Did you know entry-level tire construction engineers, compounders and designers aren’t allowed to touch a new tire for a year or two? That’s because tire design and development isn’t part of any college curriculum. The making of a competent tire engineer isn’t the job of a college professor. That task belongs to veteran tire company technical types who serve as mentors.
That means only experienced engineers and chemists are making decisions about the tires you sell. These seasoned vets constantly make running changes in tire technology to improve such things as noise, vibration and harshness, as well as handling characteristics, tread life, braking, water dispersion, and even better gas mileage. And the process never stops.
In this article, the first of four, you’ll get a dose of “plain English” explanations about tire pieces and parts. Tire buyers count on you to explain the complexities of a tire. With all of the advanced technology we’ve seen in recent years, and all of the accompanying acronyms, we’ve lost touch with the basics. How, after all, can you explain to a customer why a tire performs as it claims if you don’t understand more than the acronyms?
A tire must not only look like it can get the job done; it must have the guts to perform the tough work it is asked to do. It must equal or exceed the design intention of every engineer who gives it life. And it must do so with its basic components: the bead, the sidewall, the belt package, the tread compounds and the tread design.
We’ll begin with a close look at the bead and sidewall areas of the tire. In future issues, we’ll address the other primary components.
The Bead Area
In the simplest language, the bead is a loop of high-tensile steel cable coated with rubber. Its primary mission is to provide the muscle a tire needs to remain seated on the rim flange and to pass along the forces between the tire body plies and the wheel.
Sometimes called the bead bundle, the bead must also be tough enough to handle the forces encountered by tire mounting machines.
Typically, a bead bundle is comprised of about one pound of large monofilament steel cords. The cord is coated with rubber and then wound into a properly sized loop based on the designed wheel diameter. The resulting bundle is then wrapped with a ribbon of rubber-coated ribbon material. Depending on the tiremaker or the product, the resulting bead hoop can be square, rectangular, octagonal or oval in shape.
Of course, it’s impossible to talk about the bead bundle without mentioning the tire’s body plies. Keep in mind that body plies run from bead to bead, looping down and around the bead bundle, which holds them in place.
In most cases, a passenger tire casing has one or two body plies, which can be made of polyester, steel or nylon. We’ll talk more about the casing later, but it’s important to note how the bead bundle fits into the overall tire design as it relates to body plies.
At The Apex
Next, let’s look at the bead apex filler – a hard or soft rubber compound that envelopes the bead and extends up into the sidewall. If the tire is a high profiler designed to provide a boulevard ride, the bead apex filler will be softer. However, on a low-profile ultra-high performance tire, the bead apex will be much harder and extend further into the upper sidewall area for added stiffness.
Generally speaking, a low-profile tire with a stiffer sidewall (and a harder bead apex filler) rides rougher but delivers better handling. A softer sidewall (with a softer compound bead apex filler) provides a softer, more comfortable ride.
Another function of the bead apex filler is to create a smooth contour for the body plies around the bead wire in the lower sidewall area.
The remaining component in the bead area is the bead chafer, or chafer strip. Its mission is to protect the bead area from rim chafing, mounting/dismounting damage and to prevent the tire from rotating on the rim. Chafer strips are made of a hard, durable compound rugged enough to withstand the forces working against it.
In quick review, the bead area of any tire is made up of a bead bundle, a bead apex filler and a bead chafer. Each has a separate job, yet each piece must rely on the other to function the way tire construction engineers intended.
The Sidewall
Now that we have the tire firmly attached to the wheel, the bead wire well protected, and the body plies safely wrapped around the bead, let’s move up to the sidewall.
Tire sidewalls vary in thickness from the shoulder area to the bead area. In the thinnest part, typically in the middle to upper area, most sidewalls are between 6- and 15-mm thick – about 1/4- to 5/8-inch thick. The differences are dependent upon tire application – thinner for ride comfort street tires (S- or T-rated), thicker for off-road light truck tires that require significantly stronger sidewalls.
You should also know that the sidewall and bead areas of a tire represent about 30% of a tire’s total weight. Multiple sidewall plies are typically a blend of natural and synthetic (butadiene) rubber.
Keeping that in mind, a sidewall’s primary mission is to transmit force from the ground to the vehicle via the wheel. Inflation pressure holds the sidewall out where it’s supposed to be, allowing it to help carry the load.
The sidewall is also responsible for maintaining lateral stability as hard cornering and/or braking forces are transmitted through the sidewall to the bead.
