|MadSci Network: Engineering|
|MadSci Network: Engineering|
An excellent question! This is exactly what engineers and designers have to ask themselves whenever they start to build something new. As a Materials Scientist, one of my responsibilities is to develop the materials that go into new products such as electric cars.
As a practical matter, before you actually go out an build a car I would suggest looking into radio controlled model car kits available from your local hobby store. The engineers who made the cars have already gone through the process of selecting materials to meet the needs of the cars from a larger set of possible materials than you will have. They have made the parts for you and developed and installed all the controls that you need to run the car. But do not be daunted if you want to make your own car from scratch! It can be done!
There are several electric car races held in America and internationally. An electric car race is held each year as part of the Cleveland Grand Prix at Burke Lakefront Airport. There are also an annual international solar powered car race that across Australia. Many of the crews are from universities that you might attend in the future. Some high schools also compete.
For these cars, the everything must be very light and strong. High strength aluminum alloys, carbon fiber reinforced plastics, and other high tech and aerospace materials typically are used for the body and frame of the cars. They also have a set of batteries for cloudy days. As with the requirements for the new electric cars showing up in California, these have to have as much electrical storage capacity as possible in a very light battery,
From your question, you seem more interested in a model car. Guess what? Many of the same concerns facing engineers designing the large cars are being faced by you as well!
For a fourth through sixth grader, the best materials for you to use are really governed by what you can obtain and work with. Even within that constrain there are a large number of options. The first thing to look at is what you need for a model car powered by batteries.
The frame needs to be lightweight and strong enough to support the weight of the batteries, wiring, motor and body. It also needs to be tough enough to take the pounding of running over rough terrain and hitting something. Finally it needs to be stiff enough so that it does not bend too much. For the frame your materials' options would include plastics, wood and metals.
Metals are the hardest to work with, but depending on the size and complexity of your model may be your best option. To save weight use small diameter tubing and small I-beams rather than solid pieces.
Plastic is easier to cut and join. Lexan, plexiglass and other plastics are available at your local hardware store. They tend to be highly break resistant and can be easily painted. They can be joined with special adhesives or simply by heating depending on the specific plastic you buy. They are not as stiff as a metal, so you will need to use thicker sections. CPVC piping (found in the plumbing section) may be a good option for the frame as well. It also has the advantages of a large number of fittings for joining the pipe together in various combinations.
Wood is a good material for the frame as well. It is readily available at the hardware store, can be cut, shaped and joined using nails, screws and adhesive, and is actually stronger per pound than steel. The type of wood you use is important as well. Typical white pine is soft and not as strong as hardwoods. Hardwoods such as maple, poplar and oak are more expensive, but tend to have fewer defects and knots.
The body needs to be lightweight, impact resistant and, if you want to paint the car, paintable. The ability to repair the car body should also be considered. The options are identical to the frame; plastics, metals and wood are still the best options. Here though thinner materials are typically used because the body carries only its own weight.
For plastics, commercially made model kits will use thermoplastics (plastics that can be softened by heat) because they can be made into a complex shapes using die casting. In die casting the plastic is softened, forced into a metal mold under pressure, and then quickly cooled to make the parts. If you look at any of the plastic model kits, you can still see the runners where the plastic came into the molds. You can also buy thin thermoplastic sheets from a local plastics supplier and make parts by heating the plastic with a heat gun, hair drier, quartz lamp or similar heat source. NEVER USE A FLAME TO HEAT PLASTICS! THEY WILL CATCH FIRE IF YOU DO! Be sure you ask the supplier if you can paint the plastic! Some plastic such as polyethylene cannot be painted; that is why you fill see them being used for painting equipment such as trays. Holes can be repaired using fiberglass repair kits from local automobile supply stores.
Fiberglass automobile repair kits can also be used to make the body of the car. The woven fiberglass cloth can be shaped on a mold to the shape of the body. The resin, a plastic, is then painted on, over and through the cloth to form a fiberglass composite. While not as high tech as aerospace composites, it shares the advantages of increase strength and stiffness with them. It can also be painted using auto body paints.
Metal is tougher to work with but is less likely to be seriously damaged from everyday handling. Aluminum is lightweight, easy to shape and form and available from many metal supply companies as thin sheet. Prime the metal with a primer designed to be used on aluminum (available at your local hardware or paint store) and then paint over it with model paints or enamel paints.
