Seat and vehicle

Abstract

A seat back of a seat includes: a seat back frame; a front side elastic member positioned on a user side; a rear side elastic member disposed more rearward than the front side elastic member is; and a pressure receiving member disposed at a rear of the rear side elastic member. The rear side elastic member has a lower resilience than the front side elastic member. The pressure receiving member supports the front side elastic member and the rear side elastic member while being supported by the seat back frame.

Claims

1. A seat comprising a seat back, the seat back including: a seat back frame; a front side elastic member positioned on a user side of the seat back frame; a pair of rear side elastic members which have a lower resilience than a resilience of the front side elastic member, and which are disposed more rearward than the front side elastic member; and a pressure receiving member disposed behind the rear side elastic members, the pressure receiving member configured to, while being supported by the seat back frame, support the front side elastic member and the rear side elastic members, wherein: the rear side elastic members are configured to suppress vibrations of the seat back frame or vibrations of the pressure receiving member from being transmitted to the front side elastic member, the pair of rear side elastic members comprises a first rear side elastic member disposed at a first side of a center of the seat back in a width direction, and a second rear side elastic member disposed at a second side of the center of the seat back in the width direction, and a gap is provided between the first rear side elastic member and the second rear side elastic member.

2. The seat according to claim 1, wherein the front side elastic member is formed of a first urethane material having a first resilience, and the rear side elastic members are formed of another urethane material having a lower resilience than the first resilience of the first urethane material of the front side elastic member.

3. The seat according to claim 1, wherein the rear side elastic members comprise at least one rubber sheet.

4. The seat according to claim 1, wherein a plate, which is harder than the front side elastic member and the rear side elastic members, is disposed between the front side elastic member and the rear side elastic members.

5. The seat according to claim 1, wherein the seat comprises a side support disposed at a side of one of the rear side elastic members, and a gap is provided between said one of the rear side elastic members and the side support.

6. The seat according to claim 1, wherein the seat comprises a side support disposed at a side of one of the rear side elastic members, and the side support includes: a user side elastic member positioned on the user side and having a side support first resilience; and a frame side elastic member which has a lower resilience than the side support first resilience of the user side elastic member, and which is disposed between the user side elastic member and the seat back frame.

7. The seat according to claim 6, wherein the front side elastic member and the user side elastic member are configured from a same material, and the rear side elastic members and the frame side elastic member are configured from a same material.

8. A vehicle comprising a seat having a seat back, the seat back including: a seat back frame; a front side elastic member positioned on a user side of the seat back frame; a pair of rear side elastic members which have a lower resilience than the front side elastic member and which are disposed more rearward than the front side elastic member; and a pressure receiving member configured to, while being supported by the seat back frame, support the front side elastic member and the rear side elastic member at a rear of the rear side elastic members, wherein: the pair of rear side elastic members are configured to suppress vibrations of the seat back frame or the pressure receiving member from being transmitted to the front side elastic member, the pair of rear side elastic members comprises a first rear side elastic member disposed at one side of a center of the seat back in a width direction, and a second rear side elastic member disposed at another side of the center of the seat back in the width direction, and a gap is provided between the first rear side elastic member and the second rear side elastic member.

9. A seat comprising a seat back, the seat back comprising: a seat back frame; a front side elastic member positioned on a user side of the seat back frame and having a first resilience; a rear side elastic member disposed behind the front side elastic member and having a lower resilience than the first resilience of the front side elastic member; and a pressure receiving member disposed behind the rear side elastic member, the pressure receiving member configured to, while being supported by the seat back frame, support the front side elastic member and the rear side elastic member, wherein: the rear side elastic members are configured to suppress vibrations of the seat back frame or vibrations of the pressure receiving member from being transmitted to the front side elastic member, the seat comprises a side support disposed at a side of the rear side elastic member, and a gap is provided between the rear side elastic member and the side support, and the front side elastic member abuts against the side support when the seat back is viewed from above.

10. The seat according to claim 9, wherein the side support includes: a user side elastic member positioned on the user side and having a side support first resilience; and a frame side elastic member which has a lower resilience than the side support first resilience of the user side elastic member, and which is disposed between the user side elastic member and the seat back frame.

