Tire performance evaluation method and tire design method

Abstract

To simply evaluate wet performance at a low temperature environment.SOLUTION: At first temperature 0 to 10°C and second temperature 20 to 40°C, fluorescent liquid 34 is mediated between a transparent plate 32 and a tread land 20 with an uneven surface 32A equivalent to a real road surface, and the tread land 20 is contacted on an uneven surface 32. An excitation light is irradiated to the fluorescent liquid 34, brightness distribution of fluorescence emitted by the fluorescent liquid 34 is measured, and a contact area of the tread land 20 relative to the uneven surface 32 is measured. Wet performance of a tire in a low temperature environment is evaluated on the basis of ratio of a contact area AL measured at the first temperature to a contact area AH measured at the second temperature.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a tire performance evaluation method and a tire design method. [Background technology]

[0002] Conventionally, a method for measuring tire ground contact has been to place a tire on a transparent, smooth, flat plate, photograph the tire's contact surface from the back side of the transparent plate, and evaluate the ground contact from the obtained image of the tire. However, the ground contact when the tire is pressed against the flat plate is different from the ground contact of the tire on an actual road surface. Therefore, for example, Patent Documents 1 and 2 evaluate the ground contact using a transparent resin road surface with irregularities that simulate an actual road.

[0003] Furthermore, Patent Document 3 discloses that by devising a prediction formula using the contact area with road surface irregularities, tan δ, and other physical properties at temperatures below 0°C and above 20°C, it is possible to predict the temperature dependency of a tire's friction coefficient, thereby shortening the tire development period.

[0004] However, with conventional methods, there are many design parameters linked to tire performance, making it difficult to identify the cause and improve design elements when problems arise during actual prototype development. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-240681 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-84428 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-103618 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when evaluating braking performance on wet roads (hereinafter referred to as wet performance) using an actual vehicle, a tire may perform roughly the same as a comparative tire at room temperature, but its performance may deteriorate significantly in a low-temperature environment. To identify and improve the cause of this deterioration, a simple method for evaluating wet performance in a low-temperature environment is required. However, evaluating wet performance in a low-temperature environment using an actual vehicle requires considerable time and cost. Therefore, a simpler method for evaluating wet performance in a low-temperature environment is needed.

[0007] An object of an embodiment of the present invention is to provide a tire performance evaluation method that can easily evaluate wet performance in a low-temperature environment. [Means for solving the problem]

[0008] The present invention includes the embodiments shown below. [1] A tire performance evaluation method comprising the steps of: placing a fluorescent liquid between a transparent plate having an uneven surface with unevenness equivalent to an actual road surface and a tread land, and grounding the tread land against the uneven surface at a first temperature of 0°C or higher and 10°C or lower; and a second temperature of 20°C or higher and 40°C or lower, the second temperature being 15°C or higher from the first temperature; irradiating the fluorescent liquid with excitation light; measuring the luminance distribution of the fluorescence emitted from the fluorescent liquid; measuring the contact area of ​​the tread land with respect to the uneven surface; and evaluating the wet performance of a tire in a low-temperature environment of 0°C or higher and 10°C or lower based on the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature. [2] The tire performance evaluation method according to [1], wherein the contact surface of the tread land protrudes radially outward from the tread reference contour line in a tire meridian cross section.

[0009] [3] A tire design method comprising the steps of: placing a fluorescent liquid between a tread land and a transparent plate having an uneven surface with unevenness equivalent to an actual road surface, the tread land being brought into contact with the uneven surface, irradiating the fluorescent liquid with excitation light, measuring the luminance distribution of the fluorescence emitted from the fluorescent liquid, measuring the contact area of ​​the tread land with the uneven surface, and setting a difference ΔE' in the storage modulus of the tread rubber between the first temperature and the second temperature and / or a difference in elevation of the contact area of ​​the tread land based on the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature. [4] The tire design method according to [3], wherein when the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature is smaller than a certain value, the tread rubber is changed to a rubber having a smaller difference ΔE' in storage modulus. [5] A tire design method according to [3] or [4], wherein the difference in height of the contact area of ​​the tread land is reduced when the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature is smaller than a certain value.

[0010] [6] A tire design method for a plurality of tires, at a first temperature of 0°C or higher and 10°C or lower, and a second temperature of 20°C or higher and 40°C or lower that is 15°C or higher from the first temperature, the method comprising: placing a fluorescent liquid between a transparent plate having an uneven surface with unevenness corresponding to an actual road surface and a tread land, bringing the tread land into contact with the uneven surface; irradiating the fluorescent liquid with excitation light; measuring the luminance distribution of the fluorescence emitted from the fluorescent liquid; measuring the contact area of ​​the tread land with respect to the uneven surface; determining a ratio of the contact area measured at the first temperature to the contact area measured at the second temperature; determining a relationship among the obtained ratio of the contact areas, a difference ΔE' in the storage modulus of the tread rubber between the first temperature and the second temperature, and a difference in elevation of the contact surface of the tread land; and when designing a tire, setting the difference ΔE' in the storage modulus and the difference in elevation of the contact surface of the tread land based on the relationship within a range where the ratio of the contact areas satisfies a predetermined condition. [Effects of the Invention]

