Design Rationales

Project Design:

We brainstormed and researched on many designs to find out how to make and adapt a mousetrap car for distance, yet light, stable and secure. We came up with many designs and all sorts of exotic mousetrap car designs, even one with a 1.5L plastic bottle. We usually come up with the criteria first before designing a product like for the bridge-making project. But building a mousetrap car is entirely new to us such that we have to just scribble down designs to seek and come up with these criteria. We finally need our mousetrap car to be stable and, with enough friction between its wheels and the ground and the tension in its leverage to drive it forward to its maximum distance.

After our criteria's were set, each of us came up with our own design. We chose Afiq's design because it meets all the criteria exactly. During the construction of the prototype, we want a light frame. Every gram we can shave off of our car's frame is a little further our mousetrap will be able to drive our car forward. For this we decided to use Lego beams. They are light, size-friendly and very convenient to put our axels in for our wheels, accompanied by duct tape to secure them to the mousetrap.

We then need wheels that have a large circumference. Large wheels have greater rotational inertia than small wheels. In layman's terms, this means that once they start rolling, they're harder to stop rolling. This makes large wheels perfect for distance. They will accelerate less quickly than smaller wheels, but they'll roll much longer and they'll travel a greater distance overall. It is also important to take the weight of the wheels into account — any unnecessary weight will slow our car down or lead to added friction. In addition, wide wheels can even have a small negative effect our car's drag due to air resistance. From this, we used CDs for the rear wheels. They're large, thin, and extremely light. But the CDs cant work alone as there is not enough friction between it and the ground. If the wheels slip against the ground, when the trap is sprung, energy is wasted. The mousetrap works to make the wheels turn, but you are not able to achieve the distance as the mousetrap isn't being driven forward.

We need a friction-inducing material to accompany the rear wheels to reduce their slippage. Thus, we cross sectioned long balloons and attached it the circumference of the wheels. This creates traction and allows more grip.

For the spring, we need extended leverage. Since the hammer of the trap is fairly short, if our car isn't carefully constructed, it can pull on the string too rapidly, causing the wheels to slip and energy to be lost. For a slower, steadier pull, we resource popsicle sticks from the last engineering project (bridge-making) to extend the leverage by the hammer. This gives us a sturdy yet light lever. We made sure our level did not bend at all under the stress of the string as this represents wasted energy.

The way the mousetrap is powered is that the lever transfers its energy to the axel to turn the wheels. But if the end of the string is stuck on to the axel after the lever is fully extended, the string stops the axel from turning, which stops the wheels from turning and cause the car to come to a halt. We came up with a hook string-release mechanism that ensures the string detaches from the axle after the leverage is fully extended. This causes the inertia of the car to continue to drive itself forward even after the source of power is used up.


When testing the car, we found out when the leverage is fully extended, it comes in contact with the front wheel, cause the car to come to a halt.

We then change the size to a smaller rubber front wheels but the leverage still came in contact with the wheels.

With no more smaller rubber wheels, we used small plastic wheels instead. The leverage does not come in contact with the front wheels now and the car can go much further.

After more testing we realise that the front wheels did not have much grip on tarmac and a lot of energy is need to spin the front wheels multiple times. We then decided to reduce the number of front wheels to 1 in the middle. We used a 10-diameter Lego Motorcycle wheel with tyres so that there is grip on the ground. This is also because the bigger the wheel, the tougher it is top stop, as mention above. To make sure that the leverage does not come in contact with the front wheel, we disposed of the right side of the leverage as highlighted in red and left only one side of the arm.

The car is finally able to travel much further than before without interruption.

With the planning of designs and various tests we finally managed to come up with a car that is not only light but also able to reach incredible distances.

Leave a Reply