Abstract:
A tire tread  10  for a pneumatic tire. An external surface  12  of the tread having at least one groove  14  for enclosing and channeling water during use of the tire on wet pavement. The respective groove  14  having at least two surfaces defining a channel. The two surfaces including two side surfaces. A series of peaks  22  and valleys  24  extending across at least one surface. In a preferred embodiment, the peaks  22  and valleys  24  being on two opposing surfaces and having identical wavelengths. The peaks  22  and valleys  24  on a first side surface  16  being 180 degrees out of phase from the peaks  22  and valleys  24  on the second side surface  18.

Description:
TECHNICAL FIELD 
     This invention relates to the tread of a pneumatic tire and, more particularly, to the ability of a groove within the tread to eject water. 
     BACKGROUND ART 
     Tire designers are continually striving to improve tire performance. One goal in improving tire performance is to improve the traction between the tire and the road surface in wet conditions. When a vehicle is travelling on a wet road surface at high speeds, hydroplaning of the tires can occur. Hydroplaning is caused by the tire pushing water in front of it as it advances along a road surface. As the tire continues to push the water in front of it, the back pressure of the water increases and progressively lifts the tire ground-contact area off of the pavement. This back pressure is a function of the depth of the water and the speed of the tire. Eventually, with sufficiently deep water and tire speed, the back pressure lifts the tire off of the road surface. When a tire is hydroplaning, there is no traction between the tire and the road surface and thus, control of that tire is lost. 
     To prevent hydroplaning, tire designers are continually attempting to improve the ability of a tire to eject or channel water away from the tire. U.S. Pat. No. 5,503,206 discloses a tire having improved wet traction to avoid hydroplaning. The tire that is disclosed in this patent has an annular aqua channel and lateral grooves that direct water from the footprint to either the shoulder area or the aqua channel of the tire where it is ejected away from the tire. 
     Providing grooves for the water to flow through is the first step in improving a tire&#39;s wet traction. The second step in ensuring that the tire can efficiently eject the water from these grooves. As the tire travels along the road surface, each groove within the tire ground-contact area forms a channel that is enclosed on all sides. 
     Since each groove within the tire ground-contact area forms a channel, to roughly estimate whether the water flow through each groove is laminar or turbulent, the groove section located in the tire ground-contact area can be analogized to a pipe. The determination of whether flow through a pipe is laminar or turbulent flow is determined by calculating the Reynolds number Re. The Reynolds number Re for flow though a circular pipe is calculated from the equation: Re=ρDv/μ, where ρ is the density of the fluid, D is the diameter of the pipe, μ is the dynamic viscosity of the fluid, and v is the velocity of the water. Where the groove and road surface combination does not approximate a circular pipe, the diameter D can be replaced by the hydraulic diameter dh, where dh=4F/U, where F is the cross-sectional area of the opening and U is the perimeter distance around the opening. Generally, if the Reynolds number Re is greater than 2320, then the flow is expected to be turbulent. For example, the flow of water at a temperature of 5° C.(40° F.) through a 1 cm wide groove on a tire traveling 29 meters per second (approximately 65 mph), estimated using the circular pipe formula, has a Reynolds number Re of 190,789. Thus, the water flow through the groove of a tire travelling at this speed will be turbulent. 
     Turbulent flow contains eddies or vortices, as shown in FIG. 1 As a result of these eddies, the drag along a surface is higher for turbulent flow than for laminar flow. This drag, known as skin friction drag, decelerates the flow along a surface and forms a boundary layer. Since the flow in the boundary layer is decelerated, the overall flow is reduced. 
     U.S. Pat. No. 4,706,910 discloses a flow control device that reduces skin friction on aerodynamic and hydrodynamic surfaces. The reduced skin friction is achieved by modifying the micro-geometry of the surfaces by adding riblets or large eddy breakup devices. 
     U.S. Pat. No. 4,750,693 discloses a device for reducing the frictional drag on a surface of a body in a flowing medium. The surface is provided with an asymmetrical microstructure in the form of a grooved profile. 
     U.S. Pat. No. 4,865,271 discloses a wall surface with an array of small longitudinal projections or riblets for reducing drag across the surface. The riblets modify the boundary layer flow over the surface to reduce the surface drag. 
