833 Tread bite

Bite is obtained by selecting a pattern which divides the tread into many separate elements and providing each element with a reasonably sharp well defined edge. Thus as the wheel rotates these tread edges engage with the ground to provide a degree of mechanical tyre to ground interlock in addition to the frictional forces generated when transmitting tractive or braking forces.

The major features controlling the effectiveness of the tread pattern in wet weather are:

1 drainage grooves or channels,

2 load carrying ribs,

3 load bearing blocks,

4 multiple microslits or sipes.

The removal of water films from the tyre to ground interface is greatly facilitated by having a number of circumferential grooves spaced out across the tread width (Fig. 8.22(a)). These grooves enable the leading elements of the tread to push water through the enclosed channels made by the road sealing the underside of the grooves. Water therefore emerges from the trailing side of the contact patch in the form of jets. If these grooves are to be effective, their total cross-sectional area should be adequate to channel all the water immediately ahead of the leading edge of the contact patch away. If it cannot cope the water will become trapped between the tread ribs or blocks so that these elements lift and become separated from the ground, thus reducing the effective area of the contact patch and the tyre's ability to grip the ground.

To speed up the water removal process under the contact patch, lateral grooves may be used to join together the individual circumferential grooves and to provide a direct side exit for the outer circumferential grooves. Normally many grooves are preferred to a few large ones as this provides a better drainage distribution across the tread.

Tread ribs (Fig. 8.22(a and b)) Circumferential ribs not only provide a supportive wearing surface for the tyre but also become the walls for the drainage grooves (Fig. 8.22(a and b)). Lateral (transverse) ribs or bars provide the optimum bite for tractive and braking forces but circumferential ribs are most effective in controlling cornering and steering stability. To satisfy both longitudinal and lateral directional requirements which may be acting concurrently on the tyre, ribs may be arranged diagonally or in the form of zig-zag circumferential ribs to improve the wiping effect across the tread surface under wet conditions. It is generally better to break the tread pattern into many narrow ribs than a few wide ones, as this prevents the formation of hydrodynamic water wedges which may otherwise tend to develop

Fig. 8.22 (a-d) Basic tyre tread patterns

with the consequent separation of the tread elements from the road.

Tread blocks (Figs 8.22(c and d) and 8.23(a and b)) If longitudinal circumferential grooves in the middle of the tread are complemented by lateral (transverse) grooves channelled to the tread shoulders, then with some tread designs the drainage of water can be more effective at speed. The consequences of both longitudinal and lateral drainage channels is that the grooves encircle portions of the tread so that they become isolated island blocks (Fig. 8.22 (c and d)). These blocks can be put to good use as they provide a sharp wiping and biting edge where the interface of the tread and ground meet. To improve their biting effectiveness for tractive and braking forces as well as steering and cornering forces, these forces may be resolved into diagonal resultants so that the blocks are sometimes arranged in an oblique formation. A limitation to the block pattern concept is caused by inadequate support around the blocks so that under severe operating conditions, the bulky rubber blocks tend to bend and distort. This can be partially overcome by incorporating miniature buttresses between the drainage grooves which lean between blocks so that adjacent blocks support each other. At the same time, drainage channels which burrow below the high mounted buttresses are prevented from closing. Tread blocks in the form of bars, if arranged in a herringbone fashion, have proved to be effective on rugged ground. Square or rhombus-shaped blocks provide a tank track unrolling action greatly reducing movement in the tread contact area. This pattern helps to avoid the break-up on the top layer of sand or soil and thus prevents the tyre from digging into the ground. Because of the inherent tendency of the individual blocks to bend somewhat when they are subjected to ground reaction forces, they suffer from toe to heel rolling action which causes blunting of the leading edge and trailing edge feathering. Generally tyres which develop this type of wear provide a very good water sweeping action when new, which permits the tread elements to bite effectively into the ground, but after the tyre has been on the road for a while, the blunted leading edge allows water to enter underneath the tread elements. Consequently the slightest amount of water interaction between the block elements and ground reduces the ability for the tread to bite and in the extreme cases under locked wheel braking conditions a hydrodynamic water wedge action may result, causing a mild form of aquaplaning to take place.

For tread block elements to maintain their wiping action on wet surfaces, wear should be from toe to heel (Fig. 8.23(a)). If, however, wear occurs in the reverse order, that is from heel to toe (Fig. 8.23(b)), the effectiveness of the tread pattern will be severely reduced since the tread blocks then become the platform for a hydrodynamic water wedge which at speed tries to lift the tread blocks off the ground.