11. The seat according to claim 9, wherein the front side elastic member and the user side elastic member are configured from a same material, and the rear side elastic member and the frame side elastic member are configured from a same material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an exploded perspective view in which a seat according to a first embodiment of the present invention is shown schematically, viewed obliquely from the front;

(2) FIG. 2 is a plan view in which part of the seat of the first embodiment is simplified;

(3) FIG. 3 is a plan view in which part of a seat of a second embodiment is simplified;

(4) FIG. 4 is a plan view in which part of a seat of a third embodiment is simplified;

(5) FIG. 5 is a plan view in which part of a seat of a fourth embodiment is simplified;

(6) FIG. 6A is a plan view showing a testing device for testing a vibration reduction effect, and a workpiece, and FIG. 6B is a rear view showing the testing device for testing a vibration reduction effect, and the workpiece; and

(7) FIG. 7 is a view showing vibration characteristics corresponding to a first comparative workpiece and a second comparative workpiece, and to the workpiece of FIGS. 6A and 6B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. First Embodiment

A-1. Configuration

(8) [A-1-1. Overall Configuration]

(9) FIG. 1 is an exploded perspective view in which a seat 12 according to a first embodiment of the present invention is shown schematically, viewed obliquely from the front. FIG. 2 is a plan view in which part of the seat 12 of the first embodiment is simplified. It should be noted that in FIG. 2 and later-mentioned FIGS. 3-5, a second skin member 56, third skin members 641, 64r, and so on, shown in FIG. 1 are omitted. As shown in FIG. 1, the seat 12 is disposed on a floor 14 of a vehicle 10, and includes a seat cushion 20, a seat back 22, side supports 24l, 24r, a slide mechanism 26, and a reclining mechanism 28.

(10) The seat cushion 20 includes a cushion frame 30, a first elastic member 32, and a first skin member 34. The seat back 22 includes a seat back frame 50, a second elastic member 52 (a front side elastic member), a third elastic member 54 (a rear side elastic member), and the second skin member 56. The side supports 24l, 24r include side support frames 601, 60r, fourth elastic members 621, 62r, and the third skin members 641, 64r. The cushion frame 30, the seat back frame 50, and the side support frames 601, 60r configure a seat frame 70.

(11) The first elastic member 32, the second elastic member 52, and the fourth elastic members 621, 62r are of a resin material such as a urethane material, for example. Moreover, the third elastic member 54 is of a material of lower resilience than that of the second elastic member 52 (here, a low resilience urethane material). The first skin member 34, the second skin member 56, and the third skin members 641, 64r are made of synthetic leather, for example.

(12) The slide mechanism 26 slides the seat cushion 20, the seat back 22, the side supports 24l, 24r, and the reclining mechanism 28 in a front-rear direction of the vehicle 10, in response to operation of a user 500 (FIG. 2) as an occupant.

(13) The reclining mechanism 28 changes an inclination of the seat back 22 and the side supports 24l, 24r, centered on a spindle 80, in response to operation of the user 500.

(14) [A-1-2. Seat Back 22]

(15) (A-1-2-1. Seat Back Frame 50)

(16) The seat back frame 50 supports the second elastic member 52 and the third elastic member 54 (FIG. 1). As shown in FIG. 1, the seat back frame 50 includes a lower frame 100, an upper frame 102, two side frames 1041, 104r, a lower-side support bar 106d, an upper-side support bar 106u, and a pressure receiving member 108.

(17) The lower-side support bar 106d and the upper-side support bar 106u extend in a left-right direction of the vehicle 10 between the side frames 1041, 104r, thereby supporting the pressure receiving member 108. The pressure receiving member 108 supports the second elastic member 52 and the third elastic member 54 from the rear (a back side), and receives a pressure from a back of the user 500. The pressure receiving member 108 is supported by the seat back frame 50 (specifically, the lower-side support bar 106d and the upper-side support bar 106u). A position of the pressure receiving member 108 is substantially a center in the left-right direction of the seat 12.

(18) (A-1-2-2. Second Elastic Member 52 and Third Elastic Member 54)

(19) As shown in FIG. 2, the second elastic member 52 is disposed closer to a user 500 side than the third elastic member 54 is, to the rear of the user 500. The second elastic member 52 mainly has its resilience set considering a level of sitting comfort of the user 500, and there may be utilized as the second elastic member 52 an elastic member employed in a standard vehicle.

