Suspension arm

This invention relates to a suspension arm comprised of a bent arm member, and a pair of bearing members joined to end portion thereof. The arm member of the conventional suspension is made of the aluminium alloy and manufactured by the forging. So, the arm member of hollow construction and has a bent portion is hardly manufactured by the forging. In view of this, in the present invention, the arm member 1 made of the aluminium alloy and has hollow construction is manufactured by the extrusion, and bent before a pair of bearing members 2,3 are joined to each of end portions.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a suspension arm used in a suspension mechanism
 mounted on a vehicle.
 2. Related Art
 Conventionally, in a suspension mechanism mounted on a vehicle, a
 suspension arm connected with an another connecting bar in a vibration
 preventing condition and forms a link mechanism has been used. This
 suspension arm is manufactured by or made of mainly a ferrous material and
 formed by a press working or a forging. Recently, for lightening the
 suspension arm, a light metallic materials such as an aluminium alloy and
 the like formed by the forging have been adopted.
 By the way, the suspension arm can have various shapes corresponding to an
 attached location thereof to the vehicle. For example, an upper arm
 attached to an upper portion of the suspension mechanism and a lower arm
 attached to a lower portion of the suspension arm have bent shapes such as
 an arch shape, V-shape or U-shape. However, when these bent suspension are
 formed by forging the aluminium alloy, the suspension arm of hollow shape
 is hardly formed, which makes further lightening of the suspension arm
 difficult.
 In addition, when the suspension is comprised of an arm member and a pair
 of bearing members attached to both ends of the arm member, if whole of
 the suspension arm including the bearings are formed by the forging, a
 bearing to bearing distance between the bearing members provided at the
 both ends of the arm member requiring an especially high dimensional
 accuracy, can not be maintained in high accuracy. As a result, various
 cutting working become necessary after the forging for maintaining the
 dimensional accuracy, which introduces increase of a working man-hour and
 a manufacturing cost.
 SUMMARY OF THE INVENTION
 The present invention is made in view of the above mentioned circumstances,
 and therefore intends to provide a suspension arm for the suspension
 mechanism which can avoid the increase of the working man-hour and the
 manufacturing cost, and also can further lighten the weight thereof.
 For achieving the above purpose, in the invention recited in the claim 1, a
 suspension arm is comprised of 1) an arm member formed into a pipe shape
 by extruding an aluminium alloy, and having at least a part thereof a bent
 portion, and 2) a pair of bearing members made of an aluminium alloy and
 joined to each of end portions of said arm member respectibely. The bent
 portion of said arm member is formed by bending said arm member after said
 pair of bearing members are joined to each end portion of said arm member.
 According to this invention, the arm member which constitutes a main part
 of the suspension arm is manufactured by the light aluminium alloy and
 formed into a pipe shape of hollow constructions, so that an amount of the
 aluminium alloy is greatly decreased to lighten the suspension arm
 sufficiently, and a high rigidity is maintained. In addition, since this
 suspension arm is formed by the extrusion, only a small post working such
 as cut working and the like is required. Further, the bent portion of the
 suspension arm is formed after the paired bearing members are attached to
 the both ends of the suspension arm, in other words, the bearing members
 can be attached to the straight suspension arm, which makes the attach
 working of the bearing members to the arm member easy. Thus, this
 invention can avoid the increase of the working man-hour and the
 manufacturing cost, and can lighten the weight of the suspension arm
 sufficiently.
 The arm member of this invention is formed into the pipe shape by extruding
 the aluminum alloy. The material of the aluminium alloy is not limitative,
 and the aluminium alloy of Al--Mg--Si system such as A6061T6 or 6NO1 and
 the like are preferably used, from a view point of strength and
 anti-corrosion character. As an extruding method of the arm member, a hot
 extruding method, which is a popular forming method of the aluminium
 alloy, is preferably used. As a pipe shape of the arm member, a
 cylindrical shape, rectangular shape or irregular hollow shape in which a
 plurality of independent, parallel hollow portions are formed, can be
 used.
 The arm member of this invention has at least a part thereof, a bent
 portion bent into the arch shape. This bent portion can be formed over the
 arm member such as a semi-circle or quater circle, or can be locally
 formed at a central portion of the arm member such as U-shape or V-shape.
 This bent portion is formed by bending the arm member by a known pressing
 apparatus easily, after the bearing members are attached to the both end
 of the arm member. As a result, the bearing members can be attached to the
 straight arm member easily.
