Machine Design: Dos and Don'ts: Chapter 21
The mouse trap powered car:One day my son came home with a mouse trap. I think he was about 11 years old at that time. The teacher had told the class that they had to design a car that was powered by a mouse trap. There could be no motors or springs or other power-assist mechanisms, otherwise the project would be disqualified.
Each student had to work on his own. The class was not divided into small groups as was the case in some of the previous projects.
This had been an on going project over the years at this high school. The object was simple. The car that went the furthest won first prize and all the bragging rights. In addition the winning car was placed in a show case at the high school, with the winners name, proudly displayed.
My hat off to whoever came up with this brilliant idea. It was a simple idea, yet not as easy as it sounded. Since all the previous winning designs were already on display, the kids had a pretty good idea about what to do. The problem was if the kids followed the existing winning designs, they would end up with the same results.
I followed my son's progress with great interest. He prototyped a few designs but wasn't too happy with the results. He needed some wheels. So I made some light weight wheels using some 1/8 inch plastic. He didn't like these wheels at all.
He finally started using AOL disks. We kept getting these dumb AOL disks in the mail by the hunderd. These were perfectly round, stiff and light weight. Plus they added some color to the project. For the rear wheels he used 2 LPs that he bought at the local thrift store. Finally , the project was done and he was kind-of-glad that this nightmare was over.
Confession time:
I must confess I couldn't stop thinking about this mouse-trap powered car. Designs, calculations, ratios etc., flooded my mind. I wanted to "guide" my kid in the right design direction. I gave him hints and suggestions about using pulleys and how they worked. I think I drove him nuts. It was my wife who finally suggested that I stay out of the kid's project and let him design it on his own. "How else will he learn" she said.
She was right. It took a great amount of self-control on my part to keep my mouth shut from that point on. And I learnt a valuable lesson about raising kids. Let the kids learn from their own mistakes. (Easier said than done...sigh).
The Day of the competition:
On the day of the competition, we all looked at the entries. It was obvious that 90% of the entries were not designed by 11 year old kids. Some frames were made on CNC controlled milling machines. There was no way an 11 year old could have produced these results. The proud fathers had this "look" on their faces, some were even smirking....
and the competition began.....
How to spot a winner...
In a corner, all by himself, stood a skinny little kid, with thick horn rimmed glasses. He was all alone. Neither his mother nor his father had shown up for this competition. He was being raised by a single mother....His mother was probably working that day...
He was cradling his Mouse-trap car in his arms. The whole car was made out of styrofoam, including the wheels. He had carved the whole thing using nothing more than a sharp utility knife. There was something different about this car....
Everybody had mounted the mouse trap on top of their contraptions. This skinny kid had his mouse trap mounted at the bottom of his car. On one side of his car, he had mounted a a small section of a comb. This comb was held to-gether by two clothes pins. The comb acted like a limited slip differential..and released the power in small increments.
When the mouse trap was released, at the starting gate, the sudden power burst made the tires slip on the slick floor. Almost 50% of the power transmitted was wasted in slippage and by now the mouse spring had travelled over half its arc. Most entries had this "tire " slippage problem. In addition the weight of the car played a significant role in distance covered.
The skinny kid bent down, cranked the mouse trap and let his car go. It started to move, real slow. Unlike all the other entries, where the contraptions took off like bats out of hell, this car started out real slow. And then gently it started to gain speed. None of the power was wasted. Every ounce of the spring tension was transmitted to the the wheels in small measured doses. The tires did not slip. No energy was wasted.
The car kept going....it just kept going. There was total silence in the hall. Suddenly every body realized that this baby wasn't stopping anytime soon. The car was pretty ugly, yet there was a certain elegance in its simplicity and design. This kid had used nothing more than a carving knife, some styrofoam and 2 clothes pins...
He had put to shame all the CNC designs and the fancy paint schemes. The kid set a new record for distance travelled by a mouse-trap powered car. And he had designed this all by himself. His design was a radical departure from anything done previously.
Genius is a class unto itself.
