mousetrap car design basics

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Mousetrap Cars: How to Build a Fast Racer

January 24, 2012

Learn all the secrets to building your own record setting mousetrap powered dragster. You cannot build a winning mousetrap racer until you know the basics.

While there are no perfect design or plan for the ultimate speed-trap racer there are some common design elements that most record-setting speed-trap racers will all have in common. This article outlines many of those common design elements found on most record-setting mousetrap racers. Keep in mind that as you build your first mousetrap dragster you will want to find a harmonious balance between as many of these design elements as possible without over exaggerating any one concept. You will need to do a lot of testing and adjusting in order to perfect your vehicle. Even when two people build the same mousetrap racer following the exact same steps there are no guarantees that the two mousetrap racers will travel at the same speed. This is why testing is so important, you will not know what the problems are with your design until you experiment with your vehicle and see how it performs. Making small changes and adjustments to your racer in order to see how the changes will effect your vehicle's performance. The only way to truly understand how to build a winning mousetrap car is to ask yourself the question "what if I change this?" and then make the change in order to see what happens. A good engineer knows 99 ways something will not work and one way that it will work great, find that one way!

Common Design Elements of a Speed-Trap Racer

  • high torque gearing
  • shorter length lever arm
  • smaller diameter drive wheels
  • wheels with good traction
  • reduced friction bearings
  • light weight components

Speed-trap Racer: Most record-setting mousetrap racers will have small drive wheels compared to distance racers, larger drive axles, and shorter lever arms.

Speed-Trap Racers and Friction

With speed-trap racers friction is not as big of an issue as it is with long-distance traveling mousetrap cars; but never the less, every effort should be made to reduce and eliminate as much friction as possible. Friction is the force that acts against the motion of all moving objects. We cannot eliminate friction totally but we can try to reduce it as much as possible. As a general rule of thumb, the more moving parts a device has the greater the force of friction acting against the system. The first step in deceasing friction happens in the planning of your mousetrap vehicle. As you plan your project think about all the points of friction that your design will have and then look for ways to decrease the points of friction. Because the number of moving components increases the friction point look for ways to simplify the design as much as possible. Using a lot of gears and pulleys may be a great way to change the pulling the torque but this will also lead to more points of friction that will have to be address, look for simplicity.

Axle Points

The number one point of friction on any mousetrap car is where the axle system comes in contact with the frame of the vehicle. In most cases an axle will be in direct contact with the frame and there is a lot that can be done to decrease the friction at this contact point. Test spin your mousetrap racers wheels in order to see how friction-free they spin. If the wheels stop spinning fairly quickly then you will need to find ways to reduce the axle friction.

Axle friction depends on:

  • The diameter of the axle
  • The pressure on the axle by the bearing
  • The types of materials used for the axle and bearing

bearings: The number one point of friction on any mousetrap car is always at the bearings. Find ways to reduces the friction at the bearing points.

Air Resistance

Imagine you are in a pool of water, you will find that it is easy to walk around in the water but once you try moving fast or even running in the water you immediately feel the resistance of the water pushing against your motion; this is the same experience a mousetrap vehicle will have as it tries to push it's way through the air. The faster a mousetrap racer moves the more air resistance will be pushing back against the vehicle's motion. Design your mousetrap racer so that it can slice through the air with the least amount of resistance; design your vehicle to be aerodynamic. Try sanding and painting wood frames in order to cut down on the air resistance.

Factors that effect the air resistance:

  • the speed of the vehicle
  • the shape of the vehicle

air resistance: design your mousetrap car to slice through the air and to be aerodynamic in order to decrease the force of friction.

Gearing

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. A good speed-trap racer will have a very high rate of energy consumption and will be geared to use it's stored potential energy before it crosses the finish line.

Gearing is controlled by the following:

  • the length of the lever arm
  • the diameter of drive wheel and/or drive axle set-up

bonus tip: speed-trap racers will use short lever arms and be geared to use all their energy before they cross the finish line.

