23.3.
Tyre Construction
Different parts of the tyre are illustrated in Fig. 23.9. A tyre is made from rubberized fabric piles over a rubber liner and the edges of the piles are wrapped around a wire bead, which holdsthe tyre to the wheel rim. The fabric piles are covered with a rubber compound tread and a different rubber compound for the side walls. The tyre is cured in a mould to vulcanize the parts into a single unit and form the tread design.23.3.1.
Major Components
The carcass, beads, side walls, and tread are the major components of a tyre cover.Carcass. This is a horseshoe-shaped inner lining of the tyre and is made up of a number of layers of textile cord piles. The carcass forms the backbone of the tyre construction and the tread, bead, and walls all are moulded on to these cord piles.Bead. This forms the inner edge of the tyre, and locates and centralizes the cover on to the wheel rim. It has the rigidity and strength required to support the carcass. To achieve this endless wire core is moulded circumferentially trough the bead.Side Wall. This constitutes the outside rubber covering of the carcass between the bead and the tyre tread. The amount of protection provided to the carcass and the stiffness of the tyre during deflection depends on the thickness of side wall.
Fig. 23.9. Sectional view of a tyre. Tread. This forms the part of the tyre, which contacts the road surface when the wheel rolls. It is a rubber compound and its pattern design considerably influences the tyre’s gripping, road-holding ability and working life.23.3.2.
Cross (Bias)- and Radial-ply Tyre Features.
Many textile cords are criss-crossed and embedded to provide strength to the cross-ply tyre in the rubber. These cords are arranged in layers, usually referred to as plies, which perform two jobs. Firstly they have to make the walls strong enough to contain the air pressure and yet leave them as supple as possible for deflection. Secondly they have to support the tread. These two requirements conflict each other because to obtain sufficient bracing of the tread, the ply must be reasonably stiff, which then means that the walls has to be rigid.Radial plies perform only one job that is to make the wall of the tyre strong enough to contain the air pressure. They do not support the tread. So they do not require to be criss-crossed, instead are laid readily following the natural profile of the tyre. This provides a supple but strong wall, which is desirable. To support the tread, a layer of rayon or steel cords forming a belt is placed underneath it. The sole purpose of this belt is to brace the whole of the tread firmly flat down and open on to the road.The greatest difference in tyres lies in the cord material and the way it is put into the tyre. The cross-bias cord angle runs from 30 to 40 degrees. This provides a cross-cord side wall, which gives required strength to transfer acceleration and braking torque. The tyres have gone from four-ply to two-ply construction. The strength of the ply is the result of the weight of the cord rather than the number of plies. Two-ply tyres are as strong as four ply tyres, because the tyre cord denier is larger than used in four ply tyres. Two-ply tyres run cooler, are more flexible to absorb shock from road irregularities, and apply greater self aligning torque to the steering system after a turn.Bias-ply allows the tyre to squirm as it moves through the tyre foot print or contact patch. The tread is pushed together as it goes into the foot print. This stores energy in the rubber. As it comes out of the root print, the tyre rapidly expands and goes beyond the neutral point into a stretched position. Closing and opening of the tread as it moves through the contact patches is one of the major causes of normal tyre wear.In belted tyres the tread stability and reduction of squirm results in upto 100% improvement in tyre running compared to bias-ply tyres. By holding the tread shape, belted tyres run cooler, improve fuel consumption, improve traction, and double blow-out resistance when compared to bias-ply tyres. Belted tyres do not flex as easily as bias-cord tyres, so that more road shock is transfer into the wheels and suspension system in this tyre, and hence the wheel spindles, knuckles, and suspension system are required to be stronger.Radial-ply belted tyres have been built having radial ply cord angles run from 88 to 90 degrees and the belt cords run from 12 to 20 degrees. The radial cord provides a soft side wall, which produces a softer ride than belted bias tyres. The belt (steel wires, Fiberglass, or rayon) around the radial cords holds the tread shape through the contact patch or foot prints. Radial belted tyres, as a result of a lower slip angle, provide more cornering power and less wear than bias belted tyres. These tyres produce a harsh ride at low speed, require a high steering effort, especially when parking and are expensive. The steel-belted radial tyre has less tread flexing so it rolls easier, thereby improving fuel consumption. With the radial belted tyres, loss of tyre-to-road adhesion occurs suddenly, with little warming, especially on wet surfaces. Tyre construction is shown in Fig. 23.10.
