MouseTrap Car

So you want to build a mousetrap powered vehicle. It's easy if you know how.

While you probably will not have the same eclectic collection of parts that we had to choose from, you will have to deal with the same fundamental laws of physics that we had to deal with.

The car we ended up with was the third in a series of attempts to improve the “mileage” we obtained with the earlier models.

My son's physics instructor passed out mousetraps and the following guidelines. It turned into a father-son construction process. We both learned a lot.

The Guidelines

  1. The spring from the mousetrap is the only power source that can be used to propel the racer.
  2. Rubber bands, catapults, or any other device may not be used to start the racer.
  3. You may not push the racer to get it started.
  4. You may disassemble the mouse trap and use the parts in any form that meet the above criteria.
  5. There is no size requirement nor minimum number of wheels.
  6. The race must maintain contact with the floor.

Grading

  1. Construction of a racer that meets the criteria above: 50 points
  2. Construction of a racer that meets the criteria above and travels 1 meter: 70 points
  3. You can earn 5 points for each meter over the minimum that your racer travels. The distance will be broken into 2 decimeter increments. Example: A racer that travels 4.3 dm will score 87 points.

Our final vehicle...

Mouse Trap Car

The body is 50 by 2 centimeters.

The mouse trap bar was cut near the far side of the spring and bent straight. It was then inserted into the antenna. The string attached to the antenna tip winds around the drive wheel. As the spring raises the antenna, the string spins the drive wheel and the car goes forward.
Trap detail
Drive wheel detail

We...

  1. Used mechanical advantage with the middle tire on the drive axle. We tried just wrapping the string around the axle, and the car would barely move. Adding a larger drive hub improved our pulling power.
  2. Reduced friction by minimizing the surface area of the support that contacts the drive axle. We originally used a Lego ™ strut to support the car on the rear axle. It was about a centimeter wide on each strut, so this large contact area slowed the vehicle tremendously. We switched to Erector Set™ parts which were drilled out to accommodate the axle. This reduced the contact area thereby reducing friction.
  3. Increased friction by using a rubber middle tire for winding the string. We tried just a plastic wheel and a plastic wheel with a rubber tire on it. Although the plastic wheel was slightly larger in diameter than the rubber tire, the slippage of the string due to the lower friction coefficient of the plastic outweighed that gain.
  4. Increased friction by waxing the string with candle wax. By waxing it, the string had better pull on the rubber tire.
  5. Reduced mass by using a simple light stick for the body section. Our first attempts, though they looked like fine furniture, were simply too heavy. Reducing the mass also reduced the friction at the axle supports.
  6. Used the longest lever available to extend the mouse trap arm as far as we could. The tip traversed a greater distance allowing more wheel wraps of string to play out. We cut an antenna off a broken boom-box and used that for our lever.
  7. Reduced friction by applying Molykote® a molybdenum disulfide based powdered lubricant to the axles, wheels and mouse trap spring.
  8. Reduced shock by using a bit of sponge as simulated cheese. This reduced the car hopping as the lever arm slapped home.
  9. Learned that alignment of the axles and mounts was critical to reduced friction and increased performance. Our first attempts included hand drilled holes for the axle mounts. Try as we might, they were not square. We resorted to the Erector Set™ brackets as a better solution.

Results...

The car travels 6.5 meters. That is five and a half meters above the 1 meter minimum and should fetch 97 points. Although your materials will be different, you will have do deal with the limited power of the spring and overcome friction, exploit mechanical advantage, and minimize mass to have a successful mousetrap race car.

Created on ... August 22, 2004
Mr. Phillip Sand Hansel II & Zack