Designing a Mousetrap Powered Car for Distance Competition.

If you are trying to build a long distance traveling vehicle you will need to think carefully about the following concepts:

  1. Energy

  2. Power output

  3. Inertia

  4. Rotational inertia

  5. Friction

  6. Torque

  7. Force

1  Energy is what will move your mousetrap powered car. Energy is stored as as potential energy when the trap is set, and as the spring releases unwinds the potential energy changes to kinetic energy (energy of motion.) About thirty three percent of the kinetic energy is lost to in the form of heat and vibration. The idea is to reduce the amount of energy that is lost over a long distance and that will extend the travel distance.

2  Power output is the speed at which the energy stored in the mousetrap is released. There are really only two approaches to consider when building your vehicle for distance:

2.1  Build a fast moving car the releases its energy quickly and then coasts as far as possible.

2.2  Build a car that releases its energy slowly over the entire pulling distance, this type of car moves slowly and will not coast very far.

3  Inertia is the resistance that an object has to a change in its state of motion p=(mv)-1. The more inertia an object has the more mass it has. More force will be required to change a greater mass' state of motion. A heavy car will require more pulling force than a lighter car to achieve equal acceleration. Lighter cars will be easier to accelerate but will also have less coasting distance than a heavier car at the same speed.

4  Rotational inertia is the resistance that at round object, in this case a wheel, has to changing its state of motion (rotation.) Rotational inertia is equal to:
p=(m· 2πR
time
 )-1


The less rotational inertia that an object (wheel) has the less the torque that will be needed to change its state of rotation, so it will easier to accelerate.

5  There are three types of friction, static friction and sliding friction, and rolling friction. Static friction is caused by the rubbing of two surfaces in contact with each another. Where your axle connects to the frame of your vehicle is one place that you will find rolling friction on your car. By reducing the rolling friction with graphite powder or ball bearings you will see marked increased performance with your vehicle. Traction is sliding friction between your mousetrap powered cars wheels and the floor. Traction is beneficial, but in my experiance (since 1967) not an important factor in this type of car design. Increasing your car traction will allow for greater accelerations, but it will take more torque to make the wheels spin in the first place. See section four (4). Air resistance is not a factor unless you anticipate going over 1 m/s.

6  Torque is produced as a function of the length of the mouse trap cars lever arm (bail) and the strength of the mousetrap spring. I have been testing mousetrap springs for a few years, and cannot find any one brand that is more powerful than another. This is not to say that there is not a difference, just that you do not necessarily get what you pay for! A fifty cent mousetrap might well be just a powerful as a sixty cent mousetrap. A long lever arm has the same total force as a shorter lever arm. The difference between a long lever arm and a short lever arm is that you get more pulling force in a shorter period of time with a short arm than a long arm. But only for the first half of the springs unwinding! To maximize acceleration you might want a shorter lever arm. Keep in mind that if your arm is too short the force will be large enough to cause the wheels to spin-out and waste energy. Slow, distance-cars have long lever arms.

A Slow Moving versus a Fast Moving Vehicle

Here are some thoughts on building a long distance vehicle.
Try to design a distance car to travel extremely slowly. One of my cars that travels 50 meters may take over 5 minutes to travel that distance. The idea is to reduce the power output to a minimum and only supplying enough energy to the vehicle to overcome friction. By traveling very slowly air resistance is kept to a minimum. Although air resistance is very slight, a fast traveling car will have more air friction acting against it. A quick accelerating car will also create more heat energy in its bearings due to friction during acceleration than a slow moving vehicle. It is debateable whether ball bearings help that much. The expense is very high for new bearings A pair of junk roller blade skates can yield up to sixteen 8 mm bearings. These bearings will reduce the energy needed to travel a great distance. Building a mousetrap car for distance means minimizing energy waste and converting as much potential energy as possible into the displacement of the vehicle.
Try to build cars that have very low frictional forces acting against them and move slowly. Find a balance between the movement of the mousetrap powered car and the length of the lever arm. My cars tend to have long lever arms and large wheels. If the lever arm is too long the car will not travel the full distance. It must have enough torque to keep the mousetrap powered car going as the torque changes with spring angle.

File translated from TEX by TTH, version 3.70.
On 22 Jan 2006, 00:21.