(20) The third elastic member 54 is disposed on a rear side of the second elastic member 52 and a front side of the pressure receiving member 108, to the rear of the user 500. The third elastic member 54 mainly has its resilience set with a purpose of suppressing vibration transmission to the second elastic member 52 from the seat back frame 50 or the pressure receiving member 108. In other words, the third elastic member 54 functions as a spring element. As described above, the third elastic member 54 is of a urethane material of lower resilience than that of the second elastic member 52.

(21) [A-1-3. Side Supports 24l, 24r]

(22) The side supports 24l, 24r (the fourth elastic members 621, 62r) are disposed at the sides of the seat back 22. As shown in FIG. 2, gaps 1201, 120r are provided between the third elastic member 54 and the side supports 24l, 24r.

(23) In FIG. 2, the fourth elastic members 621, 62r are shown by solid lines, broken lines, and one dot-chain lines. This indicates a state of the fourth elastic members 621, 62r vibrating in the left-right direction. In the first embodiment, since the gaps 1201, 120r exist between the third elastic member 54 and the side supports 24l, 24r, then, even if the fourth elastic members 621, 62r vibrate to left and right, they will not contact the pressure receiving member 108 or the third elastic member 54.

A-2. Advantages of First Embodiment

(24) Due to the first embodiment as described above, the third elastic member 54 (the rear side elastic member) which is of lower resilience than the second elastic member 52 (the front side elastic member) on the user 500 side, is disposed between the second elastic member 52 and the pressure receiving member 108 (FIGS. 1 and 2). As a result, even if the seat back frame 50 or the pressure receiving member 108 vibrates, vibration transmission to the second elastic member 52 is suppressed by action of the third elastic member 54. Therefore, vibration transmitted to the user 500 decreases, so it becomes possible for the level of sitting comfort of the user 500 to be improved.

(25) Moreover, it is also conceivable for the third elastic member 54 alone to be employed, instead of employing the second elastic member 52. However, there is a risk that with the third elastic member 54 alone, the user 500, in the case of being the driver, for example, will over-relax due to a low resilience of the third elastic member 54. In the first embodiment, the second elastic member 52 is disposed closer to the user 500 side than the low resilience third elastic member 54 is (FIGS. 1 and 2). This makes it possible for a selection scope of the second elastic member 52 to be broadened.

(26) In the first embodiment, the second elastic member 52 (the front side elastic member) is of a urethane material. Moreover, the third elastic member 54 (the rear side elastic member) is of a low resilience urethane material having a lower resilience than the urethane material of the second elastic member 52. Since the low resilience urethane material is softer than the urethane material, it is difficult for left-right vibration of the seat back frame 50 to be transmitted to the urethane material (the user 500). Therefore, the level of sitting comfort of the user 500 improves.

(27) In the first embodiment, the seat 12 includes the side supports 24l, 24r disposed at the sides of the third elastic member 54 (the rear side elastic member) (FIGS. 1 and 2). Moreover, the gaps 1201, 120r are provided between the third elastic member 54 and the side supports 24l, 24r (FIG. 2). This makes it difficult for the low resilience third elastic member 54 to contact the side supports 24l, 24r when the seat back frame 50 vibrates in the left-right direction. Therefore, it becomes easy for the third elastic member 54 to undergo displacement, and it becomes possible for transmission of vibration to the side supports 24l, 24r to be suppressed and for the level of sitting comfort of the user 500 to be improved.

B. Second Embodiment

B-1. Configuration (Differences from First Embodiment)

(28) FIG. 3 is a plan view in which part of a seat 12a of a second embodiment is simplified. The seat 12a of the second embodiment basically has a similar configuration to the seat 12 of the first embodiment. Hereafter, configuring elements of the second embodiment that are similar to the configuring elements of the first embodiment will be assigned with the same reference symbols, and detailed descriptions thereof will be omitted.

(29) In the first embodiment, the third elastic member 54 of the seat back 22 is of a low resilience urethane material. In contrast, in the second embodiment, a third elastic member 54a (a spring element) of a seat back 22a is a rubber sheet. Moreover, a plate 150 which is harder than the second elastic member 52 and the third elastic member 54a, is disposed between the second elastic member 52 and the third elastic member 54a (FIG. 3). The plate 150 is made of a metal such as aluminum, for example. Alternatively, the plate 150 may be configured made from a fiber reinforced plastic (FRP).

