For this project, Hanna and I created a lego racer that would be able to travel a 4 meter course while carrying a 1.0kg weight. We were given a single motor which was powered by a PicoCricket. This motor had no internal gears, so we needed to use a gear train in order to convert the power produced by the motor into something that could move the weight without stalling.
Design Process:
Initially we struggled with how to attached the gear train to the wheels, motor, motor board, and Pico cricket, without having anything interfering with the gears’ ability to rotate. For our first iteration that contained everything necessary for the racer to potentially move, we made a gear train with lots of gears, not thinking about the gear ratio. We ended up with a ratio of 1/45. When trying to attach the motor to the gear train, we realized that an axle was blocking the connection on the bottom, so rather than trying to change the location of the axle or the motor, we flipped the gear train over and attached the motor upside down, leaving the connection free. We also ended up attaching the motor board directly to the motor to make it more compact. Finally we attached the upper platform of our racer, which held the Pico cricket and 1 kg weight. This first iteration travelled the 4 meter course in 30 seconds. Many of our gears cancelled each other out which caused unnecessary friction (and a little bit of extra weight).
Iteration 1
For our second iteration, we kept our basic design, but switched a few gears to create a gear ratio of 1:15. We still had lots of unnecessary gears and because our new gear ratio decreased the torque, the motor stalled due to the extra friction between all the gears we used. This iteration did not move.
For our third iteration, we wanted to try the same gear ratio with fewer gears. We had a difficult time finding a way to create a shorter gear train without the motor getting in the way, so we ended up adding an extra 24-tooth gear as a spacer between another 24-tooth gear and the 8-tooth gear attached directly to the motor. Although this caused extra friction, our racer travelled the course in 9 seconds.
Iteration 3
Our fourth iteration was the same as our third, but we switched the 8-tooth gear attached directly to the motor with a 24-tooth gear (and removed the 24-tooth gear that we were using as a spacer) to see if we could reduce the gear ratio to 1:5 without reducing the torque too much causing the motor to stall. This configuration of gears did not fit together as nicely as iteration 3 and the gear ratio did not allow it to move.
Iteration 4
For our fifth iteration we took our third iteration and switched the 8-tooth gear attached directly to the motor with a 16-tooth gear. In order for the 16-tooth gear to line up we had to add a ⅓ FLU plate, but because the difference between the radius of the 8-tooth gear and 16-tooth gear is only ¼ of a FLU, the spacing did not align as nicely as before and the racer did not move. Multiplying the gear ratio by two was also likely not enough torque.
Iteration 5
At this point we went back to the gear ration in our third iteration and tried using the smaller tires. We thought that having harder tires would help our lego racer move more quickly on the carpet, but this iteration took about 2 seconds longer than the medium sized tires because the smaller radius meant that for each rotation of the last axle, the racer would move a shorter distance.
Iteration 6
After the sixth iteration, we decided we were happy with our lego racer. While switching our gear ratios, we also switched to shorter axles and decided to use all black and grey legos in order to match the colors of the motor. We also changed our upper base to use as few lego pieces as we could while allowing the Pico cricket and weight to be held securely in place while the racer was moving. We also decided to have the motor turn the back wheels and have the weight closer to the back of the racer.
Final Lego Racer in Action:
Engineering Analysis and Documentation of Iterations:
Much of the process we used to chose our gear ratio was trial and error. We knew that the motor given to us had a very high power. Power is the product of the torque and the angular velocity and the motor given to us had a very high angular velocity and low torque. In order to prevent the motor from stalling under the force of the 1kg weight, we had to trade angular velocity for torque. The tires of our lego racer transformed the angular velocity from the motor and gear train into linear velocity in order to move the weight forward, so speed is directly proportional to the angular velocity. To maximize speed (angular velocity), we had to increase the torque as little as possible while allowing the lego racer to move without stalling the motor. Therefore, we wanted our gear ratio to be as close to 1 as possible to have the fastest speed. We also had to keep in mind that the more gears that we used, the more frictional force there would be between them and the more legos we used, the more weight the racer would have to move.
| Iteration Number | Gear Ratio | Speed (sec) | Notes |
| 1 | 1:27 | 30 seconds | Just put a lot of gears together without thinking about the ratio |
| 2 | 1:15 | stalled | Lots of unnecessary gear that cancelled each other out |
| 3 (& 7) | 1:15 | 8-9 seconds (varied each time we did it) | The the fewest number of gears to get the same ratio we had previously |
| 4 | 1:5 | stalled | Replaced the 8 attached to the motor with a 24 |
| 5 | 2:15 | stalled | Replaced the 8 attached to the motor with a 16 |
| 6 | 1:15 | 10 seconds | Same gear train as in iteration 3, but with the smaller tires (less surface area on ground) |
Overall I am happy with the lego racer that we made. The simplistic design made it fairly easy to make changes and added very little extra weight. Although it would occasionally lose the connection between the motor and the motorboard, it worked fairly consistently and was able to quickly move along the 4 meter course. Looking back on this project there are a few other iterations I would like to have tried, like using the wheels with the largest radius in order to increase the distance the racer would travel for each rotation. I would also like to see if we could find a way to make everything fit together using only 4 gears to create the same gear ratio (8:24 and 8:40) in order to minimize friction.
