JP6578957B2 - Tire durability test method - Google Patents

Description

  The present invention relates to a tire durability test method. More particularly, the present invention relates to a method for testing durability against loose at the end of a belt.

  The tire may be damaged when the vehicle rides over a protrusion such as a stone or a curb on the road while the vehicle is traveling. One of the damages to tires is breaker edge loose (BEL). This is a damage in which the belt cord peels off from the surrounding rubber at the end of the layer constituting the belt (referred to herein as the end of the belt). It is important to suppress this BEL for improving the durability of the tire. An evaluation method that can accurately evaluate the durability against BEL is desired.

  In order to evaluate the durability against BEL, there is a method in which a vehicle is actually run on a test course and the presence or absence of BEL is confirmed. However, this method requires a long time and a lot of trouble.

  In order to more easily evaluate the durability against BEL, there is a method of running a tire on a drum of a running test machine. In this method, the tire is loaded with a load equal to or higher than the normal load during traveling. Durability is evaluated by the running time until BEL occurs in the tire. In this method, it is important to obtain a result that accurately reflects the durability in the actual use state.

  The examination about the endurance test with respect to BEL which uses a running test machine is indicated by JP, 2005-188975, A. In this running test machine, protrusions are provided on the running surface in order to cause damage in a short time. A plurality of protrusions are arranged along the inclination direction line of the belt cord.

JP 2005-188975 A

  There is a need for a more accurate durability testing method for BEL that reflects durability in actual use.

  It is an object of the present invention to provide a method for testing durability against BEL with high accuracy.

  The test method according to the present invention is a tire durability test method. This test method uses a running test machine that runs tires on the running surface. This test method includes a step of bringing the tire into contact with the running surface, a step of applying a load to the tire, and a step of running the tire with respect to the running surface. The traveling surface includes a traveling reference surface and a slat protruding from the traveling reference surface. The upper surface of the slat includes a first surface and a second surface located inside the first surface in the axial direction of the tire. The second surface is inclined toward the traveling reference surface toward the inner side in the axial direction of the tire.

  Preferably, the angle formed by the second surface and the travel reference surface is 10 ° or more and 35 ° or less.

  Preferably, chamfering is applied to a corner of the slat that contacts the tread of the tire.

Preferably, the belt of the tire includes a maximum width layer that is a layer having the largest width among the layers constituting the belt, and a maximum width outer layer that is laminated radially outward of the maximum width layer. . When the slat comes into contact with the tire, in the axial direction of the tire, the outer end of the second surface is located outside the end of the maximum width layer, and the inner end of the second surface is the maximum width outer layer. Located inside the edge.

  Preferably, the height from the traveling reference plane to the axially outer end of the second surface is 10 mm or more and 30 mm or less.

  Preferably, the first surface is parallel to the traveling reference surface or is inclined toward the traveling reference surface toward the inner side in the axial direction of the tire. An angle formed by the first surface and the traveling reference surface is 0 ° or more and 10 ° or less.

  The travel test machine according to the present invention is a travel test machine that travels tires on its travel surface. The traveling surface includes a traveling reference surface and a slat protruding from the traveling reference surface. The upper surface of the slat includes a first surface and a second surface located inside the first surface in the axial direction of the tire. The second surface is inclined toward the traveling reference surface toward the inner side in the axial direction of the tire.

  The traveling surface of the traveling test machine used in this test method includes a traveling reference surface and a slat protruding from the traveling reference surface. The upper surface of the slat includes a first surface and a second surface located inside the first surface in the axial direction of the tire. The second surface is inclined toward the traveling reference surface toward the inner side in the axial direction of the tire. The height of the slat increases from the axially inner end to the outer end of the second surface. In this test method, an impact can be effectively applied to the vicinity of the shoulder portion of the tread by the slat having this shape. This slat can effectively distort the vicinity of the end of the belt. This slat effectively generates a looseness near the end of the belt. In this method, an accurate durability test for BEL is realized.