     U.S. Pat. No. 5,133,519 discloses a device that reduces skin friction drag caused by turbulent shear flows of a fluid over a wall surface. The device includes rearward facing microsteps that reduce the drag caused by eddies. 
     Devices that reduce skin friction drag have received a great deal of attention in recent years, especially on the surfaces of air, water, and land vehicles. The reduction of skin friction drag caused by these devices can result in increased fuel efficiency for aircraft that results in savings of millions of dollars per year. Such devices may also be used in pipelines, as suggested by U.S. Pat. No. 4,907,765. However, drag reduction devices have never been incorporated into tire technology. Although the flow of water through a tread groove may be analogous to the flow of water through a pipe, a tire designer would not look to pipe technology in designing a tread. First, the leading edge of the tire footprint attempts push much of the surface water out of the path of the tire. Secondly, for the water that does enter the grooves, there are three main distinctions between the flow of water through a tread groove and that through a pipe: (1) in a pipe, the water is in motion whereas, in a tread groove, the water is relatively stationary and the groove is in motion, (2) the water flowing through a pipe is in motion relative to all sides of the pipe; whereas, in a tread groove, the water flowing through the groove is in motion relative to only a portion of the enclosed channel since there is little or no motion of the water relative to the ground surface, and (3) in a pipe, the pipe walls remain stationary; whereas, in a tire tread, the surfaces of a groove are subject to vibrations when the tire is in motion. Even when the pressure of the water entering the groove near the leading edge of the tire footprint creates motion of the water forcing it toward the rear of the footprint, the velocity of the water across the road surface if minimal compared to that across the surface of the groove. 
     International Patent Application Number PCT/JP94/02229 to Fukato disclosed a groove in a tread surface of a tire having a continuously waved bottom surface whose top does not reach the tread surface which claims to increase the ability to discharge water while avoiding an increase in the proportion of the groove. Unfortunately, because such a groove requires a bottom surface, that groove inherently must be very wide to have any effect. The paradox is wide grooves already have the capacity to discharge large volumes of water and resist hydroplaning. Applicants present invention works efficiently on narrow “V” shaped grooves having no bottom surface or narrow bottom surfaces. Greatly increasing the value of the invention concept allowing for greatly reduced groove void volumes that are superior in water discharging than conventional grooves. 
     SUMMARY OF THE INVENTION 
     This invention provides a tire tread for a pneumatic tire. An external surface of the tread has at least one groove for enclosing and channeling water during use of the tire on wet pavement. The groove has at least two surfaces defining a channel. The two surfaces include two side surfaces. The respective side surfaces begin at the external surface of the tread and extend radially inwardly toward an axis of rotation of the tire. The two side surfaces either intersect with one another or with a bottom surface. The groove having a depth defined by an average distance from the external surface of the bead to the intersection of the two side surfaces or to the bottom surface of the groove. A median plane bisects the channel formed by the respective surfaces of the groove. The groove has a width defined by twice an average distance from the median plane to a respective side surface. 
     The tire tread is characterized by a series of peaks and valleys located on both side surfaces of the groove wherein each valley extends continuously from one side surface of the groove to the other side surface of the groove. An imaginary line or arc located on the media plane within the depth of the groove extends along the length of the groove. At least half of the valleys following imaginary lines skewed with respect to the median plane or arc by an angle or angles in the range of 45 to 90 degrees from the median plane. Each peak has a maximum depth of 15% of the groove width and a minimum depth of at least 5% of the groove width. The depth is defined as the average distance from the peak to the valley. 
     In a preferred embodiment, the peaks and valleys are on two opposing surfaces of the groove. The peaks and valleys, on the two opposing surfaces, have identical wavelengths and the peaks and valleys on a first surface are 180 degrees out of phase from the peaks and valleys on a second surface. 
     The inventor believes that the use of the claimed invention can help reduce the undesirable effects of the eddies within the boundary layer of water contacting a groove surface. As a result, the skin friction drag along the respective surface of the groove will be reduced and the flow of water from the groove should be increased. 
     Definitions 
     For ease of understanding this disclosure, the following terms are disclosed. 
     “Boundary layer” means the region close to the surface of a solid body over which a fluid flows where the fluid viscosity has an effect. The viscous effect within the region is evidenced by a reduction in velocity of the fluid as the surface is approached. 
