GB2050964A - Radial cord carcass tyres for lorries, building-site vehicles or the like. - Google Patents

Abstract

A radial cord carcass tyre has blocks arranged in two rows which are separated from one another axially of the tyre by a zig-zag-shaped groove, the rows and the groove extending circumferentially of the tyre. Each block extends transversely of the tyre and is separated from its neighbours in the respective row by transverse slots. The groove is less deep than the slots. Each block is formed from three conjoined leg portions 5, 6, 7 so arranged relatively to one another as to make a zig-zag. The leg portions of the zig-zag make included angles with the mid- circumferential plane 8 which increase as one moves from said plane towards the respective tyre shoulder. The included angles preferably fall into the ranges from 35 DEG to 45 DEG , from 55 DEG to 65 DEG and from 70 DEG to 80 DEG , respectively. The groove depth is at least three- quarters of the slot depth. <IMAGE>

Description

SPECIFICATION Radial cord carcass tyres for lorries, building-site vehicles or the like This invention concerns a radial cord carcass tyre for lorries, building-site vehicles or the like.

Such vehicles are often fitted with radial cord carcass tyres each having two circumferentially extending rows of lateral blocks separated by a zig-zag-shaped groove running around the tyre circumference; the slots separating the blocks must open into the edge of the tread.

Radial cord carcass tyres (hereafter in this description called radial tyres for brevity) for lorries, which tyres possess the above characteristics, are already known. When designing a tread pattern for such a tyre, a number of different criteria must be taken into account which can only give an indication of the quality of the tyre when they all act in combination.

Of particular significance are traction, the ability to cover (lateral traction), general road behaviour, power transfer characteristics, abrasion and wear properties. In many areas, improving one of the characertistcs leads to a deterioration of one or more of the other characteristics. In order to improve the road behaviour of a lorry radial tyre of the above type without impairing its traction, a well-known brand of tyre of the above type was used of which the groove which extended around the circumference and which separated the two rows of lateral blocks was only about one-tenth of the depth of the slots separating the individual blocks from one another.This enabled the mobility of the blocks to be reduced, thus improving the road holding, butthe improvement was only achieved at the expense of the traction and drainage characteristics of the tyre because the axially central parts of the blocks of said two rows became worn away after a low mileage, with the result that the circumferential groove separating said two rows of blocks could no longer perform its function.

Other lorry radial tyres are known where the Iteral blocks are angled in zig-zag fashion, whereby the constituent leg portions of the respective blocks formed by the zig-zag layout are at approximately the same angle to the mid-circumferential plane of the tyre; tyres of this type have satisfactory wear properties, but their traction and lateral guiding characteristics are unsatisfactory. This shortcoming is due to the fact that, with radial tyres, unlike cross-ply tyres, the pressure of the tyre on the road in normal circumstances tends to be greater in the region of the shoulder than in the area of said mid-circumferential plane. The interacting forces between the contact surface of the tyre and the ground are thus also greater in the shoulder region than in the region of said plane.This means that when the leg portions ofthe respective lateral blocks are at one and the same angle to said plane, the power transfer level in the region of said plane is necessarily lower than in the region of the shoulder.

The nearer to 90 is the angle between the leg portions of the respective lateral blocks and the m;rl~riri Imnrfin+ial nlnnn thp nrnatnr thA negative effect of this fact on the cornering behaviour of a radial tyre of this type. The cornering behaviour, which is already adversely affected at this angle, also deteriorates further in the region of said plane in relation to the shoulder region due to the reduced road pressure.

On the other hand, it is not possible to arrange all the edges of the laterial blocks art a small included angle to the mid-circumferential plane of the tyre over the whole width of the tread because traction would suffer accordingly, traction properties being improved when there are edges of leg portions of blocks arranged so as to extend axially of the tyre.

However, since a tyre with insufficient lateral guiding or cornering capacity is also unable to make use of its traction characteristics because it slides or tends to slide away to the side, the conventional solution of keeping the same angle between the leg portions of a zig-zag-shaped block and the midcircumferential plane of the tyre over the whole axial width of the tread is not in keeping with the practical properties of a radial tyre.

The conditions described above are completely the reverse in the case of a cross-ply tyre because, with this type of tyre, the pressure exerted on the road is generally higher in the region of the midcircumferential plane than in the shoulder region.

The principal object of the present invention is, therefore, to create a radial tyre for lorries or the like which represents the best possible compromise with regard to all of the requirements placed upon it, bearing in mind that the working life of a tyre of this kind must be very long and that the overall characeristics of the tyre must be retained substantially unimpaired throughout the whole of this working life.

