Vehicle-mounted seat device

A vehicle-mounted seat device includes an SB main frame, a backrest, a relay disk secured at the center in the width direction of the backrest, a plurality of radial wires for suspending the backrest such that the backrest can swing relative to the SB main frame with the relay disk as the center, an SB subframe capable of swinging together with the backrest, a headrest, a headrest stay extending downward from the headrest and mounted on the SB main frame so as to rotate around a rotation shaft; a pressure-receiving body secured on the headrest stay so as to be disposed behind and opposed to the relay disk; and a provisional connection mechanism for provisionally connecting the headrest stay to the SB main frame.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2019-161188 filed on Sep. 4, 2019, which is incorporated herein by reference in its entirety including the specification, claims, drawings, and abstract.

TECHNICAL FIELD

This specification discloses a seat device to be mounted in a vehicle.

BACKGROUND

A vehicle-mounted seat device typically includes a seat cushion for supporting the hips of an occupant seated on the vehicle-mounted seat device (hereinafter referred to as a “seated person”) and a seatback for supporting the back of the seated person. The seatback further includes a backrest on which the seated person is to lean, and a frame for supporting the backrest. JP 2018-43672A discloses a vehicle-mounted seat device including a backrest that can swing in the right-left direction. The frame disclosed in JP 2018-43672A 1 includes a seatback frame and a subframe. The subframe is connected to the seatback frame via a leaf spring so as to swing relative to the seatback frame. The subframe has a net structure including a plurality of radially spreading linear members. The backrest is disposed forward of the net structure. With this structure, the tensile force of the linear members can provide appropriate cushioning, and the backrest can swing together with the subframe relative to the seatback frame.

Note here that, at the time of rear collision of a vehicle, a seated person may suffer cervical sprain, or so-called whiplash. Whiplash is caused by an impact at the time of rear collision that pushes the body of a seated person forward of the vehicle together with the backrest while his/her head remains as it is. Consequently, the head is caused to swing far rearward relative to the body, which damages the cervical tissues. Whiplash may likely become serious, in particular, with a seat device including a backrest supported by linear members, as is in JP 2018-43672A. That is, in the case of a seat device including a thick cushion member (for example, urethane mats), the body of a seated person sinks far rearward while greatly deforming the cushion member upon receipt of an impact at the time of rear collision. This results in a smaller difference in the amount of retreatment between the body and the head. In contrast, in the case of a seat device disclosed in JP 2018-43672A in which cushioning properties are ensured by the tensile force of the linear members supporting the backrest, the body of the seated person tends to sink only by a small amount upon receipt of an impact at the time of rear collision, which likely causes the head to swing far rearward, as compared with the body.

In view of the above, conventionally, a headrest has been provided to a vehicle-mounted seat device to prevent occurrence of or reduce the damage of whiplash. A headrest is effective in preventing the head of a seated person from swinging at the time of rear collision of a vehicle and preventing occurrence of whiplash. Some such headrests incorporate an active headrest mechanism that causes the headrest to lean forward at the time of collision to thereby improve protection capability. For example, JP 2012-96773A discloses a headrest including such an active headrest mechanism. A conventional active headrest mechanism, however, is incorporated in a vehicle-mounted seat device whose backrest swings in neither the right nor left direction, and can be hardly adapted to swingable backrests. Although the vehicle-mounted seat device disclosed in JP 2018-43672A includes a headrest for supporting the head of a seated person, JP 2018-43672A does not elaborate the detailed structure of the headrest.

In view of the above, this specification discloses a vehicle-mounted seat device including a swingable backrest and capable of more reliable protection of a seated person at the time of rear collision.

