This is a continuation of application Ser. No. 09/101,055, filed on Jun. 25, 1998, now U.S. Pat. No. 6,062,282, which is a 371 of PCT/US97/07008 filed Apr. 25, 1997. 
    
    
     TECHNICAL FIELD 
     This invention relates to a directional pneumatic agricultural tire for use on farm tractors and similar agricultural type vehicles. 
     BACKGROUND ART 
     Tractor tires must have good vibration characteristics on and off the road while maintaining good traction or draw bar characteristics. Such tires must also provide for the removal of soil, mud, etc., during infield use. 
     The tractive power propelling the vehicle is primarily provided through and transmitted by large lugs that are typically oriented in a directional pattern. This directional pattern generally employs the use of what is commonly called or referred to as long bars or a combination of long bars and short bars. Typically, these patterns of lugs are designed to have two rows of shoulder lugs, one row extending from each shoulder of the tire towards the equatorial plane. The volumetric space between the lugs is commonly referred to as the soil discharge channels. These channels provide a means for compacted soil to discharge over the tire shoulder. This feature prevents the tire from packing with mud and enables the tire to maintain a self-cleaning capability. Generally, these tires having two rows of shoulder lugs are arranged such that the lugs create a V or chevron-type pattern, these patterns usually are centered about the equatorial plane. If they are not centered they are typically alternating such that the chevron is on one side of the equatorial plane and the next set of circumferentially adjacent lugs have a chevron which is on the opposite side of the equatorial plane. This alternating pattern is repeated such that there is a balancing effect of the chevrons. For the purposes of this invention, these alternating chevrons on one side or the other of the equatorial plane in a repeating fashion are considered symmetrical in that as the tire passes through its footprint, that is the portion of the tire contacting the ground surface, the soil discharge channels within the footprint typically average out such that the average volume within the channel is equal on the left side of the equatorial plane versus the right side of the equatorial plane. Such a tire demonstrating a long bar/short bar combination is exhibited in U.S. Pat. No. 4,383,567 and is commonly referred to in the commercial market place as the Goodyear DynaTorque II radial tire. 
     Another tire using a similar long bar/short bar combination is taught in U.S. Pat. No. 4,534,392. This tire is commonly referred to as the Goodyear DynaTorque Radial and the Kelly-Springfield PowerMac L/S Radial Tractor Tire. This particular tire used a combination of two long bars separated by a short bar and repeated by two long bars and this pattern is repeated on both sides of the tire. This tread pattern is such that it again exhibits a combination of chevrons that have a resultant pattern such that the soil discharge channels as the tire passes through the footprint tend to equalize. 
     The prior art tires typically had several characteristics in common. One being the employment of a large number of lugs where at least one of the lugs would always cross the centerline of the tire. These tires had several beneficial tractive performance characteristics in that they were good in most soil conditions and provided good draw bar traction. The problem that was prevalent in these types of designs is that the short bar would tend to wear out more rapidly than the long bars. The resultant effect is that an uneven wear pattern would be generated in the tire after a period of time. This meant that the farmer would perceive the tire as being irregularly worn and therefore he considered the employment of a short bar detrimental to the performance of the product. 
     In 1992, The Goodyear Tire &amp; Rubber Company introduced a new tractor tire having two sets of primary and secondary lugs. The tire was commercially identified as a DT710 and is described in U.S. Pat. No. 5,046,541. As described in the patent, this tire has good traction, vibration and cleaning characteristics. These primary and secondary lugs are shorter in length than most tractor tire lugs. The tire effectively increased the number of lugs and therefore an increase in lug surface area resulted. This increased the performance capabilities of the tire. The flexible nature of these relatively short lugs also helped reduce the soil compaction potential of the tire even though more lug surface area was employed. 
     In 1995, U.S. Pat. No. 5,411,067 taught that the tire described above as U.S. Pat. No. 4,534,392 the Goodyear DynaTorque Radial and Kelly-Springfield PowerMac L/S Radial Tractor tires could be modified by the employment of a notch in at least each of the long bars across the equatorial plane. This notch could be of partial or full depth. By notching the long bar the tire achieved increased flexibility and reduced soil compaction while further enhancing the tractive capability of the tire. This pattern had the resultant effect of the directional symmetrical patterned tires in that the soil discharge channels throughout the footprint on average from left side to right side were equal as the tire rolled through the soil. 
