Passenger Car Tires and Wheels: Development - Manufacturing - Application

By Günter Leister

Starting from the beginning, this book explains the development process of all parts related to the topics tire, wheel and tire pressure monitoring system. This is continued by the modern project management methods in the development process of the parts and the necessary tests to build up this safety relevant components. Modern methods for simulations are described. 

• Language: English
• Publisher: Springer
• Release date: Mar 5, 2018
• ISBN: 9783319501185

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© Springer International Publishing AG 2018

Günter LeisterPassenger Car Tires and Wheelshttps://doi.org/10.1007/978-3-319-50118-5_1

1. Tires

Günter Leister¹  

(1)

Schwaigern, Germany

Günter Leister

Email: [e-mail]

A tire is a composite material which has rotational symmetry, is non-isotropic, and is comprised of several rubber components which are bound together, and whose strength characteristics are determined by textile or other steel reinforcement materials. This is the formal definition of a tire as given by an encyclopedia.

A tire is tasked with far more than just supporting the weight of a vehicle. It should be able to apply lateral forces to guide the vehicle safely along curves and apply longitudinal forces to transfer engine power and brake forces onto the road. It should offer maximum possible adhesion to road surfaces under all weather conditions, have spring and damping properties, be able to support steering inputs through suitable response characteristics, and have a minimal amount of rolling resistance and road noise. With regard to longevity and durability, a tire should be able to endure considerable amounts of mileage, be as air-tight as possible, and be suitably robust against external influences. Lastly and most importantly, a tire must be safe: it should retain its specified dimensions over its lifecycle and should not become detached from the wheel rim while the vehicle is in motion.

Unfortunately, the properties listed above are not exactly free from conflict. Therefore, an optimal compromise must be reached during the development of new tires. Determining this optimal compromise becomes the task of vehicle manufacturers, while the technical implementation becomes the task of tire manufacturers [1, 2].

Modern passenger cars use steel-belted radial tires almost exclusively. Steel-belted radial tires from different tire manufacturers have few significant differences from one another, Fig. 1.1.

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Fig. 1.1

History of tire outside diameter for the Mercedes S-Class

The core profile is located above the tire bead cable and is made of synthetic rubber. The core profile influences the vertical spring stiffness and hence the comfort aspect of a tire. The core profile, like the bead reinforcement, which is made of nylon or aramid, also ensures the precision of steering and dynamic stability while driving.

The sidewall rubber is made of natural or synthetic rubber and protects the tire carcass against lateral damage and weathering. The rubberized polyester- or rayon-finished textile cord carcass is essentially the reinforcing material against the tire’s internal air pressure.

The steel cord-belt inserts are made of rubberized steel cord and help to ensure stability while driving, help to reduce rolling resistance, and increase tire mileage. The joint-less bandage made of nylon improves high-speed capabilities.

A steel-belted radial tire can be made from more than twenty different rubber mixtures. One important descriptive parameter for rubber mixtures, especially for the tire tread, is the shore hardness. Shore hardness can vary depending on the type of carbon black used and its proportion to the softening agent (plasticizer) as well as the dosing of the vulcanizing agent.

The belt from which a steel-belted radial tire earns its name consists of at least two steel cord-belt inserts. These are laid one above the other, are made of drilled or twisted steel wires, and are partly coated with brass. The belt is located below the tread and is covered by the nylon bandages. The steel wires do not run in the direction of the tread, but rather at a defined angle relative to the tread. On the sides, the belt inserts are either folded or cut.

The tire carcass consists of one or more radial layers of synthetic fibers or rayon. The sidewall serves as protection against damage to the carcass yarns (e.g. while driving over curbstones) and has a major impact on driving properties as well as passenger comfort. The properties of the sidewall vary depending on the nature of the material used as well as the geometry of the tire.

The tire carcass is surrounded by the tire’s shoulders and is largely responsible for driving characteristics. It consists of a mixture of elastomers, filler materials, oils, anti-aging materials, and vulcanizing agents. A large percentage of natural rubber in this mixture helps to reduce heat generation in the tread region. Synthetic rubbers, on the other hand, offer increased wear resistance (mileage) and grip. Carbon black and silica are the main filler materials used for a tire’s tread and help to increase wear resistance while at the same time stiffening the tread. Diluting oils serve to increase the workability of the mixtures. Anti-aging materials aim to prevent damage due to ozone. Vulcanizing agents—largely sulfur, but also stearic acid and zinc oxide—promote the binding activities during the vulcanization process. Another important feature of the tread is the tread profile, which largely determines noise properties as well as tire responsiveness under winter conditions, including aquaplaning and wet grip performance.

