This is how a
rubber band powered car was built for a school assignment.
It started out where a small soap box derby car wa bought to get
the progect started. Of cource, the car sat there until the last
minute when the assignment was due the next day and that is when
the construction began. The rules stipulated that only household
materials could be used except for the wheels. The construction
began by cutting two rectangle lengths of cardboard and gluing
them together to form the floorboard or frame of the car. It should
be noted that since the car was due the next morning, the only
glue used was a generic "Super Glue" that was rated for a drying
time of 30 seconds. Also, the glue was the thick version because
of the porous nature of the cardboard. (Which came from a real
strong Canadian Mist Liquor box.) The doubled cardboard made the
frame stronger and better supported the wooden dowel.
The Canopy for the car was half of a thin plastic, disposable
Coke cup that could be cut precicely using a pair of straight
cutting tin snips. The half with the logo facing upwards was used
so that it would become part of the decoration. The canopy was
not glued on till late in the construction as the dowel hole had
to be drilled.
A reasonable fat woden dowel was inserted into a drilled hole
completely through the floorboard. The dowel became the rubber
band anchor point.
The tricky part was the rear drive system. Soap box derby wheels
are supposed to spin around a fixed axle and the car is supposed
to be powered by gravity. But in this car, there needed to be
a fixed axle attatched to the rear drive wheels so that when the
rear axle spun, the rear wheels would then spin too. In order
to have an axle, A way to support the axle was needed so vertical
supports were made from additional plastic cup bottom halfs. Originally,
it was thought that the axle would go above the floorboard. But
because the little tiny black plastic wheels were so short, the
axle assembly was put beneath the floorboard for clearance. A
hole was very carefully and very slowly cut by twisting a drill
bit with the fingers so that a nice round hole would be produced
in the thin plastic cup material. This took a couple of tries
to perfect. The cup bottoms were glued opposing each other and
the axle was placed through the holes to ensure that there would
be no binding. The axle has to spin freely inside the holes of
the vertical axle supports.
Eventually, the plastic wheels were nailed and superglued to the
ends of the axle dowel after the axle dowel was inserted into
it final position. But before that, it was anticipated that the
axle had freeplay where it would shift from side to side during
operation. This would mean that the inside of the rear wheels
would rub against the floorboard of the car causing drag. And
that would be bad. So, a very carefully placed notch was cut into
the axle. A very shallow notch resembling a groove was cut to
allow for a left/right axle sway guide.
Another small piece of plastic cup bottom was glued under the
axle in such a way that a virtical piece of plastic would ride
inside of that axle groove. The plastic finger was thin and flexible
to give the axle a little play but not enough to let if go too
Rubber bands were interlooper together to form a longer rubberband.One
end of the rubber band was wrapped around the axle in the opposite
direction of the tention that would be later applied to it and
the front end of the rubberband was looped around the anchor pin
previously mentioned. The rubber band was not permanantly attatched
to the anchor pin because if it were, the rubber band would act
as a brake once the energy of the rubberband was depleted. So
instead, the rubberband was held in place by tention at first.
But as the energy of the rubberband was all used up, gravity would
allow the rubberband to fall off the anchor pin and allow momentum
to carry the car even further foreward while the spent rubberband
wound up harmlessly around the rear axle.
The front axle was glued to the floorboard while the front wheels
were allowed to spin freely around the nails they came with. (the
axles they were designed to use.) Extra cardboard was glued near
the front of the car to increase the gluing surface for the front
The initial tests of the car once everything was assembled revealed
that the car has a huge amount of power and no traction at all
because the plastic wheels were very slick. The car just sat in
one place while the rear wheels did a burn-out. To compensate
for that, the rear of the car, over the rear axle was declared
a cargo area and weight was added. Cans of Chunky soup and a 16oz
diet coke bottle (both full) were not heavy enough to improve
the traction and the car still sat in one place while the rear
wheels spun nderneath it. The problem of poor traction was overcome
by supergluing a rubber racketball to the outer rims of the small
plastic wheels. The weakest part of the rear axle was the inner
rim area there the rear wheels joined the wooden dowel because
it has the smallest surface area for gluing purposes and all of
the torque from the wheels had to pass through that point and
into the dowel. But, it helt together. The rubber of the racketballs
proviced traction and the increased diameter of the balls changed
the gear ratio positively. The car became fast and powerful enought
o carry cargo. The balls also gave the car a higher stance in
the rear for a hotrod posture.
Click photos for better view.
This car traveled
farther than any of the other cars and received an A.
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