GB2038729A

GB2038729A - Tyre tread

Info

Publication number: GB2038729A
Application number: GB7942630A
Authority: GB
Original assignee: Industrie Pirelli SpA; Pirelli SpA
Current assignee: Industrie Pirelli SpA; Pirelli and C SpA
Priority date: 1978-12-22
Filing date: 1979-12-11
Publication date: 1980-07-30
Also published as: LU82033A1; AR222058A1; FI793910A; IT7831200A0; JPS5594802A; NL7906721A; AU5407279A; SE7910490L; ES256594Y; IT1101233B; GR73135B; DE2951444A1; BR7908495A; ZA796905B; ES256594U; DK544679A; BE880625A; FR2444576A1

Abstract

A tyre tread comprises an elongate strip having a respective longitudinal groove (2) in each lateral portion of the strip constructed and arranged to partially close when the tread is subjected to tangential forces, e.g. during cornering, stiffening the tread in the contact area to reduce excessive movement of the tread in the contact area whilst maintaining the groove open at the tread surface. Partial closing is achieved by forming the groove as two tracts a first tract, which remains open during cornering, extending from the tread surface and a second tract, which closes during cornering, separated from the first tract by a sudden variation in groove width produced, for example, by a transverse step formed in one of the groove flanks or a protuberance projecting from the base of the groove. <IMAGE>

Description

SPECIFICATION Tyre tread This invention concerns improvements in or relating to tyre treads and in particular, though not exclusively, to tyre treads for gianttyres used for heavy duty automative transport that travels on roads at high speeds.

It is already known to provide tyre treads with a pattern consisting of a series of circumferential grooves, usually of zig-zag profile disposed across the tread width in such a way as to subdivide the tread into circumferential ribs. The ribs may be further subdivided into blocks by transversal grooves extending between adjacent circumferential grooves. There are generally also provided in the ribs and/or blocks slots or narrow slits having a depth more or less equal to the groove depth and which may extend into the groove flanks and also at the peaks of the zig-zag circumferential grooves.

The main aim of this type of tread pattern is to rupture the liquid film which forms, for example when it rains, between the contact area of the tyre tread and the road surface, to allow removal of the liquid which drains away through the grooves provided in the tread. In this manner the occurrence of the phenomenon known as aquaplaning is prevented and the tyre has improved road grip in wet conditions.

However, the road holding characteristics of the tyre are reduced by the provision of grooves which weaken the tread by allowing considerable movement of the ribs and/or blocks in the contact area so that the tyre and hence the vehicle tends to move transversely as regards the direction of movement.

Such movement occurs without any indication to the driver via the steering gear. Furthermore such movement results in greater and irregular wear of thetyre.

These problems are more pronounced with giant tyres used for supporting large loads where the forces have enormous values and are particularly pronounced when the vehicle takes a bend in the road.

Under such conditions the composition of the weight of the vehicle and the centrifugal force that acts upon it gives rise to a tangential force acting parallel to the tyre axis which is counteracted by a tangential friction force system localised in the lateral portion of the tyre tread which is outermost with respect to the arc of the bend. Since the force acts on a surface area smaller than that of the contact area which engages the road during movement of the vehicle in a straight line wear in the lateral portion of the tread is increased.

Furthermore a circumferential groove provided in the lateral portion tends to close under such force. If the groove width is such (even with regard to the depth of the groove) that the groove is effectively closed under this thrust then although the corresponding tread zone stiffens with a resulting increase in the stability and the steerage precision of the tyre a serious decrease occurs in the grip of the tyre on wet roads because the action of rupturing the liquid film between the tread contact area and the road surface and removal of the liquid is lost. If on the other hand the groove width is such that the groove remains open under this thrust then although aquaplaning is prevented a loss of grip occurs when cornering in wet or dry conditions due to excessive movement of the ribs and/or blocks.

It is an object of the present invention to provide a tyretread having a tread pattern which mitigates at least some of the above-mentioned problems.

According to the present invention we provide a tyre tread comprising an elongate strip having a pattern formed in that surface which is intended to contact the ground in use of a tyre incorporating the tyre tread wherein the pattern includes a respective longitudinal groove in each lateral portion of the strip and each lateral groove is constructed and arranged to define in transverse cross-section a first tract extending from the tread surface and a second tract separated from the first tract by a sudden variation in transverse groove width such that the minimum width of the first tract exceeds the maximum width of the second tract and the tract widths are such that in use of a tyre incorporating the tread the second tract closes in the contact area during cornering under normal running conditions and the first tract remains open.

