AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Bridgestone Firestone North American Tire, LLC Actual Inventor(s): Paul M. Neugebauer, John L. Turner, Stephen M. Vossberg Address for Service and Correspondence: PHILLIPS ORMONDE & FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: METHOD OF COMPENSATING FOR RESIDUAL ALIGNING TORQUE (RAT) Our Ref: 819483 POF Code: 496/462267 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1 - METHOD OF COMPENSATING FOR RESIDUAL ALIGNING TORQUE (RAT) The present application is a divisional application from Australian patent 5 application number 2002327252 the entire disclosure of which is incorporated herein by reference. BACKGROUND OF THE INVENTION Technical Field 10 The invention relates to pneumatic tires and in particular to pneumatic tires having sipes formed in certain of the tread blocks forming a pair of circumferentially extending ribs located on opposite sides of a mid circumferential plane of the tire. More particularly, the invention relates to forming the sipes at angles in the tread blocks in a slanted forward direction in 15 one of the ribs and in the opposite slanted reverse direction in the other of said ribs to make changes in the residual aligning torque (RAT) without changing the carcass or road contacting tread pattern of the tire. Background Information 20 Pneumatic tires such as used for passenger and light trucks, have a tread pattern which extends circumferentially about the tire. The tread consists of a plurality of circumferentially and laterally extending grooves which divide the tread) into generally circumferentially extending ribs formed by a plurality of either continuous or discontinuous tread blocks. Many of the tread blocks are 25 formed with very narrow slots or slits commonly referred to as sipes. These sipes are formed by thin metal strips placed in the mold, referred to as sipe blades. These sipes provide various traction and stiffness characteristics to the tread blocks which provides certain ride and handling characteristics for the vehicle. 30 Even when the ribs of a tire are spaced symmetrical with respect to the mid-circumferential plane of the tire, they can produce a moment on the tread at the contact patch with the roadway, causing a twisting force on the tire. This moment provides a tendency for the vehicle to drift left or right when the steering wheel is released. This moment or force at the contact patch is W:\DEULAH\DDM\D~isional\2002327252(divisional) 20.12.07.dc 1A referred to as residual aligning torque (RAT). The effect of this twisting moment and a detailed discussion relating to the various factors thereof and prior attempts to reduce or eliminate the same, is disclosed in detail in U. S. Patent No. 5,944, 082, the contents of which are incorporated herein by reference. 5 The tire construction and method set forth in U. S. Patent No. 5,944, 082 provides draft angles in lateral grooves of certain ribs, which angles are sloped forward on one side of the midcircumferential plane and sloped backward on the opposite side of the plane. These sloped grooves allegedly result in the tread blocks sloping forward and backward during forward motion of the 10 vehicle which induces a torque to compensate for the inherent residual aligning torque due to the tread pattern. U. S. Patent No. 5,944, 082 states that the siping in tread blocks is usually not wide enough to allow effective rolling radius changes before adjacent blocks come into contact and that the friction between sipes 15 frequently locks the sipes together cancelling the effect to be achieved. It concludes that inclined sipes on the rolling circumference of the rib are usually negligible and not predictable. However, the method of the present invention of forming the tire with oppositely sloped sipes has been found to achieve satisfactory results in affecting the residual aligning torque not believed 20 possible in the tread pattern of U. S. Patent No. 5,944, 082. The discuss of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general 25 knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. SUMMARY OF THE INVENTION The present invention provides a method of manufacturing a pneumatic 30 tire, including: providing a circumferentially extending tire pattern of a pneumatic tire with at least first and second circumferentially extending ribs, said ribs being located on opposite sides of a mid-circumferential plane of said tire, each rib containing a plurality of tread blocks separated by laterally extending grooves, W:\ELILAH\D DisionaA20327252(dislonal) 20. 