A variety of information is available on the sidewall, Fig. 1.2. This information includes the width of the tire, its cross-section measurement, tire design type, and rim size. The width is measured in millimeters, whereas the tire cross-section denotes the ratio of the height of the sidewall to the width of the tread. Modern passenger cars use tires with an R designation, for radial, meaning that the cord inserts run radially from tire bead to tire bead. In other words, they lie at an angle of 90° relative to the running direction of the tire. Rim diameter is described in inches.

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Fig. 1.2

Tire markings

(Source Michelin)

The United States Department of Transportation (DOT) demands that details regarding a tire’s structure be written on the sidewall in the form of numerical codes. Normally, the term DOT Number refers to a tire’s date of manufacture, which is written in code form on the sidewall. For tires manufactured after 2000, the week of manufacture and the tire model appear as the last four digits of the code number.

The Uniform Tire Quality Grading (UTQG) classification is a sidewall marking prescribed by consumer ordinances in the United States which contains details about the operational performance (mileage or treadwear), traction, and temperature.

Tires should have profile grooves around the entire tread periphery. Profile depth should be measured in the main grooves, which is additionally marked with tread wear indicators (TWI) in the case of modern tires.

A comprehensive overview of tire markings is given, for instance, in [3, 4] and on the websites of tire manufacturers. The most important information is also usually found in the user manual provided with new vehicles.

1.1 Tire Manufacturing

Tire manufacturers are independently responsible for the manufacturing of tires; however, automobile manufacturers must be familiar with tire manufacturers’ processes, techniques, and facilities [2]. In fact, tire manufacturing plants should be audited and approved by vehicle manufacturers. The basic processes of tire manufacturing are depicted in Figs. 1.3 and 1.4.

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Fig. 1.3

Fabrication of a tire at a tire factory

(Source Michelin)

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Fig. 1.4

Schematic of the tire manufacturing process at a tire factory

(Source Continental)

1.1.1 Mixing

One of the most basic materials used in tire manufacturing is the rubber mixture. These rubber mixtures are mixed, sprayed, rolled, and cut with special machinery. The basic rolled material is called a sheet. The ingredients are illustrated in Fig. 1.5.

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Fig. 1.5

Components of a mixture

(Source Continental)

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Fig. 1.6

The rolling process, which will later yield a rubber sheet

(Source Continental)

Before a finished sheet can leave the mixing department, a multiple-phase mixing procedure must take place. The mixture will differ depending on the intended purpose of the tire. In particular, the tread will require filler materials such as carbon black and silica to help increase abrasion resistance, and materials such as silica to help ensure high-quality compounding which thereby improves braking distances, especially on wet surfaces.

Natural and synthetic rubbers are the basic materials. Chemical additives such as antioxidants are responsible for a tire’s longevity. During the mixing process, other substances such as chalk, oil, resins, catalysts, and sulfur are also added. Depending on the type of tire or tire component, the process and additivity are varied until the desired material characteristics are obtained. The finished sheet can then be processed further (Fig. 1.6).

1.1.2 Inner Liner

The sheet for the inner liner is shaped, cut, and wound onto a transportation roller during the rolling process, Fig. 1.7. The inner layer is a thin layer of butyl rubber which is—as much as possible—impermeable to air, and forms the first layer of a tire. Normally, two different sheets provide the finished mixture for the inner layer. With the help of a calender, the mixture is formed into mixed sheets. Afterwards, the sheets are cut into dimensions appropriate for the respective tire dimension.

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Fig. 1.7

An inner liner after extrusion

(Source Continental)

This layer fulfills the task of retaining the tire’s internal air pressure.

1.1.3 Carcass

The carcass of the final tire is formed using the textile cord insert. The insert is coated with a mixed layer using a calender, Fig. 1.8.