Accordingly, the present invention consists in a radial cord carcass tyre (as defined above) having a tread comprising two circumferentially extending rows of blocks, said rows being separated from one another by a zig-zag-shaped groove extending around the circumference of the tyre, all of said blocks extending transversely of the tyre and being separated from one another by respective transversely extending slots which open into the edge of the tread; each block comprising conjoined leg portions so angled relatively to one another in zig-zag fashion that the included angles between the respective leg portion on the one hand and the mid-circumferential plane of the tyre on the other hand become progressively greater as one moves from said plane towards the respective shoulder of the tyre, the axially innermost leg portion of each block in one row being in alignment axially of the tyre with the axially innermost leg portion of a respective block in the other row, and said groove having a depth which is less than that of said slots and which is equal to at least three quarter of the depth of said slots.Each block may consist of three conjoined leg portions, the included angles between the mid-circumferential plane on the one hand and the axially innermost, middle and axially outermost leg portions on the other hand being a, 3 andy respectively, and said included angles falling into the following rangers, namely a = from 35'to 45' ss = from 55'to 65' y = from 70'to 80' The present invention will now be more particularly described with reference to the accompanying drawings, in which: Figures 1 and 2 are partial front views of a radial tyre in accordance with the present invention; and Figure 3 shows a schematic cross-section of said tyre taken on the line A - A' in Figure 1.

Referring firstly to Figure 1, a radial cord carcass tyre 1 has two rows of lateral blocks 2, 2', 3, 3', said rows being arranged parallel to each other and extending around the circumference of the tyre. The two rows of blocks are separated from one another by a circumferentially extending zig-zag shaped groove 4 placed approximately in a central position axially of the tyre. The blocks 2, 2', 3, 3' are angled in zig-zag fashion, whereby the respective angels a, ss, y formed between the constituent leg portions 5, 5', 6, 6', 7, 7' of said blocks and the mid-circumferential plane 8 of the tyre increases as one moves from said plane towards the shoulder i.e. a < ss < y.

The tread pattern shown in Figure 2 is provided by a radial cord carcass tyre 1 similar to that illustrated in Figure 1 with the addition of blind slots or recesses 9, 9' located, in each case, approximately at the junction or in the region of the junction of the constituent leg portions 5, 5', 6, 6' of the blocks in each row. Moreover, the axially outer ends of the blocks 2, 2', 3, 3' end alternately at different distances from the mid-circumferential plane 8, this giving rise to bevelled end surfaces 10, 11 which begin at approximately the same distance (measured radially of the tyre) from the respective bead.

The angles a, P and y fall within the following ranges: a = from 35'to 45' ss = from 55' to 65' y = from 70' to 80' Figure 3 shows that the base of the groove 4 is raised by about 25% of the depth of the transversely extending slots which separate the adjacent blocks in any row from one another; that is to say, the diameter of the bottom of the groove 4 is greater than that of any part of the bottom of any of said slots. This brings about a stabilisation in this region which helps to keep wear to a minimum. The base of the groove is, however, raised to such a small degree that the drainage properties are only adversely affected when the tyre has almost reached the end of its service life with regard to the depth of the tread.

On the basis of work carried out in connection with the present invention, it has been found that the principal object of the invention can be achieved completely or at least to a significant extent by angling the constituent leg portions of each block in each circumferential row in a zig-zag fashion in the manner described above. The leg portions of the blocks in one row facing or directed towards the mid-circumferential plane 8 are in alignment with the corresponding leg portions of the blocks in the other row. The groove around the circumference must have a depth, radially of the tyre, equal to at least three-quarters of the depth of the slots separating the blocks in any row from one another and, if necessary, the axially outer ends of the blocks can end alternately at different distances from the plane 8.This combination of features ensures that a tyre according to the invention has excellent lateral guiding and traction properties when used on a building site and is thus particularly suitable for use on the drive axles of building-site vehicles. Particularly significant is the fact that the good power transfer properties which can be achieved using this combination of characteristics are not attained at the expense of wear behaviour; on the contrary, the wear behaviour is extremely favourable due to the more even loading over the whole axial width of the tread.

This increase in the power transfer efficiency is due to the fact that the angle between the leg portion of the block and the mid-circumferential plane 8 is smaller in the region of said plane where the road pressure is often lower, which has an extremely favourable effect on the lateral guiding properties.

This compensates for the fact that the lateral guiding properties in the region of said plane 8 are necessarily inferior to those in the region of the shoulder due to the lower road pressure.

Although the tread pattern design resulting from the present invention leads to a somewhat reduced traction capacity in the region of the plane 8, this is of no significance; in fact, the traction capacity can be controlled due to the angle of each of the leg portions of the blocks, which angle approaches the axial direction towards the shoulder of the tyre, so that the traction is perfectly adequate to meet the practical requirements. It must be emphasised that the driving characteristics of a tyre used in off-road situations must principally meet the requirement that the vehicle does not slide away to the side, even on extreme up or down gradients or when driving across a slope.If the vehicle begins to slide away to one side, even the best straight-line traction properties are of no avail, since they no longer have any effect on the vehicle and, in this context, the present invention represents an excellent compromise.