[0011] The tire performance evaluation method according to the embodiment makes it possible to simply evaluate wet performance in a low-temperature environment. The tire design method according to the embodiment makes it possible to design a tire with good wet performance in a low-temperature environment. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a meridian cross-sectional view showing a portion of a pneumatic tire according to an embodiment. [Figure 2] A meridian cross section of the tire tread land [Figure 3] FIG. 1 is a schematic diagram showing the configuration of an apparatus for measuring a contact area in one embodiment. [Figure 4] Graph showing the change in tread contact area at normal and low temperatures [Figure 5] Graph showing the relationship between the change in tread contact area at normal and low temperatures and wet performance in an actual vehicle [Figure 6] Graph showing the temperature dependence of storage modulus E' of tread rubber [Figure 7] Diagram showing the difference in height ΔL between the tread and ground contact surface [Figure 8] A graph showing the relationship between the ratio of the tread land contact area at normal and low temperatures, the difference in storage modulus of the tread rubber at normal and low temperatures ΔE', and the difference in height of the tread land contact area ΔL. DETAILED DESCRIPTION OF THE INVENTION

[0013] The tire performance evaluation method according to the embodiment is a method for evaluating the wet performance of a tire in a low-temperature environment. More specifically, in one embodiment, the method is capable of predicting whether the wet performance in a low-temperature environment will improve or deteriorate compared to a comparative tire.

[0014] The tire to be evaluated is not particularly limited, and examples thereof include pneumatic tires such as passenger car tires. Fig. 1 is a cross-sectional view in the tire meridian direction showing the tread 10 and its surroundings of a pneumatic tire to be evaluated in an evaluation method according to one embodiment.

[0015] In the drawings, the symbol CL indicates the tire equatorial plane, which corresponds to the tire axial center. In this specification, the tire axial direction refers to the direction parallel to the tire rotational axis, and is indicated by the symbol AD ​​in the drawings. The tire radial direction refers to the direction perpendicular to the tire rotational axis, and is indicated by the symbol RD in the drawings. The tire meridian cross section refers to a cross section of the tire cut by a plane including the tire rotational axis. The tire meridian direction refers to the direction along the tire surface in the tire meridian cross section.

[0016] The pneumatic tire includes a pair of left and right bead portions and sidewalls in addition to a tread 10. The internal structure of the pneumatic tire can be a known structure, and includes, for example, a pair of left and right bead cores, a carcass ply 12 toroidally stretched between the pair of bead cores, and a belt 14 disposed on the outer periphery of the crown portion of the carcass ply 12. Tread rubber 16, which forms the tire's contact surface, is provided on the outer periphery of the belt 14.

[0017] A plurality of (four in this example) main grooves 18 extending in the tire circumferential direction are provided on the surface of the tread 10. A plurality of tread lands 20 are defined and formed in the tread 10 by the main grooves 18.

[0018] Specifically, the tread 10 is provided with a center land 20A located at the center in the tire axial direction AD, a pair of left and right quarter lands 20B, 20B located on both sides of the center land 20A, and a pair of left and right shoulder lands 20C, 20C located outside the quarter land 20B. The tread land 20 may be provided as a continuous rib around the entire tire circumferential direction, or may be provided as a block row by providing lateral grooves that cross the rib.

[0019] The tread land 20 has a contact patch 22 that protrudes outward in the tire radial direction RD with respect to a tread reference contour line 24 in the tire meridian cross section shown in Fig. 2. The protruding shape of the contact patch 22 is not particularly limited, and in this example, as shown in Fig. 2, the contact patch 22 has a convex shape in which a center 22A in the meridian direction bulges outward in the tire radial direction RD in a curved manner relative to both ends 22B, 22B.

[0020] Here, the contact surface 22 of the tread land 20 is the surface that comes into contact with the road surface when the tire touches down on a flat road surface, and is the upper surface of the tread land 20. The tread reference contour line 24 is a curve consisting of one or more smoothly continuous arcs that pass through the open ends of each main groove 18 (the edges in the meridian direction of each tread land 20).

[0021] In the cross section of the tread land 20, the height difference ΔL of the contact patch 22 (hereinafter sometimes referred to as land height difference ΔL) is not particularly limited and may be, for example, 50 to 1000 μm or 100 to 600 μm. The land height difference ΔL is the degree to which the contact patch 22 protrudes outward in the tire radial direction RD, and is the height difference between both ends 22B, 22B of the contact patch 22 and the most protruding part (the meridian center 22A of the land). More specifically, the land height difference ΔL is the distance between a straight line connecting both ends 22B, 22B of the contact patch 22 in the meridian cross section and the point at which the distance to the straight line is greatest. The width of the tread land 20 is not particularly limited and may be, for example, 10 to 60 mm.