     Each of the tires described above had several key limitations; one being that the employment of short lugs in combination with long bar lugs inherently results in a potential nonuniform treadwear problem. Alternatively, the employment of short lugs such as the DT710 although resulting in very uniform wear has bars that are substantially shorter than the typical lugs and as a result the tread although wearing uniformly is perceived by the farmer to have the potential of wearing out quickly because the lugs are substantially shorter than the conventional lugs. This in spite of the fact that there is a larger surface area in the use of the D1710 type short lugs with the resultant effect of more lug surfacing contact as the tire rolls therefore enhancing the wear and durability of this particular tire. Nevertheless, the customer perceives the potential for fast wear due to the use of short lugs. 
     SUMMARY OF THE INVENTION 
     A tractor of the present invention ( 10 ) has a drive axle having mounted thereto a pair of pneumatic agricultural tires  20 A,  20 B, each having an asymmetric directional tread pattern. 
     The tire has the unique asymmetric directional tread pattern such that the soil discharge channels between the lugs on one side of the tire are uniformly greater than the soil discharge channels created on the opposite side of the tire. This creates a unique asymmetric soil discharge channel. Additionally, the inventive tire has two rows of shoulder lugs, the lugs being of substantially equal lengths which enables the tire to exhibit very uniform wear patterns. 
     A pneumatic agricultural tire  20  having a maximum section width (SW), an axis of rotation (R), an equatorial plane (EP) centered between the maximum section width (SW) and perpendicular to the axis (R) is described. The tire  20  has a casing having a carcass  21  having one or more plies  22  reinforced with rubber coated cords  22 A and has a rubber tread  32  disposed radially outwardly of the casing. The tread  32  has first and second lateral tread edges  32 A, 33 B; the distance between the lateral tread edges  33 A, 33 B defines the tread width (TW). The tread  32  has an inner tread  34  and a plurality of lugs  50 A, 50 B, 50 C projecting radially outwardly from the inner tread  34 . The tread lugs  50 A, 50 B and  50 C have a length l l , and a width l w  the ratio of the lug length l l  to lug width l w  is at least three times, preferably at least three times. 
     The tread  32  has a plurality of shoulder lugs  50 A and  50 B. The plurality of shoulder lugs  50 A, 50 B are divided into a first row of shoulder lugs  50 A extending from the first lateral edge  33 A respectively towards the equatorial plane and a second row of shoulder lugs SOB extending from the second lateral edge  33 B. The lugs  50 A of the first row are circumferentially offset relative to the lugs  50 B of the second row. A plurality of similarly oriented central lugs  50 C are arranged in row and each central lug  50 C extends across the equatorial plane EP. The lugs of the first row of shoulder lugs  50 A are substantially aligned with the central lugs  50 C along their respective lug lengths l l , while the shoulder lugs  50 B of the second row are similar but oppositely oriented relative to the first row of shoulder lugs  50 A. The combination of shoulder lugs  50 A, 50 B and central rugs  50 C form an asymmetric chevron pattern  70 , 72  having a point  74  of the chevron  70 , 72  located between the equatorial plane EP and the second lateral edge  33 B. A primary leg  76  of the chevron  70 , 72  lies along the substantially aligned lengths of the shoulder lugs  50 A of the first row and the central lugs  50 C. A secondary leg  78  of the chevron  70 , 72  lies along the length of the shoulder lugs  50 B of the second row and the point  74  of the chevron  70 , 72 . 
     In the preferred embodiment each of the shoulder lugs  50 A of the first row are similar in shape and length. Similarly, each of the shoulder lugs  50 B of the second row are of similar shape and length. It is most preferred that both the shoulder lugs  50 A and  50 B of the first and the second row are of similar shape and length. This is believed to improve the uniform wear of this tread pattern. 