SUMMARY

A vehicle-mounted seat device disclosed in this specification includes a seatback main frame standing in a vehicle cabin; a backrest on which the back of a seated person is to lean; a relay disk secured at the center in the width direction of the backrest; a plurality of suspension tools spreading radially from the relay disk as the center, the suspension tools being for suspending the backrest such that the backrest is able to swing relative to the seatback main frame with the relay disk as the center; a seatback subframe mounted on the backrest, the seatback subframe being capable of swinging together with the backrest relative to the seatback main frame; a headrest provided above the backrest, the headrest for supporting the head of the seated person; a headrest stay connected to the headrest and extending downward from the headrest; a rotation shaft extending in the right-left direction, the rotation shaft for supporting the middle portion of the headrest stay such that the headrest stay is able to rotate relative to the seatback main frame; a pressure-receiving body secured on a lower portion of the headrest stay and disposed behind and opposed to the relay disk; and a provisional connection mechanism for connecting the headrest stay to the seatback main frame to regulate the rotation of the headrest stay and for releasing the connection between the headrest stay and the seatback main frame upon input of a force greater than a predetermined force in a direction in which the headrest stay leans forward.

As the backrest swings with the relay disk as the center, even when the backrest should swing around the front-back axis and thereby be inclined, the position of the relay disk in the right-left and up-down directions remains unchanged. With the pressure-receiving body provided behind this relay disk, even if the backrest should be inclined at the time of rear collision, the relay disk moves rearward together with the backrest, and the relay disk reliably collides against the pressure-receiving body. This causes the headrest stay to swing to lean forward with the rotation axis as the center, which enables reliable support of the head of a seated person. Consequently, the above-described structure enables more reliable protection of a seated person at the time of rear collision, while having a swingable backrest.

In this case, the provisional connection mechanism may connect the headrest stay to the seatback main frame by any of magnetic force, friction force, and a member that is breakable upon input of a force greater than a predetermined force.

With this structure, as a force greater than a predetermined force is inputted, it is possible to reliably cause the headrest stay to lean forward.

The seatback main frame may have a stay hole that is a through hole into which the headrest stay is to be inserted and that is larger than the headrest stay.

In this case, when being inclined by an amount greater than a predetermined amount, the headrest stay abuts the circumferential edge of the stay hole to thereby prevent further inclination. That is, with provision of a stay hole into which the headrest stay is to be inserted, it is possible to regulate the amount of inclination of the headrest stay.

The pressure-receiving body may be a plate equal to or larger in size than the relay disk, and the pressure-receiving body may have a recess formed on a surface thereof opposed to the relay disk, the recess for avoiding interference with the suspension tool extending from the relay disk.

As this structure can prevent interference between the suspension tool and the pressure-receiving body, it is possible to have the suspension tool strung in a more appropriate condition.

The vehicle-mounted seat device disclosed in this specification enables more reliable protection of a seated person at the time of rear collision, while having a swingable backrest.

DESCRIPTION OF EMBODIMENTS

The structure of a vehicle-mounted seat device10will now be described while referring to the following drawings.FIG. 1is a perspective view of the vehicle-mounted seat device10viewed from diagonally rearward.FIG. 2andFIG. 3are exploded perspective views of a seatback14. Note that terms such as “up/down”, “front/back”, and “right/left” in the following description refer to respective directions as viewed by an occupant seated on the vehicle-mounted seat device10(hereinafter referred to as a “seated person”) unless otherwise stated. In respective drawings, “Fr”, “Up”, and “R” respectively refer to forward, upward, and rightward directions. A “seatback” may be hereinafter abbreviated as “SB” when necessary.

The vehicle-mounted seat device10is to be mounted in a vehicle, and is to be used, for example, as a front seat, such as a driver's seat or a passenger seat. The vehicle-mounted seat device10includes as main components a seat cushion12that supports the hips of a seated person, the seatback14that supports the back of the seated person, and a headrest16that supports the head of the seated person. In view that a known conventional technique can be referred to as to the structure of the seat cushion12, the structure of the seat cushion12is not described in detail. InFIG. 1, the shape of the seat cushion12is only roughly illustrated. On the floor surface inside the cabin of the vehicle, a slide rail18is installed. The slide rail18includes a lower rail18L secured on the floor surface of the cabin and extending in the front-back direction, and an upper rail18U slidable along the lower rail18L. The seat cushion12is connected to the upper rail18U at its four corners, which makes the vehicle-mounted seat device10slidable in the front-back direction.

