A pneumatic tire includes a ground contacting portion or tread, the tread having a pattern designed to provide the tire with a desirable combination of traction, durability, ride comfort, and quiet operation. It is also desirable that the tread pattern provide the tire with an all-weather capability or a set of characteristics providing adequate performance under a variety of adverse road conditions including dry, snow, ice, rain, and mud.
The all season tire has been introduced by The Goodyear Tire and Rubber Company many decades ago and was defined by lateral extending grooves open to the side of the tread. These lateral extending grooves were oriented perpendicular to the direction of travel for at least 0.5 inches and a width of at least 0.06 inches from the open shoulder laterally inward and provided a huge improvement in snow traction, virtually reducing the need for snow tires except in the most extreme weather conditions.
Tire tread patterns designed for traction on wet surfaces, snow, and ice often feature a block type tread pattern. A block type tread pattern is characterized by a plurality of main grooves extending in a circumferential direction and a number of lateral grooves extending in a more or less axial direction. The areas of the tread between the circumferential and lateral grooves are referred to a tread blocks. Tread blocks may also be defined by the edges of the tread and by grooves having other orientations. In comparison, a rib-type tread pattern may be characterized primarily by circumferential grooves separating circumferentially continuous ribs. Tread designs may also combine rib and block patterns.
The use of blocks as elements of a tread pattern tends to increase the level of noise generated by such tires, as compared to rib-type tires. Such blocks may produce irregular wear due to their lack of stiffness in the circumferential direction of the tread. It is known in pneumatic tires having a block tread pattern that normal operation of the tire produces uneven wear of the tread blocks called heel-and-toe wear. In heel-and-toe wear, the rate of wear at the toe or trailing edge of the blocks may exceed the rate of wear at the heel or leading edge of the blocks. In normal operation, the heel of each block may strike the pavement first followed by the toe. Similarly the heel of each block may be lifted first from its contact with the pavement followed by the toe. In addition to reduced tread life, irregular and heel-and-toe wear may increase the level of noise generated by operation of the tire. Also, the cornering and braking performance of a tire with irregular and/or heel-and-toe wear may be degraded.
Another tread pattern may suppress heel-and-toe wear by providing a narrow tread block axially outside each block. The narrow block may have a surface formed to be a circular arc by setting both end parts of the narrow block lower than the adjacent tread block by 1.5 to 2.5 mm
To balance the rate of heel and toe wear, the leading edge or heel of one or more blocks may have one or more notches with a variable width in the axial direction. The width may generally decrease from a maximum at the heel to a minimum in the direction of the toe. The notches may provide the tread blocks with a variable net to gross where the net to gross increases from the heel to the toe of the blocks.
An “aqua-channel” large circumferential groove with a width 7 to 12 percent of the tread width combined with a network of generally curved inclined lateral grooves flowing over the tread shoulders may also greatly enhance wet traction. As shown in FIG. 3, the aqua-channel may be connected to curved lateral grooves and water may be directed into a large groove and into the lateral grooves to be expelled through a channel or through the lateral grooves.
These directional treads should not have the lateral grooves oriented such that water is directed to the center of the tread. Therefore, the orientation is such that the axially inner portions of a lateral groove and the leading edges and trailing edges of the tread elements must enter the footprint or contact patch prior to the axially outer portions. Accordingly, any inclination other than 90 degrees may be inclined or sloped away from the contact patch as the grooves extend axially outwardly. These constructions have been found to contribute to irregular heel toe wear in shoulder block elements. This irregular wear may be exaggerated or reduced depending on the shape of the tire's footprint or contact patch shape.
Another tread pattern may produce a footprint shape which, regardless of load, may operate in a range of footprint shape factors that permit tire treads to be optimized thereby omitting tire rotation requirements. A tire's footprint may thus be measured and a footprint shape factor (FSF) may be calculated. To measure the footprint shape, a tire may be inked and pressed against a paper or a cardboard sheet or laid on a flat hard surface at a fixed load with the tire inflated at a fixed pressure leaving the impression of the tread on the paper or cardboard surface. Alternatively, inkless procedures may include carbonless paper, pressure sensing pads, and the like. In all cases, the objective is to define tread contacting surfaces within the footprint.
Conventionally, a butterfly shaped footprint has been undesirable. Alternatively, a footprint having a shape similar to the bow of a boat have been desirable for pushing water away from the center of the tread. As shown in FIGS. 1 and 2, conventional tires may exhibit this bow shaped of footprint.
