Pneumatic tire tread

A pneumatic tire having a tread portion comprises a mid-circumferential zig-zag rib and a plurality of blocks located in the edge portions of the tread between adjacent peaks of the zig-zag rib. The zig-zag rib has amplitude of between 60-70% of the tread width and has sied faces thereon that have an exposed length of at least 40% of the tread width. By varying the width of the grooves, between the blocks and the rib, on one rotational directional side of the rib from the width of the grooves on the other rotational directional side of the rib, it is possible for the tire to have different traction grips for each rotational direction.

BACKGROUND OF THE INVENTION 
This invention relates to pneumatic tires and primarily to tires for 
vehicles used in cross-country applications. 
Typical vehicles used in cross-country applications often have four wheel 
drive and are fitted with tires that have prominent tread patterns to 
provide traction in mud and sand. As used herein "cross-country" refers to 
off-the-highway operation of vehicles. 
Typical cross-country tires are directional to provide good grip and self 
cleaning properties going forwards but this results in poor traction and 
self cleaning when the tires rotate in reverse. Tread patterns for 
cross-country tires are typically bar type treads, or block type treads 
that give good traction but compromise on lateral stability and when used 
on highway applications can be noisy and give an uncomfortable ride. Other 
tread patterns, for example as shown in British Patent Application No. 2 
042 992, have been developed having a mid circumferential zig-zag rib with 
blocks located in the edge portion of the tread on either side of the 
central rib. The blocks provide the traction for the cross-country 
applications and the central rib provides favourable traction and ride for 
highway applications. However, because these prior art tread designs are a 
compromise between cross-country and highway uses they are not as 
efficient in off-the-highway applications as they might be. 
Accordingly there is provided a pneumatic tire having a tread portion with 
a pair of lateral edges which are spaced apart by a given tread width and 
comprising a zig-zag rib at the mid-circumference of the tire and having 
an amplitude of between 60-70% of the tread width and having side faces 
thereon each of which is directed in one of the two rotational directions 
of the tire, the side faces each having a length of at least 40% of the 
tread width, and blocks located in the edge portions of the tread between 
adjacent peaks on the zig-zag rib. 
The long length of exposed side face on the central rib provides good 
off-road traction, whilst the high amplitude zig-zag rib provides for a 
good highway ride. 
Preferably said side faces on the rib are first side faces directed in one 
rotational direction, and second side faces directed in the opposite 
rotational direction, and the blocks each have two faces thereon each of 
which is substantially parallel to a respective one of the first and 
second sides faces on the rib and forming therebetween a first and second 
groove respectively, the width of each first groove being between 10-15% 
of the tread width, and the width of each second groove being between 
35-100% of the width of the first groove. 
By varying the width of the first grooves and the second grooves it is 
possible to obtain different degrees of traction for the two directions of 
rotation of the tire.