The headrest16is a member provided above the seatback14to support the head of a seated person. From the lower edge of the headrest16, a pair of headrest stays96extend. The headrest stays96are rotatably mounted on an SB main frame22, as described later.

The seatback14includes as main components the SB main frame22and an SB assembly24. The SB main frame22is a frame member standing in the cabin and made of highly rigid metal material (for example, steel). As illustrated inFIG. 2, the SB main frame22includes four connected upper, lower, right, and left fame members that together define a substantially rectangular shape when viewed from the front. Near the lower end of the SB main frame22, a reclining shaft20is provided extending in the right-left direction. The SB main frame22is swingably connected to the seat cushion12via the reclining shaft20. Specifically, the SB main frame22swings around the reclining shaft20as the center, whereby the entire seatback14can swing (that is, recline) relative to the seat cushion12.

FIG. 3is an exploded perspective view of the SB assembly24. The SB assembly24is formed by combining a backrest26, an SB subframe28, and an SB middle frame30. The SB middle frame30is a frame member secured on the SB main frame22, and is not movable in position relative to the SB main frame22. The backrest26is a thin panel member on which the back of a seated person is to lean. The backrest26is held suspended from the SB middle frame30, and can swing relative to the SB middle frame30and thus the SB main frame22. The SB subframe28is provided behind the backrest26, and can swing together with the backrest26relative to the SB middle frame30and thus the SB main frame22.

More specifically, the SB middle frame30includes a pair of side frames60extending in the up-down direction, and a support pipe62extending between the pair of side frames60. Each side frame60is placed on the outer surface of the SB main frame22and fastened to the SB main frame22with bolts. Near the lower end of the side frame60and the middle of the side frame60in the height direction, upper front restraining brackets66U and lower front restraining brackets66L (hereinafter to be referred to as “front restraining brackets66” when discrimination between upper and lower ones is not made) are secured such as by welding. The front restraining bracket66is a bracket to which the front end portion of a front-back wire82, to be described later, is secured. A specific structure of the front restraining bracket66will be described later.

The backrest26is a member for supporting the back of a seated person, and is curved in accordance with the shape of the back of a person. On the back surface of the backrest26at the center in the right-left direction, a relay disk32is secured. The relay disk32is positioned higher than the center of the backrest26in the height direction and substantially as high as the shoulder blades of a seated person. The relay disk32is a low-profile disk member whose diameter is sufficiently larger than its size in its axial direction. On the surface of the relay disk32, there are formed grooves or slits in which radial wires74, to be described later, are to be hooked. On the rear surface of the backrest26below the relay disk32, a plurality of wire guides36are disposed at intervals in the up-down direction. Each wire guide36indicates a position where to dispose a lateral wire58to be described later, and includes ribs standing from the rear surface of the backrest26.

The SB subframe28is a frame member mounted on the rear surface of the backrest26. The SB subframe28includes a pair of vertical frames42, an upper lateral frame43U, and a lower lateral frame43L (hereinafter referred to as “lateral frames43” when discrimination between upper and lower ones is not made), and a plurality of the lateral wires58. The vertical frames42are frame members extending in the up-down direction, and are mounted on the respective right and left end portions of the backrest26. The lateral frame43connects the pair of vertical frames42.

The lateral frame43projects further rearward than the vertical frame42; that is, toward the SB middle frame30, such that the lateral frame43defines a substantially C-shape in a top view. On the upper lateral frame43U, a pair of upper rear restraining brackets52U and a pair of wire-retaining end portions44are secured. Meanwhile, on the lower lateral frame43L, a pair of lower rear restraining brackets52L (hereinafter referred to as “rear restraining brackets52” when discrimination between upper and lower ones is not made) and a pair of wire-retaining end portions44are secured.

The wire-retaining end portion44is a portion where a lower wire80(the radial wire74), to be described later, is retained. The rear restraining bracket52is a portion where the rear end portion of the front-back wire82, to be described later, is secured. The lateral wires58are wire strung between the pair of vertical frames42. The lateral wires58support the backrest26from its rear side to bear the load applied from a seated person.