Inherently, when the leading and trailing edges of the footprint are not axially extending (e.g., curved or bowed), as the tire rolls, a portion of the tread contacts the ground first and laterally adjacent tread elements follow. This may cause a phenomenon known as “tread element squirm.” As the tread elements leave the treads footprint, the elements snap out of the contact patch as the pressure holding the element against the road is released. The elements lightly contacting the road are slid across the roadway wearing the element similar to sliding rubber eraser across a sheet of paper. These tread elements may have a uniform pressure distribution laterally across the tread and, more particularly, the leading and trailing edges of the footprint may be axially extending in a straight line path under all operating conditions.
A concept and methodology to define a footprint shape factor F is shown in prior art FIGS. 1 and 2. First, the maximum axial width W of the footprint may be measured. Then, the distance halfway between the maximum axial width W may be defined as the tire's centerplane CP. A distance 40% of the tread width (W) on each side of the centerplane may be located as shown as reference numerals 2, 4. A circumferential line 5, 6 may be drawn through points 2-2 and 4-4, respectively, and the lengths of lines Ls1 and Ls2 may be calculated, summed, and divided by 2 to arrive at an average shoulder length A. The footprint length LC at the centerplane may be measured. The footprint shape factor F may be the ratio of LC/LS. The footprint shape factor F of FIGS. 1 and 2 may be 1.12 at normal inflation and 100% load at a fixed pressure. At 50% load, the footprint shape factor F may be 1.50 at the fixed pressure. As shown, the footprint's shape may be very different at these different loads. In light truck tires, this variation in loading may be a greater challenge than in passenger tires.
An improvement in irregular tread wear has been be achieved by using lateral groove orientations that completely go against the conventional construction discussed above. Further, studies have confirmed the use of this tread pattern design while reducing heel toe wear dramatically in the shoulder tread elements thereby mitigating degradation in wet and/or dry traction performance.
A pneumatic passenger or light truck tire having a radially outer tread may have a plurality of tread elements defined by grooves arranged circumferentially and laterally around the tread between a pair of lateral tread edges to define a tread pattern. A plurality of tread elements may extend across the width of the tread between the lateral edges, including central tread elements and shoulder tread elements having leading and trailing tread edges. The shoulder tread elements may be arranged in two rows, one row adjacent each lateral edge. At least one row of shoulder elements may have the leading edges inclined relative to the direction of rotation of the tire having an axially outward portion of the leading edge entering and exiting a footprint contact patch prior to the axially inner portion of the leading edge of the shoulder tread elements. The leading edges of one row of shoulder elements may be oriented equally, but oppositely directed, relative to the leading edges of the other row. The leading edge of the shoulder tread elements may be inclined greater than 0 degrees or 10 degrees or greater relative to a plane perpendicular to the centerplane of the tire. The pneumatic tire may have a non-directional tread pattern wherein both rows of shoulder tread elements are directionally oriented in the same direction. The tread pattern may be directional having equal, but oppositely oriented, shoulder tread elements. The leading edges of each shoulder element may be equally oriented and the leading edge of each shoulder element may be inclined at an angle of 10 degrees or greater relative to a plane perpendicular to a centerplane of the tire.
Another pneumatic tire may have a directional tread pattern wherein the plurality of tread elements extend across the width of the tread between the lateral edges and include central tread elements and shoulder tread elements with each tread element having a leading edge and a trailing edge. A first line may extend along the leading edges of laterally adjacent central tread elements and have a generally “V” like or chevron shape laterally inward of the lateral edges extending to an apex where the apex of the “V” or chevron first enters a contact patch of the tire as it rotates in a forward direction prior to the remaining portions of the leading edges. The shoulder tread elements may be arranged in two circumferential rows, one adjacent each lateral edge, with the leading edges having an inclination directionally opposite to the leading edges of the central tread elements. A second line may extend along the leading edge of the shoulder elements and may be connected to the first line. Axially outer portions of the leading tread edge of each shoulder element may enter the contact patch prior to an axially inner portion of the leading edge of the shoulder elements. Upon exiting the contact patch, the axially outer portions of the shoulder elements may exit prior to the axially inner portions while the central tread elements may have the apex and axially inner portions of the central tread elements exit the central patch prior to axially outer portions. The directional tread may be symmetric or asymmetric about the centerplane of the tread.