[0022] In the tire performance evaluation method according to the embodiment, in order to evaluate the temperature dependency of the wet performance of a tire, a contact analysis is performed on an uneven surface equivalent to an actual road surface using a fluorescence method at a first temperature in the range of 0 to 10°C and a second temperature in the range of 20 to 40°C.

[0023] A method for contact analysis using the fluorescence method can be that described in the above-mentioned Patent Document 2 (JP 2018-84428 A). That is, a fluorescent liquid is interposed between the tread land and a transparent plate having an uneven surface with unevenness equivalent to that of an actual road surface, the tread land is brought into contact with the uneven surface, excitation light is irradiated onto the fluorescent liquid, and the luminance distribution of the fluorescence emitted from the fluorescent liquid is measured to measure the contact area of ​​the tread land with the uneven surface.

[0024] Figure 3 is a schematic diagram showing the configuration of an apparatus for measuring contact area using the fluorescence method. A transparent plate 32 having an uneven surface 32A on one side with unevenness equivalent to that of an actual road surface is placed on a transparent installation stand 30, and the tread land 20 is placed on the uneven surface 32A. A fluorescent liquid 34 is interposed between the uneven surface 32A and the tread land 20. That is, the fluorescent liquid 34 is filled on the uneven surface 32A, and the tread land 20 is placed on the uneven surface 32A via the fluorescent liquid 34.

[0025] Below the installation stand 30 are arranged a light source 36, a filter 38 that transmits and separates only light of a specific wavelength from the light irradiated from the light source 36, a dichroic mirror 40 that reflects only light of a specific wavelength, a mirror 42 that reflects the fluorescence emitted from the fluorescent liquid 34, a filter 44 that transmits and separates only light of a specific wavelength from the emitted fluorescence, and an imaging means 46 that measures the fluorescence that has transmitted through the filter 44.

[0026] The tread land 20 used for measurement may be cut out from a tire to be evaluated. The cut tread land 20 may be attached to a test jig 48 with an adhesive to form a test specimen. Alternatively, a rubber sample corresponding to the tread land 20 may be vulcanized and molded to form a test specimen. Alternatively, the tire to be evaluated may be used as is, and the tread 10 may be brought into contact with the uneven surface 32A, thereby bringing the tread land 20 into contact with the uneven surface 32A.

[0027] The tread land 20 to be measured is preferably the center land 20A and / or the quarter land 20B. When measuring using both the center land 20A and the quarter land 20B, the contact areas AL and AH described below may be calculated as the average of the contact areas measured for the center land 20A and the quarter land 20B. The land height difference ΔL may also be calculated as the average of the height differences between the center land 20A and the quarter land 20B. In the experiments described below, these average values ​​were used. Instead of using the average values, the center land 20A and the quarter land 20B may be evaluated separately.

[0028] The fluorescent solution 34 is an aqueous solution containing a hydrophilic fluorescent dye whose excitation spectrum and fluorescence spectrum have a peak wavelength difference of 100 nm or more. For example, pyranine may be used as the hydrophilic fluorescent dye. In the case of pyranine, the pH of the fluorescent solution 34 is preferably 5 to 8. The concentration of the hydrophilic fluorescent dye in the fluorescent solution 34 is not particularly limited and may be 100 to 10,000 mg / L.

[0029] The load applied when the tread land 20 is brought into contact with the ground is not particularly limited, and for example, a static load equivalent to a ground contact pressure of 100 to 800 kPa may be applied.

[0030] For example, if pyranine is used as the hydrophilic fluorescent dye, ground contact analysis can be performed as follows: An ultraviolet LED (peak wavelength 365 nm) is used as the light source 36 to irradiate excitation light, and excitation light with wavelengths of 400 nm or less is separated using a filter 38 (400 nm low-pass filter). The separated excitation light is reflected by a dichroic mirror 40 and irradiated onto the fluorescent liquid 34 interposed between the tread land 20 and the uneven surface 32A of the transparent plate 32 from the side opposite the uneven surface 32A. This causes the pyranine contained in the fluorescent liquid 34 to transition from the ground state to an excited state. The excited pyranine then returns to the ground state, emitting fluorescence. The emitted fluorescence passes through the dichroic mirror 40 and is reflected by a mirror 42. A filter 44 (400 nm high-pass filter) separates fluorescence with wavelengths of 480 nm or greater. The separated fluorescence is photographed using an imaging device 46 to obtain a luminance distribution (fluorescence intensity image).

[0031] Based on the obtained film thickness distribution, an arbitrary film thickness is used as a threshold value to perform binarization. The area where the film thickness is equal to or less than the threshold value is determined to be the area where the tread land 20 and the uneven surface 32A are in contact. This makes it possible to calculate the area where the tread land 20 and the uneven surface 32A are in contact (the contact area of ​​the tread land).