     “Eddy” or “eddies” means a vortexlike motion of a fluid running contrary to the main current. 
     “Groove” means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight, curved, or zigzag manner. Circumferentially and laterally extending grooves sometimes have common portions and may be sub-classified as “wide,” “narrow,” or “slot.” A “slot” is a groove having a width in the range from about 0.2% to 0.8% of the compensated tread width, whereas a “narrow” groove has a width in the range from about 0.8% to 3% of the compensated tread width and a “wide” groove has a width greater than 3% thereof 
     “Laminar flow” means streamline flow of an incompressible, viscous Newtonian fluid; all particles of the fluid move in distinct and separate lines. 
     “Pneumatic tire” means a laminated mechanical device of generally toroidal shape, usually open torus, having beads and a tread and made of rubber, chemicals, fabric and steel or other materials. When mounted on the wheel of a motor vehicle, the tire through its tread provides traction and contains the fluid that sustains the vehicle load. 
     “Reynolds number” is a dimensionless number that is significant in the design of a model of any system in which the effect of viscosity is important in controlling the velocities or the flow pattern of the fluid. 
     “Tread” means a molded rubber component which, when bonded to a tire casing, includes that portion of the tire that comes into contact with the road when the tire is normally inflated and under normal load. 
     “Turbulent flow” means flow in which the motion of the fluid is subjected to irregular velocities and pressures and results in motion in a random manner. Eddies are located in turbulent flow. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is depicts the flow of water through a prior art groove on a tire tread; 
     FIG. 2 is depicts the flow of water through a groove of the invention; 
     FIG. 3 is a view of an embodiment of the groove surface undulations; 
     FIG. 4 is a view of a second embodiment of the groove surface undulations; 
     FIG. 5 is a view of a third embodiment of the groove surface undulations; 
     FIG. 6 is a view of a fourth embodiment of the groove surface undulations; 
     FIG. 7 is a view of a preferred embodiment of the groove surface undulations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 depicts the flow of water through the groove  14  of a tire tread  10 . The groove  14  contains a series of peaks  22  and valleys  24 , also called eddy breakup devices. The surfaces of the groove  14  in the tire tread  10  include at least two surfaces. The groove illustrated in FIG. 2 contains three surfaces, a first side surface  16 , a second side surface  18 , and a bottom surface  20 . When the external tread surface  12  adjacent to the groove  14  contacts the road surface, the groove  14  forms a channel and encloses any water remaining on the road surface. The arrow or arrows shown in each figure depicts the direction of water flow in relation to the groove  14 . 
     When the flow of water through the groove  14  is turbulent, many eddies or vortices are present in the flow. Since portions of these eddies flow in a direction contrary to the main current, the flow creates high shear stresses on the groove surfaces and results in increased skin friction drag. Skin friction drag is the drag caused by flow of a fluid over a surface of a solid body. The peaks  22  and valleys  24  of this invention disrupt the eddies or vortices that are present along the respective groove surface. The peaks  22  and valleys  24  disrupt the eddies by disrupting the tangential flow and the reverse flow regions of the eddies. As a result, the skin friction drag along the surface of the groove  14  is decreased. This decreased skin friction drag results in an increased flow of the water near the respective surface of the groove  14  and thus, an increased flow of water from the groove  14 . 
     FIG.  3  through FIG. 6 show different embodiments of peaks  22  and valleys  24  of the invention. FIG. 3 shows the peaks  22  and valleys  24  being a plurality of sinusoidal waves. FIG. 4 shows the peaks  22  and valleys  24  forming a sawtooth configuration. Each peak in the series of peaks  22  and valleys  24  is separated by a pitch P 1  and has a depth D 1 . Both the pitch P 1  and the depth D 1  can be optimized for a given Reynolds number. Since the Reynolds number is dependant upon velocity and the groove dimensions, the pitch P 1  and the depth D 1  can be optimized for a particular speed and groove size. For example, if a tire manufacturer decides to optimize wet traction for a tire at 29 meters per second (approximately 65 mph), the manufacturer could optimize the pitch P 1  and the depth D 1  of the peaks  22  and valleys  24  to provide the greatest reduction of skin friction drag at that velocity. Generally, the pitch P 1  will be less than 40% of the groove width GW and the depth D 1  will range from a maximum of 15% of the groove width GW to a minimum of 5% of the groove width GW. Preferably, the pitch P 1  will be less than 5 mm and the depth D 1  less than 3 mm. Additionally, the pitch P 1  and the depth D 1  of the peaks  22  and valleys  24  can be varied along the length of the groove  14 . The void volume of a groove  14  having the peaks  22  and valleys  24  is preferably at least 70% of the void volume of the groove  14  with no peaks  22  and valleys  24 . 
