Lego Racer

Challenge:

With a partner (someone different from the bottle opener and windlass assignments), design a 

vehicle with a single motor, powered by a PicoCricket, that can carry a 1.0 kg weight as fast as 

possible on a 4 meter course. You will use one of the old gray rectangular motors that does not 

have internal gearing, which will force you to experiment with building your own gear trains.

Design Process:

First, we tried placing the motor at the back of the vehicle near the wheels, adding a 40t gear on the back axle and an 8t gear on the front axle. We hoped that minimizing the number of gears while still creating the largest gear ratio possible (8:40 = 1:5) would increase speed and reduce friction. We connected the axles by wrapping a chain around the gears. A picture of our first iteration appears below. 

Our vehicle went pretty fast without the weight, but when we added the 1 kg, it wouldn't move. We learned a few things from our first iteration. First, the belt added a significant amount of friction with little benefit. Second, we didn't actually need to connect the two axles. If we built a gear train from the motor to one of the axles, the motion of the motor-driven wheels would turn the other set of wheels once the vehicle was placed on the ground. Lastly, while we thought we needed to create the largest gear ratio possible in order to maximize speed and win the race, we actually needed to find a gear ratio that was a good compromise between torque and speed. Without sufficient torque, the vehicle would not be able to haul the weight.

                          

After these realizations, we decided to place the motor in the middle of the vehicle and build our gear train from it to one axle. On the other side of the motor, we built a fence to prevent the weight from falling off the car. During the journey to our final iteration, the design of our car did not change much, but the gear ratio did. We tried several different ratios, documenting the speed of each in a chart. A picture of our final gear train (ratio 1:25) appears to the right. 

Above and to the left, you can see the fence for the weight. Pictured to the right is the entire vehicle with the weight and motor attached. 

Here is a video of our lego racer in action. 

Engineering Analysis:

All of the gear ratios we tried (after deciding to make a two-wheel drive vehicle) are shown in the chart above. With the first two ratios, the car would not move.  With the last ratio, the car was very slow. Our final gear ratio was 1:25.  We chose this ratio because it was the fastest of all the ratios we tried.