Lego Racer

This week, our challenge was to build a Lego Car that can carry a 1.0kg weight across a distance of 4.0m. The supplies were a gray Lego motor without internal gearing, and many, many Legos. 

We knew that the motor has high speed and low torque. As a result, we attempted to use gear reduction to reduce the speed of the motor so that the output of the motor would provide the wheels of the Lego Car enough torque to move. 

The above graph shows that speed and torque are inversely related. We wanted to reduce the gears enough so that the output gear reached the midpoint of the graph -- where the speed and torque are both maximized. 

We used two Lego beams as the basic structure of the car. The motor was attached to one end of the beams, and the gear train and main wheels in the middle section of the beams. The last gear of the gear train was put on the same axle as the wheels. A platform was built behind the gears to hold the 1.0kg mass and the Picocricket.

At the back of the platform, we added a small wheel to support the car. We chose to use only one back wheel, a small one, because it was not attached to any gears and we wanted to minimize the mass of the entire car. 

In our first iteration of the car, we attached a 16 tooth gear to the motor, which was meshed with a 40 tooth gear. An 8 tooth gear was put on the same axle as the 40 tooth gear, and was meshed with a 24 tooth gear. Then another 8 tooth gear was put on the same axle as the 24, and meshed with another 40 tooth gear. This last 40 tooth gear was attached to the wheels of car, therefore acting as the output of the system.

The overall gear ratio for this system was 37.5:1, meaning that for every 37.5 rotations that the gear attached to the motor (16T) makes, the output gear (40T) makes one rotation.

The gear ratio is calculated by dividing the number of teeth of the output gear by the number of teeth of the input gear. 

For example, if a 16T gear is the the input gear, and a 40T is the output gear, then the gear ratio is 

40/16 => 2.5 => 2.5:1. 

The gear ratio of each pair of gears is then multiplied together.

So for our gear train, 

(40/16)x(24/8)x(40/8) 

= 2.5x3x5 

= 37.5 

=> 37.5:1

This system of gears was built into the structure of the lego car, as depicted.

The motor is the input and the output gear is attached to two large wheels. We chose to use the wheels with a larger diameter so that with every rotation of the output gear, the car car travels a longer distance.

Many bushings were used to secure the gears and wheels on the axles. We found that at first the car did not go in a straight path. We realized that the two large wheels were different distances from the center of the car, and therefore the car curved to one side. The problem was fixed and the car went straight this time.

The car was fully functional and took 13.6 seconds to travel the 4.0m. We decided to try different gear ratios to try to increase the speed. We tried the following three other gear combinations:

Because we wanted the car to go faster, we did not reduce the gears as much as we originally did. Our three different gear ratios were 15:1, 22.5:1, and 25:1. We tested each of the gear reduction systems. The times taken to travel the 4.0m for each version were 10.8s, 9.03s, and 10.2s, respectively. 

The system of gears that gave a gear ratio of 22.5:1 was chosen in the end, as it gave the car the highest speed. We used one 16T gear, one 40T, two 8T and two 24T gears. Therefore,

(40/16)x(24/8)x(24/8) 

= 22.5 

=> 22.5 : 1

In class, our car took 8.92 seconds to complete the race!

My partner and I were very pleased with our results. (I originally was working with Sophia, but then partnered with Sarah when Sophia and Sarah's original partner left the class.) Working with the Legos was very fun. However, with the small pieces of Lego, like the bushings, attaching and taking apart pieces of the racer would sometimes be difficult. We had to be very careful about firmly attaching the pieces together so that the car would not break. Also, we had to be sure that the pieces were properly aligned and fitted, as any poor fittings could lead to the racer to breaking or to traveling in a curved path or to not going at all. The gears were especially tricky to align, as the space between the two bars that provide the car's structure is very small. 

Given more time, I would test more different gear ratios to maximize the speed and torque of the output gear. The difficult part with the Lego gears is that some gears, like the 16T, don't mesh with other gears when the axle is inserted into the Lego beam, as the Lego beam has evenly spaced holes for the axles. In order to use those gears, the axle would need to be inserted in between two of the holes. Also, we tried to minimize the mass of the car by removing unnecessary pieces. However, with more time, it may have been possible to reduce the mass even more.