motorcycles tires are critical to rider safety and bike performance. The casual rider probably cares little and thinks even less about how tire companies turn raw materials into high-performance tires. During a recent press introduction at Bridgestone’s Japanese home base, Motorcycle-USA got an eyewitness account of the tire-making process during a tour of the company’s Nasu Plant.
The real beginning of the tire-making process is in the company’s engineering labs where designers use the latest technology to develop new compounds and tread patterns. After rigorous testing at the company’s nearby Bridgestone Proving Grounds, prototypes go into production at the Nasu facility. The 49-year-old plant, located in the Tochigi prefecture, is the sole motorcycle tire production plant for Bridgestone (with the company retaining 190 plants worldwide, 49 of them producing tires).
Nasu gets raw materials in the front door and delivers finished tires out the back, with Bridgestone reps claiming 6300 motorcycle tires produced daily. Bridgestone keeps a tight rein on its process, with photo and video forbidden during our guided tour. The photos for this article come from Bridgestone itself. But, we asked a lot of questions and took some notes. Here’s how the rubber meets the road.
RAW MATERIALS
The first step of tire production is mixing the raw materials into usable compounds. Bridgestone sources both natural and synthetic rubber in its compounds, with the natural rubber the milky white latex of the rubber tree grown on plantations in Thailand. The natural stuff produces an acrid smell at the production plant, as rubber’s natural fat renders during the initial mixing process. Petroleum-based synthetic rubber is clear in appearance. Each rubber type has advantageous qualities for the various parts of a tire with our guide claiming the typical Bridgestone motorcycle tire
After rubber compounds and specialized component layers completed, the assembly process begins (top) with the "green tire" completed and ready for curing (bottom).
Large chunks of natural and synthetic rubber head up a conveyer belt with workers spearing smaller chunks onto the belt to fine tune the ratios. Specialized additives are then added to enhance tire performance. Carbon black increases durability and gives the tire its coloring (early tires were all white, the same color as the natural rubber used). Sulfur acts as a vulcanizing agent, hardening the tire. Silica is important for wet grip. A host of other chemicals fine-tune the properties of the rubber compound, depending on its function. All the raw materials mix in a large vat under heat and pressure, with atmospheric variables closely monitored. The resulting rubber compound exits in long flat sheets.
EXTRUSION
The extrusion process sees the warm, malleable rubber compounds pushed through slender dies, forming long strips. The extrusions specialize depending on the particular tire components being made, including carcass and sidewall layers. Fabric and steel-belt plies are also created, with the tire component layers prepared for the final assembly. During our tour we saw the production of carcass layers as well as the insertion of steel cords that make up the tire bead.
ASSEMBLY
The assembly process takes place on a large drum. The carcass layer is rolled out, measured and cut. An internal air bladder shapes the tire as various layers are added. Tires source different layers of body plies and belts depending on its spec design. The Bridgestone tires we sampled, the BT-023, utilize a monospiral belt, with a single strand of steel wound over the diameter of the tire.
Sealed chambers control pressure and temperature as the green tire cures (top). Close up of tire after curing. Note the small nubs (AKA spew) throughout the tire's surface (bottom).
Once the internal layers are formed, the critical exterior tread layer is added. While the entire process is largely automated, the level of human interaction during the assembly stage was surprising. In particular, the application of the exterior tread layer was cut and molded together by the hands of the machine operator. The exterior layer added, the tire is removed from the drum. The result is a warm, pliable form dubbed a “green tire.”
CURING
Green tires are moved over to curing chambers. The curing phase sources three elements: heat, pressure and time. Green tires enter the mold chambers, with the tire tread pattern raised on the inside. Once sealed, an internal air bladder applies pressure from the center, forcing the green tire into the mold and imprinting the tread patters. Temperatures raise to 170 degrees Centigrade, with chemical reactions in the rubber compound vulcanizing the tire. The length of time required for the curing process depends on the tire. Most motorcycle tires at the Bridgestone facility take about 15 minutes, but tires for large trucks or buses take upwards of 40 minutes. Heavy duty earth mover tires, like the world’s largest tire that graces the Nasu Plant’s entrance (see side bar) take about one full day of cooking time.
After exiting the molds, the warm tires cool down. Small knobs, called spew, prickle the exterior of the tire. They are formed by purpose-built pinholes evenly spaced throughout the mold to allow the escape of steam and air. Many of the spew are shaved off prior to the final inspection (we saw a gigantic tub of them). But not all of them get removed, with a small amount remaining on the lips of some tires – there for future owner’s riding buddies to gauge his or her wuss factor.
The tire doesn't leave the factory before one more human step is completed, the final quality control inspection.
INSPECTION
The final check of each tire is a pair of human hands and eyes. Quality control workers check for shape and any defects, measuring and examining the finished product. Less than 0.11% don’t pass muster. As for the remaining 99.89%, they are stored on large racks and then shipped out the back door for worldwide distribution.
In the door as raw materials. Out the door as a fully functional motorcycle tire.
