My partner, Katrina, and I started by creating random gear train ratios to get an idea of what would be appropriate for our racer. Here are some examples of gear trains that we built and tested.
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| 1:25 24:40 , 8:40 , 40:24 , 8:40 |
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| 1-9 8:24 , 8:40 , 40:24 |
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| 1:125 8:40 , 8:40 , 8:40 |
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| 1-15 8:40 , 8:40 , 40:24 |
Once we made a few options, we went ahead to build the rest of the car, so we could begin testing out the ratios. Katrina focused on figuring out where to put the wheels and where to put the motor and PicoCricket. We originally felt that we would need a way to keep the weight from rolling off and given a weight that would sit flat, we decided to build a sort of container to put it in, so that it would be able to attach to the rest of the cart.
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| Katrina is troubleshooting the motor |
Regarding the wheels, we decided to use three wheels instead of four thinking that less wheels would mean less friction with the ground, especially since the motor was only driving the front wheels. For the front wheels, we decided to use the biggest wheels we had available because that meant that for one rotation, the wheel with the larger circumference would travel a farther distance. The back wheel, whose only real purpose is for support, we decided to used the smallest wheels available to use because a smaller wheel yields less friction.
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| Our first iteration: The Tank (1:125) |
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| Here's the structure that I worked on |
We did a lot of testing and recorded times for the gear trains we built earlier and soon came to a conclusion that a 1:15 gear ratio would work to best. While the other gear ratios had higher torque, they didn't have enough speed. We needed to find the ratio that would allow the lego racer to have enough torque to move across the carpeted race cart, while maximizing speed and the 1:15 gear train do just that. It was also during these trials that we realized the less gears we used the better because each gear added, added on friction to the system.
We also realized that it would be in our best interests to make the cart thinner and move the wheels closer together. The further apart the wheels were from one another and the slower the cart would be. What we decided to do, in order to make the lego racer thinner, was to make parts of the gear train vertical, therefore preserving space. This would also allow us to drive both of the front wheels on one axil instead of two, which definitely helped to reduce gear friction.
Originally, we were hesitant to make the cart to tall because we were worried it would tip over more easily. However, we remember that since we are going along a straight course, height won't be a huge factor, now if this lego racer was taking laps in a circle, this might be something we want to consider.
We also decided to do away with the container we encased the weight in. Since we had two bigger wheels in the front and one small one in the back, by placing the weight in the front, gravity would keep the weight propped up. Therefore, we don't need the container that added a lot of weight to our cart (although it did look pretty cool...).
We struggled to to create a 1:15 gear ratio with just four gears because of spacing issues, so we ended up having to use six gears instead. This definitely added a bit more gear friction and slowed us down during the race.
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| Underside |
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| Front view |
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| Our final iteration |
Come race day our lego racer definitely wasn't the fastest clocking in around 12 seconds, but that was still a pretty big improvement from our first iteration. Given more time, we would definitely try to find a way to recreate the 1:15 gear ratio with only 4 gears. That is likely the leading cause of why we took so much longer than other teams. From there, we would then also consider ways to minimize the weight on the cart, but the gear train would definitely be the priority.
The tank is an interesting design though :-)
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