Engineering at Work
As these forces push and pull their way though the sidewall and bead area, we see some of the finest engineering in the world at work. The body plies, always under compression, are assembled in such a way that the forces working against them are passed to the vehicle via the strong contact between the bead wire, the chafer strip and the wheel’s rim flange.
All of this assumes that the tires are properly inflated. Driving on an underinflated tire results in unwanted sidewall deflection. Such deflection can be more than the tire was designed to handle, resulting in too much heat generating flexing and life-shortening possibilities for the tire over time.
Acceleration also does its best to shorten tire life. Step on the accelerator, and you pull the tire components forward, bending and twisting them in the process (If you’ve ever witnessed a rear drag race tire work in slow motion, you’ve seen an extreme example of this phenomenon.). Step on the brakes, and the forces at work stress the rubber in the opposite direction.
Ultra-high performance tires handle these assignments well because of compounding and design technologies employed in the tread area, which we’ll talk about in a future Tire Tech.
To be clear, it is the sidewall and bead areas that deliver all of the real performance and driver comfort.
Sidewalls also face another force: the elements. Weather and ozone can cause cracking and weather checking. That’s why a tire’s sidewall is loaded with a host of materials like anti-oxidants, anti-ozonants and paraffin waxes.
Bounce-Back Factor
The ideal bead and tire sidewall combination offers low hysteresis (for low energy consumption), good tear strength and low heat generation. These properties and characteristics contribute to low rolling resistance, which, in turn, contributes to better gas mileage.
In tire-speak, low hysteresis represents the ability of a tire to return to its normal shape after encountering severe deflection or opposing force. Think of dropping a super ball (low hysteresis) and a ball of Play-Doh (high hysteresis). The super ball bounces very high because it doesn’t absorb the energy. Play-Doh doesn’t bounce because it absorbs all of the energy.
The rubber used in tires must fall somewhere in between, yet be a lot stronger than Play-Doh. Its job is to absorb some of the energy, which is converted into heat.
A tire’s innerliner, one of the first building steps in the production of a radial tire and the last item we’ll talk about in this installment, functions like an inner tube and is the unseen part of a tire and its sidewall.
An innerliner is made up of low-permeability rubber laminated to the inside of a radial tire. Its mission: to keep air in the tire. Typically, it is made of butyl rubber, which is not reactive with oxygen. A small amount of rubber (synthetic isoprene) is added to allow the innerliner to adhere to the body plies during vulcanization.
In the next Tire Tech, we’ll explore the role of the belt package. If you require further elaboration on what we’ve talked about in Part 1, please drop me an e-mail at [email protected].
Mini Glossary of Basic Tire Terms
Bead: The tire part made of steel wires, wrapped or reinfored by tire cords and shaped to fit the rim flange. The bead anchors the body cords of the tire to the rim so that they may resist external and internal (pneumatic) forces.
Bead area: That part of the tire structure surrounding and in the immediate area of the bead wire hoop. Consists of fabric components and shaped rubber parts to provide a tight fit to the contour of the rim flange, resistance to chafing at rim interface and flexing support for the lower sidewall.
Bead filler (apex): A rubber compound filler smoothly fitting the body plies to the bead.
Bead heel: Rounded part of bead contour, which contacts rim flange where the flange bends vertically upward.
Bead reinforcement: A layer of fabric located around the bead area outside of the body plies to add stiffness to the bead area.
Bead separation: Failure of bonding between components in the bead area.
Bead wrap: Subsequent to forming of the bead, for some manufacturers, the bead is wrapped with a fabric similar to friction tape.
Body (carcass; casing): The rubber-bonded cord structure of a tire (integral with bead) containing the inflation-pressure-generated forces.
Body ply turnup (turnup plies): Ends of body plies in a tire, which are wrapped under the bead wire bundle and extend up the sidewall.
Chafer: A layer of rubber compound, with or without fabric reinforcement, applied to the bead for protection against rim chafing and other external damage.
Flange: The upward curved lip of a wheel rim, which contacts the outer surface of the tire bead.
Flex cracking: A cracking condition of the surface of rubber resulting from repeated bending or flexing.
Flipper: A partial ply wrapped around the bead coil but not extending the full height of the sidewall.
Innerliner: Innermost layer of rubber in a tubeless tire, which acts as an inner tube in containing the air.
Innerliner separation: Separation of tire innerliner from tire carcass, resulting in air loss.
Ply turn-up: The portion of body plies passed around the bead coil.
Sidewall: The portion of either side of the tire that connects the bead with the tread.