Referring toFIG. 4andFIG. 5, various manners of stringing various wires for supporting the backrest26or regulating the position of the backrest26will now be described.FIG. 4illustrates wires strung.FIG. 5is a perspective view of a front-back position regulation mechanism50and portions therearound. InFIG. 4, the backrest26and the lateral wires58are not illustrated.

As is obvious fromFIG. 4, a plurality of wires extend radially from the relay disk32. These plurality of radially extending wires will be hereinafter collectively referred to as radial wires74. Each radial wire74functions as a suspension tool for holding the backrest26suspended. The radial wires74include one upper wire76, a pair of right and left wires78, and two lower wires80. The upper wire76is a ring wire suspended from a pulley64and passing through a hole formed on the relay disk32. With the upper wire76, the relay disk32and thus the backrest26are held suspended. Note that the support pipe62, on which the pulley64is mounted, is a part of the SB middle frame30, and the position of the support pipe62relative to the SB main frame22is fixed. Being suspended from the pulley, the backrest26can swing relative to the SB main frame22. Each of the lower wires80is a wire passing through the relay disk32so as to bend into a U-shape such that both its ends descend from the relay disk32. The respective end portions of each lower wire80are secured to the pair of wire-retaining end portions44provided to the lateral frames43.

The two right and left wires78are strung in a right-left symmetrical manner. Each of the right and left wires78is a ring wire, and is strung so as to define a substantial triangle by passing through the relay disk32, the upper end portion of the vertical frame42, and a portion of the support pipe62on the extension from the relay disk32in a corresponding right or left diagonally forward direction. As the two right and left wires78are strung in a right and left symmetrical manner, the backrest26is naturally positioned at a neutral position where the tensile forces of the two right and left wires78balance.

As described above, the positions of the relay disk32and the backrest26having the relay disk32secured thereon, relative to the SB middle frame30(and thus the SB main frame22), are restrained by the plurality of radial wires74. This restraint, however, is not rigid. That is, when the radial wires74flex or incline moderately, the backrest26and the SB subframe28mounted on the backrest26can swing relative to the SB main frame22with the relay disk32as the center.

In order to regulate the position in the front-back direction of the backrest26, one or more (four in this example) front-back position regulation mechanisms50are provided to the vehicle-mounted seat device10. As illustrated inFIG. 5, each front-back position regulation mechanism50includes a front restraining bracket66provided to the SB middle frame30, the rear restraining bracket52provided to the SB subframe28, and the front-back wire82. The front restraining bracket66is a bracket mounted on the front end portion of the side frame60of the SB middle frame30. On a part of the front restraining bracket66, a substantially cylindrical front fixing portion68is mounted extending rearward. The front end portion of the front-back wire82is secured on the front fixing portion68.

The rear restraining bracket52is a bracket to be mounted on the lateral frame43of the SB subframe28. The rear restraining bracket52includes as main components a rear fixing portion55, on which the rear end portion of the front-back wire82is secured, and an arm portion56extending from the rear fixing portion55so as to detour upward before further extending forward to be secured to the lateral frame43. The rear fixing portion55is positioned behind and substantially directly opposed to the front fixing portion68. The arm portion56is a substantially L-shaped member that is bent midway. At the bent portion of the arm portion56, a weak portion where stress is more likely concentrated than other portions is formed, as will be described later.

The front-back wire82is a wire whose front end portion is secured to the front fixing portion68and whose rear end portion is secured to the rear fixing portion55. This strung front-back wire82regulates the rearward movement of the backrest26and the SB subframe28. That is, the front-back wire82functions as a restraining member for regulating the rearward movement of the SB subframe28relative to the SB main frame22.

With the vehicle-mounted seat device10having the above-described structure, the backrest26can swing relative to the SB main frame22around the front-back axis. In this case, rightward and leftward swings caused when a vehicle is running can be absorbed not by the motion of the head of a seated person but by the motion of a body, which can stabilize the posture of the head of the seated person. Moreover, as the backrest26, on which the back of a seated person is to lean, is supported by a plurality of wires, it is possible to provide suitable cushioning even without a thick cushion member (for example, a urethane seat), which enables reduction in weight of the vehicle-mounted seat device10.

