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Mouse Trap Cars: Making a Big Wheel

January 20, 2012

Gearing determines how far and how fast a mousetrap racer will travel. Learn how to make a super large drive wheel that will increase the overall pulling distance.

The gearing of a mousetrap car determines the acceleration and the travel distance. Gearing can be adjusted to increase and/or decrease the pulling force, the torque, the travel distance, and the acceleration of a mousetrap powered vehicle. With all mousetrap cars the gearing is controlled by the length of the lever arm and/or the ratio of the drive axle and wheel set-up. Most record-setting long distance mousetrap vehicles are geared so that they have the smallest possible energy consumption rate or power output in order to maximize the pulling distance. Smaller power outputs produce less wasted energy and have greater efficiency. The amount of energy released by using a short lever arm or a long lever arm is the same but the length of the lever arm will determine the rate at which the energy is consumed called power output.

Wheels and Axles

By changing the size of the drive wheel(s) you can speed-up or slow-down your mousetrap vehicle. Larger diameter drive wheels have a greater travel distance per each turn than compared to smaller diameter drive wheels. It would make sense to use a super huge drive wheel so for each turn of this massive wheel the mousetrap vehicle travels a super distance but as the size of the drive wheel increases so does the amount of torque required to start and keep the super huge wheel turning. At some point a drive wheel can be so large that there is not enough force from the mouse trap to start and/or keep the vehicle moving. With speed-trap racers smaller wheels will have a shorter travel distance per turn but will be much easier to accelerate and will require less pulling force to achieve the same acceleration as a larger wheel. Smaller drive wheels should be used on speed-trap racer in order to increase the acceleration and larger drive wheels should be used on long-distance travelers in order to cover more linear distance per rotation.

The diameter of the axle compared to the diameter of the wheel is also very important, the smaller the diameter of the axle in comparison to the diameter of the wheel the more force that will be required to accelerate the vehicle but the greater the distance of travel per rotation. This axle-to-wheel ratio is an important part of a mousetrap vehicle's gearing that needs to be understood in order to build the perfect mousetrap racer. For long distance mousetrap racers a large diameter drive wheel with a small diameter drive axle is desired. For speed trap racers a smaller diameter drive wheel with a larger diameter drive axle is desired.

It is always important to understand the relationships between variables but never over exaggerate any one concept. The ideal size for an ultra big wheel on a long distance mousetrap racer seams to be somewhere between 10 and 15 inches. And the ideal size for a drive wheel on a speed-trap racer seams to be between 2 and 3 inches but no more than 4 inches.

bonus tip: speed-trap racers have a short lever arm, smaller diameter drive wheels, and a larger diameter drive axle in comparison to a long-distance racer.

bonus tip: long-distance racers have a longer lever arm, larger diameter drive wheels, and a smaller diameter drive axle in comparison to a speed-trap racer.

Ultra-Light 11" Big Wheel

If you are building a long-distance mousetrap powered racer then one of the most basic design concepts to understand is gearing. Gearing in the simplest of terms is how far the vehicle will travel from the amount of string that is pulled from the drive axle. With mousetrap cars the gearing can be controlled by changing the length of the lever arm and/or changing the size of the drive axle/drive wheel system. Back by popular demand Doc Fizzix's Big Wheel is a guaranteed record setter that increases the gear ratio and the travel distance. The Ultimate Big Wheel is designed to cover almost 1 meter of linear distance for every wrap of string that is pulled from its drive axle. The Ultra-light Big Wheel is made from a very light weight corrugated plastic that can be cut and trimmed to save even more weight and even more rotational inertia translating into even greater travel distance. This is one of our best selling items, don't miss out.

bonus tip: Stop using record albums, Doc Fizzix makes a special low inertia large drive wheel for use on any long-distance traveling mousetrap racer.

Making Your Own Big Wheel

The are several ways to make your own big wheel including drawing a circle with a protractor on a piece of foam board and then carefully cutting out the circle. You can even use a record album (or any other circle) to trace a wheel onto a piece of foam board and again cut it out carefully..

bonus tip: a large wheel is drawn onto a piece of foam or matt board and then carefully cut.

bonus tip: records should be avoided due to their large mass but they can be used to trace an image onto foam or matt board for cutting.

Truing-up a Wheel

If one is careful enough during the cutting process, you can get a wheel close to round but some times a wheel will have to be trued-up or made round. For those whom are more daring you can set-up a makeshift lathe and cut (or true-up) a large wheel. This can be dangerous so it should not be attempted unless you have experience with this type of set-up. Here is how it works:

step #1: a wheel is first cut from foam or matt board as described above.

step #2: The wheels is secure and tightened to a bolt using two large washers.

step #3: The set-up is placed in a drill and then the drill held steady on a table.

step #4: You will need someone to help you perform this task, have someone hold the drill steady on a table and slowly turn the drill on to a steady, medium, speed.

step #5: Using a utility knife or another carving tool that can be controlled by hand, true the wheel up by holding the knife against the wheel. Do not apply a lot of pressure. It is important to note that the wheel should start fairly round before this method is attempted. It is also important to do all cutting on the side of the wheel that is turning down and not the side going up. Also make sure to stand away from the turning wheel.

Decreasing the Rotational Inertia

The best ways to decrease rotational inertia of a wheel is to remove mass. For those who are wiling and those who are a bit more daring this trick will make sure that your mousetrap vehicle is the best that it can be. Decreasing the rotational inertia of a wheel will give your distance mousetrap racer that extra advantage over the rest of the competition and will make sure your speed-trap racer gets to this finish before the rest of the competition. The best way to decrease the rotational inertia of a wheel is to drill and/or cut mass away to give your wheel spokes. Mass can be removed with a drill and/or dremal tool by drilling and/or cutting on the outer rim of the wheel. Mass removed from the outer rim of the wheel will have a larger decrease in the rotational inertia than mass removed from the inside of a wheel.

bonus tip: mass is cut from a super big wheel using and X-acto knife.

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