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Fig. 1.8

Manufacturing of the carcass

(Source Continental)

The insert is then cut in such a way that the threads are oriented laterally to the tire’s eventual direction of travel. In this way, the threads are radially positioned. After the cut, yarns are then inserted laterally to the direction of the thread. The resulting product is then wound up for further processing. The textile fabric which is embedded in a mixed layer is positioned directly above the inner liner and acts as a reinforcing material. The strength properties of the insert are further improved by the radial orientation of the yarns.

All in all, the textile cord insert essentially determines the load capacity of the tire as well as comfort properties such as spring stiffness.

1.1.4 Bead Cable and Apex

The bead cable fixes the tire to the rim. It consists of rubber-encased steel wire which is wound to form a ring, Figs. 1.9 and 1.10.

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Fig. 1.9

Structure of the bead cable (Wulstkabel)

(Source Continental)

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Fig. 1.10

Rubberized bead cable (Wulstkabel)

(Source Continental)

The bead is covered by the extruded apex. The apex acts as a protective shell for the bead cable, and is also sometimes called the filler. The apex is produced simultaneously to the production of the bead cable, albeit on another manufacturing line. After the extrusion process, the apex is fed through rollers and then fastened to the tire bead.

In a finished tire, the apex has a strong influence on dynamic stability, steering, and comfort characteristics.

1.1.5 Belt

To make the belt, many steel wires are joined together from the rollers in the spool chamber to create a fine steel ply, Fig. 1.11.

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Fig. 1.11

Individual wires in the coil chamber

(Source Continental)

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Fig. 1.12

Calendered steel cord

(Source Continental)

This resulting ply is then encapsulated in a rubber mixture by passing it through a calender.

After this step, according to specifications, the steel cord will be cut individually into acute angles, Fig. 1.12. Then, the belt is joined to the cut edges perpendicularly, and then wound for further processing.

The steel cord belt ensures the stiffness of the tread in longitudinal and lateral directions. Because of this, improvements are seen in longitudinal force transmission, directional control, and wear resistance.

1.1.6 Tread Rubber

After the extrusion process, the tread is cut into the required length, Fig. 1.13. The tread is the part of a tire which has contact with the road surface, and therefore must meet correspondingly stringent requirements for its properties. Up to four different mixtures are generally processed. This layer of the tire is marked with a color code. Later, this layer will be added to the tire profile.

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Fig. 1.13

Tread extrusion

(Source Continental)

A tire’s tread is responsible for good grip, wear resistance, and optimal rolling resistance.

1.1.7 Assembly

The first step of final tire assembly is the trimming of the tire carcass. The core and the apex are bound to the inner layer using a construction drum. Once the sidewall is fastened, the carcass package is complete, Fig. 1.14. The belt package consists of the belt plies, bandages for high-speed tires, and the tread. It is assembled independently from the carcass package. In the last step, the carcass and belt packages are finally pushed together and are married to one another with the help of pressurized air in the assembly station. The unified, non-vulcanized assembly, colloquially called a green tire, is now ready for vulcanization, Fig. 1.15.

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Fig. 1.14

Including the material of the carcass in the construction machines

(Source Continental)

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Fig. 1.15

Green tires before the vulcanization

(Source Continental)

1.1.8 Vulcanization

Vulcanization is the last step to the tire production process. First, the green tire is placed into a baking mold which is then sealed. The mold is then heated to a temperature of more than 170 °C, and a set of bellows inflates inside the tire, pressing the green tire against the mold at a pressure of up to 22 bar. This is where the tire receives its profile, Fig. 1.16. This process is known as vulcanization in a technical context and as baking in an informal context. After vulcanization, all tire components are joined together in an undetachable manner, Fig. 1.17.

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Fig. 1.16

Tire heating mold

(Source Continental)

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Fig. 1.17

Final baked tire

(Source Continental)

1.1.9 Quality Check

A visual and sensory check is the last step to the production process. The visual inspection is carried out by a worker and is followed by machine-conducted sensory checks, Fig. 1.18. Sensory checks include measurements of diameter and width, checks for true-run properties such as balance and run-out force, as well as random X-ray checks to ensure that tire components are positioned correctly within the completed tire. It is only after these final checks that a tire can be cleared and prepared for transportation, Fig. 1.19.