 The bearing members of this invention is manufactured by the aluminium
 alloy, and can have various shape which is provided with a connecting
 portion to be connected to the arm member, or a cylindrical portion on
 which a vibration preventing member to be explained later is mounted. A
 material of the aluminium alloy which constitutes the bearing member is
 not limitative, and the aluminium alloy of Al--Mg--Si system such as
 A6063T6 and the like are preferably used, from viewpoint of the strength
 and the anti-corrosion character. The bearing member can be formed by the
 extruding, a die-casting or the forging.
 As a join method for joining the bearing member of the aluminium alloy to
 the arm member of the aluminium alloy, one of the conventional, known join
 method such as a friction agitate joining, a frictionally abutted joining,
 a MIG welding and a laser welding can be used. Especially, in this
 invention, since the bearing members are joined to the straight arm member
 which has not been subjected to the bend working, the friction agitate
 joining or the frictionally abut joining in which a building is relatively
 easy and which can maintain the sufficient joining strength is preferably
 used. Here, "the friction agitate joining" is the joining method in which
 a rotating body which rotates between a pair of opposed members with
 contacting with them is moved, and surface portions of the both members a
 deforming resistance of which is reduced by a friction heat are agitated
 to join the both members. The "frictionally abut joining" is the joining
 method in which by a relative movement between the opposed members a
 friction heat is generated at the boundary surface, and a high temperature
 portion at the boundary surface is pressed and extruded to join the both
 members.
 In the invention recited in the claim 2, an intermediate bearing member is
 joined to an intermediate portion of the arm member. According to this
 invention, since the bearing member can be joined to the portion of the
 arm member other than the both end portions, the suspension arm which is
 connected the associating member of the suspension mechanism at more than
 three portions can be obtained. Here, plural intermediate bearing members
 can be attached to plural positions on the arm member.
 In the invention recited in the claim 3, the arm member has a flat surface
 formed at a part of an outer peripheral surface thereof in extruding and
 extending in the extruded direction. According to this invention, when the
 intermediate bearing member is joined to the arm member, the joined
 surface of the arm member is flat, not curved, at least in a cross-section
 perpendicular to the extruding direction, which can heighten the joining
 responsibility of the intermediate bearing member to the arm member. In
 addition, when the arm member is formed into the pipe shape which has an
 inner peripheral surface, due to formation of the flat surface at the part
 thereof, a modulus of section and a rigidity of the arm member can be
 increased, to thereby heighten the strength and durability of the arm
 member.
 In the invention recited in the claims 4 to 6, the both end portions of the
 arm member, or the both bearing member at the both ends of the arm member
 are connected with each other by a connecting bar. According to this
 invention, a positional error occurred between the both ends of the arm
 member in bending the arm member can be corrected or absorbed when the
 connecting bar is attached between the both ends of the arm member or
 between the both bearing members. Thus, the dimensional accuracy of the
 shaft to shaft distance between the both bearing members attached to the
 both ends of the bent arm member can be increased. The connecting bar can
 be formed by a rigid body such as a metal, but is preferably formed by the
 aluminium alloy, in view of the material (aluminium alloy) of the arm
 member and the bearing members to which the connecting bar is connected,
 and lightening of the suspension arm including the connecting bar.
 In the invention recited in the claim 7, the intermediate bearing member is
 joined to the flat surface of the arm member, which heighten the joining
 strength of the intermediate bearing member to the arm member. In the
 invention recited in the claim 6, the arm member has the semi-circular or
 quater circular shape, which can absorb the vivrations in various
 direction between the both ends and the intermediate portion of the arm
 member.
 In the invention recited in the claims 9 and 10, a connecting members with
 vibration preventing fuction are attached to the bearing members at the
 both end of the arm member, or the intermediate bearing. According to this
 invention, on account of the connecting member with vibratio preventing
 function attached to the both bearing members or the intermediate bearing
 memer, the suspension arm can be connected to an another suspension arm or
 an attaching member easily. The connecting members with vibration
 preventing fuction are attached to the both bearing members or the
 intermediate bearing memer to connect this connecting member with another
 or adjacent connecting members with vibration preventing fuction. As the
 connecting member with vibration prevent, a vibration preventing bushing
 provided with a rubber elastic body for absorbing the vibration, or a ball
 joint provided with a joint shaft which has a cubic portion held
 contactingly slidably by a spherical contact with a mating member and
 which slides to change an axis of the joint shaft far absorbing the
 vibration can be used.
 In the invention recited in the claim 17, the suspension arm is
 manufactured by a step for forming straight a pipe member extruding the
 aluminium alloy, a step for joining a pair of bearing members made of
 aluminium alloy to both end portions of the arm member, and a step for
 bending the straight arm member to each end portion of which the pair of
 bearing members are joined at least at a part, to form a suspension arm
 having a bent portion on the arm member.