Fig. 23.10. Tyre Construction. A. Bias angle construction. B. Bias belted construction. C. Radial construction.
Characteristics.
Ride Comfort.
The bending of the walls of cross-ply tyres requires a shear action to change the criss-cross ply angle, which makes the walls very stiff, and hence the bounce on rough roads is not properly cushioned. Radial-ply tyre construction provides a supple wall due to the natural direction of radial-ply cords. These tyres bend readily and hence can absorb a great deal of extra bounce. The radial-ply tyre is more comfortable at higher speeds because of its shock-absorbing deflection characteristic, which is 25% greater than that of the cross-ply type. However at lower speeds the cross-ply tyre provides more smooth riding and the steering is also lighter, so that it is more suitable for parking (Fig. 23.11A).
Acceleration and Braking.
Conventional cross-ply tyres tread is affected by every movement of the walls because tread is not braced and held down on to the road. As a result of this the tread blocks are able to shift and dance about on the road surface providing a small contact area, which reduces road grip. In radial-ply tyres, the braced layers of cords act independently of the wall plies. The bracing belt follows the contour of the road during movement of the wheel providing a continuous large flat contact-patch area with the road surface. The whole of the tread pattern is fully stabilized, so that road-wheel acceleration and braking traction are improved (Fig. 23.1 IB).
Fig. 23.11. Comparison of cross-ply and radial-ply tyres. A. Ride comfort. B. Acceleration and braking. C. Cornering.
Cornering.
Cross-ply tyres do not bend sufficiently to absorb any sideways strain during cornering. The cross-plies therefore pull and lift up one side of the tread from the ground, reducing road grip and traction. Radial-ply tyre during concerning bends readily and absorbs extra strain. The tread stays firm and flat down, with its whole working area of tread pattern biting into the surface of the road (Fig. 23.11C).
Tyre Life.
With a cross-ply tyre, when the wheel rolls, the distortion of the walls tends to pull the tread away from the road surface, thereby scraping the tread blocks as road contact begins and ends. A radial-ply tyre produces a flat full-width track-laying action along the wheel-and-road interface (Fig. 23.11C). The full width tread pattern contact during cornering and the extended flat zone reduce wear during driving and stopping. This extends tyre life considerably by as much as 80%.
Fuel Consumption.
The more flexible casing of radial-ply compared to cross-ply tyres reduces the amount of energy consumed while running, so that saving in fuel consumption in the order of 5% is achieved.
Initial Cost.
The first cost of cross-ply tyres is about 20% less than for radial-ply tyres.23.3.3.
Tyre Material
The efforts are continuously being put to improve the existing tyre materials and to introduce new materials for meeting tyre requirements better. Several types of fabric cord materials are in use for tyre piles. Rayon was introduced in 1938 as a tyre cord material to replace cotton. This material was much more durable, produced a soft ride, and was more resilient and less expensive than cotton. It has tensile strength of648,110 kN/m2. Subsequently to improve its characteristics and to reduce its cost several modifications have been introduced. In 1947 two forms of nylon were introduced as tyre cord material, which has tensile strength of 841,160 kN/m and is more heat and water resistant than rayon. The nylon cord tyres have less flexibility than rayon, which tends to produce better vehicle handling. The major disadvantage is that nylon cord tyres take a set while standing, which produces flat spots causing a thump when the car first starts to roll. As the tyres warm from rolling the cords relax and the flat sport quickly disappears. To reduce the flat-spotting tendency, the nylon cord is stretched and tempered using special techniques prior to use in a tyre. In 1962 the Polyester cord havingtensile strength of 717,050 kN/m was introduced for car tyres. This material provides a soft ride and does not produce flat spot. It is less heat resistant than nylon and more heat resistant than rayon. In 1962, fiberglass was used as a special cord in tyres. But because of its low flexibility it is rarely used as a side wall cord. It was initially used in 1968 for belt between the tyre ply and tread, and provided a much longer life. Steel cords are used for belts in many radial ply tyres, which are quite rigid and have high impact strength. Two to ten piles of cord fabric may be used in tyres.Rubber, an elastomer compound, is used in tyres to blend natural and synthetic rubbers. Chemicals and filler compounds are added to rubber to produce desired characteristics. In order to increase wear and abrasion resistance of the tyre tread, large amounts of carbon black are added to the tread rubber. Traction depends on the tread rubber hardness, compounding, and tread design. Hard compounds provide good wear and poor traction whereas soft compounds give good traction and poor wear. The tread compound is chosen as compromise to provide the properties required for each tyre application. The impregnated rubber used for side wall and ply is a more flexible rubber compound than tread rubber. The side wall must be sufficiently flexible to deflect as it passes the tyre contact patch on each revolution and also to absorb any shock produced by road surface irregularities. It must have sufficient strength to transfer all the acceleration and braking torque between the wheel rim and tyre tread, and to withstand cornering forces that are applied to the automobile. Tyre rubber deteriorates with temperature and age. Rubber compounds are, therefore, varied to provide the expected service life required for the tyre.
