mousetrap cars and vehicles

Changing the Pulling Force

By Doc Fizzix // Published Feb 02, 2005

By far the biggest and easiest adjustment that can be made to affect the performance of a mousetrap powered vehicle 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.

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

Changing the diameter of either the wheel(s) or the axle controls the mechanical advantage of a wheel-axle system. When the ratios of the length of string used per turn divided by the distance traveled is less than one, the mechanical advantage is small and the car travels slow and far. When the ratios of the length of string used per turn divided by the distance traveled is greater than one, the car accelerates very quickly and uses only a small amount of string. The diameters of your drive wheel and drive axle represent your gearing or transmission.

A transmission is any device that transmits mechanical energy from one place to another. With a mouse-trap car, power is transferred to the wheels via a string wrapped around a drive axle. The ratio of the drive axle to the drive wheel represents the gearing of the vehicles or the transmission.

If you place your mousetrap vehicle on the ground and the vehicle does not start to move or it starts to move but then stops part of the way through its motion, the vehicle does not have enough pulling torque and/or the vehicles axle-to-wheel ratio is too large. If you do not have enough torque you should increase the mechanical advantage of the system by doing one of the following:

• shorten the length of the lever arm
• using a smaller diameter drive wheels, or
• make the drive axle thicker by wrapping tape around the axle

(The tips from above can also be used to increase the speed of a racer)

See the following related articles to get started:

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