Patent Description:
The following Patent Document <NUM> discloses a tire for running on rough terrain in which the tread portion is provided with crown blocks. The crown block comprises a crown block main portion, and three crown fin portions protruding in the tire circumferential direction from the crown block main portion. such crown fin portions are explained as being useful for enhancing traction performance. Patent Document <NUM>: <CIT>.

A tire in accordance with the preamble of claim <NUM> is known from <CIT>, <CIT> (document according to Article <NUM>(<NUM>) EPC) and <CIT> (document according to Article <NUM>(<NUM>) EPC). A related tire is described in <CIT> (document according to Article <NUM>(<NUM>) EPC).

When traveling on muddy ground or road with the above-mentioned tire, the mud tends to be clogged between the crown fin portions, and the clogged mud is difficult to be ejected, therefore, the traction tends to decrease.

The present invention was made in view of the above circumstances, and
a primary object of the present invention is to provide a tire for running on rough terrain which is improved in traction performance on rough terrain.

The object is solved by a tire having the features of claim <NUM>. Sub-claims are directed to preferable embodiments.

According to the present invention, a tire for running on rough terrain for which an intended tire rotational direction is specified, comprises a tread portion provided with crown blocks disposed on the tire equator,
wherein
each of the crown blocks comprises.

According to an embodiment of the invention, each of said only two crown fin portions has an outer edge in a block width direction which is located inside in the block width direction than both axial ends of the crown block main portion.

According to an embodiment of the invention, the distance in the tire axial direction between said only two crown fin portions is in a range from <NUM>% to <NUM>% of the width in the tire axial direction of the crown block main portion.

According to an embodiment of the invention, the width in the tire axial direction of each of said only two crown fin portions is in a range from <NUM>% to <NUM>% of the width in the tire axial direction of the crown block main portion.

According to an embodiment of the invention, the protruding length of each of said only two crown fin portions measured in the tire circumferential direction from the crown block main portion is not less than <NUM>% of the length in the tire circumferential direction of the crown block main portion.

According to an embodiment of the invention, the crown block main portion has a ground contacting top surface and a toe-side block edge, wherein the toe-side block edge comprises an inner edge portion extending inward in the tire axial direction from the connection portion, and an outer edge portion extending outward in the tire axial direction from the connection portion, and the shallow groove extends from the inner edge portion to the outer edge portion so as to surround said part of the crown fin portion on the connection portion side.

According to an embodiment of the invention, the width of the shallow groove is in a range from <NUM>% to <NUM>% of the width in the tire axial direction of the crown block main portion.

According to an embodiment of the invention, the crown block main portion has a ground contacting top surface and a toe-side block edge, wherein the distance in the tire circumferential direction between the toe-side block edge and a heel-side end of the shallow groove is not more than <NUM>% of the length in the tire circumferential direction of the crown block main portion.

According to an embodiment of the invention, the crown block main portion has a heel-side block edge, wherein the angle of the heel-side block edge with respect to the tire axial direction is in a range from <NUM> to <NUM> degrees.

In the tire according to the present invention, therefore, the traction performance when traveling on rough terrain can be improved by the crown blocks having the above described configuration.

The present invention is suitably applied to a tire for motorcycles, but may be applied to tires for passenger cars, heavy duty vehicles and the like.

Further, the present invention can be applied to not only pneumatic tires but also non-pneumatic tires so called airless tires.

<FIG> is a cross-sectional partial view of a pneumatic tire <NUM> for running on rough terrain as an embodiment of the present invention.

This cross-sectional view is a tire meridian cross-sectional view including the tire rotation axis (not shown) under a normal state of the pneumatic tire <NUM>.

<FIG> is a developed partial view of the tread portion <NUM> of the pneumatic tire <NUM>.

In the case of a pneumatic tire, the normal state is such that the tire is mounted on a standard wheel rim and inflate to a standard pressure but loaded with no tire load.

In this application including specification and claims, various dimensions, positions and the like of a pneumatic tire refer to those under the normal state of the tire unless otherwise noted. The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, i.e. JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), TRAA (Australia), STRO (Scandinavia), ALAPA (Latin America), ITTAC (India) and the like which are effective in the area where the tire is manufactured, sold or used.

