PT2.2 - 3 designs and decision making matrix


The ideas

Idea 1: Direct String Drive


Idea 2: Speed Gear Drive (3.125:1)


Idea 3: Speed Gear Drive (2:1)


We have decided on Idea 2, Speed Gear Drive (3.125:1).
Design Rationales
Wheels
By using CDs as wheels, the larger circumference of the discs compared to regular wheels will allow for a larger distance covered with just one turn of the axle. As discs are usually smooth, there'll be too little friction for the reaction force on the car to be enough to push it forward.
Chassis
By using the mousetrap itself as the main frame, there will be less unnecessary weight from additional materials used, therefore allowing the momentum to carry the car further
Lever arm & string type
A longer lever arm equates to a larger turning force due to the Principle of Moment, thus resulting in more turns of the axle, and longer distance covered. We decided to use duct tape as the string so that it is tangle-free and has more friction on the axle and will not slip when being released which would cause a loss of energy which means less distance.
Axels
We bought a 1m 2mm axle to use with the CD wheels. An axel with a smaller diameter would turn the CD wheels with lesser force compared to an axel with a larger diameter.
Positioning of mousetrap
The mousetrap is the main chassis of the car, so it is placed in the center, with it's wheels on the sides of it, and the lever above it. This ensures even weight distribution and that the car does not move sideways, the wheels would be as close to the mouse trap as possible to ensure that the connections between the wheels and mousetrap are as short as possible so that they do not bend or break easily which makes the car rigid
Gears
By gearing the car we can increase the speed of the car about thrice fold as we are using 3.125 to 1 ratio which would mean that with every rotation of the axle when it is spun by the mousetrap pulling the string would result in the wheels turning 3.125 times instead of one, but the ratio of the gears cannot be too great as it would result in a huge loss of energy due to friction between the gears.

There are two important factors: Speed and Distance. We could implement measures in our Mousetrap car in both aspects so that we could get as close as possible to the maximum marks.

Speed
Speed Gears are useful in increasing the speed of the wheels.
longer pulling arm is useful in more efficiently transferring the force of the mousetrap due to the Principle of Moment. Thus it results in more force acting on the gears and the wheels.

Distance
Lighten the car by using lighter materials.
Keeping the car mostly flat reduces air resistance, thus resulting in the car covering more distance.

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