Junior Solar Sprint
Building a Solar Car – Student Guide
One thing you will discover is that there is no ideal design.
Overview
In this design challenge, students will harness the power of the sun to design,
construct and evaluate a solar-powered model car of their creation. Students will
utilize the design process and undergo review by their peers to select an optimal
gear ratio and components for their car.
Winning Categories:
Speed
Craftsmanship
Technical Merit
Documentation Portfolio
Best Use of Recycled Materials
**students can win in more than one category
Vehicle specifications:
1. One solar panel and one motor per vehicle.
2. The vehicle, including any attachments, may not be larger than 30 cm. (12 in.) wide by 60 cm.
(24 in.) long by30 cm. (12 in.) high.
3. The vehicle, including the panel support system, must be structurally sound without the solar
panel or payload. The payload may not support the solar panel or function as any other
component of the vehicle’s structure. The panel must be able to be easily disconnected from
the motor and removed from the vehicle.
4. The payload: The vehicle must carry a payload (passenger) of one empty 12 oz. conventional
aluminum soda can. The can must not be altered in any way, and application of adhesives or
other materials is not allowed. It must be easily and quickly removed or reinserted.
5. The compartment: The vehicle must be designed with a compartment that secures the payload
even in the case of a roll-over “accident.” The compartment, including all components
required to secure the payload, must retain its shape with or without the payload.
6. The vehicle with its solar panel must be powered solely by the sun’s energy. No energy
storage devices.
7. If the sun’s energy is judged insufficient, two batteries will be furnished for each race to
Motor power leads should be readily accessible for easy attachment. The battery pack should
be included in the design of the vehicle.
8. The vehicle will be attached to a guide wire in the center of the lane that runs the length of
the track. The wire will be no higher than 1.5 cm above the track surface.
9. The vehicle must be of students’ own design and manufacture from current school year. Each
team from a given school must have a unique car design. An engineer’s journal or work
record portfolio demonstrating progress and originality is optional. Team members present
their vehicle to judges without adult assistance and make their own car repairs and
adjustments.
The Race Track:
Transoptions:
The race lane is 60 cm. wide and 20 meters long. The track is a hard flat rubber surface such as a
running track. The track will be oriented facing east.
WAMS:
The race lane is 60 cm. wide and 20 meters long. The track is a hard flat surface such as asphalt
(parking lot). The track will be oriented in any direction (e.g. North-South, East-West, etc.)
Conduct of the Race:
1. The vehicle will start from behind the starting line with all wheels touching the track. The
solar panel will be completely shaded by a supplied opaque material cover held above the
panel by a member of the team until the judge signals the start of the race, the team member
will remove the cover so the panel will be exposed to the sunlight.
2. Any car that leaves its lane will be disqualified.
3. Loss of payload during a race will result in disqualification.
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Six things to keep in mind when designing and
building your car.
# 1 - Chassis
Purpose
The car’s chassis is its frame. It holds all of its main parts together.
Ideas
Some possible ideas for a solar car chassis are below. Try different ideas! Try different
materials!
Weight and Stiffness
You don’t want your car too heavy. It is easier for your motor to push a light car than a
big, heavy one. You do not want to waste energy!
But something you must also keep in mind is that a light car can be pushed easily by the
wind too. Even if the wind does not blow the car over, it may make it harder to go in a
straight line.
For the solar car, stiffness is very important
What is the difference between strong and stiff? Strong means it will not break easily.
Stiff means it will not bend easily.
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Materials
Any material that is light and stiff would be appropriate.
• Balsa wood - commonly chosen material because it is lightweight and more
importantly, it is stiff.
• Foam core
• Corrugated cardboard
• Plastics
• Brass tubing
• cardboard tube
• shoe box
• soda bottle
• rigid plastic
# 2 - Wheels and Bearings
Purpose
Wheels support the chassis and allow the car to roll forward. Bearings support the wheel
while allowing them to rotate.
