Lego Racer

Hello those reading my blog! My assignment this week was to make a Lego RACE CAR! My partner, Brooke, and I needed to design a Lego car that could carry a 1kg weight as fast as possible on a 4 meter course. On Friday, our team pitted our car against the cars of everyone else in the class.

We could only use a motor that didn't have internal gearing. This meant that we needed to design a car with gears that went from smaller gears to larger gears to increase the torque of the motor. This
would allow the car to hold more weight. The hardest part of the whole endeavor was getting different gears and Lego pieces to fit in the right way.

The first iteration of the Lego Racer.

When I made the first car, I was mostly seeing how different pieces fit together. I found it difficult to get everything on the car without the wheel hitting the gear and making sure all the gears would work together. However, when I tried to turn on the engine, nothing would move. The is because my gear ratio was too small- 8.33:1. In two places, I had the same gear coupled with one another that did not increase my gear ratio. Those gears were also providing another source of friction, which would make it harder for the car to move. I also noticed that since the car was quite long, it was very unstable. I would need to make it shorter to make it sturdier. So I moved on to a second iteration.

The second time I made the car, I tried to cut down on the number of gears to decrease the amount of friction between the gears. However, only one gear was decreased from the car.  I also added a smaller gear to the motor to increase the gear ratio and create more torque. Since I had one less gear in the middle of the car, the size of the car was decreased to make it sturdier. Though, even with the decrease in length, the car was still very unsturdy. This iteration was an improvement from the first iteration because all of the gears would turn and the car would move when I turned it on. However, when I added the 1kg weight to the car, it did not have enough torque to move the car. So Brooke and I moved on to the third iteration.

For our next iteration, Brooke and I changed the design of the car, instead of modifying it like I did in the second iteration. We made the car smaller in width and shorter in length to increase the sturdiness of the car. We also added bigger wheels to the ones connected to the motor and smaller wheels to the ones in back that just rolled. The bigger wheels in back would increase the speed of the car and the gear ratio. We also thought hard on the gear ratio and ended up with a gear ration of 20.83. We made sure that the gear ratio was increased every time gears were paired together. When we tried this car out we found out that it worked! It made the 4 meter course in the 9 second mark. 

After we finished this racer, Brooke thought we could increase the torque by adding a smaller gear to the motor and increase the gear ratio. However, when we tried this, the racer went slower than the last iteration. (Unfortunately we forgot to get a picture of this.) So we went back to our third iteration and made it our final RACE CAR!!!

We then raced all the cars to see which car would be the fastest.
Time(s)        Gear Reduction
8.42              15:1

8.92              22.5:1
9.63              15.6:1
9.68              20.83:1- Our Car!
10.57            24.7:1
10.93            20.83:1

Our car wasn't the fastest, but it still made a good showing and was very close to the car ahead of ours.

Videos

Engineering Analysis
 The goal of the car was to create the optimal balance between the torque of the motor and the speed of the motor. The graph below shows the torque vs speed curve of a perfect D.C motor. The motor we used started out with maximum speed, but no torque. This would allow the car to move very fast, but doesn't allow it to hold any weight. The more gears added, the more torque the motor will have. If a million gears were added to the motor, the car would be able to hold a large amount of weight, but the car would move extremely slowly. Our goal was to put enough gears on the car to get equal torque and speed for the car. Our final gear ratio was 20.83:1. We had a 24 tooth gear attached to the motor and a 40 tooth gear. That gear was connected by an axle to a 16 tooth gear which was attached to a 40 tooth gear. That gear was connected by an axle to an 8 tooth gear which was connected to a 40 tooth gear. The gear was connected by an axle to the wheels.  (40/24)*(40/16)*(40/8)= 20.83:1 gear ratio.
It is important to note that the gear reduction was calculated without considering the size of the wheels. Since the wheels are connected to the axle and the other gears, they influence the gear ratio of the whole car. A larger wheel would increase the velocity of the car. That is why we changed the wheels attached to the gear in the final iteration of our car.

http://lancet.mit.edu/motors/colorTS1.jpg

Another design element of the car was where the wheels were placed in relation to the rest of the car. Since the axles where made out of plastic, the further the wheels were placed from the car, the more the axle would bend. In order to have the weight of the car not be solely on the axle, the wheels would need to be closer to the car. While we tried to have the wheels as close to the car as possible, the gears would interfere with the wheel. Therefore it was necessary to space the wheels further apart than we would have liked and caused some bending in the axle.

Summary
In the end I learned about gear trains and how they influence the torque of a motor. I also learned that there is a lot to consider when making a product because the size and shape of the materials used need to  influence the design of the product. I found the hardest part of making the Lego racer was making sure the gears fit together correctly. If I could improve on my design, I would try to make the gear reduction closer to 15:1 to increase the speed of the car. I would also experiment with different wheels if possible.