 According to this invention, the arm member of the aluminium alloy can be
 formed by the pipe member of the hollow construction, so that the amount
 of the aluminium alloy material can be greatly decreased to lighten the
 suspension arm, and the rigidity is maintained. Also, no additional
 working for accurately positioning the paired bearing at the both ends of
 the arm member is necessary.

PREFERRED EMBODIMENT OF THE INVENTION
 Hereinafter, preferred embodiments of the present inventions will be
 explained with reference to attached drawings. However, it is noted that
 the present invention is not limited to these embodiments, but can include
 various variation or modification within spirit thereof.
 &lt;First Embodiment&gt;
 FIG. 1 is a plan view (partially broken) of the suspension arm according to
 a first embodiment of the present invention, and FIG. 2 is across-section
 along a line II--II in FIG. 1.
 A suspension arm of this embodiment is comprised of, as shown in FIG. 1, an
 arm member 1 made of an aluminium alloy and formed into a pipe-shape by an
 extrud and bent into an arch-shape (quater circle), a first bearing member
 2 made of an aluminium alloy and joined to one end of the arm member 1, a
 second bearing member 3 made of an aluminium alloy and joined to another
 end of the arm member 1, and an intermediate bearing member 4 joined to an
 intermediate portion of the arm member 1.
 The arm member 1 is manufactured by extruding the aluminium alloy of
 Al--Mg--Si system of A6061T6 into a formed body of substantially
 cylindrical shape, and cutting the formed body by a predetermined length.
 This arm member 1 has a flat surface 1a at a part of an outer peripheral
 surface thereof as show in FIG. 2, which flat surface 1a is formed in
 extruding the arm body 1 and extending in the extruding direction to form
 a bent portion 1b. This arm member 1 is bent into the arch shape so that
 the flat surface 1a faces outwardly. Here, the bend working is carried out
 after the pair of bearings 2 and 3 are joined to the both ends of the arm
 member 1.
 The first bearing member 2 is manufactured by forging the aluminium alloy
 of Al--Mg--Si system of A6063T6, and is comprised of a cubic portion 2a,
 and a shaft-like connecting base portion 2b extending from the cubic
 portion 2a. At a tip end of the connecting base portion 2b, a
 small-diameter, pillar-like engaging portion 2c is formed. The first
 bearing member 2 is joined to the arm member 1 by the friction agitate
 joining between a step surface of the small-diameter portion of the
 bearing member 2 and an end surface of the arm member 1, with engaging the
 engage portion 2c with the arm member 1. Here, this join working of the
 bearing member 2 is carried out in the state where the arm member 1 is
 straight (not bent yet).
 The second bearing member 3 is manufactured by extruding the aluminium
 alloy of Al--Mg--Si system of A6061T6. The first bearing member 3 is
 comprised of a cylindrical portion 3a having a fitting hole (not shown)
 penetrating in an axial direction (perpendicular to a sheet surface of
 FIG. 2), and a shaft-like connecting base portion 3b extending from the
 cylindrical portion 3a. At a tip end of the connecting base portion 3b, a
 small-diameter, pillar-like engage portion 3c is formed. The second
 bearing member 3 is joined to the arm member 1 by the friction agitate
 joining between a step portion of the engaging portion 3c and an end
 surface of the arm member 1, with engaging the engage portion 3c with the
 arm member 1. Here, this joining working of the bearing member 3 is
 carried out in the state where the arm member 1 is straight (not bent
 yet).
 The intermediate bearing 4 is manufactured by extruding the aluminium alloy
 of Al--Mg--Si system of A 6061T6. This intermediate bearing 4 is comprised
 of a cylindrical portion 4a, and a connecting base portion 4b extending
 from the cylindrical portion 4a like a letter of L and having two cavities
 4c at a central portion thereof. As shown in FIGS. 1 and 2, a tip end
 surface of the connecting base portion 4b is joined to a flat surface 1a
 of the arm member 1 formed at a substantially central portion thereof by
 the friction agitate joining. The intermediate bearing member 4 is joined
 to the arm member 1 so that an axis of the cylindrical portion 4a is
 substantially perpendicular to an axis of the cylindrical portion 3a of
 the second bearing member 3. Here, the join working of the intermediate
 bearing 4 to the arm member 1 is carried out after the arm member 1 is
 bent.