FIG. 1 is a diagram schematically showing a test situation by a test method according to an embodiment of the present invention. FIG. 2 is a side view of the test situation of FIG. FIG. 3 is an enlarged cross-sectional view of a part of the tire and the testing machine.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

  FIG. 1 schematically shows a test situation according to a test method according to an embodiment of the present invention. In this embodiment, a drum type traveling tester 2 is used. FIG. 2 is a side view of the test situation of FIG. FIG. 2 shows only the tire 4 and the drum 6 of the drum-type running test machine 2. FIG. 3 is an enlarged sectional view of a contact portion between the tire 4 and the running surface 8 of the drum 6. This figure shows a cross section cut by a plane perpendicular to the circumferential direction of the tire 4. This figure shows a cross section cut by a plane perpendicular to the circumferential direction of the drum 6. In FIG. 3, the vertical direction is the radial direction of the tire 4, the horizontal direction is the axial direction of the tire 4, and the direction perpendicular to the paper surface is the circumferential direction of the tire 4.

  As shown in FIGS. 1 and 2, the running test machine 2 includes a running surface 8 on which the tire 4 runs. The traveling surface 8 includes a slat 10 and a traveling reference surface 12. The slat 10 protrudes from the travel reference plane 12. The travel reference surface 12 is a portion other than the slats 10 in the travel surface 8.

  As shown in FIG. 2, in this embodiment, when the drum 6 is viewed from the side, the shape of the slat 10 is substantially trapezoidal. In this embodiment, the shape of the slat 10 is a substantially isosceles trapezoid. As shown in FIG. 3, the upper side of the slat 10 (the outer surface in the radial direction of the drum 6) includes a first surface 14 and a second surface 16. The second surface 16 is located inside the first surface 14 in the axial direction of the tire 4 when the tire 4 is in contact with the traveling surface 8. In this embodiment, the upper surface of the slat 10 includes a first surface 14 and a second surface 16.

  As shown in FIG. 3, the second surface 16 is inclined with respect to the travel reference surface 12. The second surface 16 is inclined toward the traveling reference surface 12 toward the inner side in the axial direction of the tire 4. That is, the height of the slats 10 increases from the axially inner end to the outer end of the second surface 16.

  As shown in FIG. 2, in this embodiment, the running surface 8 includes two slats 10. These slats 10 are separated from each other by a half circumference of the drum 6. The number of slats 10 included in the traveling surface 8 is not limited to two. The number of slats 10 may be one. The traveling surface 8 may include three or more slats 10.

  As shown in FIG. 3, the tire 4 includes a tread 18, a pair of sidewalls 20 that extend substantially inward in the radial direction from the ends of the tread 18, and a carcass 22 that extends along the inside of the tread 18 and the sidewalls 20. A belt 24 positioned on the inner side in the radial direction of the tread 18 and stacked on the outer side in the radial direction of the carcass 22, and an inner liner 26 positioned on the inner side of the carcass 22. The tire 4 is a tubeless type. The tire 4 is attached to a truck, a bus or the like. This tire 4 is a pneumatic tire 4 for heavy loads.

  The belt 24 is located inside the tread 18 in the radial direction. The belt 24 is located outside the carcass 22 in the radial direction. The belt 24 reinforces the carcass 22. In this embodiment, the belt 24 includes a first layer 24a, a second layer 24b, a third layer 24c, and a fourth layer 24d. The belt 24 is composed of four layers. The belt 24 may be composed of three layers. The belt 24 may be composed of two layers. The belt 24 may be composed of five or more layers.

  The first layer 24a forms an inner portion of the belt 24 in the radial direction. The first layer 24a is laminated with the carcass 22 on the equator plane. The second layer 24b is located on the radially outer side of the first layer 24a. The second layer 24b is laminated with the first layer 24a. The third layer 24c is located on the outer side in the radial direction of the second layer 24b. The third layer 24c is stacked with the second layer 24b. The fourth layer 24d is located on the radially outer side of the third layer 24c. The fourth layer 24d is stacked with the third layer 24c.

  As shown in the drawing, the second layer 24 b has the largest width among the four layers forming the belt 24 in the axial direction. The layer with the largest width is referred to as the maximum width layer. In the tire 4, the second layer 24b is the maximum width layer. A layer laminated outside the maximum width layer is referred to as a maximum width outer layer. In the tire 4, the third layer 24c is the maximum width outer layer. Here, the end of each layer is referred to as the end of the belt 24.