Note here that a seated person may suffer cervical strain; that is, so-called whiplash, at the time of rear collision of a vehicle, depending on the structure of the vehicle-mounted seat device10. Specifically, whiplash is a disorder caused when the body is pushed forward by the seatback14while the head remains as it is at the time of rear collision, and consequently, the head swings far rearward relative to the body, whereby the cervical vertebrae, which support the head, are damaged. In a structure in which the backrest26is supported with wires, in particular, the body of a seated person sinks rearward only by an amount within the range permitted by the tensile force of the wires. Consequently, the head of the seated person tends to swing far relative to his/her body in the case of a structure in which the backrest26is supported with wires, as compared with a structure including a thick cushion member.

To address the above, in this specification, a headrest16is caused to lean forward and the backrest26is caused to sink far rearward at the time of rear collision in order to prevent occurrence of or reduce the damage caused by whiplash. This will be described below in detail.

As described above and illustrated inFIG. 2, the pair of headrest stays96extend from the lower end of the headrest16. Each headrest stay96is a metal bar member. The lower end portion of each headrest stay96is bent forward, as illustrated inFIG. 2. On the middle portion of the headrest stay96, a shaft hole is formed for insertion of a rotation shaft90therethrough. As illustrated inFIG. 1, the headrest stay96extends in the right-left direction, and is secured on the SB main frame22. The headrest stay96is securely mounted on the SB main frame22so as to rotate via the rotation shaft90passing through the shaft hole.

On the top surface86of the SB main frame22, a stay hole88is formed for insertion of the headrest stay96.FIG. 6is a schematic perspective view of the stay hole88and a portion therearound. As is obvious fromFIG. 6, the stay hole88is sufficiently larger than the headrest stay96with a space left between the outer circumferential surface of the headrest stay96and the inner circumferential edge of the stay hole88. To regulate the movement of the headrest stay96in the stay hole88(specifically, the rotation of the headrest stay96around the rotation shaft90), the headrest16is provisionally connected to the SB main frame22by a provisional connection mechanism92. The provisional connection mechanism92is a mechanism for connecting the headrest stay96to the SB main frame22to regulate the rotation of the headrest stay96and for releasing the connection upon input of a force greater than a predetermined force to the headrest stay96in a direction in which the headrest stay96leans forward.

In this example, the provisional connection mechanism92magnetically connects the headrest stay96to the SB main frame22. Specifically, in this example, the headrest stay96is made of magnetic member, and a stationary magnet92ais secured on the top surface86of the SB main frame22to magnetically attract the rear end surface of the headrest stay96. Upon application of a force greater than a predetermined force, the headrest stay96leans forward against the magnetic attracting force. Note that the above-mentioned structure of the provisional connection mechanism92is only an example, and can be arbitrarily modified. For example, the stationary magnet92amay be secured on the headrest stay96, rather than on the SB main frame22, and the top surface86of the SB main frame22may be made of magnetic material. Still alternatively, magnets may be provided to both the headrest stay96and the SB main frame22.

The provisional connection mechanism92may provisionally connect the headrest stay96to the SB main frame22, using any another connector other than a magnet. For example, as illustrated inFIG. 7A, the rear end surface of the headrest stay96may be connected to the rear end edge of the stay hole88through spot welding92b. In this case, the strength of the spot welding is such that it is broken with application of a force greater than a predetermined force. As an alternative configuration, as illustrated inFIG. 7B, the size of the stay hole88in the front-back direction may be sufficiently larger than that of the headrest stay96, and a narrower portion92chaving a width slightly smaller than that of the headrest stay96may be formed in the intermediate portion of the stay hole88. As a still alternative configuration, as illustrated inFIG. 8A, a fitting portion92dmay be formed on the rear end edge of the stay hole88and on the rear end surface of the headrest stay96so that the fitting portions92dare fit to each other to thereby regulate the mutual movement by friction force. As a yet alternative configuration, as illustrated inFIG. 8B, a spring92emay be provided to push the headrest stay96onto the rear end edge of the stay hole88.

