Pneumatic tyre

Abstract: A pneumatic tyre for a vehicle comprising a ground contacting tread region extending between sidewalls (3) and through two tread contact edges (TCE), and bead regions (11) disposed radially and axially inward of the outer tread surface having a curvature in the axial direction when the tyre is mounted on a wheel rim and normally inflated which has a continuously decreasing radius (R') from a point P which is at an axial distance SP from the tyre circumferential centreline (C) equal to 20% of the distance from the tread centre to the adjacent tread contact edge, the tread region being provided with tread grooves characterised in that the tread thickness progressively reduces from the tread centre to the tread contact edge and the depth (d) of the tread grooves decreases continuously from the tread centre to the tread contact edge.

Description: This invention relates to a pneumatic tyre and particularly to a tyre for a vehicle having three or more wheels. Such vehicles, unlike two-wheeled motorcycles, are not banked over in use and therefore the tyres are not subject to running at very high camber angles. Thus tyres on a four-wheeled passenger vehicle for example will normally operate at a maximum camber angle of 5°. Accordingly conventional tyres for such vehicles have relatively flat ground contacting treads which are designed to use the entire width of the tread and to present a substantially constant road contact area, both in shape and size, under all motoring conditions. Therefore such conventional tyres have tread patterns which have a substantially uniform pattern depth over the tread surface, to provide water clearance from the contact area,. In this way the pattern and thus the wet grip of the tyre is preserved as the tread wears evenly across its width. Variable depth tread grooves are known for example from EP-A-0064934. This discloses a racing tyre having a extremely wide flat tread of uniform thickness. Reduced groove depths in the shoulder regions of the tyre increase block stiffness and thus prevents overheating of the tyre during high speed running in dry conditions. Recently a tyre for a passenger car has been proposed by EP-A-0658450 which does not have the conventional flat tread. This new tyre comprising all of the features of the preamble of claim 1 has a reinforced tread region having substantial curvature in transverse cross-section. In straight running the contact area between the tread and the road is relatively narrow in comparison to that of the conventional tyre and is substantially less than the width of the tyre. In tests this new tyre has shown improved characteristics which are believed to be due in part to the ability of the contact area to move across the tread and/or to increase in transverse width in response to changes in load. The Inventor has investigated the wear of the above-mentioned new tyre and discovered that it does not wear as a conventional flat tread tyre. Thus the pattern of the new tyre is not maintained during the life of the tyre. Accordingly it is the object of the present invention to provide a tyre of the proposed type which maintains its tread pattern throughout the life of the tyre. This object is achieved by a pneumatic tyre according to claim 1. Further aspects of the present invention will become apparent from the following description by way of example only of one embodiment and in conjunction with the following drawings in which:- Figure 1 shows a schematic diagram of a tread pattern of a tyre according to the present invention; Figure 2 shows a schematic diagram of a transverse cross-section of the tyre of Figure 1 taken in the direction of A-A; and Figure 3 is a schematic diagram showing the distribution of pattern depth in a tyre according to the invention. The tyre of Figures 1 and 2 is a 195/45R16 passenger vehicle tyre for a 6½" x 16" wheel rim. The tyre comprises a tread region 1 reinforced by a breaker assembly 2 and a pair of sidewalls 3. A single ply of radially extending cords provides a carcass reinforcement 4 which extends between bead regions 11 of the tyre and is wrapped at each edge from the inside to the outside in the axial direction around a bead core 5. The edge region 6 of the ply 4 is turned up radially outwards to provide a ply turn-up. A rubber apex 7 is positioned on top of the bead core 5 in a conventional manner. A chafer 8 is wrapped around the bead 5 inwards of the ply 4 to provide a cushion for the ply around the bead core 5 and apex 7. A clinch strip 9 is also positioned around the bead. The tyre carcass has a substantially curved cross-sectional shape to match the breaker package 2. The tread region has a gradually decreasing radius of curvature Rc moving outward from the tyre circumferential centreline C. More specifically the tread region has a radius of curvature Rc which continually decreases to either side of the centreline axially outwards of a point P on the tread surface which is at a distance SP equal to 20% of the distance from the centreline C to the tread contact edge TCE. The tread thickness reduces continuously from the tread centre to the tread contact edge TCE. Here the tread thickness is the distance between the bottom of the tread or sub-tread component and the tread outer edge measured normally to the tread outer surface. Accordingly in this embodiment the tread thickness distribution is:- t i 9.2 9.0 8.4 7.3 6.0 4.8 3.9 2.8 i 30 40 50 60 70 80 90 100 wherein t i is the tread thickness in mm at a position displaced by i mm along the tread surface from the tread centre C. The tread region 1 is provided with a tread pattern on the tread surface 10 comprising diagonal grooves G extending from the tread centre axially outward. These grooves may be 'U' shaped of 'V' shaped in cross-section, both of which are common in the art. The depth d of the groove varies inversely with its displacement GD from the tread centre C. In this particular embodiment the groove depth d decreases linearly from the tread centre C to the tread contact edge TCE. The groove depth d at the tread contact edge TCE is 60% of the maximum groove depth d at the tread centre C. This pattern depth distribution is illustrated in Figure 3 which shows the graph of groove pattern depth d expressed as a percentage of the maximum groove depth at the tread centre plotted against the axial displacement GD expressed as a percentage of the maximum displacement SW / 2 which is half of the tyre section width SW. From Figure 3 it can be seen that in this embodiment the tread contact edge TCE is displaced from the tread centre C by a distance WC equal to 92% of the half section width SW / 2. A tread pattern depth and tread thickness distribution of the above-described kind has been shown to preserve worn pattern depth differentials and pattern appearance throughout the life of the pattern.

Claims: A pneumatic tyre for a vehicle comprising a ground contacting tread region extending between sidewalls (3) and through two tread contact edges (TCE), and bead regions (11) disposed radially and axially inward of the outer tread surface, the latter having a curvature in the axial direction, when the tyre is mounted on a wheel rim and normally inflated, which has a continuously decreasing radius (Rc) from a point P which is at an axial distance SP from the tyre circumferential centreline (C) equal to 20% of the distance (WC) from the tread centre to the tread contact edge, the tread region being provided with tread grooves (G) and having the tread thickness (ti) reducing continuously from the tread centre to the tread contact edge (TCE), characterised in that the depth (d) of the tread grooves decreases linearly with axial displacement from the tread centre and the groove depth (d) at the tread contact edge (TCE) is 60% of the groove depth (d) at the tread centre wherein the tread contact edge (TCE) is axially displaced from the tread centre by a distance (WC) of 92% of the half section width ( SW / 2). of the tyre. A pneumatic tyre according to claim 1, characterised in that the tread thickness in the tread centre is 9.2mm. A pneumatic tyre according to claim 1 or claim 2, characterised in that the tread thickness at the tread contact edge is 2.8mm.

Inventors: CORNER MICHAEL RAYMOND, CORNER, MICHAEL RAYMOND
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1996
Main classification: B60C11
IPC codes: B60C11, B60C3