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The rules are very tight...as a matter of fact, the kit the our boys received (their pack derby was last weekend) had everything in it to compete. The "axles" had grooves already cut into the block of wood for the nail. You have to use the supplied wheels. No shaving the width of the tires. You can smooth the wheels where they touch the car to "deburr" them. Powder graphite helps.
An engineer's observation...the cars that had their weight concentrated toward the back were faster (as someone already said here). Simply physics...covert potential energy to kenetic energy. All cars start at the same height (nose height) with stops in at the top of the track holding the nose of the car. The further you can place the center of mass of the car, the higher your potential energy will be.
You can see the cars with less kenetic energy start to slow down at the bottom of the track. They then have to go another 30 or 40 feet. The ones that were well designed seemed to maintain speed all the way to the finish line.
All that said, it was our boys' first derby, and they really are not that competitive yet so I held back on the help. I cut the shapes they wanted, and they sanded until they got bored or tired.
I wanted them to fail (be as competitive as they know how..not how much I know) this time so they'd be more motivated next year. They give all the kids a trophy (crappy PC sheet..teaches them ntohing). The younger one got in the car afterward and said..."is this trophy for competing or for being competitive." Well, he was not happy when he realized it was for just competing...next year we'll engineer the cars!
You should have seen the parents at this thing...guys were working on there cars in the field house at the last minute...no sign of his kid(s)...but cordless power tools all around. You can work on your car after weigh in...you don't have to submit it on first weigh in. I'm as competitive as the next guy, but things like that takes away from the kids experience I say. Pine derby is not something to overshadow your kid's efforts. Have fun and let then do the work.
There is a "Master" competition where the dads and moms can build a car to compete.
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