The standard pressure is the air pressure for the tire specified by the same organization in the Air-pressure/maximum-load Table or similar list, i.e. the "maximum air pressure" in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tire Load Limits at Various Cold Inflation Pressures" table in TRA or the like.

In the tire meridian cross-section of the tire <NUM>, the radially outer surface of the tread portion <NUM> is curved in an arc shape which is convex toward the outside in the tire radial direction.

The tread portion <NUM> is provided with a directional tread pattern for which the tire rotation direction N is specified.

The tread portion <NUM> in the present embodiment is provided with a plurality of crown blocks <NUM> disposed on the tire equator C.

Each of the crown blocks <NUM> comprises a crown block main portion <NUM> and crown fin portions <NUM>.

The crown block main portion <NUM> is formed in a v-shape bent convexly toward a tire circumferential direction opposite to the intended tire rotation direction N, namely, toward the toe side in the intended tire rotation direction N.

The crown fin portions <NUM> protrude from the crown block main portion <NUM> toward the toe side in the intended tire rotation direction N.

In such crown block <NUM>, the crown fin portions <NUM> suppress the crown block main portion <NUM> from collapsing toward the circumferential direction opposite to the intended tire rotation direction N, and the crown block main portion <NUM> exerts an essential mud digging power to improve the traction performance of the tire.

For each of the crown blocks <NUM>, only two crown fin portions <NUM> are formed. Thereby, the region where mud is easily clogged, that is, the region between the crown fin portions <NUM> becomes one place for each crown block <NUM>, and the clogging of mud is reduced. As a result, the crown block main portion <NUM> can exert its edge effect at a high level.

The tread portion <NUM> in this example is further provided with middle blocks <NUM> located axially outside the crown blocks <NUM>, and shoulder blocks <NUM> located axially outside the middle blocks <NUM>.

In the present embodiment, as shown in <FIG>, on each side in the tire axial direction of each of the crown blocks <NUM>, one middle block <NUM> is disposed adjacently. But, the blocks <NUM> to <NUM> are separated from each other by a tread base portion 2R.

<FIG> is an enlarged top view of the crown block <NUM>.

As shown in <FIG>, the crown block main portion <NUM> has.

Further, the crown block main portion <NUM> has a block side wall surface <NUM> extending from the heel-side block edge <NUM>, the toe-side block edge <NUM> and the circumferential edges <NUM> to the above-mentioned tread base portion 2R.

Each of the circumferential edges <NUM> in this example extends straight and substantially parallel to the tire circumferential direction.

Here, the expression "substantially parallel to the tire circumferential direction" means that the inclination angle with respect to the tire circumferential direction is in a range from <NUM> to <NUM> degrees.

In the present embodiment, each of the heel-side block edge <NUM> and the toe-side block edge <NUM> is inclined to the heel side in the intended tire rotation direction N toward both outer sides in a block width direction in parallel to the tire axial direction from the center in the block width direction.

The heel-side block edge <NUM> has the toe-side end 13e positioned on the most toe-side in the intended tire rotation direction N. The toe-side block edge <NUM> has the toe-side end 14e positioned on the most toe-side in the intended tire rotation direction N. The toe-side ends 13e and 14e in the present embodiment are located on the tire equator C.

The angle θ1 of the heel-side block edge <NUM> is preferably not less than <NUM> degrees, more preferably not less than <NUM> degrees, but preferably not more than <NUM> degrees, more preferably not more than <NUM> degrees with respect to the tire axial direction.

Here, the angle θ1 is that of a straight line drawn between the toe-side end 13e and the intersecting point between the heel-side block edge <NUM> and one of the circumferential edges <NUM>.

As shown in <FIG>, the heel-side block edge <NUM> is inclined with respect to the tire axial direction continuously from each circumferential edge <NUM> to the toe-side end 13e to the same direction toward the toe side in the intended tire rotation direction N.

The heel-side block edge <NUM> comprises two first outer portions 13a extending from the respective circumferential edges <NUM>,.