[0032] In this embodiment, the contact analysis using the above-mentioned fluorescence method is carried out at a first temperature of 0 to 10° C. and a second temperature of 20 to 40° C. Then, wet performance in a low-temperature environment of 0° C. or higher and 10° C. or lower is evaluated based on the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature.

[0033] The first temperature is set to 10°C or less to evaluate the wet performance of the tire in a low-temperature environment. The fluorescent liquid 34 is an aqueous solution containing a small amount of a hydrophilic fluorescent dye such as pyranine, and can be considered to have properties similar to those of water, and will freeze below freezing. Therefore, the first temperature is set to 0°C or more. The first temperature is more preferably between 0°C and 5°C.

[0034] The second temperature is set to 20°C or higher, based on room temperature (22°C), to provide a temperature range where the rubber is soft and provides good ground contact. Furthermore, since the focus of this embodiment is on the deterioration of ground contact due to a decrease in rubber elasticity at low temperatures, the difference between the first and second temperatures is set to 15°C or higher to achieve a wide range of temperatures and increase the change in ground contact. The upper limit of the second temperature does not need to be set too high, and is set to 40°C or lower because the test uses water.

[0035] In one embodiment, the contact area measured at a first temperature may be designated as AL, and the contact area measured at a second temperature as AH, and the ratio of AL to AH (AL / AH) may be calculated. In other words, the temperature dependency of contact performance is evaluated based on the contact area at a second temperature, which includes room temperature. The larger this ratio AL / AH, the more the reduction in contact area in low-temperature environments is suppressed, and wet performance in low-temperature environments tends to improve. Conversely, the smaller this ratio AL / AH, the more the reduction in contact area in low-temperature environments is suppressed, and wet performance in low-temperature environments tends to deteriorate.

[0036] For example, if the index X is set to (AL / AH) × 100, a judgment may be made that an index X of 95 or greater and 100 or less is "good performance," an index X of 90 or greater and less than 95 is "pass," and an index X of less than 90 is "fail."

[0037] In tire development, even if a tire exhibits the same excellent wet performance as a comparison tire in a normal temperature environment (20-40°C), its wet performance may deteriorate compared to the comparison tire in a low temperature environment (0-10°C). Because a full-scale evaluation requires a considerable amount of time and cost, it is necessary to evaluate wet performance in low temperature environments using a simpler laboratory evaluation. In light of this, we have found that the contact area ratio determined by the above-mentioned fluorescence method correlates with the evaluation of wet performance in a full-scale vehicle.

[0038] Specifically, during the development of a certain tire, an on-vehicle test was conducted between the developed tire and a comparative tire. The developed tire exhibited similarly good wet performance to the comparative tire in room temperature environments. However, the performance of the developed tire differed significantly from that of the comparative tire in low-temperature environments, resulting in significantly worse wet performance than the comparative tire. Generally, wet performance tends to improve at lower temperatures within the range of approximately 0 to 40°C. However, the developed tire's performance improvement at low temperatures was small, resulting in poorer wet performance in low-temperature environments compared to the comparative tire. Furthermore, when wet performance was evaluated in the on-vehicle test after a prescribed break-in period, it was confirmed that the developed tire showed less tread surface wear after break-in in low-temperature environments than the comparative tire.

[0039] Further investigation was carried out on the assumption that this phenomenon is due to road contact at low temperatures. In general, rubber hardens at low temperatures and the contact area decreases. If this decrease becomes significant, the tire's road contact will change significantly depending on the temperature range, and it can be predicted that the tire will not be able to perform stably over a wide temperature range.

[0040] Therefore, first, bench tests (laboratory evaluations) were conducted in which the tires were rolled and braked on a flat road surface at room temperature (22°C) and at a low temperature (5°C). As a result, in the laboratory evaluation, the contact patch decreased at low temperature compared to room temperature, but there was no significant difference in the rate of reduction in contact patch between the developed tire and the comparison tire, which differed from the results of the actual vehicle evaluation.

[0041] In response, a contact analysis was performed on an uneven surface equivalent to an actual road surface using the above-mentioned fluorescence method for each of the developed tire (evaluation target tire) and the comparison tire, and the index X was calculated. As a result, as shown in Figure 4, the index X for the comparison tire "Tire A" was 99.3, while the index X for the development tire "Tire B" was 86. Here, tires A and B are summer tires with a tire size of 205 / 55R16 and similar tread patterns. The conditions for the contact analysis were a first temperature of 5°C, a second temperature of 22°C, and a contact pressure of 350 kPa. An aqueous solution of 0.1 g of pyranine powder dissolved in 100 mL of water was used as the fluorescent liquid 34.