     It is believed preferable that the circumferentially adjacent chevrons  70 , 72  have a circumferential overlap (O) as measured by axially extending lines  84 , 86 , the overlapping (O) distance between these lines  84 , 86  at the extremes of the circumferential overlap being at least 25% of the total circumferential extent (T) of a chevron  70 , 72  enables the tire  20  to achieve extremely uniform ride and handling characteristics. The shoulder lugs  50 A, 50 B each have an axially outer end  53  and an axially inner end  51 . The axially outer ends  53  of the shoulder lugs  50 A of the first row are circumferentially offset relative to the axially outer ends  53  of the shoulder lugs  50 B of the second row as measured as the distance X circumferentially between axial lines  90 , 92  tangent to the extremes of the axially outer ends  53 . The circumferential offset distance X is at least 75% of the circumferential distance between the axially inner  51  and axially outer ends  53  of the shoulder lugs  50 A of the first row. In a preferred embodiment of the tire  20 , the tire  20  has a net gross ratio as measured around the entire circumference of the tire of less than 35%, preferably about 22%. 
     In order to maintain this open tread pattern, each tread lug  50 A, 50 B, 50 C is spaced a minimum distance(s) of 1.5 the lug width (lw) from an adjacent lug  50 A, 50 B, or  50 C. The central lugs are spaced a minimum distance of 1.5 times the central lug width from an adjacent central lug such that the central lugs do not overlap in the circumferential direction. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The following is a brief description of the drawings in which like parts bear like reference numerals and in which: 
     FIG. 1 is a perspective view of the preferred tire according to the present invention. 
     FIG. 2 is a plan view of the preferred tire according to the present invention. 
     FIG. 3 is a fragmentary view of the tread portion of the preferred tire according to the present invention. 
     FIG. 4 is a cross-sectional view of the preferred tire taken along lines  5 — 5  of FIG.  2 . 
     FIG. 5 is a plan view of a portion of the contact patch of the preferred tire according to the present invention. 
     FIG.  6 A and FIG. 6B are fragmentary view of a tread portion of a first and second embodiment of the inventive tire. 
     FIG. 7A and 7B are schematic view of the tire according to the invention mounted to a vehicle. 
     Definitions 
     “Aspect ratio” of the tire means the ratio of its section height (SH) to its section width (SW) multiplied by 100% for expression as a percentage. 
     “Axial” and “axially” means lines or directions that are parallel to the axis of rotation of the tire. 
     “Bead” means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chafers, to fit the design rim. 
     “Belt reinforcing structure” means at least two layers of plies of parallel cords, woven or unwoven, underlying the tread, unanchored to the bead, and having both left and right cord angles in the range from 17 degrees to 27 degrees with respect to the equatorial plane of the tire. 
     “Carcass” means the tire structure apart from the belt structure, tread, undertread, and sidewall rubber over the plies, but including the beads. 
     “Circumferential” means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction. 
     “Design rim” means a rim having a specified configuration and width. For the purposes of this specification, the design rim and design rim width are as specified by the industry standards in effect in the location in which the tire is made. For example, in the United States, the design rims are as specified by the Tire and Rim Association. In Europe, the rims are as specified in the European Tyre and Rim Technical Organization-Standards Manual and the term design rim means the same as the standard measurement rims. In Japan, the standard organization is The Japan Automobile Tire Manufacturer&#39;s Association. 
     “Design rim width” is the specific commercially, available rim width assigned to each tire size and typically is between 75% and 90% of the specific tire&#39;s section width. 
     “Equatorial plane (EP)” means the plane perpendicular to the tire&#39;s axis of rotation and passing through the center of its tread. 
     “Footprint” means the contact patch or area of contact of the tire tread with a flat surface at zero speed and under normal load and pressure. 
     “Inner” means toward the inside of the tire and “outer” means toward its exterior. 
     “Lateral edge” means the axially outermost edge of the tread as defined by a plane parallel to the equatorial plane and intersecting the outer ends of the axially outermost traction lugs at the radial height of the inner tread surface. 
     “Leading” refers to a portion or part of the tread that contacts the ground first, with respect to a series of such parts or portions, during rotation of the tire in the direction of travel. 
     “Net-to-gross ratio” means the ratio of the normally loaded and normally inflated tire tread rubber that makes contact with a hard flat surface, divided by the area of the tread, including non-contacting portions such as grooves as measured around the entire circumference of the tire. 