As illustrated inFIG. 2, a pressure-receiving body98is secured on the leading end portion of the headrest stay96. The pressure-receiving body98is a plate member disposed behind and opposed to the relay disk32. The pressure-receiving body98is larger in size than the relay disk32, and is disposed near the relay disk32.

The movement of the headrest stay96having the above-described structure will now be described while referring toFIG. 9andFIG. 10.FIG. 9andFIG. 10are schematic cross sectional views of the headrest stay96and portions therearound. In particular,FIG. 9relates to a normal state, andFIG. 10relates to the time of rear collision. As illustrated inFIG. 9, in a normal state, the headrest stay96remains provisionally connected to the top surface86of the SB main frame22with the stationary magnet92a. The pressure-receiving body98is disposed behind and opposed to the relay disk32with a slight space left therebetween. Note that as the backrest26swings with the relay disk32as the center, the position of the relay disk32relative to the SB main frame22and thus the pressure-receiving body98remains unchanged even if the backrest26should swing. That is, the relay disk32always remains opposed to the pressure receiving body98.

In contrast, at the time of rear collision, the vehicle-mounted seat device10moves forward together with the vehicle body. Then, the body of a seated person sinks rearward together with the backrest26by an amount in the range permitted by the tensile force of the radial wires74and the lateral wires58. Consequently, as illustrated inFIG. 10, the relay disk32, secured on the backrest26, collides against the pressure-receiving body98. Due to the collision, the pressure-receiving body98as well moves rearward, whereby the headrest stay96rotates with the rotation shaft90as the center so as to lean forward. This causes the headrest16to move forward to thereby restrain the rearward movement of the head portion. Consequently, the amount of rearward swing of the head relative to the body is reduced. This can effectively prevent occurrence of or reduce the damage of whiplash.

As described above and illustrated inFIG. 9, each of the lower wires80extends diagonally rearward and downward from the relay disk32. When the pressure-receiving body98is disposed behind and opposed to the relay disk32, the lower wire80may possibly interfere with the pressure-receiving body98. In view of the above, as illustrated inFIG. 11, a recess100may be formed on the surface of the pressure-receiving body98that is opposed to the relay disk32to thereby avoid interference with the lower wire80. In addition, the height of the position where the backrest26is disposed can be changed depending on the body-build of a seated person. In this case, the pressure-receiving body98may have a sufficiently large size, as compared with the relay disk32; that is, large enough for the pressure-receiving body98to remain directly opposed to the relay disk32should the height of the relay disk32be changed following change in height of the backrest26.

Although in the above description the pressure-receiving body98is a plate member, the pressure-receiving body98may have any other shape and structure without limitation, provided that the relay disk32can collide against the pressure-receiving body98. For example, the pressure-receiving body98may be a bar member straddling between the leading end portions of the pair of headrest stays96. Alternatively, the pressure-receiving body98may be either a separate member from the headrest stay96or a part of the headrest stay96. That is, the tip end portion of the headrest stay96may be used as the pressure-receiving body98. Still alternatively, the headrest stay96may be made from a single member or a combination of a plurality of members.

The front-back position regulation mechanism50in this example will now be described in detail. As described above, the front-back position regulation mechanism50includes the rear restraining bracket52to be secured relative to the SB subframe28, the front restraining bracket66to be secured relative to the SB main frame22, and the front-back wire82strung between the respective restraining brackets52,66.FIG. 12is a perspective view of the rear restraining bracket52. As described above and illustrated inFIG. 12, the rear restraining bracket52includes the substantially L-shaped arm portion56and the rear fixing portion55provided to the leading end portion of the arm portion56. Further, the arm portion56includes a thin panel main portion56a, and a railing portion56bstanding from the respective sides of the main portion56a. Provision of the railing portion56benhances the strength of the arm portion56. The rear fixing portion55is a substantially cylindrical member mounted on an end portion of the arm portion56. Alternatively, the rear end portion of the front-back wire82may be secured directly to an end portion of the arm portion56without providing such a cylindrical member. In this case, the end portion of the arm portion56functions as the rear fixing portion55.

