Pneumatic tyre

Abstract: A pneumatic tyre comprises a tread portion provided with a plurality of longitudinal grooves, extending continuously in the tyre circumferential direction and having a bottom (4) and a pair of sidewalls (5) defining corners (6) therebetween, the corners (6) being rounded by a circular arc whose radius is not more than 2 mm, the inclination angles  of the sidewalls (5) being in the range of not more than 15 degrees with respect to the normal direction (N) to the tread surface line, the bottom (4) being provided with a plurality of ribs (10) which extend in the longitudinal direction of the groove substantially parallel with each other so that the bottom has a wavy configuration having no angled corner and the heights (H) of the ribs measured from the deepest point of the longitudinal groove being in the range of from 0.5 to 1.0 mm.

Description: The present invention relates to a pneumatic tyre and in particular to the tread. In general, pneumatic tyre treads are provided with circumferentially extending longitudinal grooves for the purpose of draining water in the ground contact patch. If the groove width is increased, the drainage increases and wet performance improves. In practice, however, due to many restrictions, it is difficult to increase the groove width. On the other hand, to prevent the groove bottom from being cracked, the corners between the groove bottom and sidewalls are rounded and thus grooves have generally a U or V-shapes cross section. If the groove width is increased near the groove bottom and the cross-sectional shape approaches a rectangle, then the cross-sectional area increases and the drainage may be increased. However, stress is liable to concentrate at the corners and the durability greatly deteriorates. In practice, U or V-shaped grooves are usually employed. German Patent DE 42 20 677, corresponding with the preamble of claim 1, discloses a tyre having a wide circumferential groove and a large number of sub-main grooves each slightly inclined to the tyre circumferential direction which have parallel protuberances or ribs at the bottom of the wide grooves having a radius of curvature chosen to control the flow of rubber across the tyre mould when the pattern is formed to reduce corrugation of the belt layer and thus improve tyre uniformity. It is an object of the present invention to provide a pneumatic tyre, in which longitudinal grooves are provided with a generally rectangular cross-sectional shape to improve drainage and wet performance without deteriorating the resistance to cracking. Accordingly the present invention provides a pneumatic tyre as set out in claim 1. Further features of the invention are in the dependent claims. Embodiments of the present invention will now be described in detail, by way of example only, in conjunction with the accompanying drawings in which: Fig.1 is a plan view of a tyre according to the present invention showing an example of the tread pattern; Fig.2 is a cross sectional view of a longitudinal groove showing the basic groove configuration; Fig.3 is an enlarged cross sectional view showing an example of the ribs; Fig. 4 is a cross sectional view of a non-inventive exemple of the ribs; Figs. 5 and 6 are cross sectional views each showing an example of the ribs; and Fig.7 is a cross sectional view showing the ribs of a reference tyre used in a comparison test. In the drawings, the tyre 1 is a pneumatic tyre for passenger cars. The tyre 1 comprises a tread portion 2, a pair of axially spaced bead portions, and a pair of sidewall portions extending between the tread edges and the bead portions. The tyre is reinforced with a bead core disposed in each of the bead portions, at least one carcass ply extending between the bead portions, and belt cord plies disposed radially outside the carcass and inside a rubber tread. As shown in Fig.1, the tread portion 2 is provided with a plurality of longitudinal grooves 3 extending continuously in the circumferential direction and axial grooves 20. The axial grooves 20 extend from an axial position near the tyre equator C to the tread edges E while describing a gentle curve. In this embodiment, the longitudinal grooves 3 are a straight groove, and two such grooves 3 are disposed