Wood is again a good choice. Pine and balsa woods are two options. Pine is stronger and will not be easily damaged in an wreck. Balsa wood is much lighter, but also much more easily damaged. Both can be easily painted.
Once you have the frame made, you need to select the batteries to put inside the car. A battery should be a light, but it also must have as much electrical energy stored in it as possible. For an engineer, the term used to describe the ratio of electrical energy to the weight is specific storage capacity. It is typically measured in Watt-hours per kilogram or something similar. The ideal is to get as high a value as possible without making the battery too expensive to use.
The batteries sold in the local stores are not the lightest in existence, but they are readily available and contain a surprising amount of electrical energy. The size you chose will be governed by the space you make available for the batteries in the car and the electrical needs of the motor. For a faster car, use smaller batteries such as AAA size to minimize weight. For a car that needs a lot of torque for climbing over obstacles or needs to run for a long time, use a large battery such as a D cell or even a 6 volt lantern battery. If you plan to use the car frequently, rechargeable nickel-cadmium (NiCad) batteries will cost more initially but can be reused up to 600 times. They have a slightly lower voltage, so a small amount of speed will be sacrificed.
You can also step up by going to a hobby store that sells radio controlled (RC) model cars and buying a battery for one of these cars. These batteries tend to be a little lighter, have a higher voltage, may last longer between charges than NiCad batteries and are rechargeable.
Wiring is easy and straight forward for your car. Copper is the second best metallic conductor (silver is the best), and it can be readily purchased at hardware electrical supply stores. The only real question is the diameter or gauge of wire that you need to use. Think of a straw. A small straw is very hard to suck through while a large straw is easy. Electrical wire works the same way. Large diameter wire is easier for electricity to pass through than small diameter wire. There is one trick to buying wire. The wire is sized by gauge rather than by the inch or millimeter. To complicate things, the larger the gauge number, the smaller the wire.
For a small motor such as you are using, anything between 16 gauge and 22 gauge should work okay. Best performance will be achieved with a large diameter (lower gauge number) wire.
The motor is probably the worst waster of energy in your model car. Friction is the only thing that will come close to it. If you look at a motor there is a part made up of a wound copper wire that rotates between two or more magnets housed in the outer casing of the motor. The best way for en engineer or designer to get the most out of your motor, you need to maximize the strength of the magnets in the motor. The other considerations are how much torque or twisting force can the motor generate and how fast can it turn. These are controlled by the design on the motor.
The low end of magnets are simply magnetized iron. By putting the iron in a strong magnetic field the domains or little magnets inside the iron line up to give the piece a magnetic field. The field is weak and the iron can be easily demagnetized.
One of the first big breakthroughs in magnets were the iron-cobalt magnets that had stronger magnetic fields and did not demagnetize easily. These are probably the type of magnets that will be in the motor you buy.
The latest magnets are made from iron-neodymium. They are extremely strong. Two one-inch square, quarter-inch thick magnets will come together when held about a foot apart with a loud clap. One manufacturer is investigating using them as an emergency braking device for trains. They are also being investigated for use in industrial and automotive motors since the stronger the magnets are the more efficient the motor will be. You might be able to find some motors made with these magnets.
Depending on where you live an electrical supply or hobby store might have motors. If not, Edmund Scientific has them. Their on-line store is to add motors soon, but in the meantime you can get a copy of their print catalog for ordering a motor.
The speed and torque of the car will depend on the gears you use. The motor can be directly connected to the wheels, but the speed of the car will be limited by how fast the motor can turn. By using gears, you can increase the speed that the tires rotate at and therefore the speed of the car. The price you pay is the amount of torque you can generate to accelerate the car quickly.
Gears are typically made of plastic or metal. Gears made from nylon and other plastics are lightweight and easily made, but are not as durable as metal. Some are probably used in your family car. Metal weighs more, but it is less likely to deform and will generally last longer. It is the only choice when you want to apply a large force such as when you want to use a motor on a crane to lift a heavy load.
These may be the hardest items to find. Your hobby store may have gears available for sale. Train shops can also stock them. You might be able to find them through local machine shops as well.
Hopefully this gave you some ideas of the options available to you. Just like a real engineer you will have to assess the your needs, determine the options available to you, and ascertain what you can afford. If you enjoy this type of thing, engineering is a good career where you can do these types of things on a regular basis.
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