Dear Car Talk Guys,
I was appalled to hear you tell an inquisitive den mom that the weight of her son's Pinewood Derby car should have nothing to do with the speed of said vehicle to the rubber-bumpered finish line! Before getting into the science of the situation, I must address the moral and ethical dilemma of parental involvement also raised during this call.
The official rules for Pinewood Derby state that the cars must be
made by the individual Scout with minimal parental involvement.
Anyone who has been to a Pinewood Derby, however, knows that this rule
is routinely flouted. Additionally, anyone attending a Pinewood
Derby can immediately tell whose father has been out of town during
the previous week. The wheels of this Scout's car invariably fall off
halfway down the track, sending miniature car parts crashing into the
watching crowd. Such an incident can have an extremely devastating
effect on a young Scout's motivation and self-confidence. The trick
to successful Pinewood Derby participation, therefore, is to disguise
the high level of parental involvement required to survive this annual
trial by fire, while simultaneously building your son's confidence
and crushing the others fathers' pitiful attempts to beat your car to
the finish line.
The method I use is called "gnomes and camouflage." A week prior to the race, you gather all of your supplies, give minimal instruction and tell your son to "have at it." Each night, after everyone is asleep, you get up and secretly "enhance" your son's efforts. (This is the gnome part!) After about five days of this you have a pretty good-looking car. Now comes the camouflage part! You get some paints together, give them to your son and tell him to "decorate" the car. (Some fathers should leave the house during this part). After "decoration" you have what looks like an 8-year-old's creation, but in reality is a finely crafted tool of parental warfare. Now you can take the car to the race and compete against all the other fathers while the boys chase each other around the building and pretty much ignore the race, until it's time to give out trophies. You know that you have had a successful year when your son gets a trophy and you go home with the prize for "Least Apparent Parental Influence."
Now to address the weight issue. After rummaging through some of my old college textbooks--which, by the way, my wife has been begging me to throw away for the last 12 years but I knew would come in handy some day--I offer the following explanation.
In a nutshell, the main forces acting to halt a Pinewood Derby car's forward motion are the kinetic friction force resulting from the contact of the axle nails with the plastic wheels and the occasional impact of the wheels with the track's guardrail. The force working to overcome this kinetic friction force is the car's momentum, which is directly proportional to its weight. Therefore, the lighter the car, the less force working against friction, and the quicker the car's velocity decreases to zero. The placement of the weight is also important, because this determines the stability of the car. A stable car will strike the guardrail fewer times than an unstable car, thus reducing the forces acting to slow the car. I have found that evenly distributing the weight over the rear two-thirds of the car works best. Hope this helps.
Sincerely,
Gary Brown from Shreveport, LA
