TO MAKE YOUR PINEWOOD DERBY CAR THE FASTEST
There is no limit to the number of different designs for Pinewood Derby cars. You can stay within the rules and still come up with a totally original shape. No matter what body style you choose, the following tips will help you build a really fast car. You'll also find tips to on how to give your car a high gloss finish.
Use as many of the following suggestions as possible. The more tips that you take advantage of, the more likely your car is to be a contender for the championship.
Cub Scouts will be way ahead of their classmates at school when it comes to physics if they take the time to learn what it is that makes their Pinewood Derby car go and what makes it slow. There are three main forces acting on your PWD car.
The force that makes your car go is gravity. Gravity pulls the car straight down (just try dropping your car if you don't believe me). The track prevents the car from going straight down. The car must follow the slope of the track as gravity pulls it down. The force of gravity is stronger on an object that weighs more. Hold a five pound bag of sugar in one hand and a five ounce Pinewood Derby car in the other and see if you can tell which one has the greater force of gravity pulling on it. The rules say that your car can not weigh more than five ounces. To maximize the force of gravity on your car, you want it to weigh as close to five ounces as possible without going over.
There are two forces that slow your car down. One force is air friction. As your car moves it must push air out of the way. The air does not want to be moved, so it pushes back at your car. That slows your car down. The resistance of the air to the motion of your car is called air friction. To reduce the effect of air friction you should give your car an aerodynamic shape, so that it slices through the air. Some examples of things that have an aerodynamic shape are raindrops and airplanes.
The second force that holds your car back is friction with the track. Your car reduces the friction of the track by using wheels. Without wheels your car wouldn't go very far. Wheels are humankind's greatest invention for reducing the friction of objects moving over surfaces. Using wheels does not eliminate all the friction. Each wheel must move around and rub against an axle that does not turn. And the outer edge of the wheel is slowed down by every bump and flaw in the wheel and the track. This is called rolling friction.
You can reduce the effect of the wheel rubbing against the axle by polishing the axle surface making it smooth and shiny. Also, you can put a lubricant between the axle and the wheel. The lubricant makes the axle slippery. You can remove lumps and bumps on the outside edge of your wheels by sanding them lightly (not too much, the rules don't allow changing the shape of the wheels).
The following sections give detailed suggestions on how to give your car an aerodynamic shape, how to add weight to your car, and how to reduce rolling friction.
SLICE THE AIR
An airplane wing has a perfect aerodynamic shape. A cross-section of an airplane wing looks like this:
If you put wheels on this shape, it would look like this:
The problem with the airplane wing is that it tries to go up, and you want your car to go down. Real life race cars are low in the front and high in the back. That makes the air hold them down to the race track while reducing air friction.
You can use a similar shape. Experiment with different shapes by drawing them on paper until you come up with one that you are happy with. Then draw the shape on the side of the wooden block that will become your car. Use a pencil so that you can erase mistakes.
Once you have drawn your shape on the side of the wooden block. You can cut the rough outline using a coping saw. A coping saw is a hand tool that is easy enough for a Cub Scout to use without the hazards of a band saw. Clamp your wood block in the padded jaws of a vice before cutting it with the coping saw.
After you've cut the rough shape, you'll want to fine tune the shape with a wood file or coarse sand paper. Then smooth out the surface with fine sand paper. When you are done shaping your car, it's time to add the weight. Don't do the final sanding and painting until after you have the weight just right.
GET THE LEAD OUT
You can buy several different types of lead weight for your car at the local hobby shop. One type is 3/8 inch diameter cylinders. You can drill 3/8 inch holes in your car, slide a length of this cylindrical lead into each hole and cover the ends of the holes with wood putty or spackle.
Another type of lead weight is flat and molded into sections that can be broken apart. Holes are molded into the lead to accommodate screws that will attach it to your car. Some have a peel-and-stick adhesive on the back to attach the weight.
You may want to hollow out an area on the bottom or top of your car in which to locate this lead and then cover it over with wood putty. BBs, shotgun pellets, fishing sinkers, coins, screws, and washers are some other things you may want to use to stick to or hide inside your car to increase its weight.
Once you have the shape of your wood block cut out, find a scale that will accurately weigh five ounces. You can borrow one from an office or lab, or you can ask the clerk at the Post Office if you can use his scale for a minute. Put your wood block and all four wheels and axles on the scale. Add pieces of lead until the weight is close to five ounces but not over. After you've determined how much lead you need, attach it inside or on your car. Then cover up the weight, if that is the look you want for your car. When you've finished drilling and cutting, adding weight, applying wood putty, sanding and painting, weigh your car again. At this point you will need to make some final adjustments to make sure that your car is as close as possible to five ounces but not over. If you need more weight, you may want to screw some small screws into the bottom of your car. If your car is overweight, you may want to drill some holes in the bottom.
SHINE 'EM UP
You need to complete your cars paint job before you attach the wheels. If you want a high gloss finish, you must sand your car very smooth. Start with coarse sand paper to get the shape just right, then sand the entire car with a medium grit paper. Finish up with fine and then extra fine sand paper. For a good finish you should give your car two coats of sanding sealer before painting. Lightly sand the car with the finest sand paper you can find after each coat of sanding sealer. Three or more light coats of spray paint will give your car a nice glossy appearance. After each coat is completely dry, you can use steel wool to smooth out any bubbles or rough spots before applying next coat.
To reduce friction between your wheels and axles, you will want to remove all rough spots and polish your axles to a mirror finish. The best way to do this is to clamp your axles in the chuck of an electric drill. Secure the drill by clamping it in the padded jaws of a vice.
A parent or other adult must operate the electric drill when polishing axles. Also safety glasses or goggles should be worn when working with power tools. With the drill secure in the vice, turn on the drill and lock the trigger in the on position. To remove burrs or mold lines on the head of the axle hold a fine file against the axle as it spins.
Dip a thin strip of 600 grit silicone carbide sandpaper in water and wrap it around the spinning axle for a few seconds.
Then wrap a thin strip of crocus cloth around the axle while it spins for several seconds.
For the finishing touch, dip a wet strip of cloth in pumice and wrap it around the spinning axle. This should give the axle a mirror like finish.
Remove any burrs or rough spots on the center and outside edges of your wheels with fine sand paper. Now you are ready to lubricate your wheels and axles. The best lubricant for Pinewood Derby wheels is graphite powder. You can find graphite powder at the same hobby shop that sells lead weights for your car. Dip your axles in the graphite powder and stir them around in it. Then pour the graphite down the center hole of your wheels. Slip each axle through a wheel. Now you are ready to tap your axles into place by driving the nails into the grooves in the body from the ends with a tack hammer. Leave enough space between the wheel and the body so that the wheel spins freely.
MORE PHYSICS FOR EXTRA CREDIT
Once your car gets started down the track and reaches the level portion of the track, gravity is no longer helping to move your car forward. The thing that now keeps your car moving is Newton's First Law of Motion. Newton was a genius who figured out rules that describe the way things move. Using Newton's laws we can predict what will happen to moving objects and design machines that work.
Newton's First Law of Motion says that an object that is moving will continue to move in the same direction and at the same speed unless a force acts on it. This means: a force must push or pull on the object to move it faster, slow it down, or change it's direction of motion. Your Pinewood Derby car will continue to move along the track until the forces of friction slow it down and stop it, or it bumps into the box at the end of the track (the box exerts a very strong force on the car).
Click here to see the official Pine Wood Derby Rules
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