     “Normal inflation pressure” refers to the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire. 
     “Normal load” refers to the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire. “Radial” and “radially” means directions radially toward or away from the axis of rotation of the tire. 
     “Radial-ply tire” means a belted or circumferentially restricted pneumatic tire in which the ply cords, which extend from bead to bead are laid at cord angles between 65° and 90° with respect to the equatorial plane of the tire. 
     “Section height” (SH) means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane. 
     “Section width” (SW) means the maximum linear distance parallel to the axis of the tire and between the exterior of its sidewalls when and after it has been inflated at normal pressure for 24 hours, but unloaded, excluding elevations of the sidewalls due to labeling, decoration or protective bands. 
     “Tire design load” is the base or reference load assigned to a tire at a specific inflation pressure and service condition: other load-pressure relationships applicable to the tire are based upon that base or reference. 
     “Trailing” refers to a portion or part of the tread that contacts the ground last, with respect to a series of such parts or portions during rotation of the tire in the direction of travel. 
     “Tread arc width” (TAW) means the width of an arc having its center located on the plane (EP) and which substantially coincides with the radially outermost surfaces of the various traction elements (lugs, blocks, buttons, ribs, etc.) across the lateral or axial width of the tread portions of a tire when the tire is mounted upon its designated rim and inflated to its specified inflation pressure but not subjected to any load. 
     “Tread width” means the arc length of the tread surface in the axial direction, that is, in a plane passing through the axis of rotation of the tire. 
     “Unit tread pressure” means the radial load borne per unit area (square centimeter or square inch) of the tread surface when that area is in the footprint of the normally inflated and normally loaded tire. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now referring to FIG. 5, a tire is shown in cross-section view generally as reference numeral  20 . The pneumatic tire  20  has a carcass  21  having one or more carcass plies  22  extending circumferentially about the axis of rotation of the tire  20 . The carcass plies  22  are anchored around a pair of substantially inextensible annular beads  24 . A belt reinforcing structure  26  comprising one or more belt plies  28  is disposed radially outwardly from the carcass plies  22 . The belt plies  28  provide reinforcement for the crown region of the tire  20 . A circumferentially extending tread portion  32  is located radially outwardly of the belt reinforcing structure  26 . 
     A sidewall portion  33  extends radially inwardly from each axial or lateral tread edge  33 A, 33 B of the tread to an annular bead portion  35  having the beads  24  located therein. 
     The carcass plies  22  preferably have textile or synthetic cords  22 A reinforcing the plies  22 . The cords  22 A are preferably oriented radially. Most preferably, the cords  22 A are made of polyester or nylon material. Typically, the tire  20  may have two, three or four plies  22 , each construction increasing in load carry capability as a function of the number of plies. 
     The belt reinforcement structure  26  preferably includes at least two belts  28  reinforced by synthetic cords of rayon or aramid. 
     Now referring to FIGS. 1-5, a tire  20  according to the present invention is illustrated. The tire  20  according to the present invention has a unique tread  32 . The tread  32  has a first tread edge  33 A and a second tread edge  33 B. Disposed between the tread edges  33 A, 33 B is an inner tread  34  and a plurality of lugs  50 A, 50 B and  50 C extending radially outwardly from the inner tread  34 . 
     As illustrated in FIG. 4 each lug  50 A, 50 B and  50 C has a radially outer surface  58 , a first edge  52 , second edge  54  and a centerline  63  between the first and second edges. Each lug  50 A and  50 B extends generally axially inwardly from an axially outer end  51  to an axially inner end  53 . Each lug  50  intersects the equatorial plane EP and has an orientation substantially aligned with the lugs  50 A as shown. 
     As illustrated in FIGS. 6A and 6B the radially outer surface  58  when viewed from the contact patch has a polygonal shape. The surface  58  when encompassed by a rectangle  65  exhibits the approximate orientation of the lug  50 A, 50 B, 50 C. For purposes of this invention the centerline  63  of the lugs  50 A, 50 B or  50 C is approximated by a line extending substantially parallel to the first and second edges  52 , 54  and being generally equidistanced between these edges. 