B-2. Advantages of Second Embodiment

(30) Due to the second embodiment, the following advantages can be achieved, in addition to or in place of those of the first embodiment.

(31) That is, in the second embodiment, the third elastic member 54a (the rear side elastic member) is a rubber sheet. As a result, attachment work becomes easy.

(32) In the second embodiment, the plate 150 which is harder than the second elastic member 52 (the front side elastic member) and the third elastic member 54a (the rear side elastic member), is disposed between the second elastic member 52 and the third elastic member 54a (FIG. 3). This makes it possible to stabilize a relative positional relationship of the second elastic member 52 and the third elastic member 54a.

C. Third Embodiment

C-1. Configuration (Differences from First and Second Embodiments)

(33) FIG. 4 is a plan view in which part of a seat 12b of a third embodiment is simplified. The seat 12b of the third embodiment basically has a similar configuration to the seats 12, 12a of the first and second embodiments. Hereafter, configuring elements of the third embodiment that are similar to the configuring elements of the first and second embodiments will be assigned with the same reference symbols, and detailed descriptions thereof will be omitted.

(34) In the first embodiment, the third elastic member 54 of the seat back 22 is of a low resilience urethane material, and in the second embodiment, the third elastic member 54a of the seat back 22a is a rubber sheet. In contrast, in the third embodiment, a third elastic member 54b (a spring element) of a seat back 22b is a plurality of coil springs. The coil springs have a lower resilience than the urethane material of the second elastic member 52. Alternatively, from a viewpoint of suppressing vibration transmission to the second elastic member 52 from the seat back frame 50 or the pressure receiving member 108, it is also possible for the coil springs to be configured having a higher resilience than the urethane material of the second elastic member 52. Moreover, it is also possible for a different kind of spring (for example, a plate spring) to be employed instead of the coil springs.

(35) Similarly to in the second embodiment, the plate 150 which is harder than the second elastic member 52 and the third elastic member 54b, is disposed between the second elastic member 52 and the third elastic member 54b (FIG. 4).

(36) The side supports 24l, 24r of the first and second embodiments include the fourth elastic members 621, 62r as elastic members (FIG. 1). In contrast, side supports 24la, 24ra of the third embodiment include fifth elastic members 1601, 160r (frame side elastic members), in addition to the fourth elastic members 621, 62r (user side elastic members). As shown in FIG. 4, the fourth elastic members 621, 62r are positioned closer to the user 500 side, and have a distance to the user 500 which is shorter, compared to the fifth elastic members 1601, 160r. In other words, although the user 500 sometimes leans on the fourth elastic members 621, 62r via the third skin members 641, 64r, the user 500 does not lean on the fifth elastic members 1601, 160r.

(37) As described above, the fourth elastic members 621, 62r are configured from a similar material to the first elastic member 32 and the second elastic member 52. In contrast, the fifth elastic members 1601, 160r are configured from a similar material to the third elastic member 54b. In other words, the fifth elastic members 1601, 160r are of lower resilience than the fourth elastic members 621, 62r, and, specifically, are configured from a low resilience urethane material. Alternatively, the fifth elastic members 1601, 160r may be configured from a rubber sheet, similarly to the third elastic member 54a of the second embodiment.

C-2. Advantages of Third Embodiment

(38) Due to the third embodiment, the following advantages can be achieved, in addition to or in place of those of the first and second embodiments.

(39) That is, in the third embodiment, the third elastic member 54b (the rear side elastic member) as the coil springs is disposed between the second elastic member 52 (the front side elastic member) and the seat back frame 50 (FIG. 4). This makes it possible for vibration transmission to the second elastic member 52 from the seat back frame 50 or the pressure receiving member 108 to be suppressed by bending of the third elastic member 54b.

(40) In the third embodiment, the seat 12 includes the side supports 24la, 24ra disposed at the sides of the third elastic member 54b (the rear side elastic member) (FIG. 4). The side supports 24la, 24ra include: the fourth elastic members 621, 62r (the user side elastic members) positioned on the user 500 side; and the fifth elastic members 1601, 160r (the frame side elastic members) that are of lower resilience than the fourth elastic members 621, 62r and are disposed between the fourth elastic members 621, 62r and the seat back frame 50.

