Pinewood Derby track question: Engineering advice wanted

[Morey St. Denis]

"it's a fallacious argument and the mass could be anywhere." Believe lower is likely better for conservation of energy and stability considerations.

I have no particular expertise with Pinewood Derby races, nor have I competed. I am however often invited to advise and judge regional science entrants, and certain STEM competitions such as FIRST. Figure it's how I can assist in my own small way to furthering the educational opportunities of youth...

Agreed that the change in pitch position of the vehicle is a miniscule factor as it undergoes a positive pitch displacement. But by deliberately attaching a mass concentration distant from body center, some energy of the system must be spent in raising that extended mass portion against the force of gravity as the body pitches up. The work thereby extracted is proportional to that optional mass concentration, its distance from a more neutral polar position and the sine function of the change in slope of the track from start to timed finish position. I fail to see how the principal of conservation of momentum could be employed to advantage in this positive pitch rotation. Perhaps if the pitch displacement were negative such as in an outside parabolic loop with gravity assist, but with useful loops you eventually pass full circle and likely must eventually pull out, canceling most all but the vertical change in height from beginning to end and trading potential energy of position for kinetic energy and momentum.

Agreed that random trial and error can sometimes deliver some useful empirical insights in many "real world" or field trial situations. Whenever time, effort and expense are potential considerations you are well advised to employ the scientific method; which is to form a educated hypothesis, then devise a series of tests to confirm or deny that theory. It goes without saying that you are well served to first analyze the situation using the best scientific tools at your disposal, such as the basic physics & applied mechanics in this particular example.

[Jeff Heil]

My son placed in the top 3 cars of his den every year in scouts and was 2nd overall in the pack his last year in Cub Scouts. He designed and did most of the work on his cars (he was afraid of the bandsaw the first two years) every year and he took ownership of how successful he was in the pinewood derby. I researched and provided guidance. I always made him sketch out his design on the block.

Here's what we found worked best for us to be competitive:

I agree with the others that a car that rides against the rail runs straighter and faster. We always set one front wheel up 1/16th to not touch the track, (approx. 25% less rolling friction). We balanced the car on a steel rule to get the center of gravity (COG) about 3/4" forward of the rear axle. We found using tungsten weights allowed more control of the COG than less dense weights. We would bore a 3/8 hole down the center of the block before cutting out the shape of the car and slide the round weights back and forth as needed.

My son spent lots of time with a small file taking the burr off the nails and emery cloth and wet sandpaper polishing the axels (just chucked them in the drill press). We always put a dab of super glue on the ends of the nails to keep the wheels attached to the car. Saw many cars over the years loose their wheels if they jumped the track. We made a short test track from scrap 1/4 plywood to test cars to make sure they ran straight and true without wobbling.

Great lesson for my son was learning the more effort and time he put into his cars, generally the more successful he was. Also fun shop time together. Our pack had pretty clear rules against not shaping the wheels or using anything but the pine block and stock wheels to keep it fair and not allow someone to buy aftermarket machined wheels.

Our pack gave every kid that built a a car a medal for building a car and entering the race and trophies for the top cars. I am not a fan of participation awards, but we made sure to frame the context as a "car builder award" and making the effort to enter the race.

I always asked the kids about how they built their cars at check-in and on race days, and could always tell the pride of participation and appreciation of the process of getting a car ready to race vs the kids that had parents that built the car for them or did not allow them to touch the car. If the dad carried in the car to weigh in and the son was not allowed to touch the car until they got to the scale it was pretty obvious who built it.

I still have my pinewood derby cars from 35 years ago that I built with my dad. Good memories.