 The above mentioned suspension arm is assembled into the suspension
 mechanism of the automobile and used. In assembling, the cubic portion 2a
 of the first bearing member 2, the cylindrical portion 3a of the second
 bearing member 3, and the cylindrical portion 4a of the intermediate
 bearing member 4 are connected to an another suspension arm or an
 attaching member via the connecting member with vibration preventing means
 such as a vibration preventing bushing and the like (not shown). Thus, the
 suspension arm operates as one component of the suspension link mechanism,
 with absorbing the vibration between this suspension arm and the adjacent
 suspension arm by the connecting member with vibration preventing
 function.
 As mentioned above, in the present suspension arm, the arm member 1 which
 is the main component thereof is made of the light aluminium alloy and
 formed into the pipe shape of hollow construction, so that the necessary
 amount of the aluminium alloy material is greatly reduced to lighten the
 arm member 1, and the high rigidity is maintained. Since this arm member 1
 is formed by the extrusion, only small post working such as the cut
 working is necessary. The bent portion of the arm member 1 is formed after
 the first bearing member 2 and second bearing member 3 are joined to the
 both ends of the arm member 1, that is, the first and the second bearing
 members 2 and 3 are joined to the straight arm member 1, the joining
 operation of the both bearing members 2 and 3 to the arm member 1 becomes
 easy. According to the present suspension arm, sufficient lightening can
 be realized with avoiding the increase of the working man-hour and the
 manufacturing cost.
 In addition, in the present suspension arm, since the intermediate bearing
 member 4 is joined to the intermediate portion of the arm member 1, the
 suspension arm can be connected to another suspension arm and the like at
 three points. Further, the arm member 1 of the present suspension arm has
 the flat surface 1a at the part of the outer peripheral surface thereof,
 the connecting base portion 4b can be joined to the flat surface 1a which
 is not curved in the cross-section perpendicular to the axis of the arm
 member 1. Accordingly, the joining responsibility of the intermediate
 bearing member 4 to the arm member 1 is heightened. Finally, the arm
 member 1 is formed into the pipe shape having the flat surface 1a at the
 outer peripheral surface, the modulus of section and the rigidity of the
 arm member 1 is increased to thereby realize the heighten of durability
 and strength.
 &lt;Second Embodiment&gt;
 FIG. 3 is a plan view (partially broken) of the suspension arm according to
 a second embodiment of the present invention, and FIG. 4 is a
 cross-section along a line IV--IV in FIG. 3, according to a first
 embodiment of the present invention.
 A suspension arm of this embodiment is comprised of, as shown in FIG. 3, an
 arm member 11 made of an aluminium alloy and formed into a pipe-shape by
 an extruding and bent into an arch-shape (U-shape); a first bearing member
 12 made of an aluminium alloy, joined to one end of the arm member 11, and
 to which a vibration preventing bushing 15 is attached, second bearing
 member 13 made of an aluminium alloy, joined to another end of the arm
 member 11 and to which a vibration preventing bushing 16 is attached; and
 an intermediate bearing member 14 joined to an intermediate portion of the
 arm member 11, and to which a ball joint 17 is attached.
 The arm member 11 is manufactured by extruding the aluminium alloy of
 Al--Mg--Si system of A6061T6 into a formed body of substantially
 cylindrical shape, and cutting the formed body by a predetermined length.
 This arm member 11 has a flat surface 11a at a part of an outer peripheral
 surface thereof, which flat surface 11a is formed in extruding the arm
 member 11 and extending in the extruding direction. This arm member 11 is
 bent into the U shape so that bent portions 11b are formed and the flat
 surface 11a faces outwardly. Here, the bend working is carried out after
 the pair of bearings 12 and 13 are joined to the arm member 11.
 The first bearing member 12 is manufactured by extruding the aluminium
 alloy of Al--Mg--Si system of A6061T6. This first bearing member 12 is
 comprised of a cylindrical portion 12a which has a circular fitting hole
 extending axially (laterally in FIG. 3), and a connecting base portion 12b
 extending from an outer peripheral of the cylindrical portion 12a and
 having a cavity 12c therein. At a tip end of the connecting base portion
 12b, a small-diameter pillar-like engaging portion 12d is formed. The
 first bearing member 12 is joined to the arm member 11 by the friction
 agitate joining between a step portion of the engaging portion 12d and an
 end surface of the arm member 11, with engaging the engaging portion 12d
 with the arm member 11. Here, this joining working of the bearing member
 12 is carried out in the state where the arm member 11 is straight (not
 bent yet).
