Science Fair Project: Stopping Power

It is science fair time all across America.

Which also means that it’s time for parents all across the country to answer the question, “Dad/Mom, what should I do for my science fair project?”

This is a surprisingly hard question to answer because most of us are not scientists and don’t know anyone who is. Even if we did have a regular cup of coffee with a scientist, his or her work would probably not apply to a seventh grader that gets B’s in math class.

So let me offer a project idea that is fun, safe, has real world applicability and doesn’t cost very much. With a few supplies and a basic 100ft tape measure you can design a project to show the critical safety importance of good tire tread.

As a tire wears the tire’s ability to perform in rain and snow is reduced. With only 2/32″ of remaining tread depth, resistance to hydroplaning in the rain at highway speeds has been significantly reduced, and traction in snow has been virtually eliminated. (Yes, since this is a science fair project we could reduce the fraction to 1/16 of an inch, but tread depth law and practice uses 32nds of inches as the scale of measurement.)

When tires are worn down to 2/32 of an inch you can see the tread wear indicator bars. These bars are federally mandated since 1968 and require all tires for sale in the U.S. to have indicators molded into their tread design which run across the tread pattern from the outside shoulder to inside shoulder. Wear bars are designed to visually connect the elements of the tire’s tread pattern and warn drivers when their tires no longer meet minimum tread depth requirements.

Testing the reduced stopping power of tires with exposed tread bars is an easy, fun and exciting science experiment that can be accomplished in one afternoon with a few days of preparation. You will need a:

Set of tires with exposed wear bars.
100 ft measuring tape.
Car.
Stick of sidewalk chalk.
Licensed driver for the car.
Four 5-gallon buckets of water.

Finding tires with exposed tread bars sounds like the hardest part of the experiment, but is actually pretty easy. You can find a dealer eager to sell you worn out tires for next to nothing by visiting a local Auto Recycling Center (that’s what we are calling ourselves these days, but you can find us all over the country listed as Salvage Yards, Wrecking Yards, Junk Yards or Junkyards). You will also want to make sure that you buy four steel wheels that match your car’s bolt pattern. Most centers have a pile of steel wheels so with a little elbow grease and you’ll find what you need. Get the tires mounted and you are ready to test. You won’t need to get the tires balanced because you will not be driving on them except at the test site. One word of caution. You will be stopping abruptly on these tires to measure stopping power. So the tires need to have 2/32 of tread left on them. Completely bald tires or tires with exposed belts may not hold up during the test and could blow out under heavy stopping loads. Also, dry rotted tires where the rubber breaks up when you pick it with your fingernail will not work.

Arrange with a local church or shopping center to use their parking lot. Churches work particularly well, because on many weekday afternoons the large parking lots are completely empty. Your licensed driver will accelerate the car to 30 mph and then fully depress the brakes when they cross the “stop line”. So mark a stop line on the parking lot with about 100 yards of advance to come up to speed and another 100 yards to decelerate. (It won’t take this far to stop but having some extra room is safe.)

Once you have the course marked pour the water onto the stopping area of the course.

Then conduct five test runs. The licensed driver will hold at 30 mph until arriving at the stop line, then fully depress the brakes until the car comes to a complete stop. Turn the car off. Get out and measure the distance from the stop line to the front tires of the car. Repeat until you have data for five “good tire” stops.

After you have your control data, change all four tires on the car, one at a time. Follow the manufacturer’s guidance in the owners manual for changing a tire. Make sure to engage the parking brake and do not let your foot or any other body part get underneath a car on the jack.

Now conduct five test stops on the worn down tires. You will want to make sure the course is still wet because we are measuring the tires stopping ability in wet conditions when lack of tread causes the greatest performance decrease. Since water can’t be compressed, your worn tires may not have enough tread depth to allow water to escape through the tire’s grooves. If the water can’t escape fast enough, your vehicle’s tires will be forced to hydroplane (float) on top of the water, losing traction.

Upon completing the five stops on worn tires, DO NOT DRIVE HOME ON THESE TIRES. You will need to change back to your good tires for a safe ride home.

This experiment should give solid set of data for analyzing and plotting. The student will need to do additional research on traction and friction along with gaining a basic understanding of water’s incompressibility to write the research paper, but at least they won’t be growing mold in your garage. And maybe if we can get some kids excited about science that has a clear and direct benefit we can make a small movement toward building the next set of American engineers that will build our dream cars of tomorrow.