The first intermediate portions 13c are inclined at a larger angle with respect to the tire axial direction than the first outer portions 13a and the first inner portion 13b.

In the present embodiment, each of the first outer portions 13a and the first intermediate portions 13c extends in a straight line, and the first inner portion 13b extends in a V shape.

The difference (θ1c-θ1a) between the angle θ1a of the first outer portion 13a and the angle θ1c of the first intermediate portion 13c, each with respect to the tire axial direction, is preferably not less than <NUM> degrees, more preferably not less than <NUM> degrees, but preferably not more than <NUM> degrees, more preferably not more than <NUM> degrees.

The difference (θ1c-θ1b) between the angle θ1b of the first inner portion 13b and the angle θ1c of the first intermediate portion 13c, each with respect to the tire axial direction, is preferably not less than <NUM> degrees, more preferably not less than <NUM> degrees, but preferably not more than <NUM> degrees, more preferably not more than <NUM> degrees.

The toe-side block edge <NUM> comprises an inner edge portion 14A and two outer edge portions 14B.

The inner edge portion 14A extends axially inwardly from connection portions K between the crown block main portion <NUM> and the two crown fin portions <NUM>.

The two outer edge portions 14B respectively extend axially outwardly from connection portions K between the crown block main portion <NUM> and the two crown fin portions <NUM>.

The inner edge portion 14A includes the toe-side end 14e.

The inner edge portion 14A extends in a V shape.

In the present embodiment, the outer edge portions 14B respectively extend to the circumferential edges <NUM>.

In the present embodiment, each of the outer edge portions 14B extends in a straight line.

In the present embodiment, each of the two crown fin portions <NUM> is formed in a parallel quadrilateral shape in the top view of the block. As shown in <FIG>, the crown fin portions <NUM> in this example each have.

The outer edge 11e and the inner edge 11i in this example extend in parallel with the tire circumferential direction.

The first edge 11a and the second edge 11b in this example extend in parallel with the adjacent outer edge portion 14B. The inner edge 11i, the outer edge 11e, the heel-side edge 11a and the toe-side edge 11b define the radially outer surface 11A of the crown fin portion <NUM>.

As shown in <FIG>, in the present embodiment, the radially outer surface 11A is located radially outside the ground contacting top surface <NUM> of the crown block main portion <NUM>. However, the radially outer surface 11A may be located at the same radial position as the ground contacting top surface <NUM> of the crown block main portion <NUM>.

The outer edges 11e of the two crown fin portions <NUM> are located inside in the block width direction than the both ends 10e in the tire axial direction of the crown block main portion <NUM>. As a result, the deformation of the crown fin portions <NUM> is ensured, and the effect of ejecting mud is enhanced.

The distance La in the tire axial direction between the two crown fin portions <NUM> is preferably set in a range of not less than <NUM>%, more preferably not less than <NUM>%, but not more than <NUM>%, more preferably not more than <NUM>% of the width W1 in the tire axial direction of the crown block main portion <NUM> as shown in <FIG>.

Since the distance La is not less than <NUM>% of the width W1, mud ejection becomes smooth.

Since the distance La is not more than <NUM>% of the width W1, the collapse of the crown block main portion <NUM> can be effectively suppressed.

The distance Lb in the tire axial direction between the outer edge 11e of each crown fin portion <NUM> and the adjacent axial end 10e of the crown block main portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the width W1 in the tire axial direction of the crown block main portion <NUM>,.

As a result, the deformation of the crown fin portions <NUM> is ensured, and the effect of ejecting mud is enhanced.

The width W2 in the tire axial direction of each crown fin portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the width W1 in the tire axial direction of the crown block main portion <NUM>.

Since the width W2 is not less than <NUM>% of the width W1, it is possible to effectively suppress the collapse of the crown block main portion <NUM>.

Since the width W2 is not more than <NUM>% of the width W1, excessive increase in the rigidity of the crown fin portion <NUM> is suppressed, and the effect of ejecting mud is maintained.