  Although not shown, each of the first layer 24a, the second layer 24b, the third layer 24c, and the fourth layer 24d is composed of a large number of cords arranged in parallel and a topping rubber. Each cord is inclined with respect to the equator plane. The inclination direction of the cord of the second layer 24b with respect to the equator plane is the same as the inclination direction of the cord of the first layer 24a with respect to the equator plane. The inclination direction of the cord of the third layer 24c with respect to the equator plane is opposite to the inclination direction of the cord of the second layer 24b with respect to the equator plane. The inclination direction of the cord of the fourth layer 24d with respect to the equator plane is the same as the inclination direction of the cord of the third layer 24c with respect to the equator plane. In each layer, the absolute value of the angle formed by the cord with respect to the equator plane is 15 ° to 70 °. The cord material is steel.

The durability test method for BEL according to an embodiment of the present invention is as follows:
(1) A step of bringing the tire 4 and the running surface 8 of the running test machine 2 into contact with each other,
(2) a step of applying a load to the tire 4;
(3) a step of causing the tire 4 to travel with respect to the running surface 8; and (4) a step of observing the BEL.

  In the step (1), the tire 4 is set on the running test machine 2. The tire 4 is mounted on a regular rim and filled with air. The tire 4 and the running surface 8 of the drum 6 are brought into contact with each other. FIG. 3 shows a state where the tire 4 is in contact with the slat 10 of the running surface 8. As shown in the figure, the tire 4 and the running surface 8 are brought into contact with each other so that the slat 10 comes into contact with the tire 4. The axial position of the tire 4 on the drum 6 is determined so that the slat 10 contacts the tire 4.

  In the step (2), a load is applied to the tire 4. The tire 4 is pressed against the running surface 8 of the drum 6. An arrow F in FIG. 2 is a load applied to the tire 4.

  In the step (3), the drum 6 is rotated in the direction of arrow A. Along with this, the tire 4 rotates in the direction of arrow B. As a result, the tire 4 travels on the travel surface 8. The traveling speed of the tire 4 is determined by the rotational speed of the drum 6.

  In the step (4), the presence or absence of BEL is observed for the tire 4 traveled in (3). This observation is performed by visually checking the tire 4. This observation may be performed by confirming a photograph taken with an X-ray inspection apparatus. In addition, the travel time until BEL occurs is measured. It is judged that durability with respect to BEL is so high that driving time is long.

  Hereinafter, the function and effect of the present invention will be described.

  The traveling surface 8 of the traveling test machine 2 used in this test method includes a traveling reference surface 12 and a slat 10 protruding from the traveling reference surface 12. The upper surface of the slat 10 includes a first surface 14 and a second surface 16 positioned inside the first surface 14 in the axial direction of the tire 4. The second surface 16 is inclined toward the traveling reference surface 12 toward the inner side in the axial direction of the tire 4. The height of the slats 10 increases from the axially inner end to the outer end of the second surface 16. In this test method, an impact can be effectively applied to the vicinity of the shoulder portion of the tread 18 by the slat 10 having this shape. The slat 10 can effectively distort the vicinity of the end of the belt 24. The slat 10 effectively generates looseness in the vicinity of the end of the belt 24. In this method, an accurate durability test for BEL is realized.

  As shown in FIG. 3, the belt 24 includes a plurality of layers. In actual use, the BEL is near the end 28 of the maximum width layer (in this embodiment, the end 28 of the second layer 24b) and the end 30 of the maximum width outer layer (in this embodiment, the end 30 of the third layer 24c). ). In this specification, the BEL that occurs near the edge 28 of the maximum width layer is referred to as the outer BEL. The BEL that occurs near the edge 30 of the maximum width outer layer is referred to as the inner BEL. In the endurance test by the running test machine 2 so far, the inner BEL can be reproduced, but it is difficult to reproduce the outer BEL. In the endurance test so far, the vicinity of the end 30 of the maximum width outer layer near the tread surface 32 is distorted more than the vicinity of the end 28 of the maximum width layer, so that the inner BEL occurs before the outer BEL occurs. This is probably because of this. On the other hand, in an actual use state, a large distortion also occurs in the vicinity of the end 28 of the maximum width layer during cornering or the like. For this reason, it is considered that the outer BEL occurs.