DETAILED DESCRIPTION OF INVENTION 
With reference to FIG. 1 and FIG. 2, there is illustrated a cross-country 
tire 10, preferably of radial type construction, for mounting on 
cross-country four wheel drive vehicles. The tire 10 has a tread portion 
11 extending circumferentially thereabout, with a pair of lateral edges 12 
and 13 spaced apart by tread width TW. 
The tread width TW for the purposes of this invention, is defined as as the 
axial distance across as measured from the footprint of the tire when 
inflated to a design pressure and subjected a rated load. The tread 
portion 11 is connected by sidewalls 14 to a pair of beads 15. For the 
purposes of this invention axial and axially refer to the axis of rotation 
of the tire, and radial and radially refer to directions perpendicular to 
the axis of rotation of the tire. 
The tread portion 11 comprises a zig-zag rib 16 extending circumferentially 
about the tire and centered axially with respect to the 
mid-circumferential plane M--M of the tire. For the purposes of this 
invention the mid-circumferential plane is a plane perpendicular to the 
axis of rotation of the tire, disposed midway between the sidewalls of the 
tire. The zig-zag rib 16 has an amplitude X of between 60-70% of the tread 
width TW and preferably 62% of the tread width TW. The amplitude is the 
axial distance across the tread between circumferentially adjacent peaks 
22 on the zig-zag rib 16. The rib 16 has a width W of between 15-20% of 
the tread width TW and preferably 20% of the tread width TW, when measured 
perpendicularly of the zig-zag center line L of the rib 16. The segments 
17 of the zig-zag rib are arranged such that the zig-zag center line L of 
the rib always intersects the mid-circumferential plane M--M at an angle 
of 45.degree.. 
The rib 16 because of its zig-zag nature has radially projecting side faces 
18 and 19 thereon that are respectively first side faces 18 directed in 
one rotational direction for the tire, and second side faces 19 that are 
directed in the opposite rotational direction for the tire. The side faces 
18 and 19 have lengths S.sub.1 and S.sub.2 respectively measured parallel 
to the zig-zag center line L, of at least 40% of the tread width TW, but 
preferably about 50% of the tread width. The zig-zag rib 16 is interrupted 
by a plurality of spaced shallow grooves 31 which are perpendicular to the 
rib center line L. The shallow grooves 31 are between 1/3 and 2/3 of the 
tread depth. As used herein the "tread depth" or "full depth of the tread" 
is understood to mean the height of the side faces 18 and 19 of the rib as 
measured in a radial direction on a newly molded tire made according to 
the present invention. 
A series of blocks 21 are located in the edge portions of the tread portion 
11. In each respective edge portion, each block 21 is located between 
adjacent peaks 22 of the rib 16 that are in that respective edge portion. 
The blocks 21 each have a substantially triangular ground contacting 
surface and each has two radially projecting faces 23 and 24 thereon. The 
blocks 21 each have blades 33 and 34 (also known as sipes) that extend to 
almost the full depth of the tread. The blades 33 and 34 are parallel with 
the faces 23 and 24, respectively, and are joined together in an 
arrow-like formation pointing towards the mid-circumferential plane M--M. 
The faces 23 and 24 on each block 21, are opposite to the side faces 18 
and 19 of the rib 16 and form therebetween a series of circumferentially 
alternating linked first and second grooves 25 and 26 respectively. The 
first groove 25 has a width `a` of between 10-15% of the tread width and 
the second groove 26 has a width `c` of between 35-100% of the width of 
the first groove. 
In this embodiment of the invention, the first and second grooves 25 and 26 
have equal widths `a` and `c` (as measured perpendicularly to the center 
line of the groove) which are approximately 15% of the tread width TW. 
The blocks 21 are spaced circumferentially at the lateral edges 12 and 13 
of the tread by a distance `b` which is between 100% to 200% of the width 
`a` of the first grooves 25, and preferably 165% of width thereof. The 
first and second grooves 25 and 26 between each block 21 and the rib 16 
are progressively enlarged towards the respective tread edge 12 or 13 to 
open into the spaces `b` between adjacent blocks 21. 
Because the first and second grooves 25 and 26 both have the same width, 
the tire will have substantially equal traction in the forward and reverse 
directions of rotation. The wide circumferential spacing between the 
blocks 21 gives good self cleaning properties for the tread portion. For 
each direction of rotation in the footprint of the tire, the mud or sand 
will hit the respective side face 18 or 19 of the rib 16 which is facing 
in that direction and the mud or sand will be deflected along the side 
face and out of the footprint of the tread via the space `b` between the 
blocks 21. 
A further embodiment of the invention is illustrated in FIG. 3 which is a 
fragmentary view of the tread portion 111 of a second tire according to 
this invention. Because of the similarity between FIG. 3 and FIG. 2 only 
those parts which are different will be given new reference indicator 
numbers. 
The tread portion 111 has a tread width TW and a zig-zag rib 16 which is 
identical with that described for FIG. 1 and FIG. 2. Blocks 121 are 
located in the edge portions of the tread portion 111 between adjacent 
peaks 22 of the rib 16 that point towards the same lateral edge of the 
tread portion. The blocks 121 are of modified shape when compared with the 
blocks 21 previously described. 
For each block 121 the first groove 25, between the radially projecting 
face 23 on the block 121, and the opposite radially projection side face 
18 on the rib, has a width `a` as previously described. However, the 
length of the face 23 on the block 121 has been increased so that the 
other radially projecting face 24 on the block and its opposite side face 
19 on the rib have a second groove 126 therebetween which has a width `c` 
which is 50% of the width `a` of the first groove 25. 
Also the grooves 25 and 126 are not progressively enlarged towards the 
spaces `b` between adjacent blocks 121 but are opened out from corners 127 
and 128 by a series of steps 131, 132 and 133, and 134 respectively. The 
steps 131 and 133 have radially projecting surfaces that align with the 
faces 18 and 19 respectively on the rib 16. The space `b` between the 
corners 127 and 128 of adjacent blocks 121 is 95% of the width `a` of the 
first grooves 25. 
Because the first grooves 25 have a width `a` which is wider than the width 
`c` of the second grooves 126, the tire will have different traction grip 
for each of the two directions of rotation. 
For example, when the tire rotates for forwards movement of the vehicle on 
which it is mounted then as the side faces 18 on the rib 16 advance 
against the ground, the mud, sand, etc will move down the wider first 
grooves 25 along the faces 18 and exit the tread portion through the 
circumferentially spaces `b` between the blocks. Similarly when the tire 
rotates in the reverse direction, the mud or sand will move down the 
narrower second grooves 126, along the faces 19 and also exit the tread 
through the spaces `b`. Because the second grooves 126 are only 50% of the 
width of the first grooves 25, the dirt clearance is not as efficient and 
the tire will have less traction grip in this direction of rotation than 
in the other. 
Tests have shown that the traction grip of the tire in the reverse mode 
vary between 60% and 85% of the traction grip in the forward mode 
depending upon the condition, in which the tire is operating. The closer 
spacing `b` between the blocks 121 of the embodiment showin in FIG. 3 as 
compared with the spacing between the blocks 21 of the embodiment shown in 
FIG. 2 slightly reduces the clearance for evacuating mud and sand between 
adjacent blocks, but alignment of the surfaces 131 and 133 on the block 
121 with the side bases 18 and 19 of the rib helps to offset this effect 
and also provides effective extensions to the side faces 18 and 19 of the 
rib. 
In order to make mud clearance more effective the first and second grooves 
25 and 126 can be progressively enlarged as previously shown for FIG. 1 
and FIG. 2. 
The tread portion 111 could be altered to give the same traction grip for 
the vehicle moving in the reverse direction as in the forward direction. 
This can be simply achieved by making width `a` of the first grooves 25 
narrower than the width `c` of the second grooves 126, for example the 
relationship between the widths of the first and second grooves `a` and 
`c` described above could be reversed for the tread to one side of the 
mid-circumferential plane M--M. 
It is obvious that the tread portion shown in FIG. 3 could be modified in a 
number of different ways. The relative widths of the first and second 
grooves can be varied so that the width of the second groove lies between 
35%-100% of the width of the first groove. This is achieved by displacing 
the face 24 on the block 121 to widen or narrow the groove 126. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the invention, it will be apparent to those 
skilled in the art that various changes could be made without departing 
form the scope of invention.