Mousetrap Cars
By Bill Kuhl
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.
http://www.scienceguy.org
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!