The Lever Arm

Changing the length of a mouse trap's snapper (or lever arm) is how you control a mousetrap vehicle's acceleration and/or travel distance. Different lengths of lever arms can be used to increase and/or decrease the pulling force and change the amount of string that can be pulled off the drive axle. Changing the length of the lever arm does not change the total energy and/or the torque produced by the mouse trap but it does change the pulling force applied to the drive axle. Longer lever arms will have less pulling force than shorter lever arm but longer lever arm will pull more string from the drive axle than shorter lever arm. Changing and/or attaching a lever arm to the mousetrap is the number one way to control a mousetrap vehicle's performance.

Shorter lever arms have more pulling force and produce greater acceleration. But, if the lever arm is to short the pulling force will be so larger that the drive wheels will spin out at the start. The goal with any speed trap racer is to use as short a lever arm as possible or just before the point where the drive wheels spin out.

bonus tip: the length of the lever arm determines the pulling force and the length of string that can be wrapped around the drive axle. Short lever arms use less string but produce more force.

Wheels and Axles

By changing the size of the drive wheel(s) you can speed-up or slow-down your mousetrap vehicle. Larger drive wheels have a greater travel distance per each turn compared to smaller drive wheel. As the size of the drive wheel increase the torque required to start a wheel turning also increase. At some point a drive wheel can be so large that there is not enough force from the mouse trap to keep the wheel turning. 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 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 part of a mousetrap vehicle's gearing that needs to be understood in order to build the perfect racer for the contest at hand. For long distance 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 a drive wheel on a speed-trap racer is between 2 and 3 inches but no more than 4 inches.

bonus tip: speed-trap racers should have a smaller drive wheel and larger drive axles. This this picture the drive axle is built up with tape to increase the torque.

Power Output

Power output is the rate at which energy is being used. Heat and sound are forms of friction that will eventually absorbs all the energy from your mousetrap racer and cause it to come to rest. Higher rates of energy output produce more heat and sound compared to smaller energy outputs. With all speed-trap racers the objective is to get to this finish line as-fast-as possible and this means higher rates of energy output. The perfect speed-trap racer will be geared to use all of its starting potential energy as fast as possible and long before the finish line. Speed-trap racers will not be as efficient and the forces of friction will be much higher when compared to a long-distance mousetrap car. Design your speed-trap racer to have a high energy output. Design your mousetrap racer to get to it's top speed as soon as possible.

bonus tip: design your speed-trap racer to use all it's energy before the finish line.

Inertia

Inertia is the resistance that an object has to a change in its state of motion, the more inertia an object has the more force that will be required to change is state of motion off the start line. A heavy mousetrap vehicle will required more pulling force than a lighter mousetrap vehicle in order to achieve an equal acceleration. Lighter mousetrap vehicles will require less energy at the start and can use longer lever arms for more pulling distance. Always use lightweight components when building a mousetrap vehicle.

bonus tip: lightweight mousetrap vehicles require less pulling force off the start.

Rotational Inertia

Rotational inertia is the resistance an object has to changing its state of rotation. For an object that is not rotating we commonly talk about its inertia or its mass; the more mass the object has, the more resistance the object will have 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 and its point of rotation means the greater the rotational inertia of the object (large wheels have more rotational inertia). The more the rotational inertia an object has the more 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 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 you 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 not as big of an issue as it would be in a maximum travel-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 to 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) will need to accelerate the racer as fast as possible the wheel will need to be able to grab the road surface 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.

Traction

The maximum acceleration of a mousetrap vehicle will depend on the amount of grip or traction the racer's wheels have on the floor. Traction is a form of friction that allows object to move. In order to walk forward your foot needs to be able to push against the floor and the floor has to push back. It is the friction or traction between the bottom of your feet and the floor that provides the grip you need to move. It is also the friction that exists between the road and a wheel that keeps the wheel from spinning out thereby moving the car along the road. Traction is a major issue when it comes to building a super fast speed-trap racer. If there is not enough traction between the drive wheels and the floor then your mousetrap racer will not be able to accelerate as designed. Use drive wheels that provide a lot of traction on the floor.

bonus tip: use drive wheels on speed-trap racers that have a lot of traction.

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.

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