle leaves the 0.75 m track before reaching the 8.50 m line, or a run where the vehicle does not reach the 8.50 m line.
  3. Tier 3: A run with construction violations.
  4. Tier 4: A vehicle with no completed runs. This tier receives only participation points.

Resources

Several sections of the Scrambler wiki
Mark Rober's Video feat. Troy
Two Mousetraps Reversal Method


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