The transverse width of the first and/or second tracts may be variable as considered in the direction of groove depth or else may be constant. The groove may be symmetrical with respect to a single median plane or one or both tracts may have its own plane of equatorial symmetry or there may be no plane of symmetry at all.

The sudden variation in axial width may be produced by a transverse step in one or both of the groove flanks which may have no depth as considered in the direction of groove depth. Where the step has a depth this should not exceed one third of the overall groove depth. Alternatively the sudden variation in axial width may be produced by a protuberance projecting from the base of the groove which subdivides the second tract into two tracts.

Concerning the geometric and dimensional characteristics of the groove it is preferred that the ratio between the maximum transverse width and maximum depth of the groove is comprised between 0.2 and 1.0 while the ratio between the maximum transverse widths of the first and second tracts is comprised between 2 and 10. Moreover the ratio between the depths of the first and second tracts should be comprised between 0.3 and 0.7. Finally the distance between the equatorial plane of the tyre tread and the meridian plane of a longitudinal band containing the lateral groove should be comprised between 45% and 85% of the distance between the equatorial plane of the tyre tread and the lateral edge of the tyre tread.

The tyre tread according to the present invention may be formed as a separate component suitable for use in building or retreading a tyre or it may be formed integrally with a tyre.

Thus according to a further aspect of the present invention we provide a tyre having a tyre tread according to the present invention.

The tyre may be either radial or cross-ply.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings wherein: Figure 1 is a plan view of a portion of a tyre tread according to the present invention; Figure 2 is a transverse cross-section of the tread shown in Figure 1; Figures 3, 4 and 5 each show in transverse cross-section three different configurations of lateral groove for the tread according to the present invention, and Figures 6, 7 and 8 are sections on the lines Vl-VI, VII-VII and VIII-VIII respectively of Figure 1.

The tyre tread shown in Figures 1 and 2 of the accompanying drawings comprises an elongate strip (part only shown) having an overall transverse width L. A pattern is formed in that surface of the tread which in use provides the ground contacting surface of a tyre incorporating the tread.

The pattern comprises two wide longitudinal grooves 1 of zig-zag profile disposed in a central portion of the tread and two narrower longitudinal grooves 2 also of zig-zag profile disposed one either side of the central portion in lateral portions of the tread. The grooves 1,2 delimit a wide central longitudinal rib 3, two intermediate longitudinal ribs 4 and two lateral longitudinal ribs 5 which are the so-called shoulder ribs.

Recesses 7 are provided at the peaks of the grooves 1 and slots 6 having the profile shown extend from the recesses 7 into the central ribs 3.

The existence of the slots 6 assists in rupturing a liquid film between the tread contact area and road surface and directing the liquid to the wide grooves 1 for drainage.

Each groove 2 is positioned within a longitudinal band 10 of the tread defined by longitudinal planes r and s. Each band 10 has a meridian planep and the planes are separated by a distance 1. The distances L and 1 are such that the ratio between the transverse width from the meridian plane m of the tread and the meridian planep of a band 10 equivalent to 1/2 and the transverse width from the meridian plane m of the tread to the edge of the tread equivalent to L12 is comprised between 0.45 and 0.85.

The construction of the grooves 2 is more clearly shown in Figures 6,7 and 8 from which it will be seen that each groove 2 comprises a first tract extending from the tread surface and a second tract separated from the first tract by a transverse step formed in one of the groove flanks and projecting towards the other groove flank to produce a sudden variation in groove width. The groove flanks defining the first tract are inclined towards one another so that the maximum width of the first tract is at the tread surface and the minimum width is at the step.

The groove flanks defining the second tract are substantially parallel to one another and the tract has a uniform width substantially less than the minimum width of the first tract.

Cavities 8 are provided in the groove flanks defining the first tract at the groove peaks (Figure 1) and recesses 9 are provided in the stepped groove flank defining the second tract between successive groove peaks (Figure 1) for purposes to be described later.

Referring now to Figures 3,4 and 5 there are shown in transverse cross-section alternative constructions for the grooves 2. In Figure 3 the first and second tracts are separated by a respective transverse step formed in each groove flank and the groove is symmetrical about a plane t. Each step extends between two points 11, 12 in the associated groove flank and has a depth d as considered in the direction of groove depth. As shown each step is inclined at a relatively large angle with respect to the adjacent portions of the associated groove flank defining the tracts to produce a sudden variation in the groove width.

