Mousetrap Car
Posted Tue Jul 5, 2005 in
Engineering
One of Young Son’s projects for this summer is the construction of a mousetrap car. Of course, I learned of this just last week, late, and the capability test of the car is Tuesday, today. There is nothing like a little stress to add spice to an engineering problem.
One of my objectives for him this summer is that Young Son learn to be responsible to adults other than his regular-term teachers, his mom and me. There are always trade-offs, and in this case it was learning, late, that he had this project to complete.
I never built a mousetrap car. This is a first for me, too.
Wife did some ‘net research last week. Thousands of sites present mousetrap cars. One site even offers kits for their construction. Of course, that would reduce the value of the learning experience, so I eschewed that possibility.
Saturday Young Son and I went exploring at Lowes for materials. We bought a couple of mousetraps, a section of solid round stock, and some hardware for use in fastening the axles to the body and to the wheels. Young Son wanted to use CD’s for wheels, which is a good idea. We used 1/4 L faucet washers for the inserts in the wheels, but the round stock I purchased on Saturday was too small for the center hole of the faucet washers.
Sunday afternoon I returned to Lowes and purchased a 3/16-inch diameter stick of round stock for axle material. I also purchased another package of screw-eyes for axle bearings because my original purchase was too small.
We began assembly of the first car. This exercise is engineering in its purest form—the empirical. Young Son has no idea how to measure the spring constant nor how to execute a force analysis to determine how the car will react to the torque generated by the spring. So, we forged ahead.
I used a hand drill to make pilot holes in the mousetrap frame for the screw-eyes. Young Son fastened them into the holes. We tried to be careful with the front bearings because the car must run straight. He marked off the axle stock and we cut it using a hacksaw. (That was challenging.) He mounted the faucet washers into the CD’s using some gaffer’s tape to shim the washers for a tight fit. I applied super glue to the joints to ensure they would stay solid. We used flat washers and more 1/4 L faucet washers as stand-offs to keep the wheels from rubbing against the body.
Supper interrupted the engineering project and materials were put away.
It was Monday (4 July 2005) before we were able to return to the project. He cut the fat part from two balloons to serve as the tires for the rear wheels. The CD’s were too slippery for good traction.
A bit of string was used to transfer spring torque to the rear axle. His first attempt was to use a slipknot to fasten the string to the rear axle. Two problems ensued, the first being insufficient friction between the string and the axle. The solution was another bit of gaffer’s tape affixed to the axle. When he triggered the trap, the rear axle turned, but the knot snagged after the string was extended and the rear wheels backlashed against the string. The solution was to remove the string from the rear axle and use friction only to keep the string taut against the rear axle.
Those problems solved, the car was fully assembled and prepared for the first test run. Young son set the trap and wound the string on the rear axle. He placed the car on the floor and triggered the trap. The car rolled about 8 feet, pulling strongly to the left. We set the car on the table and checked the front wheels. Only the left wheel made contact with the table top—he had a tricycle. I suggested he move the left wheel between the bearings to move the contact point more central to the axis of the car. After a couple more runs, he had the car running nearly straight and it traversed the width of our living room, about 15 feet.
We celebrated by eating hamburgers on 4 July 2005, Independence Day. A few images of the event are on my gallery.
I’ve never even heard of a mousetrap car. My dad the CE would have loved it.
— Linkmeister 6 July 2005, 22:43 #Heh heh heh… I’ll bet he would! They are kinda fun to build. A lot can be done with just a mousetrap. It’s a real challenge.
— ruminator 7 July 2005, 17:57 #i am building a mousetrap car in my Physical Science class and i wanted to know how it moves.
— Alicia 12 December 2005, 11:55 #The force of the spring has to be transferred to the driving wheel(s). In our case, we connected a bit of string from the bail to the rear axle and wound it around the rear axle. With a bit of work, we were able to get enough friction to make the string turn the axle.
— ruminator 12 December 2005, 11:59 #