In order to prevent the crown block main portion <NUM> from collapsing, the protruding length Lc (shown in <FIG>) in the tire circumferential direction of each crown fin portion <NUM> from the crown block main portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>% of the length L1 (shown in <FIG>) in the tire circumferential direction of the crown block main portion <NUM>.

However, if the protruding length Lc is excessively large, the effect of ejecting mud may deteriorate, therefore, the protruding length Lc is preferably not more than <NUM>%, more preferably not more than <NUM>% of the length L1.

As shown in <FIG>, the connection portion K between the crown block main portion <NUM> and each crown fin portion <NUM> is provided with a shallow groove <NUM> for promoting deformation of a part of the crown fin portion <NUM> on the connection portion K side. Thus, such shallow grooves <NUM> help to smoothly eject the mud clogged between the crown fin portions <NUM>.

The shallow groove <NUM> extends from the inner edge portion 14A to the outer edge portion 14B so as to surround the above-said part of the crown fin portion <NUM> on the connection portion K side.

The shallow groove <NUM> extends along a part of the outer edge 11e, the heel-side edge 11a, and a part of the inner edge 11i. Thus, the shallow groove <NUM> extends in a U shape convex toward the heel side in the intended tire rotation direction N.

The shallow groove <NUM> allows the crown fin portion <NUM> to deform as if independent from the crown block main portion <NUM>, therefore, the traction performance on rough terrain is further improved.

<FIG> is a cross-sectional view taken along line A-A of <FIG>.

As shown in <FIG>, the groove depth d1 of the shallow groove <NUM> from the ground contacting top surface <NUM> of the crown block main portion <NUM> is not less than <NUM>%, preferably not less than <NUM>%, but not more than <NUM>%, preferably not more than <NUM>% of the radial height H1 of the crown block <NUM> from the tread base portion 2R to the ground contacting top surface <NUM>.

Further, as shown in <FIG>, the width W3 of the shallow groove <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the width W1 in the tire axial direction of the crown block main portion <NUM>.

As a result, the above-mentioned action is effectively exhibited, and the crown fin portions <NUM> and the crown block main portion <NUM> secure rigidity and generate a larger shearing force against mud and soil.

It is preferable that the distance Ld measured parallel to the tire circumferential direction from the heel-side end 18e of the shallow groove <NUM> to the toe-side block edge <NUM> is not more than <NUM>% of the length L1 in the tire circumferential direction of the crown block main portion <NUM>.

Thereby, the rigidity in the tire circumferential direction of the crown block main portion <NUM> is maintained.

As a result, the effect of suppressing the collapse of the crown block <NUM> when contacting with the ground is highly exhibited.

The width W1 in the tire axial direction of the crown block main portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the developed tread width TW.

As shown in <FIG>, the developed tread width TW is the distance measured in the tire axial direction between the tread edges Te when the tread portion <NUM> is unfolded flat.

<FIG> is a cross-sectional view taken along line B-B of <FIG>. As shown in <FIG>, the block side wall surface <NUM> of the crown block main portion <NUM> includes.

The first side wall surface <NUM> comprises a radially outer portion 16a and a radially inner portion 16b.

In the cross-sectional view, the radially outer portion 16a extends substantially straight, radially inwardly from the heel-side block edge <NUM>, while inclining toward the toe side in the intended tire rotation direction N with respect to a straight line drawn normally to the ground contacting top surface <NUM> from the heel-side block edge <NUM>.

Here, the expression "extend substantially straight" means to extend parallel to not only a perfect straight line having an infinite radius of curvature but also an arc line having a radius of curvature of at least <NUM>.

The radially inner portion 16b extends from the radially outer portion 16a to the tread base portion 2R while curving in an arc shape in the cross-sectional view. such first side wall surface <NUM> can deeply dig into mud or soft ground.

The radially inner portion 16b is formed with a single radius of curvature in the present embodiment.

Such radially inner portion 16b relaxes the stress concentration acting on the radially inner portion 16b, and further improve the traction performance. However, the radially inner portion 16b may be formed by a multi-radius curve.

The second side wall surface <NUM> in the present embodiment comprises a first portion 17a, a second portion 17b, and a third portion 17c.