  As described above, in this test method, the second surface 16 is inclined toward the traveling reference surface 12 toward the inner side in the axial direction of the tire 4. The height of the slats 10 increases from the axially inner end to the outer end of the second surface 16. The slat 10 can effectively distort the vicinity of the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 3, the symbol θ is an angle formed between the second surface 16 and the travel reference surface 12. The angle θ is preferably 10 ° or more. By setting the angle θ to 10 ° or more, the slat 10 can effectively distort the vicinity of the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized. In this respect, the angle θ is more preferably 15 ° or more. The angle θ is preferably 35 ° or less. By setting the angle θ to 35 ° or less, the second surface 16 can contact the tread 18 with an appropriate contact area. The slat 10 can efficiently generate the outer BEL. In this respect, the angle θ is more preferably 30 ° or less.

  As shown in FIG. 3, the outer end 34 of the second surface 16 is preferably located outside the end 28 of the maximum width layer in the axial direction. The inner end 36 of the second surface 16 is preferably located inside the end 30 of the maximum width outer layer. Thereby, this slat 10 can give moderate distortion to the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 3, the double-headed arrow To represents the axial distance between the outer end 34 of the second surface 16 and the end 28 of the maximum width layer. The distance To is preferably 1 mm or more, and preferably 10 mm or less. Thereby, this slat 10 can give moderate distortion to the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 3, a double-headed arrow Ti is an axial distance between the inner end 36 of the second surface 16 and the end 30 of the maximum width outer layer. The distance Ti is preferably 0 mm or more, and preferably 10 mm or less. Thereby, this slat 10 can give moderate distortion to the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 3, for easy understanding, the distance To and the distance Ti are defined in a state where the tire 4 and the slat 10 of the running surface 8 are in contact with each other. Actually, when the tire 4 is in contact with the slat 10, the tire 4 is deformed. The amount of deformation varies depending on the size of the slat 10, the internal pressure of the tire 4, the load applied to the tire 4, and the like. The position of the end 28 of the maximum width layer and the position of the end 30 of the maximum width outer layer also vary accordingly. In the present invention, assuming that the tire 4 is incorporated in a normal rim, has a normal internal pressure, and is not loaded, the position of the end 28 of the maximum width layer and the position of the end 30 of the maximum width outer layer are the distance. Used to define To and Ti. In this state, assuming that the tire 4 and the slat 10 are in contact, the distances To and Ti are defined.

  In FIG. 3, the double-headed arrow T <b> 2 is the axial width of the second surface 16. The width T2 is preferably 10 mm or more, and preferably 30 mm or less. Thereby, this slat 10 can give moderate distortion to the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 3, the double-headed arrow H is the height from the travel reference surface 12 to the outer end 34 of the second surface 16. The height H is preferably 10 mm or more. By setting the height H to 10 mm or more, the slat 10 can effectively give an impact to the vicinity of the shoulder portion of the tread 18. This method can efficiently test the durability against BEL. In this respect, the height H is more preferably 15 mm or more.

  When the slat 10 gives an excessive impact to the tire 4, damage other than BEL may occur. For example, excessive impact can lead to a loose carcass ply. If looseness occurs in the carcass ply, the durability against BEL cannot be evaluated.

  In this test, the height H is preferably 30 mm or less. By setting the height H to 30 mm or less, the impact applied to the tire 4 is maintained at an appropriate magnitude. In this test method, the looseness in the carcass ply is suppressed. This method can efficiently test the durability against BEL. In this respect, the height H is more preferably 25 mm or less.

  The corners of the second surface 16 and the first surface 14 of the slat 10 are in contact with the tread 18. If this corner is sharp, it may happen that this corner damages the tread 18. As a result, chunking may occur in which the rubber on the surface of the tread 18 is torn off. This is not limited to the corners of the second surface 16 and the first surface 14 of the slat 10, and the same thing can happen for all corners that contact the tread 18 of the slat 10. When chunking occurs, durability against BEL cannot be evaluated.