In this example, a weak portion102is formed at the bent portion of the arm portion56. The weak portion102is a portion where stress is likely concentrated when a compressing force is applied in the front-back direction. In this example, as illustrated inFIG. 12, a notch is formed on the railing portion56bat the bent portion to thereby constitute the weak portion102. The weak portion102may have a different structure. For example, the main portion56aat the bent portion may be thinner than other portions, or a notch may be formed in the main portion56aat the bent portion, to thereby constitute the weak portion102.

The movement of the front-back position regulation mechanism50having the above-described structure will now be described while referring toFIG. 13.FIG. 13is a schematic side view of the front-back position regulation mechanism50. The upper illustration inFIG. 13relates to a normal state, while the lower illustration relates to the time of rear collision. As shown in the upper illustration inFIG. 13, in a normal state, the front fixing portion68is directly opposed to the rear fixing portion55, and the front-back wire82is strung between the respective fixing portions68,55. The SB subframe28including the lateral frame43is regulated by the front-back wire82so as not to move rearward by an amount in excess of a predetermined amount.

At the time of rear collision, the SB main frame22moves forward together with the vehicle body, while a seated person remains where he/she is. In other words, the seated person moves relatively rearward relative to the SB main frame22. The backrest26and the SB subframe28as well, which support the seated person, move rearward relative to the SB main frame22. In the case where the rear restraining bracket52is sufficiently rigid, however, the relative rearward movement of the SB subframe28is regulated by the front-back wire82, and the body of the seated person does not sink sufficiently. In this case, only the head of the seated person moves rearward relative to the body, which likely causes whiplash.

In this example, the weak portion102is provided to the arm portion56of the rear restraining bracket52, as described above. Consequently, when the SB subframe28moves rearward relative to the SB main frame22and a load is thereby applied to the arm portion56, stress will be concentrated on the weak portion102. Thus, the weak portion102bends, as shown in the lower illustrated inFIG. 13; the arm portion56thereby bends into a V-shape; and the rear fixing portion55is displaced forward relative to the leading end portion of the arm portion56. In this case, the SB subframe28is allowed to move rearward relative to the SB main frame22by an amount corresponding to an amount by which the rear fixing portion55is displaced forward. This resultantly increases the amount by which the body of a seated person sinks at the time of rear collision. The increase in the amount by which the body of a seated person sinks and the readiness with which the body moves together with the head can accordingly effectively prevent occurrence of or reduce the damage of whiplash. Note that as the buckling load of the weak portion102is smaller than a load which the front-back wire82can tolerate against tension, or a maximum tensile load, the weak portion102is bent and deformed before the front-back wire82is broken by tension. This prevents the body of a seated person from limitlessly moving rearward at the time of rear collision.

In this example, provision of the weak portion102in the arm portion56induces deformation of the arm portion56. The arm portion56, however, may have a different structure, provided that the structure causes the rear fixing portion55to move forward relative to the front end portion of the arm portion56to be thereby deformed upon input of a rearward load greater than a predetermined load. For example, as illustrated inFIG. 14, the arm portion56may include a reference portion104secured relative to the SB subframe28and a movable portion106extending rearward from the rear end of the reference portion104. In this case, the movable portion106is slidable relative to the reference portion104.

FIG. 15is a schematic cross sectional view along line A-A inFIG. 14. As illustrated inFIG. 15, each of the reference portion104and the movable portion106has a substantially C shape that is open upward. The movable portion106is smaller than the reference portion104and disposed inside the reference portion104. Guide shafts108extend from respective side surfaces of the movable portion106. As illustrated inFIG. 14, a guide slit110is formed on each of the side surfaces of the reference portion104so at to extend in the front-back direction for insertion of the guide shaft108therein. Only the front and rear end portions of the guide slit110are large, and the middle portion of the guide slit110is slightly smaller than the outer diameter of the guide shaft108. Thus, the guide shaft108remains in the rear end portion of the guide slit110when no large load is applied, as shown in the upper illustration inFIG. 14.

