Solar-Powered Cars for Junior Solar Sprints

Background Information for Teachers

Building solar cars for the Junior Solar Sprint creates a hands-on opportunity for students to learn about many scientific and engineering concepts, ranging from solar energy, forces, mechanical efficiency, automotive design, and the engineering design process. This background section lays out some important features of the car that you and your students can explore, but feel free to tailor the lesson and topics to what works best in your classroom. You can also tie this lesson in with a unit about alternative/renewable energy sources.

Solar power is power we get from the sun. Unlike fossil fuels, which can cause significant pollution and emit greenhouse gases when burned, solar power is clean and renewable. However, solar power is not without its drawbacks—it is not available at night or on a cloudy day.

Solar panels convert sunlight into electrical energy, which can be used to power machines like motors. They must be connected to the motor by wires which form a circuit. A motor can be used to drive the wheels of a solar-powered car (Figure 1). In order to build a solar car, you need to be familiar with the basic parts that make up the car:

• The chassis is the car's frame, or body.
• The axles are straight, rigid rods that support the wheels.
• The bearings connect the axles to the chassis and allow them to spin.
• The motor converts electrical energy from the solar panel into spinning motion. It has a metal shaft that spins.
• The transmission connects the motor's shaft to one of the axles (it is called a transmission because it "transmits" power from one place to another).

Image Credit: Ben Finio, Science Buddies / Science Buddies

A solar panel, motor, axles, gears and wheels are purchased online from Solar Made kits or Pitsco kits. The chassis of the solar power car is made from a rectangular piece of cardboard cut to a slightly smaller length than the solar panel. Attached to the bottom of the cardboard chassis are two plastic tubes similar to straws that will act as axles for four plastic wheels. A motor with two alligator clip leads is taped to the top of the chassis next to a rear wheel that has a gear which will interlock with a gear on the motor. Three paperclips are bent and taped to the top of the chassis to support the solar panel, two at the rear and one at the front.

Figure 1. A solar-powered car (top) and the car with the solar panel removed (bottom) so you can see the other components. This car was built using a kit that supplies the solar panel, motor, axles, gears, and wheels (see Materials section). The chassis is a piece of corrugated cardboard, and straws act as the bearings. Paper clips hold the solar panel in place on top of the chassis.

There are several different types of transmissions that you can use for a solar car (Figure 2):

• In a friction drive, a disk on the motor shaft rubs directly against another disk on the axle.
• In a belt drive, a pulley on the motor shaft is connected to a pulley on the axle by a belt (e.g. a rubber band).
• In a gear drive, a gear on the motor shaft meshes with a gear on the axle.

Image Credit: Ben Finio, Science Buddies / Science Buddies

Different transmissions change the way a motor transfers its energy to a wheel. In a friction drive a motor spins a smooth disk that is in contact with a smooth disk attached to the axle, the friction between the two disks allows the motor to spin the axle and wheels. In a belt drive a belt is wrapped around a disk on a motor and a disk on an axle, as the motors disk spins the belt is pulled along and rotates the disk on the axle. In a gear drive the motor and axle have gears instead of smooth disks. When the motor spins its gear, the force is transfered to the axles gear causing it to spin as well.

Figure 2. Different types of transmissions.

There are different engineering and physics concepts your students need to take into account when designing and building a solar car. Some of the factors are listed here. You can decide whether and to want extent you want to cover these topics in your class.

• Mass and stiffness: what material(s) will you use to make your chassis? Different materials have different densities (mass per unit volume) and stiffnesses (resistance to bending/flexing). In general, you want a chassis to be stiff and not too flexible. Your motor will have a hard time moving a very heavy chassis. However, if a chassis is too light, it could be blown around easily by the wind, or its tires might slip because there is not enough friction with the ground (see next point).
• Friction is the force that resists two surfaces sliding against each other. Sometimes friction is bad—you want your bearings and axles to have as little friction as possible, so the axles can spin freely. However, sometimes friction is good—you want your tires to have a lot of friction with the ground so they do not slip.
• The gear ratio or transmission ratio is the ratio between the diameter of the drive gear on the motor shaft and the driven gear on the axle. This number tells you how many times the driven gear will rotate for each rotation of the drive gear. For example, if the driven gear is five times bigger in diameter than the drive gear, then whenever the drive gear completes one full rotation, the driven gear will complete 1/5 of a rotation. (Note that this concept also applies to friction drives and belt drives.)
• Adjusting the gear ratio allows you to adjust the speed of your motor, measured in rotations per minute or RPM, and the torque of the motor, or how "hard" it spins. There is a trade-off between these two quantities (if you increase the RPM, you decrease the torque, and vice versa). You may need to experiment to find out what gear ratio makes your car go the fastest.
• The angle of the solar panel relative to the sun's rays affects how much electrical power it produces. It will produce the most power when the panel is perpendicular to the sun's rays.