The first portion 17a extends radially inwardly from the toe-side block edge <NUM>.

The second portion 17b extends radially inwardly from the first portion 17a and is inclined more gently than the first portion 17a.

The third portion 17c extends radially inwardly from the second portion 17b to the tread base portion 2R.

In the present embodiment, each of the first portion 17a and the second portion 17b extends linearly in the cross-sectional view.

And the third portion 17c is curved in an arc shape.

For example, the third portion 17c is formed in an arc shape concave toward the heel side in the intended tire rotation direction N.

<FIG> shows the vicinity of the middle block <NUM>.

As shown in <FIG>, each middle block <NUM> is inclined to the heel side in the intended tire rotation direction N, while extending from the inside to the outside in the tire axial direction, and comprises a middle block main portion <NUM> having a parallelogram shape, and a middle fin portion <NUM> protruding toward the toe side in the intended tire rotation direction N from the middle block main portion <NUM>.

The middle block main portion <NUM> has a ground contacting top surface <NUM>, a heel-side middle edge <NUM>, a toe-side middle edge <NUM>, and a pair of circumferential middle edges <NUM> extending from both ends of the heel-side middle edge <NUM>, respectively, toward the toe side in the intended tire rotation direction N.

The pair of circumferential middle edges <NUM> is an axially inner circumferential middle edge 25a adjacent one of the crown blocks <NUM>, and an axially outer circumferential middle edge 25b adjacent to one of the shoulder blocks <NUM>.

The circumferential middle edges <NUM> in this example extend linearly.

In the present embodiment, the middle block main portion <NUM> is provided with the two middle fin portions <NUM>.

The axially outer middle fin portion 21A is directly connected to the middle block main portion <NUM>.

The axially outer edge <NUM> of the axially outer middle fin portion 21A and the axially outer circumferential middle edge 25b are formed by a single straight line.

The axially inner middle fin portion 21B is connected to the middle block main portion <NUM>, and along the junction between them, a middle shallow groove <NUM> is formed.

The axially inner edge <NUM> of the axially inner middle fin portion 21B and the axially inner circumferential middle edge 25a extend in line across the middle shallow groove <NUM>.

since the deformation of the axially inner middle fin portion 21B is promoted by the middle shallow groove <NUM>, the mud clogged between the middle fin portions <NUM> can be smoothly ejected.

The middle shallow groove <NUM>'s edge 30a positioned on the heel side in the intended tire rotation direction N and extending in the longitudinal direction forms the above-mentioned toe-side middle edge <NUM> in this example.

The groove width w4 of the middle shallow groove <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the groove width W3 (<FIG>) of the shallow groove <NUM> of the crown block <NUM>.

The groove depth of the middle shallow groove <NUM> (not shown) is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the block height of the middle block main portion <NUM> measured from the ground contacting top surface to the tread base portion 2R.

The axial width W5 (<FIG>) of the middle block main portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the developed tread width TW.

<FIG> is a top view of one of the shoulder blocks <NUM>. As shown, the shoulder block <NUM> in this example is formed in a generally quadrilateral shape, more specifically trapezoidal shape in its top view.

The ground contacting top surface 7a of the shoulder block <NUM> has an axially outer edge <NUM>, an axially inner edge <NUM>, a toe-side edge <NUM>, and a heel-side edge <NUM>.

The axially outer edge <NUM> extends in the tire circumferential direction, and in this example, forms a part of the tread edge Te.

The axially inner edge <NUM> extends in the tire circumferential direction in this example.

The toe-side edge <NUM> extends in parallel with the tire axial direction from the axially outer edge <NUM> toward the axially inner edge <NUM> in this example.

The heel-side edge <NUM> extends from the axially inner edge <NUM> to the axially outer edge <NUM> while inclining with respect to the tire axial direction, for example, toward the intended tire rotation direction N in this example.

Each of the shoulder blocks <NUM> is provided with a shoulder shallow groove <NUM> in this example.