Ideas
Wheels can be large, small, narrow, or wide
Here are some ideas to start you thinking:
Friction
Friction keeps things from sliding against each other. When you build your cars, there are
some parts that you want to slide easily, and there are other parts you don’t want to slide
at all.
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Tire Traction
When you have two things that must roll against each other, like a wheel rolling along the
road, friction keeps them from slipping. This type of friction is also called “traction,” and
is important to remember when building your wheels.
Heavy tires will be more difficult to move than weight in the chassis. Weight in the
chassis has to be moved forward, but the weight in the wheels has to be moved forward
and around in a circle. The heavier the wheel, the more energy it takes to get the wheel
turning.
Surprisingly, the bigger the wheel diameter (even if it is the same weight), the more
energy it takes to get the wheel turning.
Weight Distribution and Traction
Imagine your rear-wheel-drive solar car has trouble -- its back wheels are slipping. Your
partner suggests adding some rubber bands around the wheels to increase traction, and
you agree.
The rear wheel still slips some. Your other partner wants to add some weight to the car.
But it doesn’t work. You tell him you have a better idea. You move the existing weight,
and now it works! Why?
Remember that all of the force is transmitted through the driven wheels, so the moved
weight increased the traction where it was needed. Weight distribution is very important,
since you can increase traction just by moving existing weight from one part of the car to
the other.
Traction is important for transmitting the forces from the wheels to the road. If any of
your wheels are spinning rather than rolling, you probably need more traction. Traction
can be increased by adding a non-slip material around the wheels (like a tire) or by
moving weight over the driven wheels. But, remember, it is also important to have
efficient wheels, which are usually thin and lightweight.
Bearings
When you have two things rubbing against each other and you want them to move freely,
friction slows things down and wastes energy. Friction is very undesirable is in the wheel
axle. The axle must be supported and attached to the chassis, but still must be able to turn.
Look at a bicycle or a Roller Blade. Hold it above the ground and spin one of the wheels.
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Between each wheel and its center axle is a type of bearing called a “ball bearing.” The
bearing holds the wheel on the axle, but reduces the friction between them, so the wheel
can spin for a long time without slowing down.
Lubrication
Lubrication helps parts slide against each other, so it is used in bearings to reduce friction.
Some appropriate lubricants for the solar car bearings may be light oil, light grease, or
graphite powder (crushed pencil lead).
Wheel Alignment
Another problem that wastes energy is poor wheel alignment. When the wheels on your
vehicle are not lined up properly, some of the wheels must slide sideways.
One way this might happen is sketched below.
Make sure that the axle goes through the center of the wheel.
One suggestion is to use a compass, rather than tracing a circle, if you cut a circle out of a
material. The compass will show you where the center of the circle is.
Taking time to align the wheels carefully the first time will make a huge difference in
how well your car runs.
Materials
Wheels
• Anything round, or things which can be cut into circular shapes
• Thin plywood balsa wood
• Foam core
• Stiff plastic sheet
• Cardboard tubes
• Toy/model wheels
• Tin can
• Tape spool
• Thread spool
• Brass tube
• Plastic pipe
• Wood dowels
Traction
• Things that are rough or rubber-like
• Rubber o-rings
• Rubber bands
• Rubber sheet
• Cloth tape
• Silicone or other caulking materials
Axle
• Stiff, narrow and round materials
• Nails
• Brass rod
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• Brass tubing
• Coat-hanger wire
Bearings
• Screw eyes/eyebolts
• Brass tubing
• Wood, aluminum, etc. (with a hole drilled into it)
• Brackets with screw holes pre-drilled
• Holes drilled directly into the chassis
# 3 - Power Source: The Solar Panel and Electric Motor
Purpose
The purpose of the solar panel is to capture energy from the sun and to turn this energy
into electrical energy. The electric motor then uses this electrical energy to power the
wheel of the solar car.
How the Solar Panel Works?
When you look at the picture above, you might ask, “How does the solar panel turn the
sun’s energy into electric energy?”