The excellent lateral guiding and traction properties in the middle region are further enhanced by the fact that the leg portions of the blocks facing the mid-circumferential plane 8 are arranged in align ment with the blocks on the opposite side. This results to a certain degree in an extension of the edges, so that once the edge of a leg portion of a block has gripped or become dug into the ground, this is continued as the tyre rolls along; this is particularly advantageous in the case of steep ramps, since it means that the high lateral guilding force is provided over a relatively wide tread area.

The lateral thrust resistance is also enhanced by the fact that, with the design based on this invention, relatively compact imprinted ribs are created in the ground, providing a certain degree of additional support for the blocks.

Since muddy ground conditions are often encoun tered on building sites, the self-cleaning effect of the tread pattern exemplified in Figures 1 and 2 of the drawings is also of considerable importance. This characteristic is also enhanced by the fact that the angle between the leg portions of the blocks and the plane 8 becomes progressively greater as one moves away from said plane towards the shoulder of the tyre.

In order further to improve traction in the region of the tyre shoulder, the blocks can end alternately at different distances from said plane 8. This feature, which is actually well known on tyres of this type, creates in effect a toothed edge to the tread, thus increasing the grip of the tyre.

In order to increase the lengths of the edges of the blocks in the axially central region of the tread, which edges are particularly important for lateral guiding properties, it is advantageous if the blocks in the two rows have blind slots or recesses as extensions of the respctive segments of the zig-zagshaped groove around the circumference of the tyre, which blind slots or recesses slant in the same direction. This results in a system of parallel block edges which are inclined in the same direction as the leg of the block facing the mid-circumferential plane, their length being equal to one third of the axial width of the tread; this system of edges enhances the lateral guiding properties and thus the power transfer capacity.

In order to improve the self-cleaning properties of the grooves opening into the edge of the tread, the axially outer ends of the blocks can have bevelled surfaces alternately inclined at different angles as clearly illustrated in Figure 2, all the surfaces beginning at the same distance, radially of the tyre, from the bead. In this way, every other block end projects out in front of the intervening block ends to a certain degree, thus forming a contact surface which helps to force any dirt out of the transversely extending slots.

As employed herein, the terms "radial tyre" and "radial cord carcass tyre" mean a pneumatic tyre with a carcass consisting of at least one ply or layer of rubberized cord fabric extending from bead to bead, in which the textile or steel cords of said fabric run parallel to each other and are laid in radial planes which contain the rotational axis of the tyre or which form small angles (not more than 10 ) with such planes. Apart from this, radial tyres based on this invention also generally have a reinforcing belt or breaker structure (not shown in Figure 3) which can be composed of several layers of rubberized cord fabric. The word "textile" used above has the meaning conventionally attributed to it in the tyre industry (namely, non-metallic) and comprises, by way of example, organic materials such for example as cotton and rayon, and fibres of synthetic polymers such for example as polyamide fibres and polyester fibres.

Claims (7)

1. A radial cord carcass tyre (as defined above) having a tread comprising two circumferentially extending rows of blocks, said rows being separated from one another by a zig-zag-shaped groove extending around the circumference of the tyre, all of said blocks extending transversely of the tyre and being separated from one another by respective transversely extending slots which open into the edge of the tread; each block comprising conjoined leg portions so angled relatively to one another in zig-zag fashion that the included angles between the respective leg portion on the one hand and the mid-circumferential plane of the tyre on the other hand becomes progressively greater as one moves from said plane towards the respective shoulder of the tyre, the axially innermost leg portion of each block in one row being in alignment axially of the tyre with the axially innermost leg portion of a respective block in the other row, and said groove having a depth which is less than that of said slots and which is equal to at least three quarters of the depth of said slots.

2. A tyre as claimed in Claim 1, wherein each block consists of three conjoined leg pOrtions, and wherein the included angles between the midcircumferential plane on the one hand and the axially innermost, middle and axially outermost leg portions on the other hand are a, ss and respectively, and wherein said included angles fall into the following ranges, namely a = from 35'to 45' ss = from 55' to 65' y = from 70'to 80'

3. A tyre as claimed in Claim 1 or Claim 2, wherein each block is formed with a blind slot or recess which is an extension of a segment of the zig-zag-shaped groove, which blind slot or recess slants in the same direction as said segment.

4. A tyre as claimed in any one of the preceding Claims, wherein the axially outer ends of the blocks have bevelled surfaces of which alternate ones are inclined at a different angle from that at which the others are inclined, all of the bevelled surfaces beginning at the same distance, radially of the tyre, from the respective bead.

5. A tyre as claimed in any one of the preceding Claims, wherein the axially outer ends of alternate ones of said blocks are at a distance from the mid-circumferential plane of the tyre which is different from that of the axially outer ends of the others of said blocks from said plane.

6. A radial cord carcass tyre (as defined above) constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 3 or Figure 2 of the accompanying drawings.

7. Any features of novelty, taken singly or in combination of the embodiments of the invention hereinbefore described with reference to the accompanying drawings.