 A vibration preventing bushing 15 joined to the first bearing member 12 is
 comprised of an inner cylindrical metal piece 15a which has a hollow hole
 extending laterally in FIG. 3 and which is formed into a pipe shape, and a
 cylindrical rubber elastic body 15b vulcanizedly attached to an outer
 peripheral surface of the inner cylindrical metal piece 15a. This
 vibration preventing bushing 15 is attached to the first bearing member 12
 by vulcanizedly attaching the outer peripheral surface of the rubber
 elastic body 15b to the inner peripheral surface of the cylindrical
 portion 12a.
 The second bearing member 13 is manufactured the same material and the same
 manner as the first bearing member 12, and is comprised of a cylindrical
 portion 13a, and a connecting base portion 13b having a cavity 13c and an
 engaging portion 13d therein. The second bearing member 13 is joined to
 the arm member 11 by the friction agitate joining between the step portion
 of engaging portion 13d and an end surface of the arm member 11, with
 engaging the engaging portion 13d with the arm member 11. The cylindrical
 portion 13a of the second bearing member 13 is joined to the arm member 11
 so that the axis of the cylindrical member 13a is substantially parallel
 to the axis of the cylindrical portion 12a of the first bearing member 12.
 Here, this joining working of the bearing member 13 is carried out in the
 state where the arm member 11 is straight (not bent yet).
 A vibration preventing bushing 16 attached to the second bearing member 13
 is comprised of a cylindrical metal piece 16a and a rubber elastic body
 16b, and is attached to the second bearing member 13 by vulcanizedly
 attaching the outer peripheral surface of the rubber elastic body 16b to
 the inner peripheral surface of the cylindrical portion 13a. Here, the
 vibration preventing bushings 15 and 16 are disposed coaxially.
 The intermediate bearing 14 is manufactured by forging the aluminium alloy
 of Al--Mg--Si system of A 6063T6, and is comprised of a cylindrical
 portion 14a, and a connecting base portion 14b extending from the
 cylindrical portion 14a. As shown in FIGS. 3 and 4, a tip end surface of
 the connecting base portion 14b is joined to a flat surface 11a of the arm
 member 11 formed at a substantially central portion thereof by the
 friction agitate joining. Here, the join working of the intermediate
 bearing 14 to the arm member 11 is carried out after where the arm member
 1 is bent.
 A ball joint 17 attached to the intermediate bearing member 14 has a joint
 shaft 17a including a cubic portion contactingly slidably held by a
 spherically contact with a holder (not shown) contained in the cylindrical
 portion 14a, and extending perpendicular to a sheet surface of FIG. 3. The
 ball joint 17 absorbs the vibration by the displacement of the axis of the
 joint shaft 17a due to contacting sliding of the cubic portion.
 The above mentioned suspension arm is assembled into the suspension
 mechanism of the automobile and used. In assembling, the vibration
 preventing bushing 15 of the first bearing 12, the vibration preventing
 bushing 16 of the second bearing member 13, and the ball joint 17 of the
 intermediate bearing member 14 are connected to an another suspension arm
 or an attaching member. Thus, the suspension arm operates as one component
 of the suspension link mechanism, with absorbing the vibration between
 this suspension arm and the adjacent suspension arm by the vibration
 preventing bushings 15, 16 and the ball joint 17.
 As mentioned above, in the present suspension arm, the arm member 11 which
 is the main component thereof is made of the light aluminium alloy and
 formed into the pipe shape of hollow construction by the extruding, so
 that the same advantage as the first embodiment, for example, the
 necessary amount of the aluminium material is greatly reduced to lighten
 the arm member 1, and the high rigidity is maintained, can be obtained. In
 addition, since in the present suspension arm, the vibration preventing
 bushings 15, 16 and the ball joint 17 are attached to the arm member 11 in
 advance, it can be connected with another suspension arm easily.
 &lt;Third Embodiment&gt;
 FIG. 5 is a plan view (partially broken) of the suspension arm according to
 a third embodiment of the present invention, and FIG. 6 is a cross-section
 along a line IV--IV in FIG. 5, according to a third embodiment of the
 present invention.
 A suspension arm of this embodiment is comprised of, as shown in FIG. 5, an
 arm member 21 made of an aluminium alloy and formed into a pipe-shape by
 extruding and bent into an arch-shape (quater circle), a first bearing
 member 22 made of an aluminium, joined to one end of the arm member 1, and
 to which a ball joint 25 is attached; second bearing member 23 made of an
 aluminium alloy, joined to another end of the arm member 21, and to which
 a vibration preventing bushing 26 is attached; and an intermediate bearing
 member 24 joined to an intermediate portion of the arm member 21, and to
 which a ball joint 27 is attached.
