how to make a mousetrap car

The mouse trap's snapper arm and torque

By Doc Fizzix // Published Feb 02, 2005

By far the biggest and easiest adjustment that can be made to affect the performance of a mouse trap powered racer is to adjusting the length of the "snapper arm" or as we call the "lever arm". When you turn a water faucet, open a door, or tighten a nut with a wrench, you exert a turning force called a torque. Torque is the rotational counterpart of force. You apply a force to make an object move or accelerate in a particular direction. You apply a torque to make an object turn or rotate.

A torque is produced when a force is exerted with leverage. For example, a doorknob is on the opposite side of the door from the hinges in order to increase the leverage. By placing the doorknob far from the turning axis of the hinges, less force is required when you push or pull on the doorknob. Less effort is required to use a long wrench than a short wrench to loosen a nut because you have more leverage with a long handle. The distance from the turning axis to the point of contact is called the lever arm when the force is applied perpendicularly. Therefore, torque is the product of the lever arm and the force that tends to produce rotation.

With mousetrap vehicles, the most common method of propulsion is to attach a string to the mouse trap's lever and then wrap the string around a drive axle. The lever arm applies a force to the drive axle through the pulling string that in turn causes a torque to be produced about the drive axle. By changing the length of the mouse trap's lever arm you will change the pulling force and the torque turning the drive axle. If you look closely at a mousetrap you will see that the mouse trap's spring push on the lever arm very close to the pivot point of the snapper arm causing a torque about any lever arm. When you increase the length of the snapper arm, the pulling string is move further and further away from the base of the snapper arm and it gets harder and harder for the mouse trap's spring to push on the lever arm (assuming the string is wound around the drive axle). This is the same principle as trying to open a door close to the hinge. The longer the lever arm extension that is used, the less force that is transferred through the string to the drive axle and the less torque that is produced. Shorter lever arms produce more torque.

The key to a quick starting racer is the proper relationship between the tension on the string and the tire grip on the floor. If the pulling force of the string is too large, the tires will spin-out at the start wasting the mouse trap's stored energy and thus decreasing performance. The torque acting on the drive axle should be at a maximum for all speed-trap vehicles but not large enough to cause the wheels to spin off the start line. By adjusting the length of the mouse trap's lever arm, you can vary the force that is applied to the wheels of your car. Long lever arms decrease the pulling force while short arms increase the pulling force.

The same concept of torque applies to the ratio of the drive axle to drive wheel relationship. Adjustments can be made to the size of the drive axle to change the torque, see the article on axles and wheels - the gearing

See the following related articles to get started:

• basic mousetrap vehicle propulsions (how to make it move)
• how to make a good distance traveler
• how to make a good speed-trap racer