Natural and Synthetic Rubbers.
Synthetic rubbers have been developed as substitutes for natural rubber. There are several synthetic rubbers used for tyre construction in addition to the natural rubber as presented below.
Natural Rubber (NR).
Natural rubber provides good wear resistance and excellent tear resistance. It also offers good road holding on dry roads but provides only a moderate grip on wet surfaces. Additionally it has low heat build-up, but this merit is contrasted by high gas permeability. Also the resistance offered by this material for aging and ozone deterioration is only fair. The side walls and treads have been made from natural rubbers but nowadays it is normally blended with other synthetic rubbers to exploit their desirable properties by minimizing their shortcomings.
Chloroprene (Neoprene) Rubber (CR).
This synthetic rubber was one of the earliest to compete with natural rubber and is made from acetylene and hydrochloric acid. This rubber compound has good wear and tear resistance with a reasonable road surface grip. A major limitation is its inability to bond with the carcass fabric due to which a natural rubber film is provided between the cords and the Neoprene covering. Neoprene rubber provides moderately low gas permeability and does not indicate signs of weathering or aging throughout the service life of the tyre. If blended with natural rubber it becomes suitable for side wall covering.
Styrene-butadiene Rubber (SBR).
It is probably the most widely used synthetic rubber for the manufacturing of the tyre. This rubber compounds are made from styrene (a liquid) and butadiene (a gas). Styrene-butadiene rubber (SBR) forms a very strong bond to fabrics and it provides a very good resistance to wear, but exhibits poor tear resistance compared to natural rubber. One remarkable feature of this rubber is its high degree of energy absorption or high hysteresis and low resilience, which allow exceptional grip characteristic, especially on wet surfaces. Because of the high heat build up, this rubber is used only for the tyre tread while the side walls are normally made from low hysteresis compounds, which provide greater rebound response and run cooler. Blending SBR and NR provides the best properties of both synthetic and natural rubber to be utilized so that only one rubber compound is used for some types of car tyres. The high hysteresis characteristic of SBR is partially achieved by adding an extra high styrene content and a large proportion of oil, the overall effects being to increase the rubber plastic properties and to lower its resilience (i.e. reduce its rebound response).
Polyisoprene Rubber (IR).
This rubber exhibits very similar characteristics as natural rubber, however provides improved wear and specifically tear resistance. It has an additional advantage of an extremely low heat build up with normal tyre flexing. This rubber material is blended with natural rubber and styrene-butadiene rubber to produce tyre treads with very high abrasion resistance. This material is highly suitable for heavy duty application such as track tyres where high temperatures and driving on rough terrains are a problem.
Ethylene Propylene Rubber (EPR).
This rubber provides the major advantage of mixing it with large amounts of cheap carbon black and oil without loosing its rubbery properties.Depending upon the compound composition it gives excellent abrasive ageing and ozone resistance with varying road holding qualities in wet weather. It has also varying skid resistance on ice. A great disadvantage with this rubber compound is that it bonds poorly to cord fabric. Therefore, rubber compounds containing EPR have not proved to be successful as a material for tyre till date.
Polybutadiene Rubber (BR).
This compound provides very high wear resistance and a high resilience i.e. a low hysteresis level. It is exceptionally stable with temperature changes. When blended with SBR in the correct proportions, its wet road holding ability reduces slightly but considerably improves its ability to resist wear. Due to its high resilience (large rebound response), the road holding in wet weather becomes relatively poor. For its application in tyres it is normally mixed with SBR in the proportion of 15 to 50%. It is, however, expensive to produce.
Isobutene-isoprene (butyl) Rubber (IIR).
