Tyre sizing

The generally accepted system for indicating car tyre dimensions is to quote the approximate cross-sectional width of the tyre, followed by the height to ­width ratio (Aspect ratio), type of construction and rim diameter. In addition both the load index and speed rating maybe inserted adjacent to the tyre size designation, or the speed rating maybe inserted before the type of construction in a Radial tyre, or before the rim diameter in the case of cross ply tyre.Different symbols and terms used to designate a tyre are as follows

PLY RATING:
It is an index of tyre casing strength. It means the equivalent strength of the carcass, in terms of the tyre been manufactured with rayon/cotton plies. It does not represent the number of plies in a tyre.

CODIFICATION
a) BIAS TYRES:
Bias Tyres may be codified as
6.4 D 13 82 S
Nominal cross sectional width (inches)
Bias construction
Rim Diameter
Load index i.e. tyre load carrying Capacity (TLCC) of 475 Kg
Speed rating (180 km/hr)

B. RADIAL TYRES
Radial tyre can be codified as
P 185/70 R 13 84 H
Passenger Tyres
Nominal cross sectional width (mm) Aspect ratio (%)
Radial Construction
Rim Diameter (inch)
Load index i.e. tyre load carrying capacity (TLCC) of 500 Kg.
Speed rating (210 Km/hr)

Note: - Rim Diameter is expressed for both cases in inches. Nominal cross sectional width is expressed in "mm" for passenger tyres and in "inches" for Light Commercial Vehicles (L C V).
Design procedures based on sound mechanical principles have become a necessity to meet the ever-increasing demands of performance placed upon the tyre of today.
In previous years there was a tendency to consider tyre design somewhat as a black art. Tyre development during the early years consisted principally of improving materials, developing methods of component assembly, shaping materials to form a tyre and improving vulcanization techniques. The increasing service demands through the years have led to increase analytical studies of the finely balance system of forces in a tyre for the sake of design improvements. The tyre engineer is in the unique and probably unenviable position of working with materials that are assembled and shaped in a form, quite different from the finished product.
The actual design process requires a combination of engineering skills and procedures as well as the influence of professional judgment based on experience. Engineering decisions fall into three areas, namely application or service demands to be made in the tyre.

Common marking on tyres
· Tyre size designation
· Manufactures name
· Brand name and extensions
· Ply rating /load range
· Single /dual Maximum load and corresponding inflation pressure
· Collaborators name
· Mould number
· Drawing number
· Serial number and date of production
· Tread wear indicator
· Radial
· Carcass ply material
· Tubeless /tube type
· Made in
· Use radial tube only
· Regroovable
· Visual alignment indicator
· Direction of rotation
· Standard marking(ISI,DOT,ECE)
· Number and types of plies
· Safety warning
· Quality grades

TIRE TERMINOLOGY

PLY RATING - The term “ply rating” is used to indicate an index to the load rating of the tire. Years ago when tires were made from cotton cords, “ply rating” did indicate the actual number of plies in the carcass. With the development of higher-strength fibers such as nylon, fewer plies are needed to give an equivalent strength. Therefore the definition of the term “ply rating” (actual number of cotton plies) has been replaced to mean an index of carcass strength or a load carrying capacity.

RATED LOAD- This is the maximum allowable load that the tire can carry at a rated inflation pressure.

RATED PRESSURE - Rated pressure is the maximum inflation pressure to match the load rating. Aircraft tire pressures are given for an unloaded tire; i.e, a tire not on an airplane. When the rated load is applied to the tire, the pressure increases by 4% as a result of a reduction in air volume.

OUTSIDE DIAMETER - This measurement is taken at the circumferential center line of an inflated tire.

SECTION WIDTH- This measurement is taken at the maximum cross sectional width of an inflated tire.

RIM DIAMETER - This is the nominal diameter of wheel/rim on which the tire is mounted.

SECTION HEIGHT - This measurement can be calculated by using the following formula:
Section Height = Outside Diameter - Rim Diameter/2

ASPECT RATIO- Measure of the tire’s cross section shape. This can be calculated by the following formula: Aspect ratio = Section Height /Section Width

FLANGE HEIGHT - This is the height of the wheel rim flange.

FLANGE DIAMETER - The diameter of the wheel including the flange.

FREE HEIGHT - This measurement can be calculated by using the following formula:
Free Height = Outside Diameter - Flange Diameter/2

STATIC LOADED RADIUS - This is the measurement from the center of the axle to the runway for a loaded tire.

LOADED FREE HEIGHT - This measurement can be calculated by using the following formula:
Loaded Free Height = Static Loaded Radius - Flange Diameter/2

TIRE DEFLECTION - A common term used when talking about aircraft tires is the amount of deflection it sees when rolling under load. The term % Deflection is a calculation made using the following formula: % Deflection = Free Height - Loaded Free Height/Free Height
Aircraft tires are designed to operate at 32% deflection, with some at 35%. As a comparison, cars and trucks operate in the 17% range.

