TIPS ON HELPING YOUR CHILD BUILD GRAVITY-POWERED RACER
PHOTO CAPTION: This car missed first place by just one inch. We still had fun!
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, Colo.
DEAR BRIDGET: So you feel 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've built some screaming machines, but I've had a few slackers as well. I'm more than happy to share some of my secrets.
Your biggest enemy in this car race is friction, which occurs when two moving parts rub against one another. Since these cars don't have 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 some Girl and Boy Scout councils have strict rules about what you can and can't do to your car.
The second enemy is balance. The kits often provided to you by your troop leaders need to have weight added to them, which 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.
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 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- not the entire hub - contacts the wood.
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's 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 you don't drive the nails too tight. If this happens, the wheels won't spin freely on the axles. Use powdered graphite to lubricate the wheels.
I like to add weight using pieces of solder. I create a 3/16-inch-deep recess that is 1 1/4 inches wide by 3 inches long between the saw-cut axle channels on the bottom of the wood car. It's 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 an accurate postal scale so 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.
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 toward a goal is much more fulfilling and long lasting. - - - Got a question for Tim? Call him toll free any Saturday from 10 a.m. to noon ET at (888) 737-1450. Need to see an old column? If so, visit Ask the Builder on the Web: askthebuilder.com. Send for Builder Bulletin No. 311, listing 10 friction beating pinewood derby secrets and other tips on building derby cars. Please send $3 and your name and address to Tim Carter, c/o INSERT LOCAL PAPER NAME HERE, P.O. Box 36352 Cincinnati, Ohio 45236-0352. c 2000 TIM CARTER BUILDER, INC. DISTRIBUTED BY TRIBUNE MEDIA SERVICES, INC.