This kind of rubber has exceptionally low permeability to gas. In practice it retains air in tubes ten times longer than the natural rubber, so that it has been extensively used for tyre inner tubes and for inner linings of tubeless tyres. Unfortunately it does not blend with SBR and NR unless it is chlorinated. Its resistance to wear is good and it has a high hysteresis level so that it becomes more plastic like than rubber to distortion. It provides good road grip for both dry and wet conditions. Its desirable properties are generally improved when mixed with carbon black. Due to its high hysteresis it does not readily give out energy to the surroundings so that tyre treads made from this material do not generate noise in the form of squeal.Merits and Limitations of Natural and Synthetic Rubbers. Some cross-ply tyres are completely made using only one rubber compound from tread to bead. The severity of the carcass flexure with radial-ply tyres dictates to use different rubber composition for various parts of the tyre matching their properties to the functional requirement of the part (i.e. tread, side wall, inner lining, bead, etc.)Side walls are usually made from natural rubber alone or blended with polybutadiene rubber (BR) or styrene-butadiene rubber (SBR) or to a lesser extent Neoprene or Butyl rubber. The properties required for side wall material include a resistance against ozone and oxygen attack, a high fatigue resistance to prevent flex cracking and good compatibility with fabrics and other rubber compounds when moulded together.Tread wear fatigue life and road grip depends, to a large extent upon the surrounding temperatures, weather conditions including dry, wet, snow or ice bound, and the type of rubber used. For more clarity a comparison is made with natural rubber and styrene-butadiene (SBR), possibly the most important synthetic rubber. At low temperature, styrene-butadiene (SBR), wears more than natural rubber but at higher temperatures it wears less than natural rubber. As the severity of the operating condition of the tyre increases, SBR tends to wear more compared to NR. The fatigue life of all rubbers reduces with the increase of the degree of cyclic distortion. For small tyre deflection SBR provides a better fatigue life but for large deflections NR offers a longer service life. The NR offers better skid resistance on ice and snow. But as temperatures rise above freezing, SBR offers an improved resistance to skidding. However it depends to some extent on the amount of oil extension (plasticizer) provided in the blending in both NR and SBR.Following are the two typical examples of rubber compositions suitable for tyre treads : (a) High styrene butadiene rubber 31%
23.3.4.
Tyre Tread
In a pneumatic tyre a cushion of air trapped between the well of the wheel rim and the toroid-shaped casing known as the carcass supports the wheel load. The tread, a thick layer of rubber compound, is wrapped around the outside of the tyre carcass to protect the carcass from damage due to tyre impact with the irregular contour of the ground and the abrasive wear as the tyre rolls along the road. During rotation of the wheel the tread provides driving, braking, cornering and steering grip between the tyre and ground. Tread grip may be defined as the ability of a rolling tyre to continuously develop an interaction between the individual tread elements and the ground. Tyre grip must be available under a variety of road conditions such as smooth or rough hard roads, dry or wet surfaces, muddy tracks, fresh snow or hard packed snow and ice, and sandy or soft soil terrain.The main function of a tyre tread pattern is (i) to provide a path for drainage of water trapped between the tyre contact patch and the road, and (ii) to provide tread to ground bite when the wheel is subjected to both longitudinal and lateral forces under various driving conditions.23.3.5.
Tread Bite
Bite is obtained through a tread pattern, which divides the tread into many separate element, and each elements has a reasonably sharp well defined edge. As the wheel rotates these tread edges engage with the ground providing tyre to ground interlock and also develop frictional forces during transmission of tractive or braking forces.Following are the major features of tread pattern that control the effectiveness of the tyre in wet weather:(i) Drainage grooves or channels. (ii) Load carrying ribs.(Hi) Load bearing blocks. (iv) Multiple microslits or sipes.
Tread Drainage Grooves.
If water is trapped between the tread ribs or blocks, the tread elements life become separated from the ground, so that the effective area of the contact patch reduces, so also the tyre’s ability to grip the ground. A number of circumferential grooves placed across the tread width (Fig. 23.12A) facilitate the removal of water films from the tyre to ground interface. These grooves help the leading elements of the tread to push water through the enclosed channels formed between the road and the underside of the grooves. Water therefore emerges in form of jets from the trailing side of the contact patch. The total cross-sectional area of tyre should besufficient to channel all the water immediately ahead of the leading edge of the contact patch away.
