In my engineering class, we were divided into teams and tasked with the project of creating rat trap powered vehicles. We were given a time of approximately two weeks. We then had two competitions between the other teams.
The first was a tractor pull, where the vehicles were hitched together and let go. Whichever one pulled the other won.
The other competition was a drag race, where the vehicles were placed side by side and set free and whichever one crossed the finish line first won.
Our vehicle took undisputed first place in the drag race and tied for third (out of six) in the tractor pull.
The wheels, though functional, were hastily made, due to us running out of time.
This isn't so much a how to as it is a documentation of how it was done.
Step 1: The Circle
The main feature of our vehicle was our circle. We were concerned with keeping the force on the axle constant, as most designs lose force over time. We knew that in order to keep the force constant, the string which was to turn the axle always had to be perpendicular to the arm of the rat trap. Keeping that in mind, we came up with the solution of the circle.
Step 2: The Axles
In our previous project, which was also a vehicle, we had huge problems with axle friction. We decided in this project that we would focus on reducing the friction by as much as possible. The solution was to use a bike axle, since it had a ball bearing system and thus produced low friction. The other wheel just needed to be free spinning, and we used some metal with surprisingly low friction to do the job.