The Rubber Plant

While some sport riders only consider the state of their tires when a frayed cord peeks out from well-worn tread, others will endlessly discuss the merits of various brands, tread patterns and models over a midride coffee. Serious folk-with names like Bostrom and Roberts, who look for every last ounce of grip and life from their tires-agonize over a myriad of compounds, constructions and profiles at each race venue. We took a trip recently to Tonawanda (near Buffalo), New York, home of Buffalo Wings and Dunlop's motorcycle, ATV and light-truck tire factory, to find out just what the ingredients in a tire are, how they all fit together and how that affects a bun's performance.

Green tires after they are hand-assembled. These are bias-ply cruiser tires with white sidewalls. From here, a high-pressure and high-temperature curing process gives the tires their shape.

In the process of constructing a tire, some steps are amazingly complex, yet others are astonishingly simple. What is evident throughout the procedure, however, is the staggering number of variables that can be tweaked and juggled to produce a subtly different end result. Our tour guides for this excursion: Tom Roles, quality assurance manager for the Buffalo plant, and the man practically every top AMA Superbike pilot talks to when he has tire questions, Jim Allen. Let's begin.

Rubber

A key ingredient in any tire is, of course, rubber. However, a surprisingly small amount (less than 5 percent) of natural rubber is employed in motorcycle tires and mostly synthetic polymers are used. The rubber is mixed with other ingredients to enhance certain characteristics: carbon black gives a tire its color and strength, silica is employed to improve wet-weather grip, cobalt salts and various resins will help adhesion, and antioxidants extend the tire's shelf life. Additionally, activators, sulfur, oils, tackifiers, and peptizers (and other items, up to 20 in all for each compound) help the process along.

Non-JLB tires can be cured using a two-piece mold. The drum-shaped green tires are given their shape by high-pressure steam and cured for approximately 14 minutes.

These ingredients are mixed in a Banbury mixer, a giant, three-story food processor (Martha Stewart would be proud) which warms and blends the mixture in 500-pound batches. Each batch goes through the mixer approximately three times (five times for racing compounds), then is extruded out of the bottom in 30-inch wide by one-quarter-inch thick strips. The Dunlop plant utilizes six Banburys, four employed for uncured rubber, the remaining two for cured. Whereas the light-truck tires made at the Dunlop plant utilize just two compounds of tread rubber, there are 20 different compounds for the various motorcycle tires.

The basic tread of a tire is formed from two rubber compounds mixed together; not like a dual-compound tire, although it's easy to understand how such tires could be made once the process is seen in action. The base rubber, even after being mixed several times in the Banburys, is broken up and blended several times again at a temperature of 150 degrees Fahrenheit. The final tread mixture is extruded through a die at 200 degrees F to its precise final form, cut to the correct length and weighed to ensure correct volume. Treads at this particular plant are cut and spliced at 25-degree angles, whereas treads made in the British Dunlop plants are butt-spliced (cut perpendicular). This explains why U.S.-built D207GP tires cannot be run in reverse rotation, as was the fashion with the original British 207s a number of years ago-the angled splice could lift under reversed loads.

Following the curing process, tires are inflated and allowed to cool to ambient temperature.

Cooling of the warm treads is crucial, because rubber has memory, and each tread unit meanders through a 450-foot long water tank as it slowly cools to room temperature. This entire process (mixing and extruding) is employed for the sidewall treads, and for the very thin liner part of the tire-the "tube," if you will. An interesting note: Street tires generally have a one-piece strip that is molded into both the main tread and sidewall treads later in the process, but race tires are built with separate sidewall treads. This is so that when different compounds of tread rubber are used, the sidewall stiffness remains constant and does not affect the motorcycle's suspension action.

Plies

The carcass of a tire, the part that gives it strength and shape, consists of a number of plies of woven material. Dunlop uses nylon and aramid in its Sportmax and D208GP buns, which are woven into a pattern at the company's Utica, New York, plant. The denier (11 different deniers are utilized in Dunlop's range of motorcycle tires), type of material and number of cords all play a part in determining a tire's ride qualities. In addition, each individual cord consists of numerous separate strands twisted together, and the amount of strands and how tightly they are twisted can affect ride quality also. In a process called calendering, the woven plies are spread out at an exact spacing and rubber is added to lock the cords into place. The resultant three-foot wide, 5000-foot long rolls of impregnated weave-in which the strands are oriented lengthwise-are then cut at the appropriate angle and spliced back together in widths appropriate for the tire's construction.

