Note: My car was featured in the September 2011 issue of Popular Mechanics
http://www.popularmechanics.com/home/how-to-plans/backyard-genius-2011-the-years-10-best-diy-creations-jet-propelled-pine-wood-derby-car

This is a 3D printable motor for an 'outlaw' class Pinewood derby car. My son's cub scout pack has run an outlaw class for the last couple years. I've made other cars (http://sliptonic.com/?p=113) but this year I wanted to make as much as possible 'printable'. The trigger was inspired by a medieval crossbow mechanism. I welcome comments on improving it.

http://www.youtube.com/watch?v=8d4W5kBPek8

First a note: My name for this car was 'Bad Idea' and it probably is. It's completely illegal in regular pinewood derbies, possibly dangerous, and will certainly scare every mother in the room. You've been warned. If you decide to build one, please take extra safety precautions like the note mentioned below and maybe make everyone wear safety glasses.

The motor uses the following printed parts:

Tankblock: holds 2 12-gram C02 cartridges at a downward angle. A channel between and below the cartridges lets a stiff wire pass underneath from the front bumper to activate the trigger mechanism.

Trigger block: Holds the parts of the trigger mechanism is alignment.

Pawl: The pawl is pushed toward the back of the car when sitting at the starting gate. It rotates on a common axle with the hammer and keeps the sear from rotating.

Sear: The sear grabs the hammer and releases it when pressure is removed from the pawl.

Hammer: Pulled forward by springs, the hammer does the work of piercing the cylinders. The hammer has a large cylindrical opening to be loaded with weight. A brass rod rests in a groove in the hammer and connects the springs. As the pins pierce the cylinders, the rod pulls out of the groove. This lets the hammer fall back and clear the way for the gas to escape.