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Fig. 1.18

100% final checking

(Source Continental)

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Fig. 1.19

Pretzel-storage for transportation

(Source Continental)

1.2 Tire Development Process

The tire development process is an extremely complex one. For a new vehicle model, it begins with tire design. At this stage, tire specifications are defined based on criteria such as the vehicle axle load, axle design, maximum speed, brake installation space, and market positioning of the vehicle.

General tire performance has improved drastically over the last several decades with respect to all characteristics, Fig. 1.20. A minimum of three to four years before market introduction of a new vehicle, intensive studies are carried out using prototype tires with the support of development partners in the tire industry. At this stage, tires are tested by both tire and vehicle manufacturers. Normally, several development cycles are necessary before the required specifications are fulfilled.

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Fig. 1.20

Performance trends of passenger car tires in the last 25 years

(Source Continental)

Every vehicle manufacturer has its own set of priorities when it comes to the technical specifications of tires. For example, tires for Mercedes-Benz vehicles are developed with the highest standards for vehicle safety. These standards include excellent driving stability and exceptional comfort without disregarding rolling resistance. Safety-specific requirements include, for instance, high-speed tests on a rolling test rig with maximum wheel load, maximum possible wheel camber, and reduced air pressure. Additionally, the maximum speed index of the tire is exceeded by up to two levels. Tests such as this ensure that the structural strength of approved tires have far greater safety reserves than is required by international standards.

1.2.1 Geometry and Load Capacity

At first glance, the geometric shape of a tire is approximately that of a torus, a shape which is characterized by an internal diameter, outer diameter, and width. The internal diameter is give in inches and the width in millimeters, while the outer diameter is described indirectly as the ratio of height to width as a percentage. The actual dimensions, including the corresponding tolerances, are clearly described in the standards of the European Tyre and Rim Technical Organisation (ETRTO). These dimensions are also important for the certification of vehicles, in that they must be appropriate so as not to interfere with the vehicle bodywork while the vehicle is in motion.

After the tire is mounted to the rim, properties such as rim width and offset gain a special significance, Fig. 1.21. The offset denotes the distance between the middle of the tire and the inner contact area of the rim, that is, the part of the rim which attaches to the wheel hub. The offset changes the width of the track for the entire vehicle but does not change the tire properties. Rim width affects tire contour in such a way that every half inch of rim width accounts for approximately 5 mm of tire width. Through varying the rim offset and rim width, comfort and handling properties can be changed.

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Fig. 1.21

Influential parameters of wheel geometry offset and rim width

(Source Continental)

The previously referenced ETRTO is a group comprised of tire, wheel, and tire valve manufacturers whose goal is to promote the harmonization of European national standards to achieve interchangeability of tires, wheels, and tire valves throughout Europe. Dimensions, load and air pressure tolerances, and guidelines for usage are also defined jointly through the ETRTO. International equivalents to the ETRTO are the Japanese Automobile Tyre Manufacturers Association, Inc. (JATMA), the Tire and Rim Association, Inc. (TRA, USA), and the Australian Design Rules (ADR). Recommendations and specifications are agreed upon between these various national organizations and are ultimately adopted as standard design guidelines in the International Standards Organization (ISO). The ISO guidelines define standard tire dimensions along with the corresponding load, air pressure, and test conditions.

The ETRTO defines the specific diameter and width tolerances for standardized tire dimensions. These tolerances are relatively large, and the specification manuals of vehicle manufacturers often prescribe narrower tolerances. These narrower tolerances act as a safety margin and are calculated for tires on an individual basis based on deformations while in motion, air pressure, and residual deformation. This ensures that ETRTO standard dimensions are not exceeded while a vehicle is in motion and over the lifetime of the vehicle. New tires should be as close as possible to standard ETRTO contour definitions to achieve visually appealing wheel arches, Fig. 1.22.

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Fig. 1.22

Tire dimensions

The ETRTO restrictions also help to ensure accuracy among systems which depend on wheel rotational speed as an input signal, such as an anti-lock braking system (ABS), electronic stability control (ESC), and indirect tire pressure warning system. In the case of all-wheel-drive vehicles, it is especially important to know the differences between front and rear axle wheel rotational speeds to ensure that stresses at the differentials do not become too large.

With regard to vehicle design, tire dimensions are used for packaging studies and to determine wheel envelope, wheel arc, and axle free space requirements of a new vehicle. Centrifugal force can result in a radial widening of up to 8 mm, something which