Meanwhile, when a rearward force greater than a predetermined force is inputted into the arm portion56at the time of rear collision, as shown in the lower illustration inFIG. 14, the guide shaft108overcomes the narrower portion of the guide slit110to move to the front end portion of the guide slit110. In the above, the movable portion106as well, connected to the guide shaft108, moves forward relative to the reference portion104. Consequently, the rear fixing portion55moves forward relative to the front end portion of the arm portion56. This movement increases the amount by which the backrest26can sink, which prevents occurrence of and reduces the damage of whiplash.

Note that in this example there are provided four front-back position regulation mechanisms50; namely, two around the height of the chest of a seated person and two around the height of the hips. The four front-back position regulation mechanisms50may all have the same structure or different structures. For example, the structures of the front-back position regulation mechanisms50may be different from one another such that the front-back position regulation mechanism50provided around the height of the hips starts deforming earlier and deforms to a greater extent than the front-back position regulation mechanism50provided around the height of the chest. This will be described below while referring toFIG. 16.

FIG. 16is a schematic side view of the backrest26and the SB subframe28. When a large distance is ensured for the hips of a seated person to sink at the time of rear collision, impacts at collision can be absorbed. This enables more reliable prevention of occurrence of whiplash. In addition, when the hips are retreated further than the chest to thereby have the body of the seating person further upright, it is possible to extract maximum capability of a seatbelt and an airbag. In this view, in order to increase the amount of sinking of the hips and portions therearound, the upper rear restraining brackets52U and the lower rear restraining brackets52L may have different structures.

Specifically, the arm portion56of the lower rear restraining bracket52L (hereinafter referred to as a “lower arm portion56L”) may be configured so as to start deforming upon input of a small load, as compared with the arm portion56of the upper rear restraining bracket52U (hereinafter referred to as an “upper arm portion56U”). In the case where each arm portion56has such a structure as is illustrated inFIG. 13, this is achievable by forming a larger notch on the weak portion102of the lower arm portion56L than that on the weak portion102of the upper arm portion56U. As an alternative configuration, the lower arm portion56L may be made from material less rigid than that of the upper arm portion56U. Further, in the case where each arm portion56has such a structure as is illustrated inFIG. 14, the lower arm portion56L may be configured so as to start deforming at earlier timing than the upper arm portion56U, by adjusting the amount by which the middle portion of the guide slit110is narrowed.

Still alternatively, the lower arm portion56L may be configured so as to be deformed to a greater extent at the time of rear collision than the upper arm portion56U. In the case where each arm portion56has such a structure as illustrated inFIG. 13, this is achievable by making the height h1of the lower arm portion56L higher than the height h2of the upper arm portion56U. In addition, in the case where each arm portion56has such a structure as is illustrated inFIG. 14, the guide slit110of the lower arm portion56L may be longer than the guide slit110of the upper arm portion56U.

By setting different amounts of deformation and different values of load which causes the arm portion56to start deformation, depending on the height where the arm portion56is disposed, as described above, it is possible to reliably prevent occurrence of whiplash. In addition, as such different settings make it possible to have the body of a seated person further upright at the time of rear collision, it is possible to enhance the effect of a seatbelt or an airbag, which enables further appropriate protection of a seated person.

Note that the above-described structure is a mere example, and there may be employed structures other than a structure in which the pressure-receiving body98for receiving an input of a load to cause the headrest16to lean forward is disposed behind and opposed to the relay disk32, or the center of swing of the backrest26, and the headrest stay96is provisionally connected to the SB main frame22. For example, although in the above description the rear restraining bracket52has the weak portion102, the weak portion102is omissible. Further, although in the above description the backrest26is held suspended from the SB main frame22via the SB middle frame30, the SB middle frame30may be omitted and the backrest26may be suspended directly from the SB main frame22. Still further, although a wire is mentioned as an example of a suspension tool for suspending the backrest26, the suspension tool may comprise any members, such as, for example, linear members, such as ropes, strings, or climbing ropes, wide band members, chains including connected rings, and so forth, other than wires.

REFERENCE SIGNS LIST