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Pinewood Derby Stories and Photos from Maximum Velocity
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PINEWOOD DERBY CAR SHOWCASE
(I am dedicating the Car Showcase this week to a car
built by Stephen Parks).
I was browsing your newsletter archives, and saw the article on the direct drive outlaw car that you built Volume 8, Issue 3
It was interesting to me because, as an engineering student, I competed in the ASME student design competition, and designed a "string transmission" for a transport device.
For last year's pinewood derby event at work, I wanted to make an electric car for the Outlaw class, and initially thought of using the same type of transmission. However, it has some issues. If the
wheels spin, then you may not have enough string to power all the way to the end, and if the wheels don't spin, then you probably should have used more power! Eventually I abandoned the idea of a string transmission, and decided to just use a gear drive. A co-worker races electric RC boats, and sourced a Lithium-Polymer battery. These batteries are expensive, but relatively light, and have almost no internal resistance. I found a few motors that looked to be a reasonable size, then built a dynamometer to test and compare them. This approach wasn't very consistent, so in the end I simply picked the motor that sounded and felt the strongest.
So far, this wasn't nearly complicated enough for me. I figured that this drag car ought to look like one, so I decided to hide the drive train, switches, etc. into a drag car model so that it would be a "sleeper". The model I chose was the Stone, Woods, and Cook Willys.
Model Box Photo
My Car
The start switch is hidden behind the grille. When it is race time, a pin fits through one of the holes in the grille and into the switch, and rests against the start post on the track. There is an arming switch underneath the car just in front of the rear bumper. There was also a cutoff switch underneath the car, so that when the car falls down onto the stop strip at the end of the track, the power is cut (well, that was the idea).
Quite a bit of work was required to transform a model kit that was intended to be stationary into a rolling, self-powered, self-guided vehicle. Here are photos of the chassis, showing the motor and gear layout. The silver rectangle is the battery pack. The gears are out of a broken DVD player.
Top of Chassis
Bottom of Chassis
I should mention that the track is not set up until the night before the day of the race (after the impound). So, testing is very difficult. But I did get the coordinator to allow a couple of practice runs. On the first run, the tires had too much traction and lifted the front end when they caught on a track joint, resulting in a
derail. But that run put enough graphite on the tires to reduce grip enough so that didn't happen again. The next run was 1.998 seconds, on 49 feet of BestTrack!
On race day, the car got a lot of interest for looking good, though some thought that it was "just a plastic model." No one expected a two second run, and it was easily faster than the other Outlaws, all of which were gravity powered. The run times gradually increased, not because of the battery, but because of graphite buildup on the tires. Going fast was not a problem with this car, but getting it stopped was. I seriously underestimated the speed at the end of the track, and didn't have a good way to stop the car. At the end of the fourth run, one of the guides that keeps the rubber wheels from contacting the center rail broke, and the car could not stay on the track after that. I took the car out of the race instead of needlessly damaging it further.
This year I'll probably reuse the motor and battery, but forgo the fancy body in favor of a much more robust frame, and I'll build a stopping device to fit onto the end of the track. I may also switch to front wheel drive to eliminate the possibility of wheelies, and I can then clean the graphite off of the tires after each run to maximize traction and maintain consistent times.