SERVICE LOAD (OPERATIONAL LOAD) – Load on the tire at max aircraft takeoff weight.

SERVICE PRESSURE (OPERATIONAL PRESSURE) – Corresponding pressure to provide proper deflection at service load.

RATED SPEED– Maximum speed to which the tire is qualified.



Bead. A ring of steel wire that anchors the tire carcass plies to the rim.

Belt. An assembly of plies extending from shoulder to shoulder of a tire and
providing a reinforcing foundation for the tread. In radial-ply tires, the belts
are typically reinforced with fine steel wire having high tensile strength.

Bias-ply tire.A pneumatic tire in which the ply cords that extend to the
beads are laid at alternate angles substantially less than 90 degrees to the
centerline of the tread. The bias-ply tire was the predominant passenger
tire in the United States before 1980 but is no longer in common use; it
has been supplanted by the radial-ply tire.

Carbon black.A very fine, nano-size particulate carbon used as a reinforc-
ing filler in rubber compounds to provide abrasion resistance and other
favorable properties.

Carcass or casing. The tire structure, except tread and sidewall rubber,
that bears the load when the tire is inflated.

Coastdown. A process in which a vehicle or test machine is allowed to
slow down freely from a high to a low speed without application of ex-
ternal power or braking.

Coefficient of friction. The ratio of friction force to normal force to
cause sliding expressed as a unitless value (i.e., friction force generated
between tire tread rubber and the road surface divided by vertical load).

High-performance tire. A passenger tire designed for the highest speed
and handling, generally having the speed symbol W, Y, or Z in the United
States.

Hysteresis. A characteristic of a deformable material such that the energy
of deformation is greater than the energy of recovery. The rubber com-
pound in a tire exhibits hysteresis. As the tire rotates under the weight of
the vehicle, it experiences repeated cycles of deformation and recovery,
and it dissipates the hysteresis energy loss as heat. Hysteresis is the main
cause of energy loss associated with rolling resistance and is attributed to
the viscoelastic characteristics of the rubber.

Light truck (LT) tire. A tire constructed for heavy loads and rough ter-
rain that is usually used on medium-duty trucks in commercial service.
LT tires are not regulated as passenger tires and are therefore not examined in this study.

Original equipment manufacturer (OEM). An automobile manufac-
turer.

Original equipment (OE) passenger tire. A tire that is provided as origi-
nal equipment on new passenger vehicles. Such tires are often designed for
particular vehicles to the specifications of the automobile manufacturer.

Passenger tire. A tire constructed and approved for use on passenger
vehicles and that usually contains the prefix P before the metric size
designation on the tire sidewall. Federal Motor Vehicle Safety Standards
and Uniform Tire Quality Grading standards are established specifically
for passenger tires.

Performance tire. A passenger tire intended to provide superior han-
dling and higher speed capabilities and generally having a speed symbol
of H or V in the United States.

Ply. A sheet of rubber-coated parallel tire cords. Tire body plies are
layered.xvi Tires and Passenger Vehicle Fuel Economy

Radial-ply construction. A pneumatic tire construction under which
the ply cords that extend to the beads are laid at approximately 90 de-
grees to the centerline of the tread. Two or more plies of reinforced belts
are applied, encircling the tire under the tread. Radial-ply tires were in-
troduced in Europe during the 1950s and came into common use in the
United States during the 1970s.

Reinforcing filler. Material added to rubber compounds to provide
favorable properties, including resistance to abrasion. The two most
common reinforcing fillers are carbon black and silica.
.
Rim diameter. The diameter of a wheel measured at the intersection of
the bead seat and the flange. The rim diameter is listed in the size desig-
nation on the passenger tire sidewall. Common rim diameters for pas-
senger tires range from 13 to 20 inches.


Rolling resistance. The force at the axle in the direction of travel re-
quired to make a loaded tire roll.


Run-flat tire. A type of pneumatic tire constructed of special materials,
supports, and configurations that allow it to travel for a limited distance
and speed after experiencing a loss of most or all inflation pressure.
While these tires usually have greater weight and resultant rolling resis-
tance, they permit the elimination of storage space and weight associated
with a spare tire and jack.

Sidewall. The portion of the tire between the bead and the tread. The
tire’s name, safety codes, and size designation are molded on the sidewall.

Silane. An organo-silicate compound that is sometimes mixed with sil-
ica to promote dispersion and bonding.

Silica. A very fine, nano-size particle, silicon dioxide, used as a reinforc-
ing filler in rubber compounding.