  In the slat 10, it is preferable that the corner that comes into contact with the tread 18 is chamfered. That is, it is preferable that the corner in contact with the tread 18 has a rounded shape. Chunking is effectively suppressed by exhibiting the rounded shape of these corners. From this viewpoint, it is more preferable that the cross section of these corners has an arc shape.

  The radius of curvature R of the arc is preferably 2 mm or more. Chunking can be effectively prevented by setting the curvature radius R to 2 mm or more. The curvature radius R is preferably 10 mm or less. By setting the curvature radius R to 10 mm or less, the slat 10 can effectively give an impact to the tread 18. This method can efficiently test the durability against BEL.

  In the tire 4 of FIG. 3, the first surface 14 is substantially parallel to the travel reference surface 12. The first surface 14 may be inclined toward the traveling reference surface 12 toward the inner side in the axial direction of the tire 4. Although not shown, when the angle formed by the first surface 14 and the travel reference surface 12 is α, the angle α is preferably 0 ° or more, and preferably 10 ° or less. As a result, the slat 10 can effectively give an impact to the tread 18. This method can efficiently test the durability against BEL.

  In FIG. 3, the double-headed arrow T is the axial distance from the outer end 38 of the first surface 14 to the inner end 36 of the second surface 16. The distance T is preferably 15 mm or more, and preferably 40 mm or less. Thereby, this slat 10 can give moderate distortion to the end 28 of the maximum width layer. With this test method, the outer BEL can be reproduced. In this method, an accurate durability test for BEL is realized.

  In FIG. 2, the double arrow W is the width of the slat 10 measured along the circumferential direction of the travel reference plane 12. The width W is preferably 10 mm or more, and preferably 50 mm or less. As a result, the slat 10 can effectively give an impact to the tread 18. This method can efficiently test the durability against BEL.

  As described above, when the drum 6 is viewed from the side, the shape of the slat 10 is preferably substantially trapezoidal. The slat 10 having a substantially trapezoidal shape when the drum 6 is viewed from the side has a sharper angle at which it contacts the tread 18 than a slat having a substantially rectangular shape. In the slat 10, the chunking of the tread 18 is effectively prevented as compared with the substantially rectangular slat. The slat 10 having a substantially trapezoidal shape when the drum 6 is viewed from the side can more effectively give an impact to the tread 18 than a slat having a substantially arc shape at this time. This method can efficiently test the durability against BEL.

  In the present specification, the normal rim means a rim defined in a standard on which the tire 4 depends. “Standard rim” in the JATMA standard, “Design Rim” in the TRA standard, and “Measuring Rim” in the ETRTO standard are regular rims. In the present specification, the normal internal pressure means an internal pressure defined in a standard on which the tire 4 depends. “Maximum air pressure” in JATMA standard, “maximum value” published in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in TRA standard, and “INFLATION PRESSURE” in ETRTO standard are normal internal pressures. In the present specification, the normal load means a load defined in a standard on which the tire 4 depends. “Maximum value” published in “Maximum load capacity” in JATMA standard, “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in TRA standard, and “LOAD CAPACITY” in ETRTO standard are normal loads.

  In the embodiment described above, the drum-type running test machine 2 is used as the running test machine. The running test machine is not limited to the drum type. A belt-type running tester may be used. Other running test machines may be used.

  Hereinafter, the effects of the present invention will be clarified by examples. However, the present invention should not be construed in a limited manner based on the description of the examples.

[Example 1]
A durability test was performed according to the steps (1) to (4). The size of the tire used for the test is 11R22.5. This tire was run on a drum type running tester. Table 1 shows the specifications of the slats provided on the running surface of the drum. When the drum is viewed from the side, the shape of the slat is a substantially isosceles trapezoid. The axial width T2 of the second surface of the slat is 20 mm. The axial distance T from the outer end of the first surface of the slat to the inner end of the second surface is 25 mm. The circumferential width W is 30 mm. In this slat, the first surface is parallel to the running reference surface. The corner of the slat that contacts the tread is chamfered. The radius of curvature R of the corner between the first surface and the second surface is 5 mm. The curvature radius R of other corners is 5 mm. The drum diameter of this testing machine is 1707 mm. The tire running conditions are as follows.
Rim used: 22.5 × 7.50
Internal pressure: 800 kPa
Load: 26.5kN
Speed: 40km / h

[Comparative Example 1]
A durability test was performed in the same manner as in Example 1 except that the angle θ between the second surface and the running reference surface was 0 ° (the slat did not have the second surface).

