This lab required us to make a self-propelling vehicle go 28 feet. The original constraints allowed for no ramps and nothing left behind save a counterweight. The group, however, had an original and creative idea, and the instructor allowed us to leave behind the majority of our mechanism. With a single axle, we had issues making our vehicle run straight every time, but guide walls along the ramp and chains used to make sure the ropes were centered helped solve the issue.
In order to create motion, we wound a string around the center of the axle, which is connected to a chain and counterweight. When released, the counterweight causes the string to unwind. The vehicle drops and continues the motion, kind of like a yo-yo.