Additional Background Links

• Junior Solar Sprint - An Introduction to Building a Model Solar Car, National Renewable Energy Laboratory
• Friction Force, The Physics Classroom
• The Engineering Design Process, Science Buddies

Prep Work (30 minutes)

Watch this video for an overview of the basics of building a solar-powered car.

https://www.youtube.com/watch?v=jBpS6W-2otk

If this is your first time doing the Junior Solar Sprint, we highly recommend building a complete car yourself. This will help familiarize you with some of the challenges your students will face.

Engage (10 minutes)

Note: if your students have already explored topics like alternative/renewable energy sources in class, then you can use that as a lead-in to this lesson.

Why would anyone want to build a solar powered car? Can you think of some advantages of having a solar powered car? What about some disadvantages?

Reasons for building a solar powered car might include being more environmentally friendly by not burning gasoline, or cheaper because you do not have to buy gas. Potential disadvantages could be that the car does not work at night, when it is cloudy, or in the shade (i.e. under tall buildings or trees).

Tell the students that today they will build their own miniature solar-powered cars.

Explore (2 hours)

1. Before starting, go over these key rules with your students:

2. Pass out copies of the student worksheet. Let students work through the part where they draw designs for their own car before proceeding.
3. Next you will walk them through steps to build a basic car. These cars are not intended to be the students' final designs. Rather, they will show the students the basic procedure for building a car and how the different pieces function. Afterwards, they can disassemble and rebuild/modify the cars as needed to match their original designs.
4. Give students about 15 minutes to look at the materials they have available, brainstorm a car design, and make a sketch of their design. They should figure out:
   1. What they will use for a chassis.
   2. How they will attach the axles to the chassis.
   3. How they will attach the motor to the chassis.
   4. How they will make sure the gear on the motor lines up with the gear on the axle.
   5. How they will attach the solar panel to the chassis.
5. Optional: attach the motor to the motor bracket (Figure 3). Since the motor is rounded, the bracket makes it easier to attach the motor to a flat surface like a piece of cardboard or balsa wood. Use the smallest screw in the kit to secure the motor to the bracket.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Motor attached to bracket.

6. Connect the alligator clips to the motor wires (Figure 4). Important: students might need help with this step. When you are done, you should be able to gently tug on the alligator clips without breaking them off the wires.
   1. Remove the red and black plastic covers from the alligator clips.
   2. Slide the plastic covers onto the wires.
   3. Use needle-nose pliers to tightly crimp the alligator clips onto the bare metal ends of the wires.
   4. Slide the plastic covers back over the alligator clips.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 4. Connecting alligator clips to motor wires.

7. Pick a small gear (the kit comes with three) and press it onto the motor shaft. Note: if you will be talking about gear ratios in class, you can encourage students to put some thought into their decision here. If not, tell them to just pick one for now, and they can experiment with different gear sizes later.
8. Build your axles (Figure 5). Students might need help with this step because it can be difficult to push the gears and wheels onto the axles.
   1. Pick a large gear (the kit comes with three—again, students can just pick one for now if you are not talking about gear ratios). Slide the gear onto one of the axles, just far enough to leave room for a wheel on the end.
   2. Slide a wheel onto the axle next to the gear.
   3. Slide a wheel onto the end of your other axle.
   4. Decide what you will use for bearings, the part that holds the axle and lets it rotate. Drinking straws work well because you can easily slide the axle through the straw, but you could use other materials (for example, a paper clip bent into a tight circle). Slide the bearings onto your axles.
   5. Slide the wheels onto the other ends of your axles.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 5. Axles with wheels, a gear, and bearings (straws).