12.07.doc 2 said tread blocks having sipes, said tread blocks having leading and trailing sidewalls symmetrical with respect to a first radial plane passing through a midpoint of said tread blocks and through an axis of rotation of the tire; varying an angle of inclination of the sipes with respect to a second 5 radial plane passing through an outermost surface of the tread block and adjacent to the sipe and through an axis of rotation of the tire to change a residual aligning torque of the pneumatic tire until a desired residual aligning torque is achieved and such that an angled sipe is formed in each of the tread blocks, each angled sipe extending for a sipe lateral width and a radial depth 10 at a substantially constant sipe angle of inclination between 20 and 150 with respect to the second radial plane passing through the outermost surface of the tread block and adjacent to the sipe and through the axis of rotation of the tire; wherein all sipes within said rib tread blocks on a first side of the mid 15 circumferential plane extend, at a first sipe angle of inclination with respect to said second radial plane for the sipe lateral width and the radial sipe depth, at least one sipe within each tread block on the first side of the mid circumferential plane extending from one side of the block and across the centre of the block to the other side of the block. 20 wherein all sipes within said rib tread blocks on a second side of the mid-circumferential plane extend, at a second sipe angle of inclination with respect to said second radial plane that is substantially equal to and opposite the first sipe angle for the sipe lateral width and the radial sipe depth, at least one sipe within each tread block on the second side of the mid-circumferential 25 plane extending from one side of the block and across the center of the block to the other side of the block; and forming a pneumatic tire such that said pneumatic tire has angled sipes in said tread pattern. Embodiments of the invention include sipes with various heights and 30 tapers to achieve a desired aligning moment and effect on the RAT, and in which the width and depth of the sipes can be varied to effect this change in RAT. W:\DELILAHODMO sional\2002327252(dslonal) 20.12.07.doc 3 Preferably, in accordance with an embodiment of the invention, the sipes are formed during molding of the tire by utilizing sipe blades in a usual mold without requiring substantial changes to the molding process or materially increasing the cost of the tire. 5 The sipes may extend in a generally lateral direction with respect to the mid-circumferential plane of the tire, perpendicular thereto or at various angles, or the method may include providing the sipes with various shapes such as zig zag, in order to achieve various changes in the RAT depending on the particular tread pattern of the tire. 10 A still further feature of an embodiment of the method is to provide the angled sipes either with a full or partial width with respect to the lateral width of the tread block, and varying the depth of the sipe to extend partially into the tread block or completely to the base of the tread block. The foregoing advantages, construction, operation and method steps of 15 the present invention will become more readily apparent from the following description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic plan view of a tire tread containing the angled 20 sipes formed by the method of the present invention; Fig. 2 is an enlarged diagrammatic perspective view with portions in section of adjacent tread blocks in two intermediate ribs having the angled sipes formed therein; Fig. 3 is an enlarged fragmentary sectioned view taken on line 3-3, Fig. 25 1; Fig. 4 is an enlarged fragmentary sectional view taken on line 4-4, Fig. 1; Fig. 5 is a further enlarged view of the angled sipes in one of the tread blocks of Fig. 4; -3a- Fig. 6 is a diagrammatic plan view similar to Fig. 1 showing the method of the present invention forming a modified sipe configuration; Fig. 7 Is a plan view similar to Fls. I and 6 showing another tread pattern having a modified sipe configuration; 5 Fig. 8 is a diagrammatic plan view of a test tire tread proving that changes in RAT are achieved with the angled sipes; and Fig. 9 is an enlarged perspective view of two of the test tread blocks of Fig. 8. 10 DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a plan view of a portion of a pneumatic tread pattern indicated generally at 1, which includes a pair of shoulder ribs 2, a pair of intermediate ribs 3 and 4 and a center rib 5. Each rib 2. is formed by a plurality of tread blocks 6 separated by generally V-shaped grooves 7. If desired grooves 7 could be 15continuously curved or have multiple angles in addition to the double angle of the V-shape groove as shown. Each intermediate rib 3 and 4 is formed by a plurality of tread blocks 9 separated by laterally extending grooves 10. Center rib 5 is formed by a plurality of circumferentially spaced tread blocks 12 separated by lateral grooves 13. Circumferentially extending grooves 11 and 14 separate rib 203 from rib 5 and rib 4 from rib 5 respectively. The particular shapes of the tread blocks shown in Fig. I can vary without affecting the concept of the invention as well as the various directions and shapes of the intervening grooves. Likewise, the various ribs can have the tread blocks integrally connected to form a continuous rib as opposed to the discontinuous figuration of the ribs as shown in 25Fig. 1. Ribs 2, 3, and 4 are spaced generally symmetrical about a mid circumferential plane 15 of the tread pattern. In accordance with the invention, sipes 17 and 18 are formed in tread blocks 9 and