8.24(a, b and c)) Microslits, or sipes as they are commonly called, are incisions made at the surface of the tyre tread, going down to the full depth of the tread grooves. They resemble a knife cut, except that instead of being straight they are mostly of a zig-zag nature (Fig. 8.22(a, b, c and d)). Normally these sipes terminate within the tread elements, but sometimes one end is permitted to intersect the side wall of a drainage groove. In some tread patterns the sipes are all set at a similar angle to each other, the zig-zag shape providing a large number of edges which point in various directions. Other designs have sets of sipes formed at different angles to each other so that these sipes are effective whichever way the wheel points and whatever the direction the ground reaction forces operate.

Sipes or slits in their free state are almost closed, but as they move into the contact patch zone the ribs or blocks distort and open up (Fig. 8.24(a)). Because of this, the sipe lips scoop up small quantities of water which still exist underneath the tread. This wiping action enables some biting edge reaction with the ground. Generally, the smaller the sipes are and more numerous they are the greater will be their effective contribution to road grip. The

Wheel locked -m-Direction of sliding

Wheel locked -m-Direction of sliding

[al Toe to heel tread wear

—Edge sweeps water

[al Toe to heel tread wear

Wheel locked —-Direction of sliding

—Edge sweeps water

Wheel locked —-Direction of sliding

(t)J Heel to toe tread wear

Fig. 8.23 (a and b) Effect of irregular tread block wear

(t)J Heel to toe tread wear

Fig. 8.23 (a and b) Effect of irregular tread block wear

Fig. 8.24(a-c) Effectiveness of microslits on wet road surfaces

normal spacing of sipes (microslits) on a tyre tread makes them ineffective on a pebbled road surface because there will be several pebbles between the pitch of the sipes (Fig. 8.24(b)), and water will lie between these rounded stones, therefore only a few of the stones will be subjected to the wiping edge action of the opened lips. An alternative method to improve the wiping process would be to have many more wiping slits (Fig. 8.24(c)), but this is very difficult to implement with the present manufacturing techniques. The advantages to be gained by multislits are greatest under conditions of low friction associated with thin water films on smooth and polished road surfaces. This is because the road surface asperities are not large and sharp enough to penetrate the thin water film trapped under plain ribs and blocks.

Selection of tread patterns (Fig. 8.25(a-l))

Normal car tyres (Fig. 8.25(a, b and c)) General duty car tyres which are capable of operating effect ively at all speeds tend to have tread blocks situated in an oblique fashion with a network of surrounding drainage grooves which provide both circumferential and lateral water release.

Winter car tyres (Fig. 8.25(d, e and f)) Winter car tyres are normally very similar to the general duty car tyre but the tread grooves are usually wider to permit easier water dispersion and to provide better exposure of the tread blocks to snow and soft ice without sacrificing too much tread as this would severely reduce the tyre's life.

Truck tyres (Fig. 8.25(g and h)) Truck tyres designed for steered axles usually have circumferential zig-zag ribs and grooves since they provide very good lateral reaction when being steered on curved tracks. Drive axle tyres, on the other hand, are designed with tread blocks with adequate grooving so that optimum traction grip is obtained under both dry and wet conditions. Some of these tyres also have provision for metal studs to be inserted for severe winter hard packed snow and ice conditions.

Fig. 8.25 (a—I) Survey of tyre tread patterns

||) Truck rough ground tyre Fig. 8.25 contd

(W Truck cross-country tyre

11} Tractor cross-country tyre

Offjon road vehicles (Fig. 8.25(i)) Off/on road vehicle tyres usually have a much simpler bold block tread with a relatively large surrounding groove. This enables each individual block to react independently with the ground and in this manner bite and exert traction on soil which may be hard on the surface but soft underneath without break-up of the top layer, thus preventing the tyre digging in. The tread pattern blocks are also designed to be small enough to operate on hard surfaced roads at moderate speeds without excessive ride harshness.

Truck and tractor off road and cross-country tyres (Fig. 8.25(j, k and l)) Truck or tractor tyres designed for building sites or quarries generally have slightly curved rectangular blocks separated with wide grooves to provide a strong flexible casing and at the same time present a deliberately penetrating grip. Cross-country tyres which tend to operate on soft soil tend to prefer diagonal bars either merging into a common central rib or arranged with separate overlapping diagonal bars, as this configuration tends to provide exceptionally good traction on muddy soil, snow and soft ice.

8.3.4 The three zone concept of tyre to ground contact on a wet surface (Fig. 8.26) The interaction of a tyre with the ground when rolling on a wet surface may be considered in three phases (Fig. 8.26):

Leading zone of unbroken water film (1) The leading zone of the tread contacts the stagnant water film covering the road surface and displaces the majority of the water into the grooves between the ribs and blocks of the tread pattern.

