mousetrap car design help

Wheels and axles - the gearing of your racer

By Doc Fizzix // Published Feb 02, 2005

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)

The larger the diameter of the driven gear or pulley, the greater the mechanical advantage or torque. By increasing or decreasing the wheel-to-axle ratio, you will change the mechanical advantage of your mouse-trap car. Keep in mind, changing the mechanical advantage does not increase the work you get from your mouse trap; it only changes the size of the force and the distance the force is applied. With distance vehicles, you want a small force over a long distance; therefore, use a large wheel with a small axle (i.e., a large wheel-to-axle ratio). A large wheel with a small axle will cover more distance per each turn of the axle when compared to a smaller wheel with the same axle. There is a trade-off for having a large wheel-to-axle ratio. The trade-off is that it takes more force to accelerate the vehicle to the same speed in the same time as a vehicle that has a small wheel-to-axle ratio, but this is okay because distance cars should not be fast in order to cut down on air resistance. It is essential that a vehicle with a large wheel-to-axle ratio has a small rotational inertia wheel, since low rotational inertia wheels will be much easier to rotate than large rotational inertia wheels.

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