Lego Racer

Compared to the previous assignments, I must admit that this lego racer project is completely unfamiliar to me. Growing up in a community with deeply rooted gender roles, a girl could either play with Barbie dolls or read many books and study very hard. I chose the latter and I never thought that one day I would play with lego, something that only boys are supposed to play with. This experience overall was very empowering and exciting for me, since I absolutely enjoyed being able to control and construct everything with my own hands.

Brainstorm

To be honest, we completely forgot about the basic brainstorming process as we did for previous projects. For previous projects, I had in mind a general shape of the products we were about to make. However, for a lego car, it was first hard to draw sketches by hand, since it has so many layers to it inside and outside of the car. Second, we had so many small pieces in our hands that I definitely felt overwhelmed and did not know where to start. I would love to read others' posts to see how they visually organized their thoughts!

Obviously, we eventually started and finished the project. Now that I think back through the thinking process, here are some main points that we considered before we started building the racer. Please keep in mind that our ideas have changed as we started building and testing the racer.

1. We wanted to power the back wheels of the car as opposed to the front wheels, as many cars are designed this way.
2. We wanted to use as few gears as possible, since more gears = more friction.
3. We somehow thought that we should put the weight closer to the back wheels.
4. We wanted to use bigger wheels in the back, because with the same angular velocity, the bigger wheels will have larger linear velocity.
5. However, we only wanted one wheel in the front to minimize friction and weight.
6. Last but not least, the motor has high speed but very low torque, so the gear reduction must be large (or the gear ratio must be small)** enough to provide enough torque to move the car, but not too high that the car moves too slowly.

** gear reduction = # of teeth of the driven gear : # of teeth of the driving gear
gear ratio = # of teeth of the driving gear : # of teeth of the driven gear

Building the Racer

Our first try was to build a structure with a working gear train. Since we wanted the weight to be as close to the back wheels as possible, the motor was put in the front. We did not worry much about the gear ratio in this iteration, because we wanted to find out the best way to connect the motor to the back wheels. It turned out that an 8-tooth gear and a 36-tooth gear can transfer the movement from above the beam where the motor sits to the beam level where the wheels are secured with an axle.

First iteration

This structure does not have a big gear reduction (36:8*16:36*36:16*40:36=5:1) and has gears that cancel each other out. This certainly creates unnecessary friction. We then decided that we will shorten the gear train. As a trade-off, the motor had to be closer to the back and the weight has to go to the front. Here is our second iteration.

Second iteration

In this iteration, the motor had some pieces at the bottom as support, but the gear box in the back was completely exposed. We put a platform in the from of the car for the weight. We did not want out car to be extremely long, because it seemed awkward and would bend rather easily with weight. As a result, we decided to put the battery on top of our motor, and we used a smaller wheel in the front, since the front wheel only serves as a support in the front to convert sliding friction to rolling friction. The gear ratio of this model is not much different from the first iteration: 16:40*24:40=1:4.17. Testing showed that it was clearly not enough to create a torque large enough to move the racer with an 1kg weight on it. As a result, out next step is to build a gear box with a larger torque.

At this point, what definitely worked was the gears used to transfer the movement from above the beam to the beam level, so we conserved that part in our next iteration. However, we were not very satisfied with the way that the battery was placed, since it was unstable, and the exposed gear box seems to be a waste of length in the car. As a result, we made some adjustments to the racer.

First of all, in terms of structure and layout, we built walls on the beams supporting the gears, so that there would be a platform for the weight. Now that the front part of the racer was empty, we moved the battery to the front, since it was much lighter than the weight and would be more secure staying at a lower position with more pegs from the lego pieces holding it to the car.

Following iterations took on a similar layout with varying gear boxes and dimensions.

As for the gear box, we decided to start with a very high gear reduction. We started with 40:16*40:8*40:8=62.5:1. It turned out to work but moved very slowing, taking 24.84 seconds to finish the 4m course. We then took out one pair of the 40:8 gear, making the new ration 12.5:1. It worked, but took 18.46 seconds, making it still much slower than the reference time (9s) we heard in class. (It's interesting to note that eventually we learned that 15:1 seems to be the best gear reduction, and reduction lower than that seemed not to work. However, with a 12.5:1 reduction, this racer moved, though strenuously.) The third try was 40:16*24:8=7.5:1, which did not work with weight on it either. At this point, we were fairly sure that the right ratio is in the interval between 10 and 20, so we decided to first calculate on paper the possible gear reduction we could get with gears available.

Gear reduction calculation with different gear combinations

With the given ratio calculation, I did a few more test runs, and finally settled down with a 40:16*40:16*40:16=15.63 gear reduction.

Testings and Changes on Structures

As we were trying to figure out the right gear reduction, we also tweaked a few things on the structure to figure out how each element would affect the speed of the racer. We consider that the impact is significant if it changes the time by at least 0.5 second.