[0042] On the other hand, wet performance of the developed tire (evaluation target tire) and the comparative tire was evaluated using an actual vehicle. In the actual vehicle evaluation, the degree to which wet performance in a low temperature environment improved or deteriorated compared to tire A was evaluated using the following index Y, using tire A as the standard. Index Y={(DLA / DL) / (DHA / DH)}×100 Here, DL represents the braking distance of the evaluation tire (here, Tire B, the development tire) in a low-temperature environment, DLA represents the braking distance of the comparison tire (Tire A) in a low-temperature environment, DH represents the braking distance of the evaluation tire in a normal temperature environment, and DHA represents the braking distance of the comparison tire in a normal temperature environment. Therefore, for Tire A, Y = 100. As a result, for Tire B, the index Y was 88. A larger value of index Y indicates a tendency for wet performance in a low-temperature environment to be improved compared to the comparison tire. Therefore, the ground contact analysis using the above-mentioned fluorescence method showed the same tendency as the evaluation of wet performance on an actual vehicle.

[0043] Here, the wet performance was evaluated using an actual vehicle by mounting the tires on a passenger car and running it through a specified break-in period on an outdoor test course. After that, the car was driven on a wet road at a speed of 100 km / h, and the braking distance was measured. Wet performance in low-temperature environments was measured at a road surface temperature of 5-10°C, and wet performance in normal temperature environments was measured at a road surface temperature of 20-25°C. The braking distance was measured 10 times and averaged.

[0044] Therefore, for several tires C to F that have the same tire size and similar tread pattern as tires A and B, we conducted ground contact analysis using the fluorescent method and evaluated their wet performance on an actual vehicle, just as we did for tires A and B.

[0045] Then, for the results of the contact analysis using the fluorescence method, the index X of each tire was converted into an index (index of change in contact area from room temperature to low temperature) with the index X of tire A set to 100. For the results of the wet performance evaluation on an actual vehicle, each tire was used as the evaluation target tire and the index Y was calculated using tire A as the standard.

[0046] The results are shown in Figure 5. It can be seen that there is a correlation between the results of the contact analysis using the above-mentioned fluorescent method (contact area change index from normal temperature to low temperature) and the results of the wet performance evaluation (index Y) in an actual vehicle. It can be seen that the larger the index X in the contact analysis using the fluorescent method, the larger the index Y in the wet performance evaluation in an actual vehicle. Therefore, the wet performance in a low-temperature environment in an actual vehicle can be easily evaluated based on the ratio (index X) of the contact area AL measured at the first temperature to the contact area AH measured at the second temperature. The larger the index X, the better the wet performance in a low-temperature environment tends to be.

[0047] In one embodiment, a ground contact analysis of an uneven surface equivalent to an actual road surface using the above-mentioned fluorescence method may be performed for each of the evaluation target tire and the comparison target tire to determine the index X, and by comparing the two indices X, it may be possible to evaluate whether the wet performance in a low-temperature environment of the evaluation target tire improves or deteriorates compared to the comparison tire. If the index X of the evaluation target tire is larger than the index X of the comparison target tire, it can be evaluated as tending to improve, and conversely, if the index X of the evaluation target tire is smaller than the index X of the comparison target tire, it can be evaluated as tending to deteriorate.

[0048] In the above evaluation method, it is preferable that the wet performance of the evaluation target tire and the comparison target tire at room temperature is approximately equivalent. If the wet performance at room temperature is approximately equivalent, the index Y can be regarded as the ratio of wet performance at low temperature DLA / DH. Since the index Y is correlated with the "contact area change index from room temperature to low temperature," which is the ratio of the index X, the wet performance at low temperature of the evaluation target tire and the comparison target tire can be simply compared using the index X. In other words, if the index X of the evaluation target tire is larger than the index X of the comparison target tire, the evaluation target tire can be evaluated as having better wet performance at low temperature than the comparison tire, and conversely, if the index X is smaller, the evaluation target tire can be evaluated as having worse wet performance at low temperature than the comparison tire. Here, the ratio (DH / DHA) of the braking distance DH of the evaluation target tire at room temperature to the braking distance DHA of the comparison target tire at room temperature is preferably 0.90 to 1.10, and more preferably 0.95 to 1.05.

[0049] Next, a method for designing a tire using the above tire performance evaluation method will be described.

[0050] The tire design method of the first embodiment measures the contact area of ​​the tread land by performing contact analysis using the above-mentioned fluorescence method at each of the above-mentioned first temperature and second temperature, and then sets the difference ΔE' in storage modulus of the tread rubber 16 at the first temperature and the second temperature and / or the difference ΔL in height of the contact surface 22 of the tread land 20 based on the ratio of the contact area AL measured at the first temperature to the contact area AH measured at the second temperature.

[0051] As shown in Figures 4 and 5, when a contact analysis using the fluorescence method is performed, the contact area generally decreases at low temperatures, resulting in a smaller index X. The following hypothesis was proposed regarding this reason: The elastic modulus of rubber is temperature-dependent, and as the elastic modulus increases at low temperatures (i.e., the difference in storage modulus ΔE' increases), contact performance deteriorates. Furthermore, when the contact surface 22 of the tread land 20 protrudes outward in the tire radial direction RD (i.e., when the land height difference ΔL is large), a difference in contact time occurs between the widthwise center 22A of the tread land 20 and both ends 22B, 22B, resulting in poor contact performance. This difference is difficult to detect in bench tests using a flat road surface as the contact target, but when the tread land 20 with a height difference ΔL is placed in contact with an uneven surface 32A as the contact target, it becomes apparent as a decrease in contact area from room temperature to low temperatures.