9. Construct your chassis. This step is fairly open-ended and depends on what materials the students decide to use. This example will use a single, flat piece of corrugated cardboard.
10. Attach your axles to the chassis (Figure 6). Important: make sure the axles are parallel. If your axles are crooked, the wheels might jam instead of rolling smoothly, causing greatly increased friction.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 6. Axles connected to the chassis with tape.

11. Connect the motor to the chassis and carefully align the gears (Figure 7). This step is important and one of the most difficult. If the gears do not touch at all or the teeth are not meshed firmly enough, the axle will not spin. If the gears are smashed together too tightly, then the axle might jam. In Figure 7, the motor is only attached with tape, which does not hold it in place very securely.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 7. Motor connected to chassis.

12. Attach supports for your solar panel to the chassis. Figure 8 shows paper clips that will be used to support the panel.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 8. Paper clips as solar panel supports.

13. Important: the solar panels are fragile. Make sure your students handle them carefully. Secure the solar panel to the chassis. Depending on how you attach the panel, you might need to connect the motor's alligator clips to the metal tabs on the back of the panel first (Figure 9). Caution: even if you are inside, if your students are working near a window on a sunny day, their motors might start spinning. Be prepared to tell them to disconnect one of the alligator clips (you do not need to disconnect both) to turn their motors off.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 9. Solar panel attached to motor wires and secured to chassis.

14. Now it is time for your students to test their cars! You can do this by going outside, or for now, holding the cars near a sunny window. Do not put the car down on the ground yet. First, just hold it in your hands, aim the solar panel towards the sun, and see if the wheels spin. If so, students can move on to the next step. If not, try these troubleshooting steps:
    1. If your motor does not spin at all:
       1. Make sure you are in direct sunlight, on a sunny day, with the solar panel facing towards the sun. Your solar panel might not generate enough power to spin the motor if you are in the shade or it is cloudy.
       2. If your motor still does not spin, double check your circuit. Make sure the alligator clips are firmly connected to the solar panel's metal tabs. Make sure the alligator clips did not come loose from the motor wires.
       3. If your circuit is OK, make sure your axle or transmission are not jammed. Does the axle spin if you twist one of the wheels by hand?
    2. If your motor spins, but the axles do not, make sure the gears are touching each other and the teeth mesh together. You might need to realign your motor or the axle to make sure the teeth of the gears mesh together firmly.
15. Once your wheels spin when you hold your car in the sun, you can test it on the ground! Spread your team out around a flat area so everyone is ready to catch the car (a well-built car can go pretty fast, but might not go straight). The solar panel is fragile, so try not to let your car crash into things. Watch your car carefully and think about things you could improve or fix.
16. After testing their cars, students should bring them back inside and work on improving their designs. They can modify the basic design as needed to fit the designs they drew on their worksheets, then re-test the cars.
17. Students should iteratively test and try to improve their cars. Emphasize that they should only change one thing at a time and then test again. Here are some suggestions for things they could change or improve:
    1. The gear ratio. What happens if you swap in a larger or smaller gear on either the motor or the axle? Remember to only change one gear at a time. Since the gears have different diameters, you might need to remove and reattach either the motor or the axle to make sure the new gears align.
       

       https://www.youtube.com/watch?v=-frF3sPyKMI
    2. The chassis. What happens if you make the chassis lighter or more aerodynamic (more streamlined)? Can you make it sturdier?
    3. The connections between parts. Early on, you might want to use tape so it is easy to remove parts or move them around if you change your design. Once you have decided for sure what parts you are using and where you want them to go, can you attach them more securely (for example, using hot glue, or screws if you are using balsa wood as a chassis)?
    4. The angle of the solar panel. Depending on how you mounted the panel to your chassis and the direction you point your car, the panel might not be pointed directly at the sun. Can you adjust the angle of the panel so it points right at the sun? This will increase the amount of power it generates. Note: if you are entering an official Junior Solar Sprint competition, you do not know which way the race track will face or what time of day it will be when you race. Consider making the angle of your solar panel adjustable so you can maximize power on race day.
       