on each side of the tyre equator C. In a meridian section of the tyre, the sidewalls 5 of each longitudinal groove 3 has an inclination angle  of not more than 15 degrees with respect to a normal line N drawn to the tread surface 2 at the intersecting point of the tread surface line and the groove sidewall line. Accordingly, the cross-sectional shape of the groove 3 is generally rectangular as shown in Fig.2. To allow demoulding of the tyre and resist stone trapping, the inclination angle  is set to be more than 0 degree, but preferably less than about 10 degrees, and the inclining directions are such that the groove width increase radially outwards of the tyre. The longitudinal grooves 3 have a groove width of 5 to 20 mm (in this example about 12 mm) at the tread surface, and a groove depth of 6 to 15 mm (in this example about 8 mm). The corners 6 between the groove sidewalls 5 and the groove bottom 4 are rounded by a circular arc 7 whose radius R1 is not more than 2 mm. If the radius R1 is more than 2 mm, the longitudinal groove 3 decreases in cross sectional area, and the water draining performance decreased. In general, by setting the corner radius R1 in the range of less than 2 mm, stress is liable to concentrate on the corners 6 and cracks are liable to occur, even though the corners 6 are rounded. In the present invention, to avoid this, the groove bottom 4 is formed with a wavy configuration. To form such a wavy configuration, the longitudinal grooves 3 are provided in the groove bottom 4 with a plurality of small ribs 10. The number of ribs 10 in each groove 3 is set in the range of from 2 to 10, more preferably 5 to 8. The small ribs 10 in each groove 3 extend parallel with each other along the length of the groove 3. For best prevention of stress concentration at the corners 6, it is preferable that the ribs 10 extend continuously in the circumferential direction. The ribs 10 have a height H of 0.5 to 1.0 mm measures from the deepest point 9 of the groove bottom 4. The widths W of the ribs 10 are set in the range of from 1.0 to 1.5 mm. If the height H is more than 1.0 mm, heat generation in the tread rubber increases which decreases high speed durability. If the widths W are more than 1.5 mm, stress concentrates at the corners 6 and cracks are liable to occur. If the height H is less than 0.5 mm and/or the widths W are less than 1.0 mm, it becomes difficult to disperse the stress and also difficult to make a mould to produce the tyre tread. If the number of ribs 10 is more than 10, the size of the ribs becomes too small, and it becomes difficult to disperse the stress and also difficult to make a mould. The ribs 10 form alternate ridges 10A and valleys 10B. In the example of Fig.3, the ridges 10A and valleys 10B have semicircular cross sections 12A and 12B. The semicircular cross sections 12A and 12B are of substantially the same single radius. The convex semicircles 12A and the concave semicircles 12B are connected at points V as inflection points. The width W of the rib 10 is defined between the inflection points V in this case. In addition to the semicircle, a sine curve, elliptical curve or the like can be used. In Fig.3, the maximum depths measured on both sides of each rib 10 are the same depth. Therefore, this depth is equal to the depth of the deepest point 9 of the longitudinal groove 3. In other words, all the valleys are the same depth. Fig.4 shows a non-inventive example, wherein the deepest points 9 reside in only the outmost valleys. Fig.5 shows still another example, wherein the deepest points 9 reside in the valleys between the ribs 10. Fig.6 shows another example of the ribs 10. In this example, the ribs 10 collectively form a configuration like a rectangular wave rather than a sine-wave-like configuration in the former examples. The ribs 10 have perpendicular sidewalls extending in the greater part of the height. In this case, the width W of the rib is defined as between the sidewalls (B). With regard to the width of the rib, if not applicable to the above-mention two cases, the width W is defined as the maximum width measured on the radially outer side of one of the deepest points of the two valleys adjacent to the rib which is not deeper than the other. Therefore, the