extend at an angle A (Fig. 5) with respect to a plane indicated at 20 in Fig. 5, which extends generally perpendicular to the tread block and in a generally radial direction through the axis of rotation of the tire. Sipes 17 in tread blocks 9 of rib 3 are slanted at the same angle, but in an equal and opposite 4 direction with respect to plane 20 to that of sipes 18 of rib 4 as shown in Fig. 4. Sipes 17 and 18 preferably will have the same width W and the same depth D. In a preferred embodiment, sipes 17 and 18 will be formed with a width in the range of between 0.015 and 0.06 inches with the preferred width being 5approximately 0.03 inches. Depth D will be within the range of 20% and 100% of the height H of the tread block. Angle A will be within the range of 20 and 150 with the preferred angle being approximately 70*. Thus, the sipes can extend completely to groove bottom surface 22 or only partially thereto without affecting the concept of the invention so long as the sipes in opposite ribs 3 and 4 have the 10same angle of inclination, but in opposite directions, and substantially the same depth and width so as to be similar, but opposite to each other. Although three sipes are shown in each of the tread blocks in Fig. 1, this number can vary so long as substantially the same number of sipes are formed in each tread block. The sipes need not extend perpendicular to.mid-circumferential plane 15 as shown in 15Fig. 1, but can extend at an angle thereto as shown in modified tread patten 24 in Fig. 6. Fig. 6 shows sipes 25 and 26 formed in tread blocks 9 extending at an angle with respect to mid-circumferential plane 15. Again, the same number of sipes will be formed in each tread block 9, but sloped in opposite directions as shown in Figs. 3 and 4. 20 Fig. 7 shows another modified tread patten 27 in which sipes 28 and 29 have a zig-zag configuration and are symmetrical to each other except that the sipes will be slanted in opposite directions with respect to those in the opposite rib as discussed above. The tread pattern of Fig. 6 also shows the formation of angled sipes 33 and 2534 in shoulder ribs 2, which again are of equal and opposite angles and depths to each other in the respective opposite ribs with respect to plane 15. Tests have been performed to prove that the angled sipes do affect the residual aligning torque shown diagrammatically in Figs. 8 and 9. Fig. 8 is a diagrammatic foot print of an actual test tire which proved that a 30change In RAT can be achieved by providing angled slpes and by varying both the depth and angle of the sipes in the tread blocks. Set forth below is Chart One 5 which was developed by Finite Element Analysis (FEA) in order to mathematically determine what, if any effect the angled sipes would have on the RAT. In the test tire of Fig. 8, sipes 40, two of which are formed in each tread block 42, were provided in shoulder ribs 43. Blocks 42 were formed by lateral grooves 44 and circumferential grooves 45. The various parameters which were considered and which could effect the change in RAT are shown in Fig. 9. These parameters include Pd (radial sipe depth); Pg (sipe gauge or width); Pb (distance of sipe base to leading edge); Pe (distance of sipe opening to leading edge); Px (sipe angle of inclination); and Pw (sipe lateral width). 10 CHART ONE Sipe Angle Compressiv Circumferential Net RAT Anticip e Force (Ibs) Direction N-rn ated Thrust/Element Result (Ibs) On Tire N-rn 15 0 63.25 0.0 0 0.0 5 62.98 1.46 4 1.89 10 62.17 2.88 8 3.69 20 Actual tests were then performed on two tires which had the parameters set forth in Chart Two and shown in Fig. 9. CHART TWO 25 Sipe Angle Measured RAT N-rn Net Angled Tie Bar (degrees) RAT Effect (N-m) -7 -5.52 -1.93 30 No Sipes -3.59 _7 -1.77 1.82 6 The FEA calculations showed that the projected sensitivity to the sipe inclination angle is about 0.37 N-m/ 2 inclination angle. The actual measured change in RAT obtained from the test results in Chart Two showed a change in RAT of 0.28 N-m /9 inclination angle. It is believed that the main reason for the Difference between the model affect and the actual affect on the test tires is in the material properties of the tread compound and the loading corrections on the tire. Most importantly, the test results show there is a definite change in RAT by providing angled sipes in the tread blocks and that various changes in RAT can be achieved by varying the sipe angle and that the amount of desired change in 10RAT can be determined to some degree of certainty by FEA as set forth in Chart One. Thus, a tire manufacturer can change the RAT of a tire easily by providing angled sipe blades in the mold and can arrive at a projected amount of change initially through FEA which then can be incorporated into an actual tire for 15subsequent testing. Thus, FEA avoids the more expensive and time consuming production of samples and actual testing of tires in order to arrive at the desired change of RAT since a change in RAT can be pre-calculated. While the embodiments in the invention has been described, the invention is not limited thereto. The claims of the invention follow. 20 7