Pinewood Derby

The pinewood derby workshop was cancelled due to inclement weather, but we will still be holding a pinewood derby work day at Fred Walls home. The workshop will be on Saturday, April 12 at Noon. In the place of the workshop, Pete Rethorn, masterracer, has put together the following tips. Thanks, Pete!

Friction
Your biggest enemy is friction. It is the most significant force slowing your car, so reducing friction is the most important thing you can do if you want a faster car. You will encounter friction in five places:

Friction between the wheels and axles.
The axles often have burrs and manufacturing marks which will dig into the wheel's hub. To fix this problem, lock each axle into the chuck of a drill, leaving the head of the axle plus about 1/2-inch of the axle itself sticking out. Polish the exposed axle with a thin strip of wet 400 emery paper. Polish both the axle shaft and the inside of the axle head. Keep the emery paper moving to avoid creating grooves in the axle. Test the axle by inserting it through a wheel and spinning the wheel with your finger. The wheel should spin smoothly and slow down very gradually. For an even better polish, you can use steel wool, metal polish, buffing compound, or jeweler's rouge after the initial polishing with emery paper.

Around the wheel's axle hole you'll often find burrs and rough edges. Carefully clean these surfaces (on both sides of the wheel) with a sharp Xacto knife and some 400 wet emery cloth.

Lube tips
1. Use graphite only. Oil damages the paint and collects dust. I'm told that the graphite works better than the new white teflon.
2. Break in the wheels by spinning them with lots of graphite.
3. Put a small drop of white glue where the axle goes into the car body and put powdered graphite on it there. That causes less friction if the wheel should rub against the car body.
4. Other than the good polishing of the axles, dump the axles and wheels in a ziplock bag with some graphite and shake them for a few days prior to the race. That way the wheel and the axles are as slick as can be.

Friction between the wheels and car body
Paint that hasn't cured completely tends to be soft and sticky. It will stick to the wheels and slow the car down. Give yourself lots of time (several days) to apply your final finish coat before you put your wheels on. Also, a single heavy coat will take longer to dry and cure, so apply several light coats instead. That soft finish will also act like a magnet for graphite and you'll never get it clean -- the graphite will permanently discolor a soft finish. If you're painting at the last minute, use laquer. Lacquer seems to give a harder, faster finish than other paints. It also hardens much faster.

Giving the wheel hub a rounded surface where is rubs against the car body reduces friction at that point by reducing the amount of surface contact.

When mounting your wheels (after the finish is thoroughly cured) don't push the axles in so tight that you bind the wheels. If you push the axles in too far, the wheels will bind against the sides of the car. Also, if you make the car too narrow at the wheel locations, the wheels will bind against the sides of the center guide rail on the track.

Friction between the wheels and the track's guide rail
A car that doesn't track straight will bounce side-to-side against the rails, dissipating considerable energy. Anything you can do to make the car run strait and smooth will help.

Friction between the wheel tread and the track surface
Out of the box, wheels may have irregularities left by the casting process (i.e. sprues, burrs, bumps, holes, etc.). You should remove these irregularities, and then carefully sand any remaining roughness with emery paper. Metal polish can restore the glossy finish to your wheels.

Be careful to avoid reshaping your wheels in any other way, or you may disqualify your car. At the very least, you should ensure the wheels are perfectly round and concentric with the axle hole and remove the imperfections left by the casting process.

Friction between the car's belly and the track's guide rail
A low-slung car might look cool, but remember that most tracks have a 3/8" strip of wood the car must straddle. Make sure your car has plenty of clearance. The most popular place to add weight to a car is the belly. Make sure it doesn't drag too.

Weight

As you shape the car, you'll remove weight in the form of wood. However, the more the car weights, the better it'll perform on the track. So, plan the car's design so you can add weight after the car's shape is completed. Weight, according to the rules, is limited to 5 ounces. Depending on how much wood you remove, you will likely need to add about 3 ounces of weight. Get your weight as close to the limit as possible. Without adding weight to the car, it will not weigh-in at the maximum allowable weight, thus it will not reach peak performance.

Because of its density, lead has been the traditional weighting material since the inception of the pinewood derby. At a density of 11.34, lead is quite dense when compared to other possible weighting materials. The relatively small amount of lead required for weighting provides the car builder greater flexibility in the car design.

An added benefit of lead is that it is very malleable. With most materials the car builder must create holes or cavities in the car body to fit the shape and size of the weighting material. However, with lead the builder can create any size hole/cavity and then shape the lead to fit the hole. Another nice feature of lead is that it can be easily drilled. Thus, the weight of the car can be easily reduced at the weigh-in by drilling out of a portion of the lead.

Warning! If used or handled improperly, lead is toxic. Therefore, follow the following rules:

• Wash your hands after handling lead (and keep your fingers away from your face).
• Don't eat or drink when handling lead.
• Keep lead away from food, water, and food preparation areas.
• Collect and properly dispose of any lead scrap.
• Melting lead must be done only outdoors by experienced adults. The fumes are toxic, and the lead can pop and/or splatter during the melting process causing severe eye and skin injuries.

Excellent options include lead fishing weights -- especially the lead wire that can be cut into short lengths to glue into holes drilled into the car body.