Fig. 23.12. Basic tyre treads patterns. A. Circumferential straight grooves and ribs with multiple sipes. B. Circumferential zig-zag grooves and ribs with multiple sipes. C. Diagonal grooves with diamond shaped blocks and central sipes. D. Diagonal bars with central vee blocks and sipes.Lateral grooves are used to join together the individual circumferential grooves so that a direct side exist can be provided for the outer circumferential grooves. Normally many grooves are preferred to a few large ones. This arrangement speeds up the water removal process under the contact patch.
Tread Ribs.
Circumferential ribs provide a supportive wearing surface for the tyre and also become the walls for the drainage grooves (Fig. 23.12A and B). Lateral ribs provide the optimum bite for tractive and braking forces whereas circumferential ribs control cornering and steering stability. For both longitudinal and lateral directional stability, ribs may be arranged diagonally. Also it may be in the zigzag circumferential form to improve the wiping effect across the tread surface under wet conditions. It is desirable to have the tread pattern with many narrow ribs than a few wide ones for better road grip.
Tread Blocks.
The consequences of both longitudinal and lateral drainage channels, used for effective drainage of water at speed, is that the grooves encircle portions of the tread forming isolated island blocks (Fig. 23.12C and D). These blocks provide a sharp wiping and biting edge where the interface of the tread and ground meet. To improve their biting effectiveness for tractive, braking, steering and cornering forces, these forces are required to be resolved into diagonal resultants and to achieve this the blocks are sometimes arranged in an oblique formation. A limitation to the block pattern concept is due to inadequate support around the blocks, which causes the rubber blocks to bend and distort under severe operating conditions. Bar shaped
Fig. 23.13. Effect of irregular tread block wear. A. Toe to heel treads wear. B. Heel to toe treads wear. tread blocks, arranged in a herringbone fashion, have proved to be effective on rugged ground. Square or rhombus-shaped blocks provide a tank track unrolling action so that movement in the tread contact area greatly reduces. This pattern helps to avoid the break-up on the top layer of sand or soil so that the tyre is prevented from digging into the ground. Since the individual blocks bend to certain extent when subjected to ground reaction forces, they suffer from toe to heel rolling action causing blunting of the leading edge and trailing edge feathering.To maintain the wiping action of the tread block element on wet surfaces, wear should from toe to heel (Fig. 23.13A). If wear occurs from heel to toe (Fig. 23.13B) i.e. in the reverse order, the effectiveness of the tread pattern severely reduces since the tread blocks then allow for the formation of a hydrodynamic water wedge, which tries to lift the tread blocks off the ground at speed.
Tread Slits or Sips.
Microslits or sipes are incisions made at the surface of the tyre tread, extending down to the full depth of the tread grooves. They reassemble a knife cut, mostly of a zigzag fashion (Fig.
Fig. 23.14. Effectiveness of microslits on wet road surfaces. A. Effective sipe wiping action on a smooth road. B. Ineffective sipe wiping action on a knobbly road. C. Close pitched sipe wiping action on a pebbled road.23.14A, B, C and D), normally terminating within the tread elements. Sometimes one end of the knife cut intersects the side wall of a drainage groove. In some designs the tread patterns use the sipes arranged at a similar angle to each other, where the zigzag shape provides a large number of edges pointing in various direction. Other designs set sipes at different angles to each other so that these sipes are effective for the wheel pointing in whichever way and for the ground reaction forces operating in whatever direction.Sipes or slits are almost closed in their free state, but as they enter into the contact patch area the ribs or blocks distort and open up (Fig. 23.14A). In this open position, the sipe lips scoop up small quantities of water that still exist underneath the tread. This wiping action produces some biting edge reaction with the ground. If the sipes are smaller in size and more in number, they are more effective for road griping. The sipes with normal spacing on a tyre tread are ineffective on a pebbled road surface as several pebbles exist between the pitch of the sipes (Fig. 23.14B), collecting water between these rounded stones, so that only a few of the stones are subjected to the wiping edge action of the opened lips. Therefore to improve the wiping process there should be many more wiping slits (Fig. 23.14C), which is very difficult to manufacturing with the present techniques.
Selection of Tread Patterns.
Normal Car Tyres.
General duty car tyres, which effectively operate at all speeds use tread blocks arranged in an oblique fashion with a network of surrounding drainage grooves so that both circumferential and lateral water release (Fig. 23.15A, B and C) are provided.
Wet Weather Car Tyres.