(41) Due to the third embodiment, the fifth elastic members 1601, 160r which are of lower resilience than the fourth elastic members 621, 62r on the user 500 side, are disposed between the fourth elastic members 621, 62r and the seat back frame 50. As a result, even if the seat back frame 50 vibrates, vibration transmission to the fourth elastic members 621, 62r is suppressed by action of the fifth elastic members 1601, 160r. Therefore, vibration transmitted to the user 500 decreases, so it becomes possible for the level of sitting comfort of the user 500 to be improved.

(42) Moreover, it is also conceivable for the fifth elastic members 1601, 160r alone to be employed, instead of employing the fourth elastic members 621, 62r. However, there is a risk that if, for example, the fifth elastic members 1601, 160r are configured to be of a low resilience urethane material when the second elastic member 52 is of a urethane material which is not of low resilience, then a feeling of discomfort will be caused in the user 500. In the third embodiment, the fourth elastic members 621, 62r are disposed closer to the user 500 side than the low resilience fifth elastic members 1601, 160r are (FIG. 4). This makes it possible for a selection scope of the fifth elastic members 1601, 160r to be broadened, by such means as configuring the material of the fourth elastic members 621, 62r to be the same as that of the second elastic member 52.

D. Fourth Embodiment

D-1. Configuration (Differences from First Through Third Embodiments)

(43) FIG. 5 is a plan view in which part of a seat 12c of a fourth embodiment is simplified. The seat 12c of the fourth embodiment basically has a similar configuration to the seats 12, 12a, 12b of the first through third embodiments. Hereafter, configuring elements of the fourth embodiment that are similar to the configuring elements of the first through third embodiments will be assigned with the same reference symbols, and detailed descriptions thereof will be omitted.

(44) In the seat backs 22, 22a, 22b of the first through third embodiments, single kinds of the third elastic members 54, 54a, 54b were disposed between the second elastic member 52 and the pressure receiving member 108 (FIGS. 1-4). In contrast, in a seat back 22c of the fourth embodiment, two kinds of third elastic members 54c, 54d are disposed between the second elastic member 52 and the pressure receiving member 108 (FIG. 5). The third elastic member 54c is a coil spring, similarly to the third elastic member 54b of the third embodiment. The third elastic member 54d is of a low resilience urethane material, similarly to the third elastic member 54 of the first embodiment. One of the third elastic members 54c, 54d may be configured as a rubber sheet.

D-2. Advantages of Fourth Embodiment

(45) Due to the fourth embodiment, the following advantages can be achieved, in addition to or in place of those of the first through third embodiments.

(46) That is, in the fourth embodiment, the two kinds of third elastic members 54c, 54d are disposed between the second elastic member 52 and the pressure receiving member 108 (FIG. 5). This makes it possible for the two kinds of third elastic members 54c, 54d to respectively reduce vibrations of different frequency regions. Therefore, a vibration reduction effect can be improved.

E. Vibration Reduction Effect of Third Elastic Member 54, and so on

(47) Next, vibration reduction effects of the third elastic members 54, 54a, 54b, 54c, 54d will be described in more detail.

(48) FIG. 6A is a plan view showing a testing device 200 for testing a vibration reduction effect, and a workpiece 202, and FIG. 6B is a rear view showing the testing device 200 for testing a vibration reduction effect, and the workpiece 202. As shown in FIG. 6B, the workpiece 202 includes a first test elastic member 210, a plate 212, and a second test elastic member 214.

(49) The first test elastic member 210 is an elastic member (of a urethane material) the same as the first elastic member 32, the second elastic member 52, and the fourth elastic members 621, 62r, and has a rectangular parallelepiped shape. The plate 212 is hard, similarly to the plate 150. The second test elastic member 214 is a rubber sheet similar to the third elastic member 54a of the second embodiment (FIG. 3). Therefore, the workpiece 202 has a similar configuration to the seat 12a of the second embodiment.

(50) Moreover, a first comparative workpiece (not illustrated) without the first test elastic member 210, the plate 212, and the second test elastic member 214, is prepared as a first comparative example. Furthermore, a second comparative workpiece (not illustrated) that includes the plate 212 and the second test elastic member 214, but does not include the first test elastic member 210, is prepared as a second comparative example.