It seems to be best to keep the weight low in the body. But be careful nothing extends below the belly which can drag on the track -- remember on most tracks, the car straddles a 3/8" thick strip of wood.

Weight distribution can affect your car's speed. Your car can correct its course more easily if it has a low rotational inertia. Make the unweighted car body as light as possible, and keep the lead weight as compact as possible -- don't spread it across the full length of the car.

One theory holds that the further back the center of gravity is, the higher the weight will be when the car is placed in the starting position on the track, and therefore, the more static energy the car will have to get it rolling down to the level portion of the track. I've seen as much as a quarter of a second difference in the same car run frontward and backward to verify this theory. However, the center of gravity must be far enough in front of the rear axles to keep the front wheels from bouncing too much over the joints and imperfections in the track. This bouncing will slow the car down. Most people recommend locating the center of gravity of the car 1 to 1-1/2 inches in front of the rear axle.

Wheel Alignment

Keep your wheels and axles straight. Crooked or misaligned wheels will cause your car to ride the track's guide rail or to shimmy. The extra friction will slow your car down. Keeping your wheels and axles straight is probably the most important step in building a faster car. Mount the wheels so all four are strait in relation to each other and perfectly perpendicular to the body. Be careful not to bend the axles, either during construction, during pre-race play, or during the race by smacking into objects at the finish line.

If you must use the slots, glue the axles in place with epoxy. I've seen many cars dismantle themselves because the slots just won't reliably hold the axles, much to the joy of those who do use glue. Wheel alignment is harder with the slots -- do the best you can.

Aerodynamics

Aerodynamic drag is a factor, but not quite as much as one would expect. Any aerospace engineer will tell you that the effects of aerodynamic drag increase exponentially with speed. At the speeds pine cars travel, drag is relatively unimportant. Other factors, listed above seem to be much more significant. That being said, a big box certainly won't help, and a sporty shape makes it look like a fast car. Other factors being the same, a streamlined shape slips though the air better. The wedge and the dome are consistently seen in the winner's circle. However, surface smoothness is not a factor at pinewood derby speeds.

While air resistance is of less importance than reducing friction and getting the car to track straight, an aerodynamic car with smooth lines certainly looks faster, and it may even race a bit faster. It may be a factor in cars racing against cars where friction has been properly addressed. A clunky looking car with no friction will invariably blow the doors off a fast-looking car with lots of drag in the wheels.

Miscellaneous

Do not make your car longer than the block of wood that came with your kit. Do not make your car wider than the block of wood plus the width of the wheels. Do not install anything forward of your block of wood nor behind. Doing any of these may affect some timing devices, starting gates, and even the ability to run on certain tracks.

Do not use any moving parts other than the wheels unless specifically allowed by your local rules.

Do not allow anything to protrude below the belly of the car -- you should have about 3/8" of clearance. Otherwise, the car's belly will drag on the track lane strip of most tracks.

Finishing

This is the artistic side of building a model car, and like many artistic things, it sometimes defies hard and fast rules. The following guidelines will be helpful, but a truly great design must spring from your own creativity and craftsmanship.

Be Careful

Neatness counts. Don't ruin a great paint job by getting bugs, fingerprints, or dirt on it. If you apply any decals, make sure they're straight and smooth. Follow the written directions of any paints or other products you use.

Be Patient

Before painting, make sure you have sanded your car smooth everywhere that shows. Sand the entire car repeatedly, using finer and finer sandpaper each time. When you paint it, apply several light layers of paint; a heavy coat of paint will often run, drip, form puddles, etc.

Paint Early, Paint Often

Most paints and finishes remain somewhat soft for a while, even after they are completely dry to the touch. Leave yourself time to apply your final finish coat well in advance of the actual race, so that it won't pick up fingerprints when you put on the wheels and adjust the weight.

A single heavy coat will take longer to dry and cure, so be sure to apply several light coats instead. With many paints, you must apply a second coat within an hour or two, otherwise you have to wait two or three days to apply a second coat. If the first coat has completely cured, you can sand it lightly with steel wool before applying a second coat.

Think in Three Dimensions

Do more to shape your car than just cutting the outline of your car with a saw. Think about how your car will look from all directions, not just the side or top. Just rounding off sharp corners with a rasp or with sandpaper can make a big difference.

Try Something Unique

The judges are more likely to notice your car if there is something different about it. Make your car "the car with the realistic (something)", or "the car that looks like a (whatever)", instead of just "car number 42".

Constructing Is Easier Than Carving

It is easier to glue on small pieces of dowel, scrap wood, etc., than it is to carve similar shapes out of the original block. For example, a formula race car has a wing in the back. It is very difficult to carve this wing and its supports in place. It is much easier to carve the wing and supports as separate pieces of wood, and to glue them onto the body of the car later. Small pieces of scrap wood should be easy to obtain at your group's derby workshop.

It is also easier to stay under the weight limit when you build instead of carving. The obvious approach when building a blocky design (e.g., a minivan, a Zamboni ice machine) is to glue another block of wood onto the one you started with, and then to carve this oversized block into the final shape. This almost guarantees that you'll have to hollow out the inside later to get your car's weight under the maximum allowed. It is easier to start by building a hollow car body from thin scrap wood.

Final Finish
After you've painted your car, several products (including floor wax and aerosol clear-coat gloss finishes) can produce a beautiful, shiny, clear finish.