groove bottoms 4 are waved by the small ribs without forming sharp corners or angled corners. As a result, stress is effectively dispersed and the groove bottoms are protected from cracking. Comparison Tests Test tyres having the following same construction and the same tread pattern of Fig.1 except for the small ribs were made by way of test, and tested for the durability. In the durability test, the test run distance was preselected and carried out on an indoor drum tester, and the groove bottom was checked. Carcass: Two plies of 1100d nylon cords turned up around bead cores Cord angle: 90 degrees to the tyre equator Cord count: 48/5cm Belt: One ply of spirally wound 1400d nylon cords Cord count: 49/5cm Tyre size: 225/50ZR16 (passenger car tyre) Rim size: 16X7JJ (standard rim) Inner pressure: 2.0 kgf/sq.cm Tyre load: 737 kgf Running speed: 80 km/h Running distance: 50000 km (increased from the usual distance of 30000km) Tire Rib No.of ribs W(mm) H(mm) Durability runable distance, crack(size) & damage Ex.1 Fig2 7 1 0.5 50000 km non Ex.2 Fig.2 7 1.5 0.5 50000 km non Ex.3 Fig.2 7 1 1 50000 km non Ex.4 Fig2 2 1 0.5 50000 km substantially non Ex.5 Fig.2 5 1 0.5 50000 km non Ex.6 Fig.2 8 1 0.5 50000 km non Ex.7 Fig.2 9 1 0.5 50000 km non Ex.8 Fig.2 11 1 0.5 50000 km non Prior - 0 - - 40000 km 0.7 mm Ref.1 Fig2 7 2 0.5 50000 km 0.2 mm Ref.2 Fig2 7 25 0.5 50000 km 0.5 mm Ref.3 Fig.2 7 1 1.5 45000 km tread separation due to heat generation Ref.4 Fig.2 7 1 2 40000 km tread separation due to heat generation Ref.5 Fig2 7 0.9 0.5 50000 km non Ref.6 Fig.2 7 1.6 0.5 50000 km 0.1 mm Ref.7 Fig2 7 1 0.4 50000 km non Ref.8 Fig.2 7 1 1.1 50000 km small tread separation due to heat generation Ref.9 Fig.7 7 1 0.5 45000 km 0.5 mm Ref.10 Fig2 1 1 0.5 50000 km 0.5 mm Ex.7: It was somewhat difficult to make the mould. The production efficiency was lower than Exs.1 to 6. Ex.8: It was difficult to make the mould. The production efficiency was lower than Ex.7. Ref.5 & 7: It was difficult to make the mould. The production efficiency is poor in comparison with Exs.1 to 3.

Claims: A pneumatic tyre comprising a tread portion (2) provided with a plurality of longitudinal grooves, each extending continuously in the tyre circumferential direction and having a bottom (4) and a pair of sidewalls (5) defining corners (6) therebetween, the inclination angles  of said sidewalls are in the range of not more than 15 degrees with respect to the normal direction (N) to the tread surface line, and said corners (6) are rounded; the bottom (4) is provided with a plurality of ribs (10) extending in the longitudinal direction of the groove substantially parallel with each other so that the bottom (4) has a wavy configuration having no angled corner, and outermost valleys between the sidewalls (5) and ribs (10) and intermediate valleys between the ribs (10) are defined, characterised in that the roundings of said corners (6) comprising a circular arc whose radius is not more than 2 mm, and in that said intermediate valleys are deeper said the outermost valleys; the heights (H) of the ribs (10) measured from the deepest point (9) of the longitudinal groove are in the range of from 0.5 to 1.0 mm. A pneumatic tyre according to claim 1, characterised in that the widths (W) of the ribs (10) are set in the range of from 1.0 to 1.5 mm. A pneumatic tyre according to claim 1 or 2, characterised in that there are five to eight ribs in each groove. A pneumatic tyre according to claim 1, 2 or 3, characterised in that the sidewalls (5) are inclined at an angle  of more than 0 degree so that the groove width increases radially outwardly. A pneumatic tyre according to any of claims 1 to 4, characterised in that said wavy configuration consists of alternate convex curves and concave curves. A pneumatic tyre according to any of claims 1 to 4, characterised in that said wavy configuration is a series of semicircles. A pneumatic tyre according to any of claims 1 to 4, characterised in that said wavy configuration consists of curved parts and flat parts, the flat parts being located on both sides of the ribs and being parallel with the radial direction of the tyre.

Inventors: IKEDA AKIO, IKEDA, AKIO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C11
IPC codes: B60C11