 The arm member 21 is manufactured by extruding the aluminium alloy of
 Al--Mg--Si system of A6063T6 into a formed body of substantially
 cylindrical shape, and cutting the formed body by a predetermined length.
 This arm member 21 is shaped into a pipe shape having a cavity of circular
 cross-section and a cavity of irregular cross-section are formed
 longitudinally thereof, and has a flat surface 21a formed in extruding the
 arm member 21 and extending in the extruding direction at a part of an
 outer peripheral surface thereof. This arm member 21 is bent into the
 U-shape so that a bent portion 21b is formed and the flat surface 21a
 faces outwardly. Here, the bend working is carried out after the pair of
 bearings 22 and 23 are joined to the arm member 21.
 The first bearing member 22 is manufactured by forging the aluminium alloy
 of Al--Mg--Si system of A6063T6, and is comprised of a cylindrical portion
 22a, and a connecting base portion 22b extending from the cylindrical
 portion 22a downwardly in FIG. 5. At a tip end of the connecting base
 portion 22b, a small-diameter, pillar-like engaging portion 22d is formed.
 The first bearing member 22 is joined to the arm member 21 by the friction
 agitate joining between a step portion of the engaging portion 22d and an
 end surface of the arm member 21, with engaging the engaging portion 22d
 with the arm member 21. Here, this join working of the bearing member 22
 is carried out in the state where the arm member 21 is straight (not bent
 yet).
 A ball joint 25 attached to the second bearing member 22 is constructed in
 the same manner as that of the first embodiment, and has a joint shaft 25a
 which includes a cubic portion contactingly slidably held by a holder
 contained in the cylindrical portion of the first bearing member 22, and
 extruding perpendicular to a sheet surface of FIG. 5.
 The second bearing member 23 is manufactured by extruding the aluminium
 alloy of Al--Mg--Si system of A6061T6. The second bearing member 23 is
 comprised of a cylindrical portion 23a having a fitting hole penetrating
 axially therein, and a connecting base portion 23b extending from an outer
 periphery of the cylindrical portion 23a in a L-shape and having two
 cavity 23c therein. The second bearing member 23 is joined to the arm
 member 21 by the friction agitate joining between a step portion of the
 engaging portion 23d and an end surface of the arm member 21, with
 engaging the engaging portion 23d with the arm member 21. Here, this
 joining working of the bearing member 23 is carried out in the state where
 the arm member 21 is straight (not bent yet).
 A tip end of the connecting base portion 23b extends downwardly in FIG. 5,
 and a vibration preventing bushing 26 is attached to the extended portion.
 This vibration preventing bushing 26 is comprised of an inner cylindrical
 metal piece 26a having a fitting hole extending laterally in FIG. 5, an
 outer peripheral metal piece 26b disposed coaxially therewith and outside
 thereof, and a cylindrical rubber elastic body 26c disposed between the
 inner and outer cylindrical metal pieces 26a and 26b. The vibration
 preventing bushing 26 is attached to the second bearing member 23 by
 pressing in the outer cylindrical metal peice 26b to the inner peripheral
 surface of the cylindrical portion 23a.
 The intermediate bearing 24 is manufactured by extruding the aluminium
 alloy of Al--Mg--Si system of A 6061T6. This intermediate bearing 24 is
 comprised of a cylindrical portion 24a, and a connecting base portion 24b
 extending from the cylindrical portion 24a and having three cavities 24c
 at a central portion thereof. As shown FIGS. 5 and 6, the connecting base
 portion 24b is joined to a flat surface 21a of the arm member 21 formed at
 a substantially central portion thereof by the friction agitate joining.
 The intermediate bearing member 24 is joined to the arm member 21 so that
 the cylindrical portion 24a is substantially parallel to the cylindrical
 member 23a. Here, the join working of the intermediate bearing 24 to the
 arm member 21 is carried out after the arm member 21 is bent.
 A vibration preventing bushing 27 attached to the intermediate bearing
 member 24 has the same construction as the vibration preventing bushing 26
 attached to the second bearing member 23, and is comprised of an inner
 cylindrical metal piece 27a, an outer peripheral metal piece (not shown),
 and a cylindrical rubber elastic body 27c. The vibration preventing
 bushing 27 is attached to the intermediate bearing member 24 by pressing
 in the outer cylindrical metal peice 27b to the inner peripheral surface
 of the inner cylindrical portion 24a. The vibration preventing bushings 26
 and 27 are disposed coaxially.
