Lego Car Version 1

Task: Build a Lego car that can carry 1kg for 3m as fast as possible, using one motor.

Cabrina and I started by making some simple gear trains, not connected to a car. We experimented with larger and smaller gears, and tried connecting the motor. We had a lot of trouble properly aligning the motor to any gears, because we were using an 8-toothed gear on the axle of the motor. After Amy told us about the belt mechanism, it became much easier to position the motor and have it be able to turn our gears. From here, we made our first car, which was extremely large and slow. It was able to carry the weight easily, but was very slow.

The gear ratio was 75:1

We then decided to decrease the gear ratio, to increase the speed, and keep decreasing it until the car would no longer run carrying 1kg. Our next iteration had a 45:1 ratio:

45:1 car

This was slightly faster, but took 26 seconds to cover 3 meters. We noticed a lot of bowing in the axles; moving the wheels closer to the chassis resulted in a 2-second improvement in the car's time (from 26 seconds to 24). 

We decreased the ratio to 27:1 and tested again. This time, the motor could not carry the weight and the gears slipped instead of turning, resulting in an awful din and no movement. We decided to move back to the 45:1 ratio. 

This is a very misleading picture. The front and back trains were not actually connected at the same time; this picture was taken in the process of switching back to the 45:1 gear train after the 27:1 failed. The front-axle train in the picture is what the car was actually running on when we tested it (i.e. the gears were actually touching, unlike in this picture).

Back on our 45:1 gear ratio, we significantly reinforced the car's body, which had been bowing in the middle under the weight. We added crossbeams across the bottom and reinforced the entire center seam of the top platform with another plate at right angles to the first two. After these reinforcements, the time again improved by 2 seconds (down to 22 seconds to run the course).

But this car couldn't be made any faster without radically changing everything about it. The size of the wheels and body, and the friction of having three gear changes, prevented us from having anything under a 45:1 gear ratio. To get a time under 20 seconds, we needed to start anew, which is what we did.