In Figure 4 the first and second tracts are again separated by a respective transverse step formed in each groove flank. In this case each step is similar to that shown in Figure 6 i.e. one having no depth as considered in the direction of groove depth. The groove flanks defining both the first and second tracts are parallel to one another and both tracts have a uniform width with that of the first tract being appreciably greater than that of the second tract. The steps are opposite one another but have different widths so that the median planes m2 and m, of the first and second tracts respectively are offset i.e. the groove has no centre of symmetry.

In Figure 5 in place of a transverse step formed in one or both of the groove flanks the first and second tracts are separated by a protuberance 13 projecting from the base of the groove. The protuberances subdivides the second tract into two tracts, one either side of the protuberance, the combined widths of which are substantially less than the width of the first tract. As shown the depths K1, K2 of the tracts either side of the protuberance 13 as considered in the direction of groove depth are unequal. An advantage of the groove construction shown in Figure 5 is that pebbles, stones or other material entering the groove during use are readily expelled.

Referring to Figure 3 the preferred geometrical and dimensional characteristics of the groove 2 are as follows;- 1) Where the step has an appreciable depth d this should not exceed one third of the overall depth Kof the groove.

2) The ratio between the maximum width a and the maximum depth Hofthe groove should be between 0.2 and 1.

3) The minimum width b of the first tract must be greater than the maximum width c of the second tract.

4) The ratio hik should be between 0.3 and 0.7.

5) The ratio alc should be between 2 and 10.

As aforementioned longitudinal grooves provided in lateral portions of the tyre tread tend to close during cornering as a result of the tangential forces which act on the lateral ribs in the contact area.

So, in the known treads, the ribs flex from the base thereof axially inwards in the contact area towards the adjacent rib causing either the groove therebetween to close with consequent loss of grip in wet conditions or excessive movement of the rib with consequent reduction in grip and increased wear in both wet and dry conditions.

In contrast the groove formed in the lateral portion of the tread according to the present invention has a configuration such that when the lateral rib 5 flexes towards the axially inner rib 4 the groove flanks defining the second, narrower tract contact one another locking the lower portions of the ribs and preventing further deformation of the lower portion of the rib 5. At the same time the first tract remains open enabling the tread to rupture a liquid film between the tread contact area and road surface and to remove the liquid so that aquaplaning is prevented and road grip in wet conditions is maintained.

Furthermore, on closure ofthe second tract not only does the first tract remain open but the effective base of the groove is raised towards the tread surface and the depth of the rib 5 which can flex is substantially reduced, by approximately half in the illustrated embodiments, so that, in practice, the rib 5 acquires considerable rigidity which substantially eliminates further movement of the rib in the contact area and thus the aforementioned problems associated with such movement.

With use the tread wears and the groove depth is reduced until the second tract extends from the tread surface. In this condition the depth of the rib 5 which can still be flexed is similar to that in the new tyre with the second tract closed and the rigidity of the rib 5 is such that in practice the rib is able to resist the tangential forces acting during cornering without flexing and the second tract remains open. The width of the second tract is considerably less than that of the first tract so that although the action of rupturing a liquid film between the tread surfce and removal of the liquid is maintained it is less effective than a new tyre. For this reason the recesses 9 are provided in the groove flank(s) of the second tract to improve the efficiency of the worn tread in rupturing the liquid film and removing the liquid.

The recesses are tapered so that the volume of the recesses 9 decreases towards the base of the groove. The cavities 8 provided in the flanks of the first tract also assist in rupturing a liquid film and removing the liquid.

The invention is not restricted to the abovedescribed embodiments, for example the sudden variation in transverse width can be achieved by a step having a double slope variation in one or both groove flanks. Where a respective step is provided in each groove flank the steps may be opposite one another as described or offset relative to one another as considered in the direction of groove depth. One or both flanks may have more than one step dividing the groove into a series of tracts. In this case not all the preferred ratios abovementioned for a groove having two tracts may be met but it is a general condition that for two successive tracts the minimum axial width of the tract nearer the tread surface should exceed the maximum axial width of the tract further from the tread surface.

In conclusion, the tyre tread according to the present invention possesses to a high degree, the qualities of steering stability in wet or dry conditions, reduced wear and also road-grip in wet conditions.