Speed rating. A letter assigned to a tire denoting the maximum speed
for which the use of the tire is rated (e.g., S =112 mph, H =130 mph).
The speed rating is contained in the tire size designation molded on the
sidewall.

Tire pressure monitoring system (TPMS). A warning system in motor
vehicles that indicates to the operator when a tire is significantly under-
inflated. Some systems use sensors in the tire to transmit pressure infor-
mation to a receiver. Some do not have pressure sensors but rely on
wheel speed sensors to detect and compare differences in wheel rota-
tional speeds, which can be correlated to differences in tire pressure.

Traction. The ability of a loaded tire to generate vehicle control forces
through frictional interaction with a road surface.

Tread. The peripheral portion of the tire designed to contact the road
surface. The tread band consists of a pattern of protruding ribs and
grooved channels on top of a base. Tread depth is measured on the basis
of groove depth. Traction is provided by the tread.

Tread compound.The general term that refers to the chemical formula
of the tread material. The compound consists of polymers, reinforcing
fillers, and other additives that aid in processing and slow degradations
from heat, oxygen, moisture, and ozone.

Tread wear life. Total miles traveled by a tire until its tread wears out,
which is usually defined as a remaining groove depth of 2/32 inch for a
passenger car tire that exhibits even wear.

Uniform Tire Quality Grade (UTQG). A passenger tire rating system
that grades a tire’s performance in tread wear durability, traction, and
temperature resistance. UTQG ratings are required by the federal gov-
ernment for most types of passenger tires and are molded on the tire’s
sidewall. The tread wear grade is a numeric rating, with a higher num-
ber suggesting longer tread wear capability. Most tires receive grades be-
tween 100 and 800. The traction grade is assigned on the basis of results
of skid tests on wet pavements. Tires are graded AA, A, B, or C, with AA
indicating superior wet traction. The temperature grade is assigned to
tires tested at various speeds to determine the ability of a tire to dissipate
heat. Tires are graded A, B, or C, with A indicating an ability to dissipate
heat at higher speeds.

Vehicle fuel economy.The average number of miles a vehicle travels per
gallon of motor fuel (typically gasoline or diesel fuel).

Viscoelastic. A viscoelastic material is characterized by possessing both
viscous and elastic behavior. A purely elastic material is one in which all
energy stored in the material during loading is returned when the load is
removed. In contrast, a purely viscous material stores no strain energy,
and all of the energy required to deform the material is simultaneously
converted into heat. Some of the energy stored in a viscoelastic system is
recovered on removal of the load, and the remainder is dissipated as heat.
Rubber is a viscoelastic material.

Wear resistance. Resistance of the tread to abrasion from use on a nor-
mal road surface.

Wet traction. The ability of a loaded tire to generate vehicle control
forces through frictional interaction with a wet road surface.

The radial-ply tire,

The radial-ply tire, on the other hand, is constructed very differently from
the bias-ply tire. It was first introduced by Michelin in 1948 and has now
become dominant for passenger cars and trucks and increasingly for heavy-
duty earth-moving machinery. However, the bias-ply tire is still in use in
particular fields, such as cycles, motorcycles, agricultural machinery, and
some military equipment. The radial-ply tire has one or more layers of cords
in the carcass extending radially from bead to bead, resulting in a crown
angle of 90 degree,. A belt of several layers of cords of high modulus of elasticity
(usually steel or other high-strength materials) is fitted under the tread, . The cords in the belt are laid at a low crown angle of approximately 20 degree. The belt is essential to the proper
functioning of the radial-ply tire. Without it, a radial-ply carcass can become
unstable since the tire periphery may develop into a series of buckles due to
the irregularities in cord spacing when inflated. For passenger car tires, usu-
ally there are two radial plies in the carcass made of synthetic material, such
as rayon or polyester, and two plies of steel cords and two plies of cords
made of synthetic material, such as nylon, in the belt. For truck tires, usually
there is one radial steel ply in the carcass and four steel plies in the belt. For
the radial-ply tire, flexing of the carcass involves very little relativemovement
of the cords forming the belt. In the absence of a wipingmotion between the
tire and the road, the power dissipation of the radial-ply tire could be as low
as 60% of that of the bias-ply tire under similar conditions, and the life of
the radial-ply tire could be as long as twice that of the equivalent bias-ply
tire . For a radial-ply tire, there is a relatively uniform ground pressure
over the entire contact area. In contrast, the ground pressure for a bias-ply
tire varies greatly from point to point as tread elements passing through the
contact area undergo complex localized wiping motion.
There are also tires built with belts in the tread on bias-ply construction.
This type of tire is usually called the bias-belted tire. The cords in the belt
are of materials with a higher modulus of elasticity than those in the bias-
plies. The belt provides high rigidity to the tread against distortion, and re-
duces tread wear and rolling resistance in comparison with the conventional
bias-ply tire. Generally, the bias-belted tire has characteristics midway be-
tween those of the bias-ply and the radial-ply tire.