     FIG. 5 is a view of an embodiment of the peaks  22  and valleys  24  where the depth D 1  of each peak  22  varies as it extends across the respective surface. This varied depth D 1  can be used to form a number of riblets  26 . The riblets  26  may be separated by valleys  24  formed by V-shaped grooves  28  or smooth grooves. The peak  22  of each riblet  26  may come to a point or may be smooth. As with the previous peaks  22  and valleys  24 , the pitch P 1  and the depth D 1  of the riblets  26  can be optimized for a particular Reynolds number. For best results the entire surface of the groove  14  should be covered with riblets  26 , as illustrated. FIG. 6 shows a view of an embodiment of the peaks  22  and valleys  24  being separated by a series of V-shaped grooves  28  where the depth D 1  of each peak  22  does not vary as it extends across a groove surface. 
     FIG. 7 is a view of a preferred embodiment of a groove  14  having peaks  22  and valleys  24 . These surface peaks  22  and valleys  24  are sinusoidal waves. The peaks  22  and valleys  24  are present on two opposing surfaces and extend from the external surface  12  of the tread  10  adjacent a first side surface  16  to an external surface  12  of the tread  12  adjacent a second side surface  18 . The groove  14  is shaped such that the surface peaks  22  and valleys  24  on the first side surface  16  are interconnected to those on the bottom  20  which are interconnected to those on the second side surface  18 . Thus, the surface peaks  22  and valleys  24  on the first side surface  16  have the same wavelength as the surface peaks  22  and valleys  24  on the second side surface  18  and the bottom  20 . The surface peaks  22  and valleys  24  of the first side surface  16  are 180 degrees out of phase from the surface peaks  22  and valleys  24  of the second side surface  18 . In this preferred embodiment, the surface peaks  22  and valleys  24  are symmetrical such that the tire containing these surface peaks  22  and valleys  24  can be non-directional. The peaks  22  and valleys  24  that are not symmetrical, such as those depicted in FIG. 4, must be located on a directional tire to work at their optimal level. 
     The groove  14 , in the preferred embodiment, has a depth defined by an average distance from the external surface  12  of the tread  10  to the bottom surface  20 . A median plane bisects the channel formed by the respective surfaces of the groove  14 . An imaginary line or arc is located on the median plane within the depth of the groove  14 . If the groove  14  runs circumferentially, then the imaginary arc will follow the curvature of the tire. At least half of the valleys  24  of the respective surface containing the peaks  22  and valleys  24  following imaginary lines that are skewed with respect to the median plane line or arc by an angle in the range of from 45 degrees to 90 degrees. In the preferred embodiment, the valleys following imaginary lines that are skewed with respect to the median plane line or arc by an angle of 90 degrees. This angle measured by transposing each respective line or arc into the same plane and measuring the angle at the intersection of the respective lines. 
     Although only a few embodiments of peaks  22  and valleys  24  have been described in this application, additional configurations are contemplated by this invention. The peaks  22  and valleys  24  can be molded into the groove  14  of the tread  10  during manufacture of the tread  10  of the tire. A further possibility is to carve the peaks  22  and valleys  24  into the tread  10  of a finished tire. 
     The use of the peaks  22  and valleys  24  in the groove  14  of a tire tread  10  can have one of two purposes. First, the peaks  22  and valleys  24  can increase the amount of water ejected from the groove  14  at a particular velocity and, thus increase the wet traction of the tire. Second, the peaks  22  and valleys  24  can allow an equal amount of water ejection from a smaller groove, thus allowing the tire designers to increase the net to gross ratio of the tire, resulting in improved dry traction, while maintaining wet water traction at a particular velocity.