These car tyres are usually similar to the general duty car tyre except that the tread grooves are made wider to allow easier water dispersion and to provide better exposure of the tread blocks to snow and soft ice without damaging much the tread (Fig. 23.15D, E and F).
Truck Tyres.
Truck tyres designed for steered axles normally use circumferential zigzag ribs and grooves (Fig. 23.15G and H) so that very good lateral reaction on curved tracks is available. On the other hand, the drive axle tyre is designed so that tread blocks have adequate grooving for optimum traction grip under both dry and wet conditions. Some of these tyres also incorporate provision for insertion of metal studs for severe winter hard packed snow and ice conditions.
Off On Road Vehicles.
OfCon road vehicle tyres normally use a much simpler bold block treads with a relatively large surrounding groove (Fig. 23.151). This arrangement permits each individual block to react independently with the ground causing biting and exerting traction on soil, which may be hard on the surface but soft underneath without break-up of the top layer, so that the tyre is prevented from digging in. The tread pattern blocks are also designed sufficiently small to operate on hard road surface without causing excessive ride harshness at moderate speeds.Truck and Tractor off Road and Cross-country Tyres. Truck or tractor tyres for off road operation generally use slightly curved rectangular blocks separated with wide grooves, which provide a strong flexible casing as well as present a deliberately penetrating grip. Cross-country tyres, for operating on soft soil, have diagonal bars either merging into a common central rib or arranged with separate overlapping diagonal bars to provide exceptionally good traction on muddy soil, snow and soft ice (Figs. 23.15J, K and L).
Fig. 23.15. Survey of tyre treads patterns. A. Car moderate speed radial. B. Car high speed radial. C. Car very high speed radial. D. Car wet weather radial. E. Car winter radial with moulded stud holes. F. Car winter radial. G. Light vehicles off I on road winter tread. H. Truck steer axle tread. I. Truck drive axle tread. J. Truck rough ground tyre. K. Truck cross-country tyre. L. Tractor cross country tyre.23.3.6.
Tyre Profile and Aspect Ratio
A tyre carcass profile considerably influences its rolling and handling characteristics. The tyre’s cross-sectional configuration determines its suitability for better performance under various applications. The aspect ratio of tyre may be defined as the ratio of the tyre cross-sectional height (the distance between the tip of the tread to the bead seat) to that of the section width (the outermost distance between the tyre walls) (Fig. 23.16). This ratio is constant for a particular tyre and is used for predicting the suitability of a tyre for an application.
Fig. 23.16. Tyre profiles with different aspect ratios. A tyre with a large or small aspect ratio is known respectively as a high or low aspect ratio profile tyre. Until about 1934 aspect ratios of 100% were used. With a better understanding and improvement in tyre construction lower aspect ratio tyres become available. Due to lowering the aspect ratio the tyre side wall height is reduced so that the vertical and lateral stiffness of the tyre increases, thereby establishing a shorter and wider contact patch.A short and wider contact patch (a) increases the load carrying capability of the tyre ; (6) generates larger cornering forces so that vehicles are able to travel faster on bends; (c) decreases the pneumatic trail so that the self-aligning torque correspondingly reduces and becomes more consistent; and (d) under certain driving conditions, reduces the slip angles generated by the tyre when subjected to side forces. Consequently the tread distortion reduces and as a result scuffing and wear decrease. Water drainage at speed becomes difficult with wider tyre contact patches, particularly in the mid tread region. Therefore it is more critical to design the tread pattern with low profile tyre on wet” roads, in case the tyre holding is to match with the higher aspect ratio tyres.The increase is vertical and lateral stiffnesses causes the following, (a) Due to an increase in vertical stiffness and a reduction in tyre deflection, less energy is dissipated by the tyre casing thereby reducing the resistance. This also causes the tyre to run continuously at high speeds at lower temperatures, which increases the tyre’s life.(b) The increased lateral stiffness of a low profile tyre increases the sensitivity to camber variations and quicken the response to steering changes.(c) The increased vertical stiffness of the tyre reduces static deflection of the tyre under load, due to which more road vibrations are transmitted through the tyre. This provides a harsher ride reducing ride comfort unless the body is further isolated from the suspension.The availability of lower aspect ratio tyres over the years was as follows : 1950s — 95%, 1962-88%, 1965-80% and about 1968-70%. Since then for special applications even lower aspect ratios of 65%, 60%, 55% and even 50% have become available.23.3.7.