[0052] To verify the above hypothesis, the difference ΔE′ in storage modulus of the tread rubber 16 and the difference ΔL in height of the contact surface 22 of the tread land 20 were measured for tires A, B, and G shown in FIG.

[0053] Here, the difference ΔE' between the storage modulus of the tread rubber 16 at the first temperature and the second temperature (hereinafter sometimes referred to as the modulus difference ΔE') can be measured by, for example, taking a rubber test piece from the tread land 20 cut out from the tire to perform the above-mentioned ground contact analysis, and using the rubber test piece with a viscoelasticity tester. The conditions for measuring the storage modulus E' are not particularly limited, but for example, the temperature may be raised at a rate of 5°C / min in the range of -80 to 40°C, while applying a static elongation strain of 10%, and a dynamic strain of ±1.0% at a frequency of 10 Hz, and measurement may be made in 1°C increments.

[0054] The land height difference ΔL may be measured using a ruler such as a metal ruler from the cross-sectional shape of the tread land 20 cut out from the tire to perform the ground contact analysis. Alternatively, it may be measured using a 3D shape measuring device.

[0055] FIG. 6 is a graph showing the measurement results of the storage modulus E' of the tread rubber 16. As shown in FIG. 6, in tire A, the difference ΔE' between the storage modulus at the first temperature and the second temperature was small, and the difference (E'(5°C) - E'(22°C)) between the storage modulus E'(22°C) at room temperature (22°C) and the storage modulus E'(5°C) at the low temperature (5°C) was 12 MPa. In contrast, in tire B, the difference ΔE' between the storage modulus at the first temperature and the second temperature was large, and the difference (E'(5°C)) between the storage modulus E'(22°C) at room temperature (22°C) and the storage modulus E'(5°C) at the low temperature (5°C) was 20 MPa. Tire G has the same composition of tread rubber 16 as tire B, and therefore has the same storage modulus E'.

[0056] Fig. 7 is a graph showing the measurement results of the height difference ΔL of the contact patch 22 of the tread land 20. As shown in Fig. 7, tire A had a large land height difference ΔL of ΔL = 520 μm. tire G had a small land height difference ΔL of ΔL = 95 μm. tires B and H had a medium land height difference ΔL of ΔL = 320 μm.

[0057] As shown in Figure 4, in tire A, the difference in height ΔL of the tread land 20 was large, but the difference in storage modulus ΔE' was small, and the ground contact was good. In tire G, the difference in storage modulus ΔE' was large, but the difference in height ΔL of the tread land 20 was small, and the ground contact was good. In contrast, in tire B, the difference in storage modulus ΔE' was large and the difference in height ΔL of the tread land 20 was moderate, and the ground contact deteriorated. From these results, it was found that the large difference in storage modulus ΔE' and the difference in height ΔL of the tread land 20 cause the ground contact to deteriorate.

[0058] To further confirm the effect of differences in storage modulus difference ΔE' on road contact, tire H was produced by changing the tread rubber 16 to a rubber compound with a small storage modulus difference ΔE' (ΔE' = 12 MPa) (see Figure 6) for tire B, while keeping the mold shape used to vulcanize tire B unchanged (thus, the height difference ΔL was unchanged). A ground contact analysis was then performed on tire H using a fluorescence method to determine the index X. As a result, as shown in Figure 4, tire H had index X = 98, indicating excellent road contact at low temperatures. In other words, by changing to a rubber compound with a small storage modulus difference ΔE', road contact could be significantly improved.

[0059] From the above, it can be seen that the combination of the difference in storage modulus ΔE' of the tread rubber 16 and the difference in height ΔL of the tread land 20 is important in adjusting the index X of the fluorescent method ground contact analysis, which correlates with the results of an actual vehicle evaluation of wet performance. For example, when using a tread rubber compound with a large difference in storage modulus ΔE', it is desirable to reduce the difference in height ΔL of the tread land 20 to prevent deterioration of ground contact. Also, when using a land shape with a large difference in height ΔL of the tread land 20, it is desirable to use a tread rubber compound with a small difference in storage modulus ΔE' to prevent deterioration of ground contact.

[0060] In this way, the index X of the fluorescent method ground contact analysis (i.e., the ratio between the contact area AL measured at the first temperature and the contact area AH measured at the second temperature) can be adjusted by the difference ΔE' in the storage modulus of the tread rubber 16 and the difference ΔL in elevation of the tread land 20. Therefore, by appropriately setting at least one of the difference ΔE' in the storage modulus of the tread rubber 16 and the difference ΔL in elevation of the tread land 20 based on the index X, the index X can be increased to improve the ground contact.

