Dave,
I'd like to disagree with your assesment of why pinewood derby cars run fast.
First, a little background: Like you, I'm a software engineer. I had access to some hardware design talent for free and so, like you, I ended up building my own hardware and software for race timing. Besides, I just couldn't see paying those companies hundereds of dollars per lane for their timers! <NERDS RULE, NERDS RULE!> :)
I was interested in exactly what made a fast car and so I did some tests on my own to help me decide what really mattered and what didn't. My results were as follows:
Based on my tests weight is the single most critical factor. In a sense you are correct when you say that weight doesn't matter because every car should be at exactly 5 oz. On the other hand if you have someone that is only at 4.7 oz, they are at a significant disadvantage. Likewise, someone at 5.2 oz is at a major advantage. Besides my own test results I would refer you to any good physics book to understand why this is true (force = MASS * acceleration).
Second only to weight is the placement thereof. Again, based on my test results I could see VERY measureable results on the exact same car by simply locating the weight differently. Again, I would refer you to any good physics book to understand why this is so. Think about how much of a drop the weight will have if it is at the front of the car vs. the back of the car. If you look at the car sitting at the starting gate you will see that the nose of the car is obviously lower than the back, right? So, a weight at the front of the car is closer to the floor than a weight at the back of the car. Now, sit a car at the finish line and look at where the two weights are with respect to the floor. You will find that both the front and the back of the car are at the same level (ie. the same distance from the floor). This means that the back of the car has "fallen" farther than the front (it was further away from the floor and now it is the same distance, therefor it fell farther than the front). Since this is true, any weight at the back of the car would fall farther than weight at the front. So, the car receives more of a "push" because there is more potential energy (more potential energy is converted to kinetic energy if it falls farther).
If you want practical proof of this do the following expirement: Construct two cars by simply pushing the axels (with wheels) into the plain block of wood as it comes from the box. Now race these two cars a few times to determine which is naturally faster. Now add about an ounce of weight to the slower car. Unless the two cars were VASTLY different the "slower" car will now be your winner.
Now play around with the weight of the second car until you get it to tie the other car most of the time. It is important that you have added a significant amount of weight to the car (at least 2 ounces) and that you added it at the front of the car. Now simply move this weight to the back of the car (either car, it doesn't matter, try both if you like). You will see that the car with the weight at the back wins handily over the car with the weight at the front. Switching the weight positions of both cars will reverse the results.
We just ran a pinewood last night for a local pack. There were two cars that were built with the weight in the back (a son and his dad). Both cars ran neck and neck in every race. I was trying to explain to others why this was so and that the position of the weight made a big difference. As a test, we had the dad run his car backward on the track. Suddenly the car that ran backward is a full car length behind. It made a 2/10 of a second difference in the time. We raced them again both the correct direction and they were neck and neck.
My only caveat is that our track is the "standard" lazy-L shaped track (high slope at the start, almost flat at the finish). If your track is significantly different (like a fixed slope throughout the track) then I can see where placement of the weight might not matter.
I had fun last year at our pack pinewood by cutting a standard block in half and using a router to hollow out the inside. I then added enough weight to the back of the car to get it to 5 ounces (it took about 3.5 ounces of lead). The center of gravity was just in front of the rear wheels. I then glued the two halves back together and put on the wheels (basic cleaning and sanding and a bit of graphite). This car easily beat every other car on the track.
Our electronics consist of a 5 volt power supply, a photo transister and a resister for each lane hooked to the PC parallel port. It doesn't get much simpler than that. Our start sensor is a standard lever switch screwed to the side of the track that reads when the start gate is released. This switch is hooked to a resister and to a pin on the parallel port. Total cost of all electronics was under $25 for a three lane track. The most expensive component is the photo transister for each lane at about $3 each from a local parts house.
As an added matter of interest, I also report the scale speed of the car at the end of the track. This number is not used in any calculations of speed / winners but is reported just as a "human interest" number. I find that cars are traveling about 100 scale miles per hour at the end of the track.
Last modified 27 May 2006