       https://www.youtube.com/watch?v=SyQQt0jzLu4
18. When students have their final car design built and tested, have them make a technical drawing of their solar car. Brief directions on how to make a technical drawing are included in the student worksheet. More information and examples are also available in our Example Technical Drawings pdf.
19. To prepare for a class-wide competition, students need to add a way for their car to hook onto a guide line that is 1.5 cm above the ground. The guide line will make sure their car goes straight and does not veer off and crash into another car during a race. The guide line will be tied between two posts, so students need to be able to hook onto the line (e.g. using a paper clip) without untying one end of the line and threading it through a hole (like an eye bolt). See Figure 10 for an example using paper clips.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 10. Chassis hooked onto guide line the red paper clip (the yellow paper clip supports the solar panel, which has been removed for this picture).

Reflect (30 minutes)

In-Class Competition

To let your students race their cars, you will need a flat, straight area in direct sunlight. The official race track will be 20 meters long with lanes 60 centimeters wide, but you can use a smaller track if you do not have that much space available.

Set up at least two parallel tracks by pulling two lengths of fishing line tightly over the length of the track, 1.5 cm off the ground, and tying it to a heavy object (e.g. the leg of a chair). Mark clear start and finish lines on the ground (using tape, objects like rocks or sticks, etc.).

To have a race, students must hook their cars onto the guide line at the start and place the cars so the wheels touch the ground. They should hold onto the cars and cover the solar panels with their hands to prevent the cars from driving forward. You can then count down 3, 2, 1, GO, and the students can let go. You could just let the students race each other for fun, but you can let them race each other in a tournament-style bracket, or time all the cars with stopwatches (take the fastest of three time trials) and rank them by time.

Competing Beyond the Classroom

Do you and your students enjoy this lesson plan and want to take your solar powered cars to the next level?

Check out the Junior Solar Sprint (JSS), administered by the Technology Student Association (TSA) through a grant awarded by the Army Educational Outreach Program (AEOP). JSS is an educational program for 5th through 8th grade students with the goal of creating the fastest, most interesting and best crafted solar-vehicle possible. Students will design, build and race solar powered cars using hands-on engineering skills and principles of science and math, develop teamwork and problem-solving abilities, investigate environmental issues and gain hands-on STEM skills.

Are you interested in registering a student team for Junior Solar Sprint? Visit Junior Solar Sprint for more information.

Assess

To assess students in this lesson, you can:

• Assess the car itself and the student worksheet. Is the car clearly thought out with the design steps well documented? Was the car well-built? Or was it hastily/sloppily constructed?
• Ask each group to give a brief presentation (or write a short written report) about their car, how it works, and the design choices they made. Do the students understand how the car works? For example, can they explain what the gears do?

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Automotive Engineer

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Career Profile

Automotive engineers design cars and all the parts that go into them. Traditional cars run on gasoline but some newer cars are hybrids or entirely electric. Real cars also have many other systems that you did not have to consider here, like a suspension to give the passengers a smooth ride, and seat belts and airbags to keep them safe in the event of a crash. If you enjoyed learning about and building the parts of a car, then you should learn more about automotive engineering. Read more

Solar Energy Systems Engineer

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Career Profile

Solar energy systems engineers evaluate locations (usually buildings and not vehicles) for installation of solar panels to provide power. How many panels does the site need? How much will they reduce a building's energy cost? How much will they reduce the building's carbon footprint? How much will it all cost to install? If finding the answers to these questions interests you, then you should learn more about solar energy systems engineering. Read more

Lesson Plan Variations

• Do you live in a cloudy climate, or is it not practical for your students to test their cars outdoors? Do not worry! Your students can still build and test a battery-powered car and apply many of the same engineering principles. Instead of a solar panel, purchase 2xAA battery packs and connect them to the motors.
• Official kits and parts for the Junior Solar Sprint are available from two vendors, Solar Made and Pitsco. A variety of options are available depending on your needs. At a minimum, you must purchase the official motors and solar panels. You can have your students build everything else (including the transmission, axles, and wheels) themselves; or you can buy kits with all the parts needed to build a complete car (including a balsa wood chassis).