Warren County, Tennessee is an unlikely spot for one of the largest truck tire plants in the world. Located about 1-½ hours southeast of Nashville, Bridgestone/Firestone’s Warren plant sits on 900 acres of wide-open farmland, nestled in the shadow of the Cumberland Mountains in southern Tennessee. Its neighbors consist of farms, garden nurseries and a few trucking depots scattered along State Route 55.

It’s appropriate, though, that several trucking firms have located near Bridgestone’s plant, because that’s where several thousand truck and bus tires are made every day. At nearly a mile in length and a football field in width, the $600-million plant required 12,000 tons of steel and 100,000 yards of concrete in its construction. A nine-story warehouse attached to the plant holds thousands of tires sorted by type by specially designed robots. Roughly 950 employees work 12-½ hour shifts, keeping the plant humming 24 hours a day.

The Warren plant is the most modern facility in the Bridgestone/Firestone network of 14 plants. A variety of robot-controlled systems minimize human contact with the tries as they are constructed – which is critical to maximizing the integrity of the tire, as the oils found on human hands can damage the rubber of tires as they are made. That’s one of the little known facts involved in tire making, as a tour given to Fleet Owner by Bridgestone/Firestone revealed.

Making tire magic

Truck tires are born in three places at once. The rubber for the entire tire is made in one location and the steel found in the tire’s sidewalls come together in two other spots.

For the tire’s casing and belts, steel cord is spun out onto a giant loom-like device called a creel – a device big enough to fill a truck trailer. The cord is threaded by hand from the creel into a rolling strip where it’s coated with rubber. Each creel contains a different thickness of steel cord, depending on what is required for a particular type of tire. The wire is then passed through a machine – sort of like a giant, old fashioned washing machine wringer – that squeezes the wire and rubber together, to coat and flatten it into long strips.

For the toe of the tire’s sidewall, steel cord is bundled into what’s called a ‘bead,’ which is also coated in rubber. Finished beads look like perfect black circles, waiting patiently for the heart of the tire to arrive.

Another part of the plant mixes the rubber for the tire components – rubber that comes from trees (natural rubber) or other factories (synthetic rubber). Both synthetic and natural rubber, carbon black, sulfur, and a precise mix of additives are dumped into a Banbury mixer, which creates thick, black ooze. This ooze is then squeezed between steel rollers to continue the mixing process and get rid of any air trapped within the rubber – a process that sounds like hundreds of firecrackers going off every few seconds. The rubber is then folded back and forth onto pallets by what’s called a ‘wig wag’ machine.

What is curing?

Another little known fact – some 14 to 16 kinds of rubber go into making a tire, to give it different capabilities, from traction to heat resistance. However, all of those different types of rubber ‘cure’ over different lengths of time. ‘Curing’ occurs near the end of the tire construction process, when the tire is ‘cooked’ at high temperatures in giant steel molds. That’s why substances called accelerators and decelerators are added to the mix so all of the types of rubber within a tire will cure at the same time.

‘Uncured’ rubber isn’t very useful for tires. It tends to get soft and sticky in hot weather, then stiff and brittle in the cold. That’s because the molecules in uncured rubber are long, disconnected strands – rather like a plate of spaghetti. Like spaghetti, the strands are not very strong, easy to pull apart, and slip past each other when warm, while clumping together when cold.

What ‘curing’ does is join the individual strands to each other, with little side links much like rungs on a ladder. This tight network of molecules formed by curing – tight like a fishing net – gives the rubber tremendous strength and resilience over a wide range of temperatures.

Construction

All the parts of a truck tire come together in what’s called the building process. Sidewalls go on the tire-building machine first, followed by the ‘innerliner’ of a tire – basically long flat sheets of rubber – which is cut and placed around a metal drum. Next comes the tire bead’s chafer reinforcement, followed by the casing ply, belt-edge inserts to position the belt edges, the tire beads, and then the tire belts. The tire tread goes on last.

The assembly of the tire components on the tire-building machine is essentially done by hand. However, lasers help the operator to precisely align all of those parts to create a tire. The tire is built from the inside out, with a computer keeping track of all the parts to make sure everything is included.

Workers that guide the process wear special gloves so when they touch the rubber, they don’t contaminate it with oils secreted from their hands. Those oils can affect the ‘tack’ or stickiness of the rubber, preventing the different sections from holding together properly.

All of these components are tracked by barcodes, by the way, so should a problem occur in the construction phase it can be traced back up the production line for easy correction. A special lab sits right on the factory floor that constantly tests the rubber coming out of the giant Banbury mixer, making sure the physical and chemical properties of the rubber are sound. If a composition is deemed incorrect, the lab can trace the formula through the production line by bar code, pulling out components made with that batch of rubber.

The tire components are then ‘stitched’ together to help the rubber bond tightly. ‘Stitching’ means using rollers to tightly press together the different rubber components. This process drives out any trapped air, with the stickiness or ‘tack’ of the different parts making them adhere to each other until they are chemically bonded in the curing process.

Once all of a tire’s components are stitched together, it’s called a ‘green tire’ and looks like one of those inflatable tubes used by children in a swimming pool. The green tire then circles around a series of conveyor belts and is sprayed with what’s called a mold release agent. That chemical makes sure the tire does not stick in the mold/curing device.

Curing, Inspecting

Robot ‘cars’ pick up the green tires and move them to the heated molds, where the tread pattern for a particular tire type is formed. The battery-powered robot cars move via special radio frequency-controlled guide wires buried under the factory floor. They have special ‘contact bumpers’ so they halt immediately if they bump into something.

Each mold cures a tire for a bit more than half an hour at over 300 degrees Fahrenheit. A bladder is inserted to push the tire tightly against the mold from the inside, while the mold presses from the outside. The metal pieces that create a tire’s tread shape – the actual ‘mold’ within the mold/curing device – cost up to $70,000 to $80,000 to create.

Once the tire is removed from the mold and cooled, it undergoes several inspections. First is a visual inspection, where workers physically look over the tire inside and out. Residue from the molding process – called ‘mold spew’ – is trimmed off at this point. The inspectors, wearing gloves, will run their hands over the entire tire as it spins slowly at the inspection station – because they sometimes can feel imperfections before they actually see them.

A magnetic sensor then checks the cord spacing in every tire and a random sample of tires is also examined by X-ray, to make sure internal cord spacing and splicing is correct.

Then there is a balancing machine that locates the ‘light spot’ on the tire and marks it with a yellow dot. Marking the light spot allows for proper balancing when the tire is put on a wheel. The light spot is where the valve stem will usually go, because that is usually the heaviest spot on the wheel with the valve stem inserted. That way, the light spot of the tire can be matched with the heavy spot of the wheel.

If any tire fails at any point in the inspection process, it is pulled from the production line, cut in half and scrapped.

Out the door

Once the tire passes all final inspections, it gets loaded into the factory’s robot-controlled warehouse, which holds thousands of tires. The bar codes again come into play, telling the robots where the tire needs to be placed. Bridgestone/Firestone also uses the bar codes to know when and what type of tires leave the warehouse, so production levels can be matched accordingly.

The final step, of course, is when the tire is shipped out of the warehouse, put on a (what else?) truck for delivery to a warehouse, a tire dealer or fleet. Here, it’s important to note that no forklifts in the traditional sense are used to load tires into a truck trailer.

Instead of using metal ‘forks’ to lift a stack of tires for loading, the forklifts use two metal plates, squeezing the tires and using the pressure to pick them up and load them. This process ensures that tires are not inadvertently ‘speared’ by a forklift, minimizing tire losses at the end of the production process.