Evaluation/Decision Matrices + Design Rationale(PT 2)



Evaluation Matrix


Decision Matrix


Design Rational


Our car consist of three main components, the wheels, drivetrain and chassis. After much discussion we voted and decided on the following:



Chasis
The chassis is the base of the car giving it the structural strength, and holding the rest of the components of the car together. We chose to use wood as it was light and inflexible, unlike our first choice, plastic cardboard which could be bent easily and lose much of its structural strength.


Lever Arm and String type
The lever arm is responsible for transferring the energy from the spring on the mousetrap to the rear axel so as to propel that car forward. The lever arm is made of satay sticks and our string type used is cotton twine to rotate the rear axel as cotton twine is not elastic, unlike raffia string, so it does not lose much energy stretching. On the rear axel is a hook which is used to hold on to the twine as we " wind up" car, this also doubles as a release mechanism of when the lever hits its limit so that the string does not coil back the other way and jerk the car backwards.


Wheels
The wheels transfer the energy to the ground using friction to push the car forward. The bigger the wheel, the greater the circumference and thus the car will be able to accelerate further, however, there is less torque when the wheel is bigger, thus we decided to use CDs as they ensured that the force generated by the spring could overcome the static friction of the car.


Axles
The axles used are satay sticks. As satay sticks are cheap but still strong enough for it not to break, we used it for our axles. Furthermore, the wheels fit the stick for it not to move freely, so when the car moves forward, the axle moves along with the wheel.


Positioning of Mousetrap
We positioned the mousetrap at the back of the car. This is because the mousetrap is heavier than most of the other components, so when it is at the back, the wheels would be pushed more towards the ground, and therefore when it spins it would not lose grip against the ground. If we put the mousetrap at the front, more force would be applied on the front wheels, but that is not where the propelled axle is, so the propelled axle would definitely lose grip and not move as far as it would be when positioned at the back.

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