# Building mousetrap cars is a very popular activity in physics classes and is a good way to learn about energy efficiency. To build a good performing mousetrap.

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Building mousetrap cars is a very popular activity in physics classes and is a good way to learn about energy efficiency. To build a good performing mousetrap car you will learn about friction and energy. Cars can be built for speed or distance, but it would be hard to build a car that would be both fast and travel a long distance because of the physics. There are kits available or you may design your own. I built several kits and then using what I had learned, built my own three-wheel design from bamboo. Mousetrap Cars http://www.scienceguy.org By Bill Kuhl

The first mousetrap car I built was the Midwest kit which is not a long distance car. The Midwest mousetrap car required that the mousetrap was tripped with a stick or pencil.

After the Midwest kit, I built two of the Doc Fizzix kits.“The Basic Kit” left is designed for distance, “Speedy” kit was suppose to be faster but appears only slightly faster to me.

I highly recommend this book which covers most aspects of mousetrap cars including the physics and applying math to improve the performance of mousetrap cars.

There were many questions about my mousetrap cars at the 2010 Frozen River Film Festival.

Speedy car with short arm only traveled around 30 feet.

The cars roll pretty easy on smooth wooden floor.

For a long distance you want a long stroke. The mousetrap moves through 180 degrees, or a 1/2 revolution. The string needs to be pulled as far as possible, which means a long arm.

The long arm added to mousetrap is known as the “lever arm”, pulls hardest up until the 90 degree position.

Force decreases as lever arm passes 90 degrees.

When the lever arm has moved 180 degrees, the string must be completely unwound and the car free to coast by kinetic energy.

The car had travelled most of the length of a small gym.

The car with the long arm went over 50 feet.

When I build kits, I try to think of ways I can improve upon the kit design. A view of the rear axle, I will look for an improved bearing.

The brass tube that runs through balsa wears a hole causing the axle to wobble. I have been experimenting with bushings created from brass tubing.

Prototype #1 Bamboo Mousetrap Car I decided I would design my own mousetrap car using bamboo skewers as the primary building material and use the lids from cottage cheese containers for wheels. The car would also have three wheels, two in the rear and one in front. This car would be very economical to build as no brass was used, the rear axle was from coat hanger wire and the front axle from left over wire from the mousetrap. Rear bearings were made from screw eyelets. This would be a car designed for distance, not speed.

This was my first prototype of my mousetrap car, because I just hot glued the wheels to the axles, the wheels wobbled badly. The car did good for distance.

The attachment point for the string on the lever arm was moved to be directly above the axle.

The rear axle was made from coat hanger wire which is smaller in diameter than the 3/16” brass tubing used in the Doc Fizzix cars.

Before reading the Doc Fizzix book, I had attached the line too far past the axle which I later read was not as efficient.

Rear axle made from coat hanger wire, bearings are screw eyes, plastic tube spacers, and cottage cheese lid wheels were hot glued to axles.

I spotted this three-wheel motorcycle, a variety of three-wheel vehicles have been built for different applications.

On this very old bike, the large diameter wheel traveled a good distance for each crank revolution by the bike rider. For a good distance mousetrap car, each turn of the driving axle should cause the driving wheels to travel a long distance.

A good distance mousetrap will have a low power output rating unlike this modified engine that has a high power output. Power is the rate of doing work, in your distance car work should be done slowly.

Friction can be a good thing also, as that is what gives a car traction to accelerate quickly and to take turns at a fast speed, the wide tires on this radio controlled race car give it good traction.

The heavy steel wheels in a railcar do not flex where it meets the rail which allows it to roll easily once the heavy load is moving. Distance mousetrap cars rolling on hard surfaces should not flex either.

An automobile tire flexes on the bottom as it turns, it has a greater rolling resistance than a hard wheel.

This hybrid car is more energy efficient than many other vehicles as it captures kinetic energy during braking and stores it to battery reserve for use with the electric assist.

I also built a basic lower cost mousetrap car from Doc Fizzix.

Contents of the Lil Moe kit. As it has a relatively short lever arm, this would not be the best long distance car.

For this kit I created brass bushing from ¼” brass tubing to fit over 3/16” brass tubing axles.

Completed Lil Moe, very few pieces in the kit.

A view from the side.

¼” brass tubing was used for spacers on rear axle to make it easier to wrap string around the axle (more room for your fingers). The car is more stable also.

I built the Torque Master mousetrap car from Doc Fizzix which uses the Jones pulley system. This is a car designed for quick acceleration and not distance. The spring from the mousetrap turns greater than 180 degrees, so this would not be legal for many mousetrap competitions.

Components of the Torque Master car, the foam wheels give the car good traction.

The “Torque Master, Jones Pulley System” can be purchased separately for your own-design car.

Completed Torque Master car, I added brass tube bushings instead of running brass axles through the wood.

Bamboo Mousetrap Car Prototype #2 I built another mousetrap car of the same dimensions from bamboo, the major changes for this car were using CD ROM’s for wheels, and ball bearings for bearings. Brass tubing was used for axles which fit the ball bearings and into the rubber washers which were used for the wheel hubs. I also wanted to see if I could make the car slightly lighter by using smaller diameter bamboo on the lever arm and on the bottom struts. This proved to be a mistake as the car frame was twisting and the lever arm had a bend in it.

On the first car, with heavier supports the frame did not twist.

CD ROM wheels with the washer hubs did not have the wobble that the cottage cheese lid wheels had with the wheels glued to axles. The CD wheels were much heavier.

Ball bearings were glued to wood blocks with CA glue, no doubt a better method could be found but I was anxious to experiment with the car.

Close-up of wheel and axle on second car. Rubber washer fits snuggly into CD ROM wheel, brass tubing used for spacers and axle, ball bearings for radio control cars used for the bearings.

Someone told me that removing the seals on the ball bearings should help the performance so I pulled off the seals.

Close-up picture of ball bearings with the seals removed, another suggestion I had read was to washout all lubricant around the bearings. At slow speeds, the lubricant causes more drag.

View of front wheel and mousetrap. The mousetrap was attached with wood screws so another mousetrap and lever arm could be easily swapped in. Another mousetrap could have a more powerful spring resulting in better performance.

This is the new framework with extra bracing to keep the car from twisting.

View from the bottom of the framework.

This is a picture of the steel framework for the forage wagon that my father’s company manufactured, notice the angle bracing.

Mousetrap competition rules might specify that a Victor mousetrap is used, pictured on the right. It can be seen in this picture that the Pic mousetrap has a smaller spring.

Small tubing cutters can often be found in hardware stores or hobby shops and work well for cutting the brass tubing.

Three different types of materials used for wheels on distance cars I have tried. CD ROM is the heaviest at 16.6 grams, CD cover at 8.2 grams, and cottage cheese lids are the lightest at 6.6 grams. Lighter wheels are better because there is less rotational inertia.

Bamboo Mousetrap Car Prototype #3 For my third prototype I went back to using the cottage cheese lids, screw eyes for bearings, coat hangar wire for rear axle, and used faucet washers in the wheel hubs. Additional bracing was added to support the front wheel. This car works well.

I found square 36” lengths of wood in building supply store where the round dowels are found that works well for the blocks the screw eyes attach to. It is good to be on the lookout for materials that will work for your projects.

New hub design was created from rubber faucet washers and plastic tubing. The wheels do not wobble now with the improvement. It is good to keep tweaking the design for performance and reliability.

On the front wheel I put faucet washers on both sides although you can only see one in this image. The front wheel runs straighter now also.

Never Stop Experimenting!

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