For instance, for a tire ‘‘P185/70 R14 87S,’’ ‘‘P’’ indicates
a passenger car tire; ‘‘185’’ is the nominal width of the cross section in
millimeters; ‘‘70’’ is the aspect ratio, which is the ratio of the height of the
sidewall to the cross-sectional width; ‘‘R’’ stands for radial-ply tire; ‘‘14’’ is
the rim diameter in inches; ‘‘87’’ is a code indicating the maximum load the
tire can carry at its maximum rated speed; ‘‘S’’ is a speed rating which in-
dicates the maximum speed that the tire can sustain without failure, S—112
mph (180 km/h), T—118 mph (190 km/h), H—130 mph (210 km/h), V—
149 mph (240 km/h), Z—149 mph (240 km/h) or more. Traction and tem-
perature capabilities are indicated on a scale fromA to C, A being the best
and C the worst. The traction rating is based on straight-line stopping ability
on a wet surface. The temperature rating is an index of the tire’s ability to
withstand the heat that high speeds, heavy loads, and hard driving generate.
Tread-wear index is an indication of expected tire life. It is rated against a
reference tire with an index of 100. For instance, a tread-wear rating of 420
means that the tire should last 4.2 times as long as the reference tire. A tread-
wear index of 180 is considered to be quite low and an index of 500, quite
high.

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MECHANICS OF PNEUMATIC TIRES

Aside from aerodynamic and gravitational forces, all other major forces and
moments affecting the motion of a ground vehicle are applied through the
running gear–ground contact. An understanding of the basic characteristics
of the interaction between the running gear and the ground is, therefore,
essential to the study of performance characteristics, ride quality, and handling
behavior of ground vehicles.

The running gear of a ground vehicle is generally required to fulfill the
following functions:

• to support the weight of the vehicle
• to cushion the vehicle over surface irregularities
• to provide sufficient traction for driving and braking
• to provide adequate steering control and direction stability.

Pneumatic tires can perform these functions effectively and efficiently;
thus, they are universally used in road vehicles, and are also widely used in
off-road vehicles. The study of the mechanics of pneumatic tires therefore is
of fundamental importance to the understanding of the performance and char-
acteristics of ground vehicles. Two basic types of problem in the mechanics
of tires are of special interest to vehicle engineers. One is the mechanics of
tires on hard surfaces, which is essential to the study of the characteristics of
road vehicles. The other is the mechanics of tires on deformable surfaces
(unprepared terrain), which is of prime importance to the study of off-road
vehicle performance.

A pneumatic tire is a flexible structure of the shape of a toroid filled with
compressed air. The most important structural element of the tire is the car-
cass. It is made up of a number of layers of flexible cords of high modulus
of elasticity encased in a matrix of low modulus rubber compounds,. The cords are made of fabrics of natural, synthetic, or metallic composition, and are anchored around the beads made of high tensile strength steel wires. The beads serve as the ‘‘foundations’’ for the carcass and provide adequate seating of the tire on the rim. The ingredients of the rubber com-
pounds are selected to provide the tire with specific properties. The rubber
compounds for the sidewall are generally required to be highly resistant to
fatigue and scuffing, and styrene–butadiene compounds are widely used
The rubber compounds for the tread vary with the type of tire. For
instance, for heavy truck tires, the high load intensities necessitate the use of
tread compounds with high resistance to abrasion, tearing, and crack growth,
and with low hysteresis to reduce internal heat generation and rolling resis-
tance. Consequently, natural rubber compounds are widely used for truck
tires, although they intrinsically provide lower values of coefficient of road
adhesion, particularly on wet surfaces, than various synthetic rubber com-
pounds universally used for passenger car and racing car tires [1.1]. For tube-
less tires, which have become dominant, a thin layer of rubber with high
impermeability to air (such as butyl rubber compounds) is attached to the
inner surface of the carcass.

The load transmission of a pneumatic tire is analogous to that of a bicycle
wheel, where the hub hangs on the spokes from the upper part of the rim,
which in turn is supported at its lower part by the ground. For an inflated
pneumatic tire, the inflation pressure causes tension to be developed in the
cords comprising the carcass. The load applied through the rim of the wheel
hangs primarily on the cords in the sidewalls through the beads.
The design and construction of the carcass determine, to a great extent,
the characteristics of the tire. Among the various design parameters, the ge-
ometric dispositions of layers of rubber-coated cords (plies), particularly their
directions, play a significant role in the behavior of the tire. The direction of
the cords is usually defined by the crown angle, which is the angle between
the cord and the circumferential center line of the tire, .
When the cords have a low crown angle, the tire will have good cornering
characteristics, but a harsh ride. On the other hand, if the cords are at right
angle to the centerline of the tread, the tire will be capable of providing a
comfortable ride, but poor handling performance.

