LEGO RACER
The goal of this assignment was to design a vehicle with a single motor, powered by a Pico Cricket, that could carry 1 kg of weight as fast as possible on a 4 meter course. The motors we used had no internal gearing, so they had very high spin and nearly no torque, which means that we needed to increase the gear ratio and sacrifice some speed for torque to move the mass.
Rinako and I began by making a box that would hold the mass, and then mounting a gear train underneath it. We drove the largest wheels and placed medium wheels in front. Originally we were going to drive medium wheels to help increase torque, but because of the size and placement of our gear train, we needed to raise our car so the gears wouldn’t drag on the racetrack.
A huge obstacle for us besides generating enough torque was the strength and security of our car; the cricket and motor repeatedly fell off because they were mounted too precariously on the back of the car. Nevertheless, we succeeded in creating a moving car after three iterations that completed the racetrack in 29 seconds.
In theory, our next step should have been to keep the same overall design and gradually step down the gear ratio with each successive iteration, to find the peak power and maximize speed. However, since our car’s structure was so problematic, we instead focused on that and changed several aspects of it, including mounting the cricket on the hinged roof. Out of all the components, I believe linking the motor gave us the most trouble, and we eventually mounted it upside-down. We also changed the gear ratio again, but the car never ran successfully. Since we had changed so many variables at once, I cannot conclude which was the cause of the car’s stubbornness.
We continued changing the gear ratio, thinking that now that there were more Lego pieces on the body, the mass was greater, so a greater torque was required. We also noticed that the wheels were buckling in towards the cart (instead of parallel to each other and perpendicular to the ground), which restricted their range of motion. Unfortunately, we couldn’t get it consistently rolling by the race.
With the additional time between the race and the blog post, we again iterated the car, starting completely from scratch and focusing on a successful gear ratio and effective mount for the motor and cricket, before building a place for the mass. Finally, we made a car with a gear ratio of 27:1 that took 22.59 seconds to go from start to finish. If I had more time, I would have liked to continue stepping down the gear ratio to see how much more speed we could get from our car. After seeing the other groups’ cars, I would also like to find a more compact design.
| ITERATION | GEAR RATIO | SPEED (s) | NOTES |
| 1 | 8/40 * 24/8 * 24/8 = 1.8:1 | 0 | 4 large tires The car was really sad… The torque needs to increase |
| 2 | 8/40 * 24/8 * 24/8 * 24/8 = 4.8:1 | 0 | 4 large tires |
| 3 | 40/8 * 24/8 * 24/8 = 45:1 |
29 |
4 large tires, then tried again with driving 4 large tires and 2 medium tires to redistribute weight. YAY! Success! The car is falling apart though, so we should modify the structure |
| 4 | 40/24 * 40/8 * 40/8 = 41.7:1 |
0 |
Drove 2 large tires, 2 small tires Changed the structure of the car… and now it doesn’t work |
| 5 | 24/24 * 24/8 * 24/8 * 40/8 * 24/40 = 27:1 |
22.59 |
Used 2 large tires Changed the structure of the car again so it’s cleaner. |
INDESTRUCTIBLE BOX
For the box, we knew the key to success was using vertical braces as written in the Building Strong LEGO Structures. Initially, we built a box with braces fitted around the outside, but when it was dropped once, the braces immediately snapped off (and dramatically exploded everywhere). For our next iterations, we put the braces on the inside of the box, so that the outside of the box was a uniform surface, and the braces were on the inside. Now, the only pieces that flew off on contact was the flat plates at either end that kept the weight bricks inside. We knew that those pieces needed to be enclosed within the vertical braces and have something over them for support, so instead of a completely closed plate, we used beams across to hold the bricks in place, so the vertical braces could still run along the inside. This required us to lock the bricks inside, which is less than ideal, but the box was completely unchanged after being dropped twice from a height of two meters.
MOTION MODULE
We built a Reacher that translates the rotational motion of the motor to rectilinear motion along the axis of the Reacher. The primary mechanism here is linkages. An 8-tooth gear connected to the motor drives a 40-tooth gear that is connected to the first linkage. Linkages are useful because they can be stored in a compact way, and then expanded to any height within the mechanism’s range of motion. This fine control is great for something like a lift in construction sites or theaters that needs to be elevated to any height.
I can’t figure out how to insert the movie? I tried to do it just like I did the other media, but it displays as a link instead of something plays in the browser.
I like the way in which your lego race car incorporated the weight into its design. I thought that it helped make sure that the weight didn’t fall of the car, which is a problem my group faced on our first attempt.
I am really interested in your indestructible box and how you made it. I found it really difficult to make one and could never get mine to work