Beads

At each edge of a tire is a wire bead, around which the plies are wrapped, and this bead holds a tire on its rim. In a method similar to calendering, a number of individual, copper-coated wires are wound in a jig, and the resulting matrix-or bead wrap-is rubberized to hold everything together while the tire is being constructed. Even this part of the process can affect a tire's performance, as the shape of the bead wrap determines the foldover of the various plies and how they wrap back into the sidewall.

Construction

After seeing all the elaborate mixers, extruders, infrared gauges and X-ray checking machines on the rubber and ply side of the factory, you would expect a similar high-tech automated assembly machine to put the pieces together. Nothing could be further from the truth, however, as each tire is individually hand-assembled on a drum. Working from the inside of the tire out, an assembler-there are just a few qualified for this job in the Buffalo plant-rolls each part of the tire onto a rotating drum, using laser pointers to keep everything lined up. First the beads are put into place, followed by the inner liner and appropriate plies, all of which are cut to length from large rolls. Next, the edges of the plies are rolled over the bead wraps to form a "green" tire, which looks like a soup can with no top or bottom.

Every tire is inflated and checked for concentricity while undergoing a load test. The spinning drum on the left simulates an 85 percent load, while the small arms on the right measure runout.

For tires that do not have a zero-degree belt around the circumference of the tire, the tread, sidewalls and a small filler strip in the bead area are then laid into place and stitched to remove any air pockets. For zero-degree buns-what Dunlop calls jointless belt (JLB) construction and uses on its rear D207 Sportmax tires-there would be too much deformation in the circumferential ply if it were to be laid on flat and then pressed into shape. For that reason, the belt-a continuous bead of three calendered strands-is wound onto a separate, tire-shaped drum in a proprietary process we were not allowed to photograph. The tread is then added and stitched into position, and the carcass portion (bead wraps and plies) is inserted and stitched after.

Curing

Non-JLB tires are encased in a two-piece mold (which looks like a large waffle iron) heated to 350 degrees F and forced into shape using high-pressure steam. JLB tires, since they are already the correct shape, require an eight-piece mold, which closes in around the tire. Curing pressure, temperature and time can be varied for different results, but the cure time is usually around 14 minutes. After curing, tires are inflated and allowed to slowly cool to ambient temperature. finishing

This scale finds the lightest point on the tire, which is marked with a dab of paint.

Once the tire is cooled, it is visually inspected, mounted and put through a load test at 85 percent of its maximum rated load. Stresses in the tire are measured during the test, and runout is checked. A precise, automated scale is utilized to find the lightest point on the circumference of the tire, at which a dab of paint is added. And voil, the finished product is packaged and shipped.

The Skunkworks

While the bulk of Dunlop's testing takes place at the company's Texas and Alabama test tracks, the Tonawanda plant has an extensive in-house facility, which includes 12 rolling road machines (each with two, four or eight positions) which can apply a set load to a tire at a given speed. Tires can be tested for endurance and speed, under various loads and at different inflation pressures; a sideways load (think flattrack) can even be added.

Anatomy of a tire's construction: click here to open a bigger (800KB) picture.

In total, more than 110 different types of tires are fabricated at the Tonawanda plant, and the factory runs virtually nonstop with 1300 employees working four shifts. Each day, 1.2 million pounds of rubber are turned in the plant, which occupies 1.8 million square feet of space. It's the nuances of the construction process that are the incredible part; the changes that can be made to the rubber compounds, the plies and their orientation, the shape of the tire and even the curing process. Here's a gem of an example: As our small group walked by the curing area, Jim Allen explained that the D208GP front tires give noticeably better performance with the simple change of not inflating them for post-cure cooling. Figure that one out.

This story was originally published in the December 2001 issue of Sport Rider.