     It is important to note that lugs have a length l l , at least three times their width l w  whereas block elements have a width greater than one-third the length of the element. A lug for purposes of this invention has a length l l  at least 10% of the section width (SW) of the tire  20 . 
     The distance along the centerline  63  between the axially outer and inner ends  51 , 53  defines the length (l l ) of the lug  50 . 
     The distance extending substantially perpendicularly between the first and second edges  52 , 54  of the lug define the lug width (l w ). The radial distance extending between the inner tread  34  and the edges  52 , 54  of the lug  50  defines the radial lug height (l h ). Preferably, the ratio of the shoulder lug width (l w ) to lug radial heights (l h ) is less than two-thirds over at least 70% of the lug length (l l ). 
     As the shown in FIGS. 6A and 6B the lugs  50 A, 50 B, and  50 C are oriented in such a fashion that the shoulder lug  50 A and the central lug  50 C form the primary leg  76  of a chevron shape  70 , 72  while the shoulder lug  50 B is oppositely inclined and forms a portion of the secondary leg  78  of the chevron  70 , 72 . The lug  50 B when connected to the inner point  74  of the chevron  70 , 72  at the end  53  of the central lug  50 C forms the entire secondary leg  78  of the chevron  70 , 72 . 
     A centerline  63  drawn between the leading edges  52  and trailing edges  54  of each lug  50 A, 50 B and  50 C established the general shape of the chevron pattern  70 , 72 . 
     In the preferred embodiment the chevron  70 , 72  appearance is similar to a pair of bird wings on the peak of a downward stroke wherein one wing is longer than the opposite wing. This arcuate shape of each leg of the chevron  70 , 72  creates soil discharge channels  80 , 82  between the lugs  50 A, 50 B and  50 C. The soil discharge channel  80  extending outward through the first lateral edge  33 A is substantially larger than the soil discharge channel  82  extending outward through the second lateral edge  33 B. Each leg, primary and secondary  76 , 78  can preferably be spaced an equal distance from a circumferentially adjacent respective primary or secondary leg  76 , 78 . In other words, the chevron patterns  70 , 72  can be uniformly repeated around the circumference of the tire  20 . 
     The total circumferential extent of the chevrons  70 , 72  is shown in FIGS.  6 A, 6 B as T. Circumferentially adjacent chevrons  70 , 72  overlap a distance O, O being at least 25% of T as measured between the axial extending  84 , 86  and  88 . The shoulder lug  50 B as shown is circumferentially offset from the shoulder lug  50 A by a distance X, X being measured as the distance between axial lines  90 , 92  as shown in FIGS.  6 A, 6 B and wherein X is at least 50% preferably about 75% of the circumferential extent of the shoulder lug  50 A as measured between lines  92  and  84 . 
     In one embodiment of the invention these tires  20  are made in two distinct molds such that the short secondary legs  78  of the chevron  70  are positioned closest to the lateral edge nearest the tractor  10  when the tires  20  are mounted as shown in FIG.  7 A. The resultant effect is that the two tires  20 A and  20 B working combination push more solid laterally away from the vehicle  10 . The lateral forces are balanced and thus offset while the displaced soil act upon the tires tending to give a resultant forward propulsion. This feature would be noticeable in very wet loose or mucky soil conditions. 
     Interestingly, for cost and performance efficiency it has been found that the tires  20  can be produced in a single mold and mounted as shown in FIG. 7B wherein the primary leg  76  of the chevron  70 , 72  is positioned such that tire  20 A has the primary leg  76  outboard of the vehicle while tire  20 B has the primary leg  76  inboard of the vehicle  10 . This means that the large soil discharge channels  82  are not working in opposite or a balanced configuration as shown in FIG.  7 A. Ordinarily one would speculate that the tires  20 A, 20 B would create a slippage moment around the vehicle  10 . Interestingly it has been found that the tire  20 B anchors the tractor  10  while tire  20 A working with tire  20 B propel the vehicle forward. 
     Historically, farm tire designers had heretofore always tried to balance the soil discharge channels  80 , in the contact patch such that the amount of soil channeled on each side of the equatorial plane (EP) was equal. This design factor has been the convention even when unequal channels were employed circumferentially. The designer always attempted to achieve this balanced loading effect by alternating the large and small channels on each side of the tread. 