Tyre Manufacturing
Tyre fabric is primarily strands of tyre cord running lengthwise in the fabric. The fabric after required processing is stretched and heated to give it uniform mechanical properties. Adhesive is used as a bonding agent between the cord and rubber compound. The treated fabric then goes to large steel rollers, called calendars, which squeeze and uncured rubber into the cord fabric to produce a sheet. This rubberized sheet is cut into strips at an angle or bias, and then reassembled into a long strip with the cords running at the required bias angle for bias-ply tyres. The cords remain straight for radial tyres. Tread rubber and side wall rubber compounds are extruded into the required shape and cut to length at an angle to provide a long tapered joint. Wire is rubber coated and rolled into the required size bundle or bead, two per tyre, for use as the tyre bead.In order to manufacture bias-ply tyre, one bead is slid over the drum and kept in position. The drum rotates as the tyre is built in layers. The first layer consists of a rubber sheet that takes place of a inner tube to seal the air. This is followed by two or more plies of rubberized cord fabric. The fabric is carefully cut to length parallel to its cords, and then lapped over the other end of the ply with the required overlap joint, to produce an enclosed cylinder shape. Adjacent plies have their bias in alternate directions to give the tyre strength. The plies are followed by belts for breakers when required. The beads are placed over the edges of the ply. The ply is then wrapped over the bead back onto itself, locking the bead bundle into place. A tread and side wall rubber strip is wrapped around the plies and trim is moulded within the side wall strip. The entire assembly is pressed together by a process called stitching. The tyre looks like an open ended barrel and can be repaired, if deviations are observed on inspection, because it is still green or uncured rubber. The manufacture of the radial-ply tyre is some what different from the bias-ply tyre. The radial tyre is built on a drum that forms the green tyre in nearly the same shape as the finished tyre.The green tyre is sent to the mould to be cured after it passes the inspection. The green tyre slides over a bladder that expands as the hot mould closes around the tyre. This bladder forces the tread surface outward into the mould. The tread pattern and identification data for the tyre are cut into the mould surface. The rubber composition determines the mould temperature, which is about 433 K, and time needed for curing is around twenty minutes. The tyre is trimmed and inspected when it comes from the mould.23.3.8.
Tyre Sizes and Designations
At present upto four pieces of information are provided through tyre marking. These include, (i) construction type, (ii) speed rating,(Hi) size, and (iv) case profile.
Construction Type.
This is indicated by a letter code and relates to the ply or ply-and-brace construction. Radial-ply tyres use the letter R and bias-belted the letter B, while cross-ply tyres carry no letter identifying construction.Speed Rating. This relates to the maximum speed at which a vehicle should be driven, that is the maximum speed of which the vehicle is capable, not the speed at which it is normally driven. Speed rating is indicated by a code letter (refer Table 23.1).
Table 23.1. Speed Marking of Tyres.
| Cross-ply: | |||
| Rim diameter | Normal (unmarked) | S | H |
| 10 degrees | Upto 120 km/h | Upto 150 km/h | Over 150 km/h |
| 12 degrees | Upto 135 km/h | Upto 160 km/h | Over 160 km/h |
| 13 degrees and above | Upto 150 km/h | Upto 175 km/h | Over 175 km/h |
| Radial-ply; | |||
| Rim diameter | SR | HR | VR |
| All sizes | Upto 180 km/h | Upto 210 km/h | Over 210 km/h |
Size.
This relates to the tyre nominal section width and wheel-rim diameter (Fig. 23.1A), which are indicated by the first and second numbers respectively. The section of radial-ply and bias-belted tyres is always quoted in millimeters, and of cross-ply tyres in inches. The rim diameter is in inches for all types of tyre. For example, “5.40-12″ indicates a nominal section width of approximately 5.4 inches and a rim diameter of 12 inches. Likewise, “155 SR-14″ indicates a nominal section width of 155 mm and a rim diameter of 14 inches.
Casing Profile.
This relates to the ratio of the section height of a casing to the section width, the measurement is taken from bead to crown (Fig. 23.1A). The ratio of these dimensions is normally 80:100, and a tyre of this form is said to have an 80% aspect ratio. Low-profile tyres with 70% and 60% aspect ratios are also available with radial-ply tyres. If there is a deviation from the normal 80%, then a further number may be added to the sidewall marking. For example 175/70 HR-13 means 175 HR-13, with a 70% aspect ratio.