[Example 2-5]
A durability test was conducted in the same manner as in Example 1 except that the angle θ was as shown in Table 1.

[Example 6-7]
A durability test was conducted in the same manner as in Example 1 except that the height H was as shown in Table 2.

[Example 8]
A durability test was performed in the same manner as in Example 1 except that chamfering was not performed on the corners of the slats contacting the tread.

[Example 9]
A durability test was performed in the same manner as in Example 1 except that the values of the distance To and the distance Ti were as shown in Table 2. In this embodiment, the value of To is negative, indicating that the outer end of the second surface is located inside the end of the maximum width layer in the axial direction.

[Damage mode and outer BEL incidence]
Each of the examples and comparative examples was tested with five tires. The tires were run on the running test machine until damage occurred, and the damage mode was confirmed. In the column of “damage mode” in the table, “O-BEL” indicates that there is a tire in which the outer BEL has occurred, and “I-BEL” indicates that there is a tire in which the inner BEL has occurred. “PLL” indicates that there is a tire in which a loose carcass ply has occurred. “TC” indicates that there is a tire in which tread chunking has occurred. The outer BEL occurrence rate is a ratio of tires in which an external BEL damage mode has occurred.

  As shown in Table 1-2, in the test method according to the present invention, external BEL can be generated in a test using a running test machine. According to this method, it is possible to accurately test the durability of the BEL. From this evaluation result, the superiority of the present invention is clear.

  The test methods described above can be applied to various tire durability tests.

DESCRIPTION OF SYMBOLS 2 ... Running test machine 4 ... Tire 6 ... Drum 8 ... Running surface 10 ... Slat 12 ... Running reference surface 14 ... First surface 16 ... Second surface 18 ... Tread 20 ... Sidewall 22 ... Carcass 24 ... Belt 24a ... First layer 24b ... Second layer 24c ... Third layer 24d ... Fourth layer 26. .... Inner liner 28 ... End of maximum width layer 30 ... End of maximum width outer layer 32 ... Tread surface 34 ... Outer end of second surface 36 ... Inner end of second surface 38 ..Outer edge of first surface

Claims (5)

A tire durability test method,
Using a running test machine that runs tires on the running surface,
Contacting the tire and the running surface;
A step of applying a load to the tire; and a step of running the tire with respect to the running surface,
The traveling surface includes a traveling reference surface and a slat protruding from the traveling reference surface,
The upper surface of the slat includes a first surface and a second surface located inside the first surface in the axial direction of the tire,
The second surface is inclined toward the traveling reference surface side toward the inner side in the axial direction of the tire ,
The belt of the tire includes a maximum width layer that is a layer having the largest width among the layers constituting the belt, and a maximum width outer layer that is laminated on a radially outer side of the maximum width layer,
When the slat comes into contact with the tire,
A test method in which, in the axial direction of the tire, the outer end of the second surface is located outside the end of the maximum width layer, and the inner end of the second surface is located inside the end of the maximum width outer layer .   The test method according to claim 1, wherein an angle formed between the second surface and the traveling reference surface is 10 ° or more and 35 ° or less.   The test method according to claim 1, wherein chamfering is performed at a corner of the slat that contacts the tread of the tire. The test method according to any one of claims 1 to 3 , wherein a height from the travel reference surface to the axially outer end of the second surface is 10 mm or more and 30 mm or less. The first surface is parallel to the traveling reference surface or inclined toward the traveling reference surface toward the inner side in the axial direction of the tire;
The test method according to any one of claims 1 to 4 , wherein an angle formed by the first surface and the travel reference surface is 0 ° or more and 10 ° or less.

Images