     The tire  20  of the present invention clearly breaks from that conventional practice, it has an asymmetric directional tread pattern that can maintain the vehicles traction even though employing an imbalanced soil discharge volume, one side of the tire having soil discharge channels  80  being substantially greater than the opposite side  82 . 
     As shown in FIGS. 3 and 5 the preferred tire  20  has both shoulder lugs  50 A, 50 B oriented similarly but opposite in hand. The centerline  63  is broken into an axially inner portion  63 A and an axially outer portion  63 B, the outer portion being inclined at an  α0, α0  being in the range of 60° to 90° relative to the equatorial plane EP. The axially inner portion  63 A is inclined at an angle  α i of about 45°. 
     The central lug  50 C has a centerline  63  divided into three parts  63 A, 63 B, 63 C. The ends  63 A and  63 B being inclined similarly at a very steep angle β, wherein β, is less than 45° relative to the equatorial plane preferably about 30°. The central portion  63 B of the central lug is oriented at an angle θ of about 45° relative to the equatorial plane. 
     As shown in FIG. 6B an alternative embodiment of the tire  20  is shown wherein the lugs  50 A, 50 B and  50 C are shown as curvilinear lugs. The lugs  50 A and  50 C follow a generally singular curvature and are generally aligned along their lengths. Shoulder lug  50 B is oriented such that the leading edge  52  intersects the point  74  of the chevron  72 . As can be more readily seen the soil discharge channels  80 , 82  similarly exhibit the volumetric imbalance taught in the preferred embodiment tire of FIGS. 1-6A. 
     A most beneficial feature exhibited in both tires is that the resultant lug lengths l l  are quite long as a result of the use of the asymmetric chevron patterns  70 , 72 . 
     When one compares the inventive tire to the prior art tires, the following is observed in a 480/80R42 (18.4R42) size: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Shoulder Lugs I t   
                 Central Lugs I t   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Inventive Tire 
                 7.87 in. (20 cm) 
                 5.74 (14.6 cm) 
               
               
                 DT710 
                 6.50 in. (16.5 cm) 
                 5.55 (14.1 cm) 
               
               
                 DynaTorque 
                 7.36 avg. (18.7 cm) 
                 NONE 
               
               
                   
                 (8.64 in. long 6.07 in. short) 
               
               
                   
               
             
          
         
       
     
     As can be seen the shoulder lugs are longer than typically can be employed while at the same time being equal in length. This means that the tire will wear uniformly while also achieving no degradation in tractive performance. As can be seen the tire according to the present invention has the lug length of the shoulder lugs greater than the average of the long bar/short bar prior art tires i.e., l l &gt;½ (L L +L s ). 
     Also due to the use of the asymmetric chevron  70 , 72  less lugs are needed to create the beneficial circumferential overlap to reduce harsh ride and vibrational problems associated with long bar/short bar tread patterns i.e., N shoulder lug  50 A and  50 B&lt;N (L L+L   S ) while N shoulder lug  50 A, 50 B+central lugs  50 C&gt;N (L L +L S ). Accordingly, the total surface areas (SA) at the center two-thirds of the tread width (TW) is greater than the total surface area in the same center two-thirds region of the prior art long bar/short bar tread pattern. This means that overall wear durability as well as improved ride characteristics are achieved by the inventive tire when compared to the prior art tires. 
     It must be appreciated that the actual shape of the individual lugs can be varied as well as their orientation without departing from the spirit of the invention. Furthermore, it is understood that the point  74  of the chevron can lie on one or the other tread half but not both.
A farm tractor has a vehicle having a pair of pneumatic agricultural tires 20 having an asymmetric directional tread 32 with lugs 50 A, 50 B, 50 C arranged in a chevron type pattern 70,72. The chevron pattern 70,72 is asymmetric having the apex or point 74 lying entirely on one-half 32 A or 32 B of the tread 32 thus creating soil discharge channels 89,82 that are greater on one tread half 32 A or 32 B relative to the other tread half 32 A or 32 B. This tread pattern permits the employment of lugs 50 A, 50 B of substantially equal lug length l l insuring more uniform wear characteristics.