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Mouse Trap Cars: Decrease Rotational Inertia

January 19, 2012

Rotational inertia effect both the travel distance and the acceleration of a mousetrap racer. Learn how to decrease rotational inertia and improve performance.

Rotational inertia is the resistance an object has to changes in it's state of rotation. For an object that is not rotating we commonly talk about the object's inertia or it's mass; the more mass an object has the more resistance the object has to any change in it's state of motion. With rotational inertia we still talk about an objects mass but we include the location of it's mass with respect to the point of rotation. The greater the distance between the average mass of a rotating object from it's point of rotation the greater the rotational inertia of the object. The more the rotational inertia an object has the greater the torque that will be required to change the objects state of rotation. A large amount of rotational inertia can have an advantage once an object is rotating because it will be harder to stop that object from rotating. Having a wheel with a lot of rotational inertia can be a big advantage when building a long distance traveling mousetrap powered vehicle; but having a wheel with a large amount of rotational inertia is a disadvantage when building a speed-trap dragster.

Mousetrap Racers for Top-Speed

If you are building a super-fast speed-trap racer then rotational inertial is one of the most important concepts you will need to understand. The secret to winning any top-speed mouse trap contest is to get to the finish line in less time than everyone else; what this really means is that your mousetrap racer will need to have a greater acceleration from the start line than the rest of the competition. Because most top-speed contest are performed over a set distance friction is usually not as big of an issue as it would be in a maximum trave distance contest. In a top-speed contest inertia is the biggest problem a mousetrap racer will have to over come in order to be successful; and for this reason, wheels must be carefully selected that have as little rotational inertia as possible. The first tip to remember is that large diameter wheels (more than 4 inches) will have way to much rotational inertia and must be avoided at all cost; try to limit the size of a drive wheels on a speed-trap racer to no larger than 3 inches (depending on the travel distance). Non-drive wheels should be as small and as light as possible. One more important concept to think about when selecting wheels for your speed-trap racer is traction. Since the drive wheel(s) need to grab the floor in order to accelerate the racer as fast as possible the wheel will need have good traction without slipping in order to rocket the mousetrap racer to the finish. Some times wheel traction can be increased by adding traction treads cut from the center of a rubber balloon (see adding traction treads for more information). Before you build your speed-trap mousetrap racer study the design of a top fuel dragsters; their rear drive wheels are designed for maximum traction and their super small lightweight front wheels have almost no rotational inertia. Try to copy the design of a top fuel dragster when building a super-fast speed-trap racer.

bonus tip: use lightweight drive wheels no larger than 3 inches and small front wheels on any speed trap racer in order to decrease the rotational inertia.

bonus tip: We manufacture a special a special wheel just for mousetrap cars that is made of a light weight urethane foam that will provide incredible traction on any surface but also has very little rotational inertia. These wheels have been used on all our record setting vehicles.

Mousetrap Racers for Distance

How does rotational inertia play into the design of a good distance-traveling mousetrap racer? When building a mousetrap car for distance the goal is to convert as much of the mouse trap's starting energy into linear displace of the vehicle over the greatest distance. The starting potential energy of the mouse trap will be needed to overcome the force of friction acting against the vehicle's travel. A mousetrap car cannot move anywhere until the potential energy of the mouse trap's spring puts energy into the wheels and changes their state of rotation from rest into motion; the more rotational inertia a vehicle has the more energy that will be consumed at the start just to get the vehicle's wheels turning.

bonus tip: although this simple mousetrap car will get the job done, it will never be a record setter.

There have been many mousetrap vehicle builders that have applied the concept of rotational inertial differently over the years. Some have designed and have built mousetrap racer that have a large and heavy wheel called a flywheel (sometimes made from an old record album) that is designed to store the energy from the mouse trap into a rotating disk. The basic concept is that the mouse trap's energy will be converted into the rotational energy of the flywheel and then the flywheel's rotational inertia will keep the vehicle traveling with a huge costing distance. This concept will work but there are some serious flaws in the application of this concept as it relates to mousetrap powered vehicles. In a perfect universe the flywheel concept would always win if we were displacing only the wheel. But we are not in a perfect universe and we not only displacing the wheel; instead, we are displace the whole car including the mouse trap. In our real world environment we have to deal with a small but important concept called friction. A mouse trap vehicle will travel further and be more efficient the slower it moves. In fact, the perfect long distance mousetrap vehicle would use energy at the exact same rate as the energy being lost to friction (this means slow moving). A flywheel will require more torque to get it rotating and this will increases the rate at which energy is consumed; the flywheel mousetrap powered vehicle will now have to depend more on it's coasting ability rather than having a maintained pulling force throughout it's travel. The flywheel set-up will encounter greater forces of friction and will have a harder time going as far as a mor efficient slow moving distance racer. A good distance vehicle will have the smallest possible pulling force, the lowest possible energy output, and the smallest possible force of friction acting against the vehicle. So if you want to build a record setting mousetrap distance racer it is always best to use wheels with the least amount of rotational inertia possible.

The Number of Wheels

Each wheel increases the rotational inertia of the system. In most cases a vehicle with less wheels will have less rotational inertia compared to a similar mousetrap car with more wheels. Depending on the rules for the contest try to design a vehicle that uses three wheels verses the same design that uses four wheels. If you are building a big-wheeled long distance racer try to use only one big wheel on the drive axle instead of two big wheels that would double the rotational inertia of the axle set-up. This same concept can be applied to speed-trap racers, try using only one drive wheel in order to cut down on the rotational inertia.

bonus tip: Each of these vehicles uses only one drive wheel in order to cut down on the rotational inertia of the system.

Ultra Stick Foam Wheels

Doc Fizzix's Ultra Sticky Foam Wheels are made from a lightweight urethane that is specially designed to grab the road and provide maximum traction for maximum acceleration. While the rest of the field is spinning their wheels at the start line, your racer will be flying across the finish in world record time. These wheels are manufactured specifically for Doc Fizzix Products and are the same wheels that were used on Doc Fizzix's world record setting mousetrap racer that traveled 5 meters in under 1.2 seconds. By far, the biggest secret to Doc Fizzix world record setting speed-trap racer was the use of the ultra sticky foam wheels that were essential in keeping his Speed-Trap Racer from spinning out at the start line and providing maximum traction to the road. Ultra sticky foam wheels are 2-inches in diameter and machined to fit a 5/32-inch axle.

bonus tip: CD/DVD spacer can be used to tapper the large hole of a CD/DVD down to a smaller axle size.

DVD Layers, The Winning Secret

DVD layers is one of those top secret tips that will definitely make your mousetrap car a winner. we manufacture a special DVD layer that is half the thickness of a normal CD/DVD and therefore it has only half the rotational inertia of a normal CD/DVD. Because our DVD layers have only half the rotational inertia of a normal CD, any mousetrap vehicle that use DVD layers for wheels instead of the regular CD/DVD will have a greater acceleration and a greater travel distance. By using DVD-layers travel distance will increase from 5-15 meters further for the same vehicle using CD wheels. Our special DVD layer package includes everything you need including wheels, axles, spacers, and thrust washers to insure that your mousetrap car is a winner. There is even a set-up guide filled with secret tips to make your project is a success.

bonus tip: DVD layers are half the thickness of normal CDs/DVDs and have half the rotational inertia. By using our DVD layers, your vehicle will travel 5-15 meters further than with regular CDs/DVDs as wheels.

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.

Decreasing the Rotational Inertia

The best ways to decrease rotational inertia of a wheel is to remove mass. For those who are wiling and for 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 being cut from a CD using a dremal tool.

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

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