Where does the process begin?

Steel coils, weighing two metric tons each, are trucked to Bridgestone Metalpha's U.S. headquarters in Clarksville, Tennessee.

The Metalpha plant produces 140 metric tons of steel cord a day. To put that into perspective, a truck tire typically uses 4.4 kilograms of steel cord - just over nine pounds - in its belts and body ply.

It takes five days to draw the coils from 5.5 millimeters thick - that's roughly the diameter of a pencil - to approximately 0.225 millimeters - about the size of an ordinary sewing needle.

The first step is welding the ends of these giant coils into one continuous steel rod. It's an art to make the perfect weld, so the ends don't break apart in the process. Then the steel coils are scaled.

Is that similar to scaling a fish?

Yes, in that you're taking off the outer skin, which is a black carbon-like material. Interestingly, the workers keep the area clean by "sweeping" the floor with large magnets.

Once the outer skin is removed, the coils begin the drawing process.

A number of evenly spaced dies draw the wire smaller and smaller through friction. Dies are changed every four days and the employees check the measurement of each die twice during each shift.

Do the dies pull the wire down to the finished size?

Not quite. It takes two more trips through the drawing process before the steel coil is the right size.

What else is going on?

In between drawing processes, the steel coil is washed, heated and cooled.  The heating process takes the wire through furnaces with temperatures up to 1,000-degrees Celsius.

Why so hot?

The incredible heat helps change the inner structure of the steel so it can be drawn to its final diameter and desired tensile strength. Without this process, the wire could develop "stress cracks," causing it to break.

After heat-treating, the wire is cooled, washed several times and then bathed in a special chemical solution. 

What does the chemical bath do?

It helps prepare the steel's outer skin to better adhere to the rubber - which, of course, won't happen until the steel filaments are delivered to the truck tire plant. Bridgestone Metalpha engineers have formulated a secret combination of chemicals that coat the steel, giving the rubber something to "stick to."

Does it look like the steel that's used in the belts?

It's bright and shiny, but it must complete a third drawing process - which is a proprietary procedure. Then the spools of steel filament - which measure about 0.225 millimeters thick - are moved to the cabling machines.

Is cabling similar to twisting?

Yes, the process is done by what looks like large mechanical spinning machines that twist up to seven strands of filament to form the steel coil. How tight, how loose and how many strands of steel used depends on the final application.

Why would it be cabled differently?

When the filaments are twisted, they need to have just the right balance of elasticity and strength depending on how the tire is used.

For example, the steel cords on long-distance highway tires are wound more tightly for durability and to help retain a stiff shape.

On the other side of the coin, steel cords on an on/off-highway tire are looser to "give" when encountering road obstacles, like uneven, rocky surfaces.

It's like choosing the right kind of rope at your hardware store. It has to be thick enough and strong enough to do the job intended, or the rope could break.

So the steel cord could break down if it's used for the wrong job?

That's why steel cord is engineered for each application.

Truck tire cord has a higher tensile strength so it can more easily withstand repeated bending and flexing, without breaking down the rubber around it.  Ordinary steel cord could fail, breaking down both the rubber and steel.

Take a look at the job of a long-distance steer tire. As the tire makes contact with the pavement, the belts are squashing into the footprint over and over again, with each rotation of the tire.

The tire may be filled to 110 psi - which is a lot of pressure pushing against the steel belts. As the tire flexes, heat is generated.

Repeat for 500 miles. Then duplicate day in and day out for the next 100,000 miles.

Is the steel cord ready once it's cabled?

Not quite. Next the steel cord is re-wound onto the customers' spools. The Metalpha plant manufacturers the steel cord for seven Bridgestone and Firestone plants, and each customer has its own color-coded spools. The spools are trucked back and forth and get re-used.  While the steel cord is re-wound, it undergoes a final inspection.

What are they looking for?

Loose wires. And the steel cord is tested for tensile strength to ensure it's the proper amount for that particular customer's application. Then the spools are packed and shipped to the tire plants - where the single coil is married to the rubber.