Method for producing lower suspension arms

Abstract

The invention relates to a method for producing a lower arm for a front axle suspension of a motor vehicle, which comprises the following steps: stamping a plate of a first metal material so as to form a first part (26) having two first branches (28, 30) defining an upper surface (34), and a first intermediate branch (36) projecting from the upper surface, the first part having a first concave edge portion (38) extending from the first intermediate branch (36) to one branch (28) of the two first branches; stamping a plate of a second metal material so as to form a second part (58) having two second branches (60, 62) and a second intermediate branch (66), suitable for being connected to the two first branches (28, 30) and to the first intermediate branch (36), respectively, the second part having a second concave edge portion (68) suitable for engaging with the first edge portion (38) so as to form a concave arm reinforcement edge (84). And when a first material having low tensile strength is selected, the first edge portion is raised so as to form the reinforcing edge, while the second edge portion (68) is raised so as to form the reinforcing edge (84) when a first material having high tensile strength is selected.

Description

[0001] Title of the invention: Method for producing lower suspension arms

[0002] The present invention relates to a method for producing a lower suspension arm for the front axle of a motor vehicle and to a lower suspension arm produced according to said method.

[0003] The lower suspension arms of a motor vehicle connect the two front wheels to the vehicle's chassis.

[0004] Each of the lower arms usually comprises a stamped flat metal part. This has two branches inclined to each other at an angle substantially greater than 90°, which two branches thus define an extrados opposite a intrados. Also, the flat metal part has an intermediate branch which extends projecting from the extrados in a direction opposite to the second branch.

[0005] A first branch of said two branches has a first end connected to the chassis of the motor vehicle, as does the intermediate branch, while the second end of the second branch is connected to the wheel support.

[0006] In addition, stamping allows the part to be formed into volume and to create a rim in the intrados and, opposite, two rims in the extrados, which two rims converge towards each other in the intermediate branch to form a free U-shaped end.

[0007] The free U-shaped end thus has two parallel edges connected together by a perpendicular bottom edge. The two parallel edges are then notched and the bottom edge extends substantially in projection. In this way, a socket can be welded, held in abutment against the two notched edges and against the projecting bottom edge.

[0008] The axis of symmetry of the socket extends substantially parallel to the mid-plane of the metal part and it intersects the first end of the first branch connected to the chassis.

[0009] The first end has an orifice whose axis of symmetry is substantially perpendicular to said mean plane of the metal part.

[0010] Thus, the lower arm is adapted to be installed on the chassis in such a way that the bushing of the intermediate branch is pivotally mounted in a direction parallel to the longitudinal direction of the vehicle, while the first end of the first branch is connected to the chassis, along a vertical axis of the vehicle. And the intermediate branch is hingedly mounted on the chassis, towards the front, while the first end of the first branch is installed towards the rear.

[0011] Reference may be made to document EP 794 075, which describes such a lower suspension arm.

[0012] Depending on the vehicle model, the lower suspension arms are subjected to varying degrees of stress. For example, when one model has wider wheels than another, the forces exerted on the lower arms are greater. This is also the case when a vehicle model is heavier due to its engine or equipment.

[0013] It is therefore necessary to reinforce the lower suspension arms. However, in modern vehicles, spaces are relatively limited and therefore it is considered to implement metallic materials with higher mechanical strength. On the other hand, the higher the tensile strength of the metallic materials, the less deformable they are and it is therefore more difficult to stamp them.

[0014] Also, a problem which arises and which the present invention aims to solve is to implement a method for producing a lower suspension arm for the front axle of a vehicle, making it possible to obtain arms of different mechanical strengths while retaining substantially the same dimensions.

[0015] For this purpose, and according to a first object, a method is proposed for producing a lower suspension arm for the front axle of a motor vehicle, comprising the following steps: a first metallic material is chosen and a plate of said first material is stamped to form a first part comprising two first branches inclined relative to each other, defining an extrados, and a first intermediate branch extending in projection from said extrados opposite one of said two first branches, said first part having a first concave edge portion extending from said first intermediate branch towards the other of said two first branches;a second metallic material is chosen and a plate of said second material is stamped to form a second part comprising two second branches and a second intermediate branch, adapted to be connected respectively to said two first branches and to said first intermediate branch, said second part having a second concave edge portion adapted to cooperate with said first edge portion to form a concave arm reinforcing edge; and when a first material having a low tensile strength is chosen, said first edge portion is dressed to form said reinforcing edge, while the second edge portion is dressed to form said reinforcing edge when a first material having a high tensile strength is chosen.;

[0016] Thus, a feature of the invention lies in the implementation of two metal parts obtained by stamping and in the assembly of these two parts to form the front lower suspension arm. Furthermore, when the metal material chosen for the first part has a high tensile strength, the concave reinforcing edge of the arm cannot be formed by the first edge portion of the first part, because the stamping depth is too great compared to the stretching limits of the metal material. Consequently, it is the second edge portion that is dressed to form the reinforcing edge. As will be explained in more detail in the remainder of the description, the second edge portion does not need to be stamped as deeply to form the reinforcing edge of the arm.

[0017] Obviously, a high tensile strength metal plate material is chosen when it is desired to increase the resistance of the suspension arm to deformation. This is the case, for example, when vehicles of the same type are equipped with wider tires or when the engine is heavier. Also, the two second branches and the second intermediate branch of the second part are connected respectively to said two first branches and to said first intermediate branch, in welding steps that will be described in more detail in the rest of the description.

[0018] Advantageously, the first and second edge portions are connected to each other. In other words, the first and second edge portions cooperate with each other to form the reinforcing edge of the arm and to ensure its mechanical resistance to deformation.

[0019] According to a particularly advantageous embodiment of the invention, said first material and said second material are identical. Thus, plates of the same material are stamped to form the first and second parts. Consequently, the parts are produced at a more advantageous cost. This embodiment is in no way limiting. In certain cases, in fact, and as will be explained in the detailed description, when said first material has a very high tensile strength, a second material having a lower tensile strength is chosen in order to obtain a sufficient height of said reinforcing edge. In this way, an arm is obtained whose arm reinforcing edge has the required height. Such an arm reinforcing edge height could not have been obtained by stamping plates of this same material with very high tensile strength.

[0020] According to a preferred embodiment, said first part has an intrados rim extending into said two first branches and an opposite extrados rim extending from said one of said two first branches to a first free end in said first intermediate branch.

[0021] Thus, the first two branches inclined relative to each other define an intrados, opposite the extrados. And the intrados is bordered by the intrados edge which extends opposite the extrados edge. Also, the first portion of the edge or the second portion of the edge extends opposite the intrados edge.

[0022] Furthermore, as will be explained in the detailed description, the second part is mounted on the first part between the flanges so that it can be welded to it. In other words, the second part extends essentially within the volume defined by the first part and its flanges. Therefore, the second part makes it possible to increase the rigidity of the first part without adding additional volume to the first part.

[0023] According to an alternative embodiment, when the first material having a high tensile strength is chosen, said second concave edge portion extends to a second free end, and said second free end comes in line with said first free end. Thus, the second concave edge portion and the extrados rim converge towards each other to become substantially parallel at their free end. As will be explained in the detailed description, these free ends have notches adapted to receive a sleeve. Also, the sleeve comes to bear in the notches and is welded there according to a welding step of the method according to the invention.

[0024] Furthermore, according to one embodiment of the invention, the first metallic