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Mousetrap Cars
by Bill Kuhl
http://www.scienceguy.org
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.
(A) 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.
(B) 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.
(C) 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.
(A) 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.
(B) The long arm added to mousetrap is known as the “lever arm”, pull is hardest up until the 90
degree position.
(C) Force decreases as lever arm passes 90 degrees.
(D) When the lever arm has moved 180 degrees, the string must be completely unwound and the
car free to coast by kinetic energy.
(A) The car with the long arm went over 50 feet.
(B) 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.
(C) 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.
(A) The attachment point for the string on the lever arm was moved to be directly above the axle.
(B) 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 well for distance.
(C) View of front fork and single wheel.
(D) Rear axle made from coat hanger wire, bearings are screw eyes, plastic tube spacers, and
cottage cheese lid wheels were hot glued to axles.
(A) 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.
(B) Before reading the Doc Fizzix book, I had attached the line too far past the axle which I later read
was not as efficient.
(C) I spotted this three-wheel motorcycle, a variety of three-wheel vehicles have been built for different
applications.
(A) 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.
(B) 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.
(C) 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.
(A) 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.
(B) An automobile tire flexes on the bottom as it turns, it has a greater rolling resistance than a
hard wheel.
(C) 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.
(A) I also built a basic lower cost mousetrap car from Doc Fizzix.
(B) Contents of the Lil Moe kit. As it has a relatively short lever arm, this would not be the best
long distance car.
(C) For this kit I created brass bushing from ¼” brass tubing to fit over 3/16” brass tubing axles.
(A) Completed Lil Moe, very few pieces in the kit.
(B) A view from the side.
(C) For this kit I created brass bushings from ¼” brass tubing to fit over 3/16” brass tubing axles.
(A) 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.
(B) Components of the Torque Master car, the foam wheels give the car good traction.
(C) The “Torque Master, Jones Pulley System” can be purchased separately for your own-design
car.
(D) Components of the Torque Master Jones Pulley system.
(A) Completed Torque Master car, I added brass tube bushings instead of running brass axles
through the wood.
(B) 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.
(C) On the first car, with heavier supports the frame did not twist.
(A) 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.
(B) 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.
(C) 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.
(A) Someone told me that removing the seals on the ball bearings should help the performance so
I pulled off the seals.
(B) 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.
(C) 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.
(A) This is the new framework with extra bracing to keep the car from twisting.
(B) View from the bottom of the framework.
(C) This is a picture of the steel framework for the forage wagon that my father’s company
manufactured, notice the angle bracing.
(A) 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.
(B) Small tubing cutters can often be found in hardware stores or hobby shops and work well for
cutting the brass tubing.
(C) 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.
(A) 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.
(B) 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.
(C) 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.
(A) On the front wheel I put rubber washers on both sides although you can only see one in this image.
The front wheel runs straighter now also.
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