Intermediate region of partial breakdown of water film (2) The middle zone of the tread traps and reduces the thickness of the remaining water

Contact patch length

Contact patch length

Fig. 8.26 Tyre to ground zones of interaction between the faces of the ribs or blocks and ground so that some of the road surface asperities now penetrate through the film of water and may actually touch the tread. It is this region which is responsible for the final removal of water and is greatly assisted by multiple sipes and grooved drainage channels. If the ribs and blocks are insufficiently relieved with sipes and grooves it is possible that under certain conditions aquaplaning may occur in this region.

The effectiveness of this phase is determined to some extent by the texture of the road surface, as this considerably influences the dryness and potency of the third road grip phase.

Trailing zone of dry tyre to road contact (3) The water film has more or less been completely squeezed out at the beginning of this region so that the faces of the ribs and blocks bearing down on the ground are able to generate the bite which produces the tractive, braking and cornering reaction forces.

8.3.5 Aquaplaning (hydroplaning) (Fig. 8.27) The performance of a tyre rolling on wet or semi-flooded surface will depend to some degree upon the tyre profile tread pattern and wear. If a smooth tread is braked over a very wet surface, the forward rotation of the tyre will drag in the water immediately in front between the tread face and ground and squeeze it so that a hydrodynamic pressure is created. This hydrodynamic pressure acts between the tyre and ground, its magnitude being proportional to the square of the wheel speed. With the wheel in motion, the water will form a converging wedge between the tread face and ground and so exert an upthrust on the underside of the tread. As a result of the pressure generated, the tyre tread will tend to separate itself from the ground. This condition is known as aquaplaning or hydroplaning. If the wheel speed is low only the front region of the tread rides on the wedge of water, but if the speed is rising the water wedge will progressively extend backward well into the contact patch area (Fig. 8.27). Eventually the upthrust created by the product of the hydrodynamic pressure and contact area equals the vertical wheel load. At this point the tyre is completely supported by a cushion of water and therefore provides no traction or directional control. If the tread has circumferential (longitudinal) and transverse (lateral) grooves of adequate depth then the water will drain through these passages at ground level so that aquaplaning is minimized even at high speeds. As the tyre tread wears the critical speed at which aquaplaning occurs becomes much lower. On very wet roads a bald tyre is certain to be subjected to aquaplaning at speeds above 60 km/h and therefore the vehicle when driven has no directional stability. Low aspect ratio tyres may find it difficult to channel the water away from the centre of the tread at a sufficiently high

Fig. 8.27 Tyre aquaplaning
Fig. 8.28 Tyre profiles with different aspect ratios

rate and therefore must rely more on the circumferential grooves than on transverse grooving.

8.3.6 Tyre profile and aspect ratio (Fig. 8.28) The profile of a tyre carcass considerably influences its rolling and handling behaviour. Because of the importance of the tyre's cross-sectional configuration in predicting its suitability and performance for various applications, the aspect ratio was introduced. This constant for a particular 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. 8.28).

Section height i.e. Aspect ratio = ——:-—— x 100

Section width

A tyre with a large aspect ratio is referred to as a high aspect ratio profile tyre and a tyre with a small aspect is known as a low aspect ratio profile. Until about 1934 aspect ratios of 100% were used, but with the better understanding of pneumatic tyre properties and improvement in tyre construction lower aspect ratio tyres became available. The availability of lower aspect ratio tyres over the years was as follows; 1950s-95%, 1962-88% (this was the standard for many years), 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.

Lowering the aspect ratio has the following effects:

1 The tyre side wall height is reduced which increases the vertical and lateral stiffness of the tyre.

2 A shorter and wider contact patch is established. The overall effect is to raise the load carrying capacity of the tyre.

3 The wider contact patch enables larger cornering forces to be generated so that vehicles are able to travel faster on bends.

4 The shorter and wider contact patch decreases the pneumatic trail which correspondingly reduces and makes more consistent the self-aligning torque.

5 The shorter and broader contact patch will, under certain driving conditions, reduce the slip angles generated by the tyre when subjected to side forces. Accordingly this reduces the tread distortion and as a result scuffing and wear will decrease.

6 With an increase in vertical stiffness and a reduction in tyre deflection with lower aspect ratio tyres, less energy will be dissipated by the tyre casing so that rolling resistance will be reduced. This also results in the tyre being able to run continuously at high speeds at lower temperatures which tends to prolong the tyre's life.

7 The increased lateral stiffness of a low profile tyre will increase the sensitivity to camber variations and quicken the response to steering changes.

8 Wider tyre contact patches make it more difficult for water drainage at speed particularly in the mid tread region. Hence the tread pattern design with low profile tyres becomes more critical on wet roads, if their holding is to match that of higher aspect ratio tyres.

9 The increased vertical stiffness of the tyre reduces static deflection of the tyre under load, so that more road vibrations are transmitted through the tyre. This makes it a harsher ride so that ride comfort is reduced unless the suspension design has been able to provide more isolation for the body.

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