Furthermore and quite unexpectedly the tread according to the present invention has been found to solve still another problem. Thus it is commonly known to those skilled in the art that, especially in giant tyres, the lateral tread portions show a type of irregular wear and tear, usually defined as 'staggered', which effect, besides conferring an ugly appearance to the tyre proving to be damaging to the qualitative image of the product and resulting in serious commerical consequences, also shortenes the tyre's service life. This type of wear and tear probably arises due to the flexional deformations of the axially outer ribs during the cornering and subsequent to which, not only the axially outer grooves close but probably the radially outer surface of the ribs also assumes a particular orientation with respect to the road surface which gives place to an irregular distribution of the tractional forces acting on the rib.

With the tyre tread according to the present invention we have found this phenomenon is greatly reduced, due perhaps to the increase in the rigidity (specified above) of the lateral ribs which, in maintaining the radially external surface of the rib correctly orientated with respect to the ground, would permit on the surface a uniform distribution of the frictional forces and consequently a uniform and regular wearing of the tyre itself.

Claims

1. A tyre tread comprising an elongate strip having a pattern formed in that surface which is intended to contact the ground in use of a tyre incorporating the tyre tread wherein the pattern includes a respective longitudinal groove in each lateral portion of the strip and each lateral groove is constructed and arranged to define in transverse cross-section a first tract extending from the tread surface and a second tract separated from the first tract by a sudden variation in transverse groove width such that the minimum width of the first tract exceeds the maximum width of the second tract and the tract widths are such that in use of a tyre incorporating the tread the second tract closes in the contact area during cornering under normal running conditions and the first tract remains open.

2. A tyre tread according to claim 1 wherein the ratio between the maximum transverse width of the first tract and the maximum depth of the groove is between 0.2 and 1.

3. A tyre tread according to claim 1 or claim 2 wherein the ratio between the maximum transverse width of the first and second tracts is between 2 and 10.

4. A tyre tread according to any one of the preceding claims wherein the ratio between the depths of the first and second tracts is between 0.3 and 0.7.

5. A tyre tread according to any one of the preceding claims wherein the transverse width of the first and/or second tracts is/are constantthrough- out the entire depth thereof.

6. A tyre tread according to any one of claims 1 to 4 wherein the transverse width of the first andlor second tracts is variable.

7. A tyre tread according to any one of the preceding claims wherein the groove is symmetrical about a median plane.

8. A tyre tread according to any one of claims 1 to 6 wherein the groove is unsymmetrical.

9. A tyre tread according to any one of claims 1 to 6 wherein the first and second tracts are each symmetrical about a respective median plane and said planes are offset relative to one another.

10. A tyre tread according to any one of the preceding claims wherein the sudden variation in transverse groove width is produced by a transverse step formed in one of the groove flanks.

11. A tyre tread according to claim 10 wherein the step has a depth not exceeding one third the overall groove depth.

12. A tyre tread according to claim 10 or claim 11 wherein a respective step is formed in both of the groove flanks.

13. A tyre tread according to any one of claims 1 to 9 wherein the sudden variation in groove width is produced by a protuberance projecting from the groove base to divide the second tract into two tracts one either side of the protuberance.

14. A tyre tread according to claim 13 wherein the tracts either side of the protuberance are of unequal depth.

15. A tyre tread according to any one of the preceding claims wherein each groove is of zig-zag profile in plan view.

16. Atyre tread according to claim 15 wherein a cavity having a depth equal to the depth of the first tract is formed in a groove flank at a groove peak.

17. Atyre tread according to claim 16 wherein a respective cavity is formed at each groove peak.

18. A tyre tread according to any one of claims 15, 16 or 17 wherein a recess having a depth equal to the depth of the second tract is formed in a groove flank between successive groove peaks.

19. A tyre tread according to any one of the preceding claims wherein the ratio between the transverse distance from the equatorial plane of the tread to the median plane of a band containing the groove and the transverse distance from the equatorial plane of the tread to the edge of the tread is comprised between 0.45 and 0.85.

20. A tyre tread according to any one of the preceding claims including at least one longitudinal groove in the central portion of the strip.

21. A tyre tread substantially as hereinbefore described with reference to Figures 1,2,6,7 and 8 of the accompanying drawings.

22. Atyre tread substantially as hereinbefore described with reference to Figures 1,2,6,7 and 8 of the accompanying drawings as modified by any one of Figures 3,4 or 5 of the accompanying drawings.

23. A tyre including a tyre tread according to any one of the preceding claims.