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Mouse Trap Cars: Making a Long Lever Arm

January 21, 2012

Longer lever arms will have a greater pulling distance but tend to bend under load and waste energy. Learn how to make a super long lever arm that will not bend.

Part of the gearing system of a mousetrap racers is 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 that is applied to the drive axle. Longer lever arms will have less pulling force than shorter lever arm but longer lever arms will pull more string from the drive axle than a shorter lever arm. Changing and/or attaching a lever arm to the mouse trap is the number one way to control a mousetrap vehicles performance.

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.

Lever Arm Materials

When selecting a lever arm it is important to think about the strength of the material you are selecting. Some materials are not strong enough to be used for long lever arms and will bend under the tension of the mouse trap's spring. If a lever arm bends under the tension of the mouse trap's spring valuable energy will be wasted long before the mousetrap vehicle even starts moving. If you look carefully at the mouse trap of a full wound mousetrap vehicle that has a bending lever arm you will notice that the mouse trap's spring is not fully compressed and therefore is not starting with all it's possible potential energy. In most cases brass tubing and brass brazing rods make excellent lever arms because of their high-strength and light-weight. Wood can also make for a very strong and light-weight lever arm if it is used properly (see making a super long lever arm). Coat hangers make poor lever arms because they tend to bend easily under tension and should be avoided. Some times you may discover after you have assembled your mousetrap vehicle that the lever arm is bending under tension in which case you should try to replace it with a different material.

bonus tip: a weak lever arm will bend under tension causing the mouse trap's spring to not fully compress. A weak lever arm will prevent a mousetrap car from starting with it's maximum possible potential energy.

How to Make a Long Lever Arm

Longer lever arms have a greater pulling distance but also have a tendency to bend and waste starting energy. From looking at the picture above you will see that the long lever arm is bending under the load of the mouse trap's spring and this prevents the mouse trap itself from fully compressing (wasting energy). If you are building a long distance traveler and are using a long lever arm (longer than 18 inches) you will want to make sure that the lever arm is strong enough that it will not bend under tension.

Doc Fizzix's Super-Long Lever Arm Set

The Super-Long Lever Arm Set includes everything you need to make a super strong and light weight lever arm including kevlar string, attachment tubes, and balsa wood rails.

the parts: everything you need to make your own super-long lever arm. Two long strips of balsa, two 6-inch brass tubes.

The two long balsa rails are glued together to form a "T" bar, the "T" bar is attached to the mouse trap's existing lever arm using a brass tube. A string is tied to the other end of the lever arm using a second coupler. The shape of the lever arm gives it incredible strength so that it will not bend even under high loads. This long lever arm kit is designed to build a 36-inch lever arm that can easily be cut to any length in order to fit any size mouse trap project.

step #1: glue the long balsa strip together to for a "T" bar as pictured.

step #2: glue the two brass tubes to the end of the super-long lever arm as pictured. Notice that the brass tube on the end that will be attached to the mouse trap sticks out 2 inches from the end of the super-long lever arm.

step #3: cut the mouse trap's snapper arm as pictures and as out line in the attaching a lever arm article.

step #4: slide the super-long lever arm onto the mouse trap's snapper as pictured.

step #4: how it looks attached to the mousetrap.

step #5: Determine the length of the lever arm that is needed and trim any excess. Attach the lever arm to the mousetrap and position the lever arm over the drive axle. In this picture, the lever arm is too long and will need to be trimmed. Cut the lever with an Xacto knife or a hobby saw.

step #5: The lever should be cut so that it just reaches the drive axle as pictured.

step #6: glue another 3" brass tube to the other end of the lever arm as pictured. Straighten the locking bar that was removed from the mousetrap.

step #7: Place the straighten locking bar into the three inch brass tube as pictured.

step #8: tie your string to the loop of the locking bar at the end of the super-long lever arm.

how it looks: the finished product shows how the super-long lever arm looks when attached to the mouse trap.

how it looks: the finished product shows how the super-long lever arm looks when attached to the mouse trap.

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