A compromise is adopted in a bias-ply tire, in which the cords extend diagonally across the carcass from bead to bead with a crown angle of ap-proximately 40 degree,. A bias-ply tire has two plies (for light-load tires) or more (up to 20 plies for heavy-load tires). The cords in adjacent plies run in opposite directions. Thus, the cords overlap in a diamond-shaped (criss-cross) pattern. In operation, the diagonal plies flex and rub, thus elongating the diamond-shaped elements and the rubber-filler.This flexing action produces a wiping motion between the tread and the road, which is one of the main causes of tire wear and high rolling resistance

Tyre Reinforcement Materials

Reinforcement materials are the cords used in the carcass and belts and bead wires. The requirements for them are

• good strength-to-weight ratio
• flexibility
• dimensional stability
• good chemical and thermal resistance
• good fatigue resistance.

Generally in agricultural tyres textile cords are used in the carcass and textile or steel cords in the belts.

As textile cords is usually used Nylon (polyamide), Rayon and Polyester. Textile cord characteristics are the following:

Nylon is the strongest of these materials. It is generally used as carcass material especially in big earthmoving tyres. The Nylon 6.6 is more common nowadays. It is more stabile than older Nylon 6. The problem is that nylon changes by the action of temperature. The cord shrinks after curing, which may cause separations in use. Shrinkage can be compensated by post curing inflation. Nylon is not suitable for highway tyres, because when the tyre gets cool, it may result in a flat spot.

Rayon is the most stabile of these materials. It is used primarily in the agricultural tyre belts. As regards to strength, it is better than polyester. High price is the only problem.

Polyester is more stabile than Nylon. It is used both in cross ply and radial tyre carcass. It suits well for driving on a highway, because the flat spotting is not characteristic to it. New types of Polyester have been developed lately and they have partly displaced rayon.

Steel cords are used as the belt material of the radial earthmoving tyres and fortification of the forestry tyres. Truck tyres have monoply construction (All steel). Different cord constructions are used for different purposes. For example truck carcass is required to be fatigue resistant. The belt cord is required to be stiff. The fortification needs to have strength and good elongation (HE).

The most important property of the bead wire is strength. Typical wire diameter is 0,9-1,5. Bead wire may also have a stronger core wire. Socalled HT quality is also used. In truck tyres may be used square shaped wire.

Tyre Basic constructions

Radial Ply Tyres, Cross Ply Tyres and Bias Belted Tyres

The carcass of a cross ply tyre actually consists of only one part, multiply crossing plies. In practice the wires are in the rear part of the tyre at an angle of 25-45^o depending on the aspect ratio of the tyre.

Radial ply tyres always have a two-piece construction including a one- or multiply-layered carcass and a multiply-layered belt structure. The cords go radially from one bead wire to another. The belts are clearly at a small angle to each other (15-20^o). In multiple-layered constructions, the wires can be set to intersect each other a little in order to regulate the features (e.g. stability).

Bias belted tyres are in principle cross ply tyres added with belts. The tyres are marked with "B". Nokian Tyres uses this marking in mining tyres. Marking B could also be used in the logging tyres (SF).

In practice, the extra reinforcements of the cross ply tyres are called brakers. They are about at the same angle as the wires.

Many arguments can be stated why radial tyres are better than cross ply tyres.

A trend is that radial ply construction will be used more in all the product areas. The first car tyre with a radial ply construction was presented already in 1948 by Michelin. After it, the radial ply construction has become general in almost every product area.

However, there are still uses, where radial structures have not been successful, like forestry, mining, and the biggest harbour tyres. The problem with the radial structures is the endurance and sidewall stability, because the sidewalls are relatively more loose compared to cross ply tyres. There have been attempts to compensate the problem by different additional components that can easily raise the price of the tyre.

Choosing a Tyre

When choosing a tyre for a certain application, following things have to be taken into consideration:

suitability of the outer dimensions to the machine

construction (radial/cross ply)

tread pattern

sufficiency of the load capacity for different purposes

stress on the surface caused by the tyre (tyre contact pressure)

behaviour of the vehicle (without suspension).

On hard surfaces, the tyre size should be chosen so that the surface pressure is the smallest possible. The pressure on hard surfaces is a function of the tyre inflation pressure (>= pressure).

In forestry, often has to be used higher recommendations for tyre pressures, because the sidewall durability is a more important factor than the load capacity. The pressure recommendations may be even 1,5 times higher than the calculated pressures.

