1. DESIGN CONSIDERATIONS

 

Chassis

The chassis of the mousetrap car is what holds it all together. All the components of the mousetrap car are fixed onto the chassis, which means that this is the backbone of the entire car. This means that it should be structurally rigid to support the components. However, the tradeoff for greater rigidity is weight. A very thick and solid chassis will be very rigid but will be heavy and slow the car down. On the other end of the spectrum, a chassis made from thin wooden tubes would be very light but could warp under acceleration.

We considered two possible designs for the chassis – a solid piece and a tubular chassis. The tubular chassis was rejected because it lacked rigidity. While its light weight would have provided a very small acceleration advantage, it had a high chance of warping and breaking under acceleration at the start gate. Thus, a solid piece 6mm thick is used.

 

Suspension

The suspension system is designed to iron out bumps and is a powerful component when it comes to the dynamics of the car. Under acceleration, the car tends to squat rearwards due to weight transfer. A good suspension system will maximize the grip in this scenario, ensuring the least amount of driving force (from the mousetraps) is lost.

We have decided against using the suspension system. This is because given the small scale of the car and the incredible force provided by the mousetraps, the system would be far too fragile and delicate. The first iteration used a beam axle suspension system, where the axle pivots about its midpoint and the wheels are attached to either end. However, under testing the mousetraps broke the suspension system. As a result, to maintain reliability, the simpler option of not using a suspension system was considered.

 

Wheels

The sizing of the wheels is an important factor to determining the outcome of a mousetrap car race. Smaller wheels have less inertia and will accelerate faster but will limit top speed. Larger wheels do the opposite – slower acceleration for higher top speed. In a way, this is like tuning the final drive in an automobile’s differential.

We chose to design the car with smaller wheels since acceleration is more important in a short course. The car will not have enough room to hit a high enough speed to compensate for lacking acceleration out of the gates. Smaller wheels make the car more agile from a standstill.

Holes were cut into the wheels to reduce weight. Too many holes would result in poor structural stiffness and too little will have unnecessary unsprung mass. We have designed the wheels to have four holes which is a good midpoint between stiffness and weight.

Depending on the surface on which the car is racing on, the wheels may or may not need rubber bands. Too little friction will cause the wheels to spin but too much grip will not make the car move at all. Thus, the surface is to be studied before the amount of grip on the wheels is to be determined.

 

Powertrain

The powertrain of the mousetrap car consists of the mousetrap, rubber bands and the axles. We chose thin steel axles and string. Thin axles have less mass (thereby less inertia) compared to thicker ones and hence take less force to be spun. String stretches much less compared to rubber bands and will reduce energy loss due to elasticity. This combination results in a drivetrain that minimizes overall energy losses.