Here are our findings:

1. Switching the front wheel from a big wheel to a small wheel does not make a huge difference on speed. However, we do prefer a smaller wheel to shorten the racer.
2. Having the motored wheels being in the front or back does not make a huge difference in our case (difference was less than half a second), but we still chose to motor the back wheels since it does make a small difference.
3. Having the wheels closer to each other makes the car move faster, because a large inter-wheel distance causes bending on the supporting axle, creating more friction. As a result, our racer changed from 7FLU wide to 5FLU wide.
4. We were concerned that the weight may fall off when the car moves, but it turned out (after a few tests) that there is enough static friction between the lego and the weight to keep it still with respect to the racer when the racer accelerates. We then did not put protecting lego pieces around the platform to support the weight, so that we would not need a bigger supporting piece or more pieces as protection and can thus reduce the weight of our racer. (Even though the weight does not have an impact on the speed significant enough in our case, we still preferred a more simplistic design.)
5. Bushings are SUPER important! I have a new understanding of the importance of bushing compared to the windlass assignment, since my ex-partner and I used none on our windlass. The appropriate bushing should use the least amount possible to secure axles in place, so that gears will work as they should when the racer moves, but not so many bushings that they would create unnecessary friction when axles spin.

Final Product and Engineering Analysis

Note that the racers are slightly different in color, but they have the same design and structure.

The fundamental physics of this project is simple. With the same power, the bigger the torque, the slower the speed. Our goal is to have just enough torque to carry the racer and an 1kg weight, so that our speed will be maximized. The greater the gear reduction, the greater the torque. Note that the actual gear reduction should also count the size of the motored wheels, since they work as gears as well. A few factors that will affect the efficiency of the structure includes friction and weight. This means that we want to use as little material as possible, and be mindful of anything that rubs against each other.

Our final racer has a gear reduction of 15.63 (40:16*40:16*40:16). The motor connects to a 16-tooth gear, which directly meshes with a 40-tooth gear. The 40-tooth gear shares the same axle as another 16-tooth gear, which meshes with another 40-tooth gear. The second 40-tooth gear shares the same axle as the third 16-tooth gear, which meshes with the third 40-tooth gear. This final 40-tooth gear shares the same axle with and thus motorizes the two back wheels.

In terms of material, our racer's body has the width of 5FLU and the length of 16FLU. The back, motorized wheels are the largest in size, and the front wheel is smaller and positioned under the battery, to reduce length and weight of the racer (does not change much on friction, since it has about the same width and patter as the bigger wheels). During the test runs before the class race, the racer finishes the 4m course at around 9 seconds on average. In class, it finished in 9.63 seconds. We suspect that a light change in the battery position shifted the center of weight and caused the racer to run side way as opposed to straight, extending the time needed to finish the course. There is also probably some variation with the person calculating the time and the reaction time of the person starting the racer.

While our racer was not the fastest in class, I am certain that it is undoubtedly the smallest and uses the least amount of lego pieces. (Simplicity is something I appreciate! This means it is easy to reconstruct and requires less material.) It finished close to the fastest racers and has the gear reduction close to the optimal reduction ratio. The gear arrangement was simple and straight forward to understand, and the gear box can be easily altered.

However, it was not perfect! I would definitely love to have had more time to figure out the balance of weight to make sure that the racer runs straight. I also noticed that because we wanted to make the racer as narrow as possible, the gears inside the racer's body had to turn against each other and the walls supporting them. This probably created friction and slowed down the racer as well. I should have found a good tradeoff between wheels close together but less friction between gears.

Final Reflection

This was not only a great process for me to learn how to work with lego, but also a great chance to learn about myself, how I think, and how I work. I have noticed that I (and probably many other people) have hands that work slower than my brain. As a result, when I start brainstorming, things spin quickly in my brain and sometimes become jammed together. Alternatively, having a more hardworking hand to write down ideas can be a great way to organize my thoughts, so that I do not miss all the ideas that my brain generate.

I also realized that I should always remember to ask for help. I stayed in the lab for 8 hours straight one day and actually spent most of my time trying to figure out a problem that I could not solve. My motor would not work consistently, causing the gears fail to spin and function. I spent a lot of time testing individual parts of the design to make sure that nothing in the connection was wrong. I also tried designs that worked before. I could not find the problem and eventually decided to email Professor Banzaert. It turned out that it was because our motor did not have enough support, so the inside of the motor did not line up well to function properly. While it was a good practice to examine each part separately and not to give up, I could have asked for help earlier. I spent a lot of time turning the motor on and off and felling after I made sure that all connections worked fine. Next time, after I try everything I can, I will make sure to remember all the resource around me. After all, there is no shame to step on the giant's shoulder!

Last but not least, here is a video of our racer running.