[0061] For example, when the ratio AL / AH of the contact area AL measured at the first temperature to the contact area AH measured at the second temperature is smaller than a certain value, the tread rubber 16 may be changed to a rubber having a smaller elastic modulus difference ΔE'. In particular, when the index X (=(AL / AH)×100) measured for a certain tire is smaller than a set value (for example, less than 90), the rubber compounding of the tread rubber 16 may be changed to a rubber compounding having a smaller elastic modulus difference ΔE'.

[0062] Furthermore, when the ratio AL / AH is smaller than a certain value, the height difference ΔL of the contact patch 22 of the tread land 20 may be reduced. In particular, when the index X measured for a certain tire is smaller than a set value (for example, less than 90), the land shape may be changed so that the height difference ΔL of the tread land 20 becomes smaller.

[0063] Furthermore, when the ratio AL / AH is smaller than a certain value, the tread rubber 16 may be changed to a rubber having a smaller elastic modulus difference ΔE', and the height difference ΔL of the contact patch 22 of the tread land 20 may be reduced. In particular, when the index X measured for a certain tire is smaller than a set value (for example, less than 90), the rubber compounding of the tread rubber 16 may be changed to a rubber compounding having a smaller elastic modulus difference ΔE', and the land shape may be changed so that the height difference ΔL of the tread land 20 becomes smaller.

[0064] The method for adjusting the difference in storage modulus ΔE' of rubber is not particularly limited, and can be adjusted, for example, by the particle size of carbon black. By reducing the particle size, the difference in storage modulus ΔE' can be increased, and by increasing the particle size, the difference in storage modulus ΔE' can be decreased.

[0065] The tire design method according to the first embodiment described above can be used at various stages in the tire development process. Preferably, it can be used after a tire prototype is manufactured and before moving on to actual vehicle evaluation, or at the design change stage where further improvements are made after actual vehicle evaluation. This leads to the early development of tires with excellent wet performance in low-temperature environments.

[0066] Next, a tire design method according to a second embodiment will be described.

[0067] In a second embodiment, the contact area of ​​the tread land is measured for a plurality of tires by performing the contact area analysis using the above-mentioned fluorescence method at each of the first and second temperatures, and the ratio AL / AH of the contact area AL measured at the first temperature to the contact area AH measured at the second temperature is calculated. The relationship between the obtained contact area ratio AL / AH, the difference ΔE' in the storage modulus of the tread rubber at the first and second temperatures, and the height difference ΔL of the contact surface of the tread land is calculated. With this relationship calculated, when designing a tire, the difference ΔE' in the storage modulus and the height difference ΔL of the contact surface of the tread land are set based on this relationship within a range where the contact area ratio AL / AH satisfies a predetermined condition.

[0068] The difference ΔE' in storage modulus of the tread rubber may be measured for a plurality of tires by taking rubber test pieces from the tread land cut out from the tires as described above and using the rubber test pieces with a viscoelasticity tester, or if data on the difference ΔE' in storage modulus of the tread rubber used in the tires is held in advance, that data may be used.

[0069] The height difference ΔL of the tread land may be measured with a ruler from the cross-sectional shape of the tread land cut out from the tire as described above for multiple tires, or if data on the height difference ΔL for the tread land of the tire is stored in advance, that data may be used.

[0070] The relationship between the above-mentioned contact area ratio AL / AH, the elastic modulus difference ΔE', and the land height difference ΔL is not particularly limited, but for example, the relationship between the contact area ratio AL / AH and each combination of the elastic modulus difference ΔE' and the land height difference ΔL may be summarized in a graph or table. The contact area ratio AL / AH may be classified into a plurality of stages (for example, a stage where the above-mentioned index X is 95 or more and 100 or less, a stage where the index X is 90 or more and less than 95, and a stage where the index X is less than 90), and each combination of the elastic modulus difference ΔE' and the land height difference ΔL may be grouped into each stage.

[0071] Figure 8 is a graph showing the relationship between the contact area ratio AL / AH, the difference in modulus of elasticity ΔE', and the difference in ground clearance ΔL. For summer tires with a tire size of 205 / 55R16 and similar tread patterns, the contact area ratio AL / AH, the difference in modulus of elasticity ΔE', and the difference in ground clearance ΔL were measured for several tires A, B, G to O, including tires manufactured by the company on the market, tires under development, and tires manufactured by other companies on the market, and the relationships between these were determined. Figure 8 is a graph showing the results, with the direction toward the lower left of the graph indicating a larger contact area ratio AL / AH and a reduced deterioration in road contact in low-temperature environments (improvement direction).

[0072] The following method can be used to set the elastic modulus difference ΔE' and the land height difference ΔL based on the relationship shown in Fig. 8. For example, consider a case where the mold shape for vulcanizing a tire is determined first, and the rubber compound for the tread rubber 16 is selected from multiple candidates. In this case, the mold shape is determined, and therefore the land height difference ΔL is also determined. Therefore, it is sufficient to select a rubber compound having an elastic modulus difference ΔE' that satisfies the requirement that the index X be 90 or more from the multiple candidates.