Following compounds are widely used in different tyre types:

Forest compounds are strongly cut resistant. Abrasion is destructive and the tyres are exposed to cutting. In tyres, chains and tracks are used much and they may gnaw the tyre. The compounds are SBR based special compounds.

Mining compounds are much like forestry compounds. The compound is in the mine under even bigger exertion than in the forest, because the conditions are very demanding (wet, sharp rock material).

Harbour compounds have a good abrasion resistance and low heat buildup. Also tear resistance is good. Typically the compounds are NR-based (NR/BR combinations). Resistance to ageing has to be taken into account.

Compounds for construction applications are like the harbour compounds. They are used a lot on the highway. Tear resistance is not required to be as high as in the case of harbour compounds.

Truck compounds resist abrasion well and produce little heat. The compounds are 100 % of NR or NR/BR combination (e.g. 80/20 % NR/BR). In on/off-road compounds SBR may be used to achieve better abrasion resistance.

Carcass Constructions

The tyre carcass is required to have extremely high strength, dimensional stability and fatigue resistance. Therefore it is important to construct the tyre so that each cord ply bears all the forces directed to it.

Cross ply construction balances well the forces between the cord plies. The reason is that under stretching the cross ply construction gives the cords more possibilities to settle. Small angle changes do not affect the cross ply construction, because the carcass tries to find its own form. Cross ply tyre constructions may have several bead wires and pockets. In radial tyres, multiwire constructions are not used, because stretching is different and it would not be possible to balance the forces between the pockets.

Tyre constructions are notified with the cord number per pocket. For a singlewire tyre, the maximum cord amount is in principle ten (6+4). In multiwire cross ply tyres the amount of pockets follow the same principle, which means that around a bead wire may be six cords at the most and around the whole bundle four cords. Also breakers can be used in tyres to increase the strength or as fortification, when the construction is given for example in the form of 6+2+2BR. The upper layers in cross ply tyres are usually cords with lower cord number (ends/10 cm) and thicker rubber than the actual carcass cords. The aim is to make it stick better on the tread and sidewall.

One type of constructions is multiply textile radial carcass. The amount of cord plies is typically 3-7. With textile carcass radial construction, it is possible to reach half of the amount of cords in cross ply construction.

The speciality among radial construction tyres is the monoply construction tyre, in which the carcass consists of one strong steel cord. With this construction, small heat build-up and long tyre life is reached.

To stabilize the tyre construction, post inflation is used. This happens immediately after curing by pressurizing the tyre (e.g. 1,3 5 standard pressure). Post inflation is used especially for Nylon carcass tyres. With post inflation, the shrinkage of the carcass is prevented when the tyre gets cool. If the tyre shrinks, it usually also stretches back to its original form in use, which may cause separations between the carcass and the surface.

Belt Constructions

The tyre belts have to stand high stress even in driving straight . The influence of the load is emphasized in driving in curves. Therefore it is very important to separate the belt ends from each other and from the carcass. The belt ends move with pumping movement under stress, and so the heat build-up of cord coating and belt edge rubber has to be low (tan d).

Belt material is either textile or steel. Textile belts are mainly of Rayon because of the good stability. Requirements for the steel cord are good stiffness and strength. Textile belts are used mainly in agricultural tyres. Steel belts are used in earthmoving tyres. Typically the number of textile belts is four, but even a bigger number can be used. The number of steel cords, on the other hand, is usually three or four (minimum 2).

Bead Area

Bead area is one of the most critical areas that affect the tyre durability, because the ply turnups often end in the flexing area the lower part of the sidewall. Therefore it is very important that the ends of the cords are staggered to each other. The cord end should not be on the apex or clinch end. By shaping the bead area it is possible to minimize the influence of flexing.

In heavy tyre constructions are often used cords of the same width, which makes the beads asymmetric: one bead with an open construction, another with a closed construction. The function of both beads has to be made certain, because their flexibility is different.

Tyre Curing methods

a) Dome Steam, Bladder Steam

Usually used in production of small tyres like passenger car tyres, bicycle tyres and smaller agricultural tyres.

b) Dome Steam, Hot Water in Bladder

This is the most common method in production of big tyres.

Often is used also circulation of cooling water in the end of the curing phase to cool the tyre before release (temperature 25-50 ^oC).

c) Nitrogen Curing

Rather new curing method. Steam or hot water in the bladder is replaced by nitrogen.

With nitrogen curing, the curing time can be shortened and problems caused by water or steam can be eliminated.

This method decreases energy consumption in curing compared to hot water or steam curing.

Curing Temperatures

Generally the bladder steam or hot water temperature is standardized. Instead, the dome steam temperature is changed according to the product. By stopping the circulation of hot water it is possible to regulate the energy that comes through the bladder.

Temperature levels in a curing process are determined by the vulcanization properties of the compound. Typically the dome steam is lower, 120-170 ^oC. Hot water in the bladder is 150-220 ^oC.