(51) The testing device 200 includes a weight 230, an acceleration sensor 232, a hammering point 234, a hammer 236, and a surface table 238. The weight 230 is disposed on each of the workpiece 202, the first comparative workpiece, and the second comparative workpiece. The acceleration sensor 232 and the hammering point 234 are disposed on the weight 230. In this state, a vibration V (hereafter, also called a vibration V3) when the hammering point 234 is struck by the hammer 236 along the front-rear direction (a direction of arrow A in FIG. 6A), is detected by the acceleration sensor 232. The first comparative workpiece and the second comparative workpiece also have their vibrations V (hereafter, also called vibrations V1, V2) detected by the acceleration sensor 232 by a similar method. FIG. 7 is a view showing characteristics of the vibrations V1, V2, V3 corresponding to the first comparative workpiece, the second comparative workpiece, and the workpiece 202 of FIGS. 6A and 6B. As described above, the vibration V corresponding to the first comparative workpiece is the vibration V1, and the vibration V corresponding to the second comparative workpiece is the vibration V2. Moreover, the vibration V corresponding to the workpiece 202 of FIGS. 6A and 6B is the vibration V3. The horizontal axis of FIG. 7 shows a frequency f [Hz] of the vibrations V1, V2, V3, and the vertical axis of FIG. 7 shows an acceleration G [m/s.sup.2] of the vibrations V1, V2, V3. The graph of FIG. 7 is obtained by performing a fast Fourier transform on a detection value of the acceleration sensor 232.

(52) As is clear from FIG. 7, a peak value of the acceleration G is reduced more in the case of the second comparative workpiece (second test elastic member 214 as a rubber sheet present but first test elastic member 210 of urethane material not present) than in the case of the first comparative workpiece (neither first test elastic member 210 of urethane material nor second test elastic member 214 as a rubber sheet present) (refer to arrow A2 of FIG. 7). This is conceivably due to the vibration V having been reduced by the second test elastic member 214 (the rubber sheet).

(53) Moreover, the peak value of the acceleration G is reduced more in the case of the workpiece 202 (both first test elastic member 210 of urethane material and second test elastic member 214 as a rubber sheet present) than in the case of the second comparative workpiece (refer to arrow A3 of FIG. 7). This is conceivably due to the vibration V having been reduced by the first test elastic member 210 (of the urethane material) in addition to the second test elastic member 214 (the rubber sheet).

(54) Therefore, a vibration reduction effect could be said to occur due to employing the first test elastic member 210 and the second test elastic member 214. Such a vibration reduction effect could be achieved also by the third elastic member 54a of the second embodiment. Moreover, a similar effect is assumed to be achieved also for the third elastic members 54, 54b, and 54c, 54d of the first, third, and fourth embodiments.

F. Modified Examples

(55) Note that the present invention is not limited to each of the above-described embodiments, and that a variety of configurations may of course be adopted based on the described content of the present specification. For example, the following configurations may be adopted.

F-1. Application Target

(56) In the first embodiment, the seat 12 is assumed to be applied to the vehicle 10 as a car (FIG. 1). However, for example, from a viewpoint of providing the third elastic member 54 (the rear side elastic member) in addition to the second elastic member 52 (the front side elastic member), the present invention is not limited to this. For example, the vehicle 10 (or means-of-transportation) may be a moving object such as a ship, an aircraft, or the like. Alternatively, it is also possible for the seat 12 to be applied to a position of other than the vehicle 10. The same applies also to the second through fourth embodiments.

F-2. Seat Back Frame 50

(57) The seat back frame 50 of the first embodiment includes the plate-like pressure receiving member 108 (FIG. 1). However, for example, from a viewpoint of supporting the second elastic member 52 (the front side elastic member) and the third elastic member 54 (the rear side elastic member), the present invention is not limited to this. For example, it is also possible for an elastic supporting member such as an S spring to be employed instead of the plate-like pressure receiving member 108 (refer to FIG. 1 of Japanese Laid-Open Patent Publication No. 2005-254962, for example).

F-3. Side Supports 24l, 24la, 24r, 24ra

(58) In the first embodiment, the side supports 24l, 24r are provided (FIG. 1). However, for example, from a viewpoint of providing the third elastic member 54 (the rear side elastic member) in addition to the second elastic member 52 (the front side elastic member), it is also possible for the side supports 24l, 24r to be omitted. The same applies also to the second through fourth embodiments.