[0073] For example, the rubber compounding of the tread rubber 16 is determined first, and then the tread land shape (corresponding to the mold shape) is set. In this case, since the rubber compounding is determined, the elastic modulus difference ΔE' is also determined, so it is sufficient to set the land height difference ΔL such that the index X is 90 or more.

[0074] The second embodiment can be preferably used at a relatively early stage of the tire development process, that is, for example, when developing a tire with good wet performance in a low-temperature environment, the second embodiment can be used to set, as a primary design proposal, a combination of the elastic modulus difference ΔE′ and the ground clearance difference ΔL that can suppress deterioration of ground contact at low temperatures in the early development stage.

[0075] In the second embodiment, it is not necessary to perform ground contact analysis using the above-mentioned fluorescence method on the development tire at this stage. That is, it is possible to set a combination of the elastic modulus difference ΔE' and the land height difference ΔL that can suppress deterioration of ground contact at low temperatures without performing ground contact analysis using the fluorescence method. A tire may be prototyped using a rubber compound with the set elastic modulus difference ΔE' and a land shape with the set land height difference ΔL, and ground contact analysis using the above-mentioned fluorescence method may be performed at this stage. In this case, if the contact area ratio AL / AH obtained by the ground contact analysis is smaller than the set value, the elastic modulus difference ΔE' and / or the land height difference ΔL may be changed according to the design method of the first embodiment to further improve ground contact at low temperatures.

[0076] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0077] 20...tread land, 22...ground contact surface, 32...transparent plate, 32A...concave and convex surface, 34...fluorescent liquid

Claims

1. a first temperature of 0°C or higher and 10°C or lower and a second temperature of 20°C or higher and 40°C or lower, the second temperature being 15°C or higher from the first temperature, a fluorescent liquid is interposed between a transparent plate having an irregular surface with irregularities equivalent to an actual road surface and a tread land, the tread land is brought into contact with the irregular surface, excitation light is irradiated onto the fluorescent liquid, and a luminance distribution of the fluorescence emitted from the fluorescent liquid is measured, thereby measuring the contact area of ​​the tread land with the irregular surface; A tire performance evaluation method for evaluating wet performance of a tire in a low-temperature environment of 0°C or higher and 10°C or lower based on the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature.

2. The tire performance evaluation method according to claim 1 , wherein the contact surface of the tread land protrudes radially outward from the tread reference contour line in the tire meridian cross section.

3. a first temperature of 0°C or higher and 10°C or lower and a second temperature of 20°C or higher and 40°C or lower, the second temperature being 15°C or higher from the first temperature, a fluorescent liquid is interposed between a transparent plate having an irregular surface with irregularities equivalent to an actual road surface and a tread land, the tread land is brought into contact with the irregular surface, excitation light is irradiated onto the fluorescent liquid, and a luminance distribution of the fluorescence emitted from the fluorescent liquid is measured, thereby measuring the contact area of ​​the tread land with the irregular surface; A tire design method, comprising: setting a difference ΔE' in storage modulus of the tread rubber between the first temperature and the second temperature and / or a difference in elevation of the contact surface of the tread land based on a ratio between the contact area measured at the first temperature and the contact area measured at the second temperature.

4. 4. The tire design method according to claim 3, wherein when a ratio of the contact area measured at the first temperature to the contact area measured at the second temperature is smaller than a certain value, the tread rubber is changed to a rubber having a smaller difference ΔE' in storage modulus.

5. 5. The tire design method according to claim 3, wherein the height difference of the contact area of ​​the tread land is reduced when the ratio of the contact area measured at the first temperature to the contact area measured at the second temperature is smaller than a certain value.

6. for a plurality of tires, at a first temperature of 0°C or higher and 10°C or lower and a second temperature of 20°C or higher and 40°C or lower, the temperature difference from the first temperature being 15°C or higher, a fluorescent liquid is interposed between a transparent plate having an uneven surface with unevenness equivalent to an actual road surface and a tread land, the tread land is brought into contact with the uneven surface, excitation light is irradiated onto the fluorescent liquid, the luminance distribution of the fluorescence emitted from the fluorescent liquid is measured, the contact area of ​​the tread land with the uneven surface is measured, and a ratio between the contact area measured at the first temperature and the contact area measured at the second temperature is calculated; The relationship between the ratio of the obtained ground contact areas, the difference ΔE′ in storage modulus of the tread rubber between the first temperature and the second temperature, and the difference in height of the ground contact surface of the tread land is obtained in advance, A tire design method, when designing a tire, based on the relationship, setting the difference in storage modulus ΔE' and the difference in height of the contact surface of the tread land within a range in which the ratio of the contact areas satisfies a predetermined condition.

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