As shown in <FIG>, the shoulder shallow groove <NUM> extends in a v shape in the top view of the shoulder block <NUM>.

such shoulder shallow groove <NUM> promotes the deformation of the shoulder block <NUM>, and helps to eject the mud clogged between the shoulder block <NUM> and the adjacent middle block <NUM>. The shoulder shallow groove <NUM> is composed of a circumferential portion <NUM> extending in the tire circumferential direction, and an axial portion <NUM> extending in the tire axial direction.

The circumferential portion <NUM> extends at an angle of not more than <NUM> degrees with respect to the tire circumferential direction.

The axial portion <NUM> extends at an angle of more than <NUM> degrees with respect to the tire circumferential direction.

In this example, the circumferential portion <NUM> extends in parallel with the tire circumferential direction from the toe-side edge <NUM> toward the heel side in the intended tire rotation direction N, and ends within the shoulder block <NUM>.

In this example, the axial portion <NUM> extends from the axially inner edge <NUM> toward the outside in the tire axial direction, and is connected to the end of the circumferential portion <NUM>. The axial portion <NUM> extends in parallel with the heel-side edge <NUM>. such shoulder shallow groove <NUM> further promotes deformation of the shoulder block <NUM>.

The groove width W6 of the shoulder shallow groove <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the axial width W7 (<FIG>) of the shoulder block <NUM>.

The groove depth of the shoulder shallow groove <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the block height of the shoulder block <NUM> measured from the ground contacting top surface to the tread base portion 2R.

The axial width w7 of the shoulder block <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the developed tread width TW.

It is preferable that the tread rubber <NUM> (<FIG>) by which the above-mentioned blocks <NUM> to <NUM> are formed has a rubber hardness of not less than <NUM> degrees, and not more than <NUM> degrees.

Here, the rubber hardness means the durometer A hardness measured at a temperature of <NUM> degrees C according to the Japanese Industrial Standard (JIS) K6253. while detailed description has been made of a preferable embodiment of the present invention, the present invention can be embodied in various forms within the scope of the appended claims.

Based on the tread pattern shown in <FIG>, pneumatic tires for a rear wheel of a motorcycle for running on rough terrain were experimentally manufactured as test tires (Working example tires Ex. <NUM> and Comprehensive example tire Ref.<NUM>). Specifications of the test tires are shown in Table <NUM>.

The test tires were tested for the traction performance, braking performance and overall performance, using a 450cc motorcycle for motocross competition having the following rim sizes and tire sizes.

In the test, the rear tire was changed, but the front tire was not changed and an identical tire was used. (tire pressure: <NUM> kPa).

The traction performance, brake performance and overall performance when the above-mentioned motorcycle was run on rough terrain covered with mud were evaluated by the test rider.

Here, the "traction performance" is an evaluation of the traction force at the time of accelerating during straight running and cornering, made by the test rider.

The "brake performance" is an evaluation of the braking force at the time of braking during straight running and cornering, made by the test rider.

The "overall performance" is an evaluation of the running stability at the time of accelerating and braking during straight running and cornering, made by the test rider.

The test rider evaluated each performance on a ten-point scale, and the results are shown in Table <NUM>.

Claim 1:
A tire (<NUM>) for running on rough terrain for which an intended tire rotational direction (N) is specified, and which comprises a tread portion (<NUM>) provided with crown blocks (<NUM>) disposed on the tire equator (C), wherein
each of the crown blocks (<NUM>) comprises
a V-shaped crown block main portion (<NUM>) which bends convexly toward a circumferential direction opposite to the intended tire rotation direction (N), and
only two crown fin portions (<NUM>) projecting from the crown block main portion (<NUM>) toward the above-said circumferential direction opposite to the intended tire rotation direction (N), and
a connection portion (K) between the crown block main portion (<NUM>) and each of the crown fin portions (<NUM>) is provided with a shallow groove (<NUM>) for promoting deformation of a part of the crown fin portion (<NUM>) on the connection portion (K) side,
characterized in that
the depth (d1) of the shallow groove (<NUM>) is in a range from <NUM>% to <NUM>% of the radial height (H1) of the crown block main portion (<NUM>).