 The above mentioned suspension arm is assembled into the suspension
 mechanism of the automobile and used in the same manner as above first
 embodiment. In assembling, the ball joint 25 of the first bearing member
 22, the vibration preventing bushing 26 of the second bearing member 23,
 and the vibration preventing bushing 27 of the intermediate bearing member
 24 are connected to an another suspension arm or an attaching member.
 Thus, the suspension arm operates as one component of the suspension link
 mechanism, with absorbing the vibration between this suspension arm and
 the adjacent suspension arm by the ball joint 25 and the vibration
 preventing bushings 26, 27.
 As mentioned above, in the present suspension arm, the arm member 21 which
 is the main component thereof is made of the light aluminium alloy into
 the pipe shape of hollow construction by the extrusion, so that the same
 advantage as that of the first embodiment, for example, the necessary
 amount of the aluminium material is greatly reduced to lighten the arm
 member 21, and the high rigidity is maintained, can be obtained.
 Especially, the present arm member 21 is shaped into the pipe construction
 having the hollow portions of the cross-section and the irregular
 cross-section, and has the flat surface 21a at the side of the hollow
 portion of irregular cross-section, the modulus of section and the
 rigidity of the arm member 21 can be increased. This is effective to
 heighten the strength and durability of the arm member 21. In addition, in
 the present suspension arm, the ball joint 25 and the vibration preventing
 bushings 26, 27 are attached to the first and second bearing members 22,
 23 and the intermediate bearing member 24 in advance, it can be connected
 with another suspension arm easily.
 &lt;Fourth Embodiment&gt;
 FIG. 7 is a plan view (partially broken) of the suspension arm according to
 a fourth embodiment of the present invention, and FIG. 8 is a
 cross-section along a line VIII--VIII in FIG. 7, according to the fourth
 embodiment of the present invention;
 A suspension arm of this embodiment is comprised of, as shown in FIG. 7, an
 arm member 31 made of an aluminium into a pipe-shape by extruding and bent
 into an arch-shape (U-shape), a first bearing member 32 made of an
 aluminium alloy, joined to one end of the arm member 31, and to which a
 vibration preventing bushing 35 is attached; second bearing member 33 made
 of an aluminium alloy, joined to another end of the arm member 31, and to
 which a vibration preventing bushing 36 is attached a tip end thereof,
 similar to the first bearing member 32; an intermediate bearing member 34
 joined to an intermediate portion of the arm member 31 to a tip of which a
 ball joint 37 is attached, and a connecting bar 38 connecting the first
 and second bearing members 32 and 33.
 The arm member 31 is manufactured by extruding the aluminium alloy of
 Al--Mg--Si system of A6061T6 into a formed body of substantially
 cylindrical shape, and cutting the formed body by a predetermined length.
 This arm member 31 is formed into the U shape to form bent portions 31b.
 Here, the bend working is carried out after the pair of bearings 32 and 33
 are joined to the arm member 31.
 The first bearing member 32 is manufactured by extruding the aluminium
 alloy of Al--Mg--Si system of A6061T6 and extending upwardly in FIG. 7.
 This first bearing member 32 is comprised of a cylindrical portion 32a
 which has a circular fitting hole extending axially (laterally in FIG. 7),
 and a connecting base portion 32b extending from an outer peripheral of
 the cylindrical portion 32a and having a cavity 32c therein. At a tip end
 of the connecting base portion 32b, a small-diameter, pillar-like engaging
 portion 32d is formed. The first bearing member 32 is joined to the arm
 member 31 by the friction agitate joining between a step portion of the
 engaging portion 32d and an end surface of the arm member 31, with
 engaging the engage portion 32d with the arm member 31. Here, this joining
 working of the bearing member 32 is carried out in the state where the arm
 member 31 is straight (not bent yet).
 A vibration preventing bushing 35 attached to the first bearing member 32
 is comprised of an inner cylindrical metal piece 35a formed into a pipe
 shape and coaxial with the cylindrical portion 32a, and a cylindrical
 rubber elastic body 35b vulcanizedly attached to an outer peripheral
 surface of the inner cylindrical metal piece 35a. This vibration
 preventing bushing 35 is attached to the first bearing member 32 by
 vulcanizedly attaching the outer peripheral surface of the rubber elastiic
 body 35b to the inner peripheral surface of the cylindrical portion 32a.
