I’m Ken Kuwako, Science Trainer. Every day is an experiment.

Science Experiments with Wild Mini 4WD Cars

Have you ever heard of Tamiya’s “Wild Mini 4WD”? The standard Mini 4WD is a sleek machine built for speed, but the Wild version is a rugged powerhouse. It has tires two to three times larger than the standard model, a sturdy body, and it effortlessly climbs over obstacles. It truly looks like the “King of Off-Road!”If you race them outdoors, the difference between the Wild version and the standard Mini 4WD becomes obvious. Looking at the Wild Mini 4WD, I noticed something interesting. Its large, rubberized wheels are built with a hollow interior, allowing them to deform and “grip” the ground to maximize friction. In short, they have incredibly strong traction.I immediately thought: “I bet this could be used for a great friction experiment!”I bought two of them and started tinkering. What I discovered was a fascinating experiment that makes the laws of physics visible right before your eyes. Today, I’m sharing this “Science Recipe” using the Wild Mini 4WD.

The Science Recipe

Objective:

We will have two Wild Mini 4WD cars compete in a “tug-of-war” while facing back-to-back. We will then investigate which Mini 4WD car wins by changing the two key factors of friction: the coefficient of friction (how rough the surface is) and the normal force (how heavy the car is).

Materials Needed:

Wild Mini 4WD cars (2 units), string (kite string, etc.), weights (mandarins or small objects work well), cleaning rag, sandpaper, plastic sheet (or clear file).

Method:

① Assemble the two Wild Mini 4WD cars. The Crucial Point: You need to modify the battery box slightly by running a wire so that the car can run on just one battery (1.5V) instead of the standard two (3V). If you use two batteries, the motor’s torque (rotational force) is too strong, and the car will wheelie and flip over the moment the tug-of-war starts, making the experiment unsuccessful.② Tie the two Wild Mini 4WDs together, tail-to-tail, with the string.③ Turn on the switches of both cars simultaneously to start the tug-of-war. If the conditions are identical, the wheels will spin in place and a balance (equilibrium) will be maintained. (Even this alone usually makes students gasp in excitement!)④ With the cars pulling, place a rag, sandpaper, or a plastic sheet under one of the machines to change the surface condition. Observe which way the cars move.⑤ Place a weight (like a mandarin orange or small block) on top of one Mini 4WD to change its mass. Observe how this affects the outcome of the tug-of-war.

For example, what happens when you put a mandarin on top?

Results:

The experiment yields results that are intuitively fascinating. The car on the smooth plastic sheet will slip and lose, but the car on the rag or sandpaper will gain traction and win. Furthermore, the car that has the added weight (like the mandarin) will clearly be stronger in the tug-of-war.

The Science Behind the Experiment: Why Does the Heavier Car Win?

In high school physics problems, you often see the condition “neglect friction” (assume friction is zero). Because of this, many people might imagine friction as the “villain” that hinders movement. However, for a car or a Mini 4WD, friction is the essential force (driving force) that allows it to move forward. Just as you slip when trying to run on ice, the tire must grip the ground (have friction) for its reaction force to propel the car forward. If you run the Wild Mini 4WD on your hand, you’ll feel the tires strongly “kick” your skin—that force is the true nature of friction.

The relationship between action and reaction forces on the wheel.

To win this tug-of-war, a greater amount of friction is required. The magnitude of the maximum static friction force can be expressed by the following simple formula:

$F = \mu N$

(Maximum Friction Force = Coefficient of Static Friction $\times$ Normal Force)

Let’s apply this formula to our experiment:

• $\mu$ (mu): Coefficient of Static Friction This represents how slippery the surface is. Placing a rag or sandpaper increases this value ($\mu$), thus boosting the friction force, and the car wins.
• $N$ (N): Normal Force This is the force the ground exerts to support the object. On a flat surface, it is proportional to the object’s weight. Adding a weight or mandarin presses the tires harder against the ground, increasing the Normal Force ($N$), which boosts the friction force, and the car wins.

Heavy case vs. light case.

When asked to predict the outcome, some students often guess, “The heavier one will lose because it’s harder to move!” However, just like in Sumo wrestling, in a pushing (or pulling) match, the heavier body is at an advantage because it can grip the ground more strongly. This experiment is highly recommended because seeing the Mini 4WDs compete right in front of them allows students to instantly grasp the concept behind the friction formula—much better than just studying the theory!

Contact and Requests

Bring the wonders and fun of science closer to home! I have compiled easy-to-understand explanations and tips for fun science experiments you can do at home. Feel free to search around for more!・The content of my science notebook has been published as a book. Find out more here・About the operator, Ken Kuwako, click here・For various requests (writing, lectures, science classes, TV supervision/appearances, etc.), click here・Article updates are posted on X!

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