Mould Materials

• Full Aluminium
• Full cast iron

Frame / Tread segment

Steel / All cast

Steel / Steel cast

Steel / Cast iron

Steel / Steel engraved

Essential Properties

• surface quality
• mould strength
• thermal conductivity
• ventilation holes
• possible to take the tyre off the mould without pattern breaks

Mould Maintenance

The precuring mould maintenance procedure includes:

• cleaning the mould (cleaning by sandblasting)
• opening the ventilation holes
• setting the equipment to meet the requirements of the tyre to be cured:
• changing the date of manufacture (DOT)
• checking or changing the LI and plyrating markings
• checking that the other markings needed correspond with the production formula for the tyre in question
• with 2-piece moulds controlling the accuracy of the alignment key so as to avoid the mould pieces becoming indented
• visual inspection of the condition of the mould before taking it to curing.

Curing bladders

• Dimensioning of the following characteristics is required:

height/width

shape

grooving (density/shape)

thickness

• Tight bead area
• Homogeneity of the material:

porosity

cuts

hardness

The Criteria for Choosing a Bladder

The dimensions and geometry of the tyre to be cured determine the bladder to be chosen for the job. Stretching occurs during the curing process both as far as the diameter and the height of the bladder are concerned. The stretching ratio of the bladder should be kept in balance. Too big an elongation not only shortens the service life of the bladder, but it may also lead to an irregular elongation of the tyre, which may cause the tyre to become asymmetric in shape. Too small an elongation may cause wrinkles to the bladder, which in turn will cause damage in the inner surface of the tyre.

The service life of a bladder depends on the elongation, bladder material, curing temperature and pressure used. The bladders are made of butyl rubber.

TYRE CURING

Curing recipes and settings

The curing recipe determines the settings to be used for the curing process. The curing recipe contains:

information on the mould, bladder, flanges as well as the tyre markings that vary by type (ply rating, LI, DOT etc.)

press settings (press type, bushing etc.)

shaping information (loading heights, shaping time)

curing program

shaping time

bladder steam / hot water filling time

bladder steam / hot water circulation time

pressure and time of dome steam applied

release time

cooling time

press opening

The operator follows a checklist to make sure all changes have been made and settings changed, and with his signature acknowledges that the setting procedure has been completed. The markings and dimensions are checked on the first tyre completed from each production series so as to get the final affirmation of the correctness of the settings.

Curing Time of a Tyre

Curing time = State of cure definition + Safety factor + Pressure release time + Press opening time

Safety Factor

Deviations in compound (reaction time)

Deviations in tyre thickness

Deviations in temperatures

Steam temperature

Hot water temperature

Variation of the temperature of the green tyre

Mould temperature

Deviations in bladder thickness

Postcuring inflation

The postcuring inflation stage involves mounting the green tyre on the flanges, inflating it and cooling it according to a predetermined and preselected procedure.

Conditions for Post Curing Inflation

Immediately after release from the press, the green tyre is mounted on the flanges either automatically or manually

The postcuring inflation pressure has to be about 20-35 % higher than what the normal inflation pressure of the tyre would be.

The inflation time has to be long enough (at least one curing cycle) depending on the size of the tyre (thickness of materials).

The distance between the flanges needs to be same as the width of the recommended rim for the tyre.

Purpose of Post Curing Inflation

• Stabilize the shape of the tyre.
• Postcuring inflation will help eliminate the influence of hot shrinkage on the tyre -> the tyre dimensions will not increase under actual operating conditions.
• Postcuring inflation is recommended especially tyres of nylon carcass construction.

Differences in the curing of radial and cross ply tyres

Cross Ply Tyres

• Cylindrical bladder is used.
• Green tyres are preshaped in a press.
• Before curing, the inside of the tyre carcass is painted - or alternatively the bladder may be processed or painted - which helps the tyre carcass slide better against the bladder during shaping.
• Considerable deformation of green tyres is possible during curing.

Radial Tyres

• Loading requirements are more precise than with cross ply tyres.
• A so called B-type bladder is used.
• Deformation during curing is small.

The most common curing faults according to the FMEA analysis

• Tread bareness
• Damaged tread
• Shoulder blisters
• Sidewall bareness
• Sidewall blisters
• Sidewall damage (steam or water leak)
• Toe cuts
• Inside bareness
• Bladder break
• Wrong green tyre
• Undervulcanized tyre
• Curing failures
• Dirty mould
• Damaged mould
• Distorted stretch
• Defective mould equipment
• Mould extraction split
• DOT missing/wrong
• Wrong post inflation pressure

Main phases of the curing process for cross ply tyres

The curing process can be divided into:

• preshaping
• loading the green tyre onto the press
• shaping: the shaping bladder is stretched evenly inside the green tyre
• curing and cooling
• offloading the tyre from the press
• postcuring inflation (if needed)

Curing press

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• The tyre is vulcanized in a curing press, which receives the energy required for the vulcanization process through the dome and bladder.
• The steam building up within the dome transmits the energy through the mould.
• Energy is transmitted from the hot water circulating in the shaping bladder through the bladder into the tyre carcass. The pressure of hot water forces the tyre carcass against the mould, which process causes the tyre tread impression to be moulded into the surface of the tyre and determines the final shape of the tyre.

