Building Small Model Cars

DEAR TIM: My mom and dad read your column each week. You help them a lot. But I need your help now. My girl scout troop has a Powder Puff derby competition each spring and we build small model cars that race down a sloped track. I would really like to get a medal for the fastest car. Looks don't mean as much to me. Can you help me? What do I need to do to make my little race car go fast? Bridget M., Denver, CO

DEAR BRIDGET: So you feel the need, the need for speed? My children are just like you. They both enjoy scouting as did I when I was a child. Each year, I help them work on their gravity powered race cars. We have built some screaming machines but I have also had a few slackers as well. I'm more than happy to share some of my secrets.

What is friction?

Your biggest enemy in this car race is friction. This happens when two moving parts rub against one another. Since these cars don't have any motors, the friction happens at each wheel and as air passes over the car. You need to do everything possible to reduce friction where the plastic wheels touch the car body and the steel axle. Keep in mind that certain girl and boy scout councils have strict rules with respect to what you can and can't do to each of the cars.

Should a model race car be balanced?

The second enemy is balance. The kits that are often provided to you by your troop leaders need to have weight added to them. This weight needs to be spread out as much as possible over the length of the car. If too much weight ends up on either the front or rear end, you increase friction on those wheels. Unbalanced cars are also very unstable on the race course and can actually jump off the track. If this happens, you will experience the agony of defeat.

How do you reduce friction?

To reduce friction at the wheels use fine sandpaper to remove any plastic burrs that might be on the inside edge of the wheel or the hub that is near the axle hole. If you are permitted to sand the hub, make it slightly rounded so that when it touches the body of the car just a small amount of plastic contacts the wood, not the entire hub. Use emery cloth to polish the shafts of the small nails that act as the car axles. Check for metal burrs under the heads of each nail at this time. Remove the burrs with a small file if this is permitted. Sand the sides of the wood car and coat the areas near each wheel with multiple coats of high gloss polyurethane after the car has been painted. It is much easier to paint the car before you assemble the wheels.

Check wheel spin on each axle before you assemble the car. You want the wheels to rotate with ease. When you attach the wheels and axles to the car, be sure that you don't drive the nails too tightly. If this happens, the wheels will not spin freely on the axles. Use powdered graphite to lubricate the wheels.

Should weight be added to the model car?

I like to add weight using pieces of solder. I create a 3/16 inch deep recess that is 1.25 inches wide by 3 inches long in between the saw cut axle channels on the bottom of the wood car. It is best to create this channel before you cut out the shape of your car. Your mom or dad can do this with a router or a sharp wood chisel. Cut out the car shape next. Sand the car and paint it at this time. Allow the paint to dry thoroughly.

I then place the car upside down on a highly accurate postal scale so that the recess area is level. Use one of the car wheels to do this if necessary. Place the remaining wheels, axles and any decals on the scale. Cut pieces of plumber's roll solder that are just the right length to fit into the recess. Make them as straight as possible and glue them into the recessed area. Glue just enough solder to take the car up to its maximum allowable weight.

Does the design of the model race car matter?

Your car design doesn't need to be fancy to win. A simple wedge shape produces minimal air friction. Be sure to cut out some material from the rear bottom of the block of wood. This excess wood can rub on the track when the car gets to the bottom of the hill. One last thing: try to remember that the car race is about working together and having fun. The thrill of victory lasts but a few minutes. The joy of competing fairly and working hard towards a goal is much more fulfilling and long lasting.