Mousetrap Vehicle

Mousetrap Vehicle is a two person building event in which teams build a vehicle powered by one or two mousetraps. The vehicle must be capable of pushing a paper cup 8 meters down a track, reversing direction, and stopping as close to the vehicle target point as possible. In previous years, the goal of the event was to travel from a Start Point to an undetermined Target Point. It is a Division C event for the 2019 season, and it has been an event in both Division B and Division C in the past. In 2019 season a practice log is also required, with at 10 practice runs and every run varying some parameter on the vehicle. Category B Safety Glasses are also required.



How It Works
One or two mousetraps are 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 object of the event in 2019 is for the vehicle to push an 8 oz paper cup that 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 the vehicle had to come to a stop behind the SP. The VTP is within a specific range based upon the competition level and will be announced by the event supervisor after the impound period is over. The track is a smooth, level and hard surface. Avoid testing the vehicle on something like hardwood that may have bumps or cracks in the surface as it can affect a test run.

Building
Many different kits and books are available in hobby stores 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. 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. 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 heavy. They can be drilled to reduce weight, but this must be carefully done to prevent and laser cut wooden wheels which can be very light but require 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,

Mousetraps
Wooden mousetraps are most commonly used because they are cheap and have a large snap range. 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. Holes are permitted to be drilled in the mousetraps in order for them to be mounted to the base of the car.

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.

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.

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

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.

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:
 * 3) a school gym
 * 4) a school cafeteria
 * 5) school hallways
 * 6) 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.
 * 7) 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.
 * 8) 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

Resources

 * Several sections of the Scrambler wiki
 * Mark Rober's Video feat. Troy