The press requires four types of motive power:

• electricity (control commands, press motion open/closed)
• compressed air (lubricators, confirmation of the control commands, other compressed air powered equipment)
• hydraulic pressure (tyre loading/offloading equipment, bladder motion etc.)
• dome steam and hot water (vulcanization energy)

Curing Press Types

1. Bag-O-Matic, BOM type, the most commonly used curing method
2. AF-type press
3. Autoclave curing method

Common press sizes are 36", 40", 55", 60", 63,5", 75", 85", 88", 91", 104" and 114".

Radial tyre building process

a) Green Tyre Forming

The first stage of building a radial tyre is to locate the beads onto a cylindrical flat forming drum, followed by a series of components which will form the base structure of the tyre and link the two beads together. The tubular shaped carcass is then transferred to the second stage.

b) Green Tyre Shaping

The second stage of construction begins by loading the tyre carcass onto a second drum, the tyre is the shaped and breaker belts are applied, followed by the sidewall and tread components.

At each stage of the construction great care is taken to apply pressure, with the view to expelling all air between each layer and to consolidate the assembly.

Radial tyre building methods

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1-Stage Assembly

The tyre carcass is built on an assembly drum. Components can be added for example in the following order:

1. sidewalls
2. chafers
3. innerliner
4. ply materials
5. belt materials / tread rubber

The assembly drum used in 1-stage assembly consists of a shaping bladder, turnup bladders and supporting bladders.

• Beads are fitted onto the drum using specific bead applicators.
• By inflating the shaping bladder, the stretching of the tyre carcass begins. Whilst this is being done, the distance between the beads shortens.
• The application of the belt and tread package takes place beforehand, and its is done on a separate drum. With the help of a transfer ring, the package is moved onto the shaping assembly drum.
• The tyre carcass is stretched and attached to the belt package.
• The tread and belt package is stitched carefully onto the tyre carcass.
• Turnup bladders are used for completing the ply turnup phase, when also the chafers and sidewalls are put into their correct places.
• Support bladders are used to ensure that the turnups are properly made all the way.
• Pressure is aspirated from the turnup bladders and support bladders.
• In the stage to follow, the sidewalls and bead area components are stitched.
• After the shaping bladder has been deflated, the tyre carcass can be taken off the drum.

Advantages:

• Suitable for long production batches.
• As far as work stages are concerned, a reasonably fast method of production.

Disadvantages:

• Suitable only for the production of open construction tyres.
• The drum required is complex and expensive.
• Changing the machine settings is a slow procedure.
• The bladders have to be made separately, which results in high operating costs.

2-Stage Assembly

1. Stage

Building the carcass follows the same procedure as used in the manufacture of cross ply tyres.

The components fitted in the assembly stage:

• innerliner
• ply materials
• beads
• insulation rubbers
• chafers
• sidewalls

The tyre is built on a regular assembly drum.

• The process starts with the fitting of the innerliner and is followed by the fitting of the carcass material and bead wires.
• Ply turnups are made either by stitching or using spring method (selection of method depends on the construction of the building machine).
• The components are stitched together.
• The chafers and sidewalls are put into their correct places, after which the materials are stitched together.
• The tyre carcass is offloaded from the drum and transferred on to the next phase.

2. Stage: Shaping

• The tyre carcass is fastened onto the shaping drum, and the bead area and the flanges are brought closer together.
• The stretching of the tyre carcass begins and the distance between the beads shortens.
• The application of the belt and tread package takes place beforehand, and it is done on a separate drum. With the help of a transfer ring, the package is moved onto to the shaping assembly drum.
• The tyre carcass is stretched and attached to the belt and tread package, after which the transfer ring is removed from the drum.
• The tread and belt package is stitched carefully onto the tyre carcass.
• The stretching pressure is released from the tyre carcass, and the green tyre is taken off the drum and moved to intermediate storage.

Advantages:

• Suitable for short production batches.
• Changes between different sizes can be made quickly.
• The drums and any other equipment required are cheaper and less complicated than those used in the 1-stage method
• Possible to apply to the production of a closed construction tyre.

Disadvantages:

• More space required for the equipment.
• Requires more material processing than the 1-stage method.

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