Mousetrap Car: how to pull weight

Mousetrap Car Report

Background and Research
James and I wanted to build the pull car, as we both had a basic concept of what it should be like. We knew that it needed to be heavy, with a large normal force, in order to pull the maximum weight without peeling out. It also needed to have a small gear ratio so the torque would be used up extremely quickly, making the car able to pull large amounts of weight.
Before we started work on the project, we looked online to find designs for other mousetrap cars and visualized real-life cars and trucks. While we were looking at multiple pictures of mousetrap cars, we found that a large portion of them used flywheels for greater tension as well as direction for the string. We would obviously not use the same design for our car and especially would not use CDs for wheels because they hardly have any friction or mass and would not sustain heavier masses. We did, however, partly derive our extended arm idea from this design.
We wanted to balance the pull of the car evenly and to create friction on all four wheels. We considered the fact that most towing trucks have a four-wheel drive train and used several other real-life examples in our planning. The larger wheels also produce greater torque because they become, in effect, the lever-arm for the axle.
(Torque = F*D*Sin(Theta).
We also wanted to prevent any vertical lift in the front of the car due to the acceleration of the rear tires so we would add weight to the front end of the car.
We extended the two bars on the mousetraps to vertical lines for greater torque (Torque = Force*distance*Sin(Theta). I decided to experiment to determine whether or not our hypothesis was correct. I duct-taped two mouse traps to the floor, one with the extended arm and the other with the standard, rectangular bar. I tied weights to both on a sled and compared the pull difference between the two traps. I found that the standard design performed more work than did the extended arm. In theory, the extended arm would produce more torque, but the arm was not strong enough to hold steady, so it bent and put more stress on the arm instead of the coil, which generates the power of the mousetrap.
We also knew that we had to build a heavy car for the pull competition to create more normal force to reduce the factor of wheel slippage. We did experiments on interactive physics and found that heavier wheels under a lighter body were more efficient than lighter wheels under a heavier body. When the added mass is placed on the wheels, the angular momentum is greater than if the mass were placed on the body because the force of the wheel’s torque is increased. When mass is placed on the body of the car, more pressure is put on the axle whereas the mass of the wheels is applied directly to the ground.
Before we built the actual car we made a model frame of the car out of Popsicle sticks, on which we experimented with the gear mechanism and overall layout of the design. We used the model for a period of time until we could decide on the materials that we would use to build the car itself.

Building Process
We chose thin Lego wheels for their traction, diameter, and ground clearance, a Lego 19cm x 6cm platform for a light but strong and large base (to which we could easily attach our gear mechanism). We also used Lego parts for the axles, flywheels and attachments, with a gear mechanism to transfer the torque of the mousetrap to the wheels. We used Legos to build the mechanism and car, primarily for convenience, because they fit together easily and made room for adjustment. The outline of the car was 25cm long x 12 cm wide.
When we designed our gear mechanism, we knew that we wanted the force of the mousetraps to be almost instantaneously turned into rotational motion in the wheels. We experimented with the gear ratios in several tests and found that the ratio 5:3:5:1 (from the wheel axis to the flywheel (near the center of the car)) performed the most work. While we were experimenting with the gear ratio we moved the mechanism from the top of the car to the bottom for more ground clearance. The car would then pull the sled slightly upward to compensate for friction and gravity simultaneously. For simplicity we moved the traps parallel to each other and glued two small Popsicle sticks to the tops of the bars for a synchronized release. We were not able to engineer a release device for the traps at opposite corners so the adjustment was even more necessary.
At this point we abandoned the four-wheel drive design and were forced to choose between a front wheel or rear wheel drive train. When we made this decision I thought back to an example of my dad’s cars. One day when his van got stuck, he explained that when his rear wheel van pulled a trailer full of wood much more easily than his front wheel van because the weight of the trailer compressed the rear tires for better traction. The front wheel van would peel out because the front was raised due to the increase of mass in the rear so the tires would lose traction on the ground. I suggested applying the same principal to our mousetrap car and the design proved to be successful. We then glued four 100g weights to the two back wheels and three 90g weights to the two front wheels.
We put one 1.5cm diameter flywheel at the very back of the platform and a second 3cm flywheel underneath the car, attached to the gear mechanism. The function of the first flywheel was simply to guide the string to the second wheel, where the string was wound and the energy sent to the gear mechanism. We were confident that we could design a car that would pull the maximum weight so we only experimented with pulling a 1000g weight. The surface we used was an old concrete floor with a higher coefficient of friction than the linoleum floor at the school.

Testing
With our car ready to go, we decided to go for the maximum weight of 100 grams. Our car easily pulled it the 2 centimeters, as we had built it to do just that. After that, for our second run, we added 500 grams. Again, the car was successful. Finally, we attempted 2000 grams and just barely got to the 2 cm mark. We did not need to do any tweaking on the car, as it didn’t break and was able to do all three tests in rapid succession.

Conclusions
Overall, we were extremely pleased with how our car turned out. We pulled twice the weight necessary, and I believe that if we had been able to construct a practical and working four-wheel drive mechanism, we could have done much more than that. If I were to change any aspect of the car it would be to add more wheels to the axels in order to provide better traction and friction. Also, I would have selected new mousetraps as the one we used had been tested quite often, and surely had lost some of their power. The new traps combined with the all wheel drive capability would have made this car a formidable pulling force. I felt that this was a very good experience, as I learned a lot about gear ratios, torque, and the best way to pull a lot of weight. It was certainly an enjoyable lab.

Photos
1). The car. (N.B. weights were removed from wheels)

2). The gear mechanism.