 The second bearing member 33 is manufactured the same material and the same
 manner as the first bearing member 32, and is comprised of a cylindrical
 portion 33a, and a connecting base portion 33b having a cavities 33c and
 an engaging portion 33d therein. The second bearing member 33 is joined to
 the arm member 31 by the friction agitate joining between a step portion
 of the bearing member 33 and an end surface of the arm member 31, with
 engaging the engaging portion 33d with the arm member 31 similar to the
 first baring member 32. The cylindrical portion 33a of the second bearing
 member 33 is joined to the arm member 31 so that the axis of the
 cylindrical member 33a is substantially parallel to the axis of the
 cylindrical portion 32a of the first bearing member 32. Here, this join
 working of the bearing member 33 is carried out in the state where the arm
 member 31 is straight (not bent yet).
 A vibration preventing bushing 36 attached to the second bearing member 33
 is comprised of a cylindrical metal piece 36a and a rubber elastic body
 36b, similar to the vibration preventing bushing 35, and is attached to
 the second bearing member 33 by vulcanizedly attaching the outer
 peripheral surface of the rubber elastic body 36b to the inner peripheral
 surface of the cylindrical portion 33a. The vibration preventing bushings
 35 and 36 are disposed coaxially.
 The intermediate bearing 34 is manufactured by forging the aluminium alloy
 of Al--Mg--Si system of A 6063T6, and is comprised of a first member 34a
 and a second member 34b.
 The first member 34a includes an arch portion 34c formed into an arch shape
 coinciding with the outer peripheral surface of the arm member 31, and a
 pair of plane portions 34d extending oppositely from both ends of the arch
 portion 34c, to have a .OMEGA.-shape. The second member 34b includes a
 cylindrical portion 34e, and a flange 34f extending from one end of the
 cylindrical portion 34e and havaing a rectangular shape coinciding with
 the plane portion 34d of the first member 34a.
 This intermediate bearing member 34 is attached to the arm member 31, by
 MIG welding with closely contacting the arch portion 34c of the first
 member 34a with the outer peripheral surface of the central portion of the
 arm member 31, and by friction agitate connecting the flange 34f of the
 second member 34b with the plane portion 34d of the first member 34a.
 Here, this connecting of the intermediate bearing member 34 to the arm
 member 31 is carried out before the arm member 31 is bent, in the straight
 condition.
 A ball joint 37 mounted to the intermediate member 34 is comprised of a
 holder 37a contained and held in the cylindrical portion 34e of the second
 member 34b, a joint shaft 37b having a cubic portion sphereically
 contacting with and slidably held by the holder 37a and extending
 perpendicular to the sheet surface of FIG. 7, and a cylindrical rubber
 cover 37c of which both ends are fixed to an opened portion of the
 cylindrical portion 34e and a central portion of the joint shaft 37b and
 covering the opened portion of the cylindrical portion 34e.
 The connecting bar 38 is manufactured by extruding the aluminium alloy of
 Al--Mg--Si system of A6061T6. Both ends of the connecting bar 38 are fixed
 to outer peripheral surfaces of the connecting base portions 32b, 33b of
 the first and second bearing members 32 and 33 respectively. The
 connecting bar 38 is attached to the arm member 31 after the first and
 second bearing members 32 and 33 are joined to the arm member 31 and the
 arm member 31 is bent.
 The above mentioned suspension arm is assembled into the suspension
 mechanism of the automobile to be used similar to the first embodiment. In
 assembling, the vibration preventing bushing 35 of the first bearing 32,
 the vibration preventing bushing 36 of the second bearing member 33, and
 the ball joint 37 of the intermediate bearing member 34 are connected to
 an another suspension arm or an attaching member. Thus, the suspension arm
 operates as one component of the suspension link mechanism, with absorbing
 the vibration between this suspension arm and the adjacent suspension arm
 by the vibration preventing bushings 35, 36 and the ball joint 37.
 As mentioned above, in the present suspension arm, the arm member 31 which
 is the main component thereof is made of the light aluminium alloy into
 the pipe shape of hollow construction by the extrusion, so that the same
 advantage as the first embodiment, for example, the necessary amount of
 the aluminium material is greatly reduced to lighten the arm member 1, and
 the high rigidity is maintained, can be obtained. In addition, in the
 present suspension arm, the connecting bar 38 is spanned between the first
 and second bearing members 32 and 33. As a result, a positional error
 occurred between the both ends of the arm member 31 in bending the arm
 member 31 can be absorbed or corrected by the connecting bar 38 when it is
 spanned and fixed to the both ends of the bent arm member 31. Thus, the
 dimensional accuracy of the shaft-to-shaft distance between the first and
 second bearing members 32 and 33 can be increased.
 In the above econd embodiment shown in FIGS. 3 and 4, the first bearing
 member 12 and the second bearing member 13 can be connected by the
 connecting member such as above conneting bar 38.