The solar panel is made of a sandwich of two materials called semiconductors. Each
material is made of millions of atoms. Remember, atoms have a positively charged
nucleus, and negatively charged electrons, which spin around the nucleus. When these
two materials are put together in a sandwich, an interesting thing happens: electrons
become pulled from the bottom half of the sandwich to the top half.
But there’s a problem. The electrons are all attached to atoms, and the atoms won’t let go
very easily. This is where the sun’s energy helps out. If we shine sunlight on these
materials, the sunlight has enough energy to knock the electrons off of the atoms. The
electrons will then be free to be pulled to the top of the sandwich.
Now if we connect wires to a motor, electrons will flow through the wire into the motor
(making it spin) and back through another wire to the solar panel where they can fill the
“holes” left in the atoms who lost their electrons.
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How to maximizing power?
Try to get the solar panel to produce more current. To produce current, more electrons
need to be forced to move inside the panel. If more sunlight hits the solar panel, more
electrons are knocked away from atoms in the solar panel and more current is then
produced!
Tilt the solar panel towards the sun. The more of the sun’s rays hit the panel, the more
current will flow and the more power will be produced.
Think of the following two cars:
Which one would have more power?
In this case, the car on the right would, because it has more sunlight hitting it.
Another idea that you might want to experiment with is using a reflector to capture more
sunlight with the solar panel.
# 4 - Gear Ratios (transmission)
Purpose
A car’s transmission transfers the power from the motor to the wheels. While doing so, it
may make the wheels spin at a different speed than the motor.
Ideas
There are different ways to transfer power from the motor to the wheels. Some popular
techniques are direct drive, friction drive, belt drive, chain drive, and gears.
Some transmissions are easier to build than others, and not all are appropriate for a solar
car.
Speed vs. Force
The simplest type of transmission is direct drive, which means the motor is connected
directly to the axle of the driven wheel.
Speed
Imagine two of your neighbors have a unicycle race. One has a regular wheel, and the
other has a very large wheel. If they both pedal the same rate, which one of them will
win?
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In both cases, each revolution of the pedal means one revolution of the wheel. BUT, one
revolution of Karen’s wheel will roll twice as far as Bruce’s. So Karen would win if they
pedaled at the same rate. If Bruce wanted to win, he would have to pedal twice as fast as
Karen.
See the picture below:
Combo 1: each rotation of the front sprocket will make the back wheel rotate once
Combo 2: each rotation of the front sprocket will make the back wheel rotate twice
Combo 3: each rotation of the front sprocket will make the back wheel rotate four times
So, combination 3 will go the fastest.
So, how can you choose the best gear ratio?
The idea is that the motor, like your legs when you ride a bike, like to go a certain speed.
First you must find the speed at which the motor gives the most power (this is usually
half the speed the motor will rotate if there is no load, or force, exerted on the motor). Try
to keep the motor turning at approximately that speed as you experiment with different
gear ratios.
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It helps if you build your car in such a way that you can change the gear ratios easily as
you experiment. (the ideal gear ratio may change if you change characteristics (size,
weight, etc) of your car)
Hint: If your car is not going very fast it can either be that the wheel speed is too slow, or
the force required to turn the wheel is too high. If so... Try a different gear ratio!
Materials
The materials you choose vary greatly depending on the type of transmission you build.
In all cases, you will need wheel like parts to put on the motor shaft and the wheel
Belt Drive
• Stiff, rubbery materials for the belt - such as a slice of inner tube or an o-ring. (make
sure your pulleys are pulled away from each other so that the belt is tight) One way to
change the gear ratio on a pulley drive is to add or remove masking tape around the
pulley, which changes its diameter.
Friction Drive
• Make sure you have enough traction on the friction disk, or it will slip. (make sure the
friction gears are pressed against each other snugly to ensure traction)
# 5 - Body/Shell
Purpose
The body or shell of a real car has several purposes. It protects the passengers from wind
and rain, it provides added safety in case of a crash, and it improves how the car looks.
But it also changes how the car performs because a well designed shell can reduce the
force of air on the car as it moves.
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While riding a bike down hill, compare how fast you can go while sitting upright, or
leaning forward. If you crouch down, the air can go over you instead of hitting you in the
chest, so you should be able to go faster. In either words, the force of the air on your body
when you crouch down is less, so you are more aerodynamic.
Ideas
Some ideas for shells are given below:
So how do you reduce the force of air on your solar car? One way might be to add a body
or shell to it that deflects the air around the car.
Materials
• Poster board
• Cardboard
• Foam core
• Stiff insulation foam
• Mylar or plastic sheet
• Paper Mache
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# 6 - Steering
An eyelet (see examples below) must be attached to the bottom of the car (our example–
example
bottom front of the chassis). A guide wire, 1 cm. from the surface of the track, will go
through the eyelet, serve as the steering mechanism, and keep the car in its lane. The
vehicle must be easily removable from the guide wire, without disconnecting the guide
wire. This is the only allowable method of steering the car. Lane changing/crossing will
result in disqualification.
Eyelet Examples
Front of Car (End View)
The vehicle must be safe to contestants and spectators (i.e. no sharp edges, projectiles,
etc).
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Troubleshooting
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If the car does not go, check the following:
Are all electric connections solid and soldered? If not, reconnect or solder and try again.
Is the sun shining? If not, wait until the sun shines or use battery packs.
Are the gears meshing freely? If not, pry the motor and glue from the chassis and reposition.
If the car goes backward, reverse the positions of the two alligator clips on the panel.
If you break a piece of balsa while constructing the chassis, use the second piece of balsa for
the chassis and use the broken piece for the panel support member.
If the wheels do not spin freely, reposition them on the axles to provide clearance between the
wheel and nylon sleeves.
If the gears or wheels spin without the axle moving, use a spot of cool-melt glue at the joint
to connect them. If you are sure of the position, you can use super glue to permanently bond
them.
If the car does not go fast enough, try different gear combinations, wheels, and chassis styles.
Try to make a car with front-wheel drive! Experiment and find out what works best!!
Helpful Hints
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Avoid Styrofoam (not durable enough)
Race track faces east
Race track is on a rubber track
The average car takes about 5-6 seconds to travel 20 m (66 ft)
Only use a small amount of Velcro to attach the solar panel (too much will break it)
Transmission Hints
1. How should I design the transmission?
Be creative. There is no one solution to the problem.
2. How should I get power from the motor to the wheels?
Experiment with several different ways such as gears, pulleys, or some other method of
drive, to get power to the wheels. Don’t be discouraged --- your first try may not work.
3. What should I know about gears?
The pitch of a gear describes the number of teeth that can be put on a 1 inch diameter
gear. Gears with different pitches will not fit together well, so the same pitch must be
used throughout the transmission. Gears in 48 and 64 pitch are the ones most often used
in slot cars. You can buy gears for the 1/24 scale at a hobby shop.
4. Where can I find parts?
Cheap, motorized toys, old cassette/8 track tape players, old motorized can openers,
recycled materials and small gear reduction boxes will have gears and pulleys that may
work in your transmission. Look for them in second hand stores, discount stores like
Target and Wal-mart, or your toy box.
5. What else should I think about as I design the car?
Think about the friction of the following components:
_ Gears moving against each other
_ The stretching or slipping of a belt
_ The tires on the track
_ All the other moving parts of the car
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Names: ______________________________________________________________________
Car Name________________________________________________________ Per _________
DESIGN YOUR OWN SOLAR-POWERED VEHICLE
It is time for you and your partners to become engineers. You and will be supplied with a
motor, wheels, axles, gears, and a solar panel. It is your job to design the car.
a) Talk with your partner and answer the following questions. What are some of the
features you think your car should have to maximize the energy produced by the solar
panel? What kind of materials should be used for the body?
Record what you think on the paper provided.
b) Draw a diagram of your car’s design below. Please use a separate page if you need
more room. Think about strategic placement of the various necessary components in
order to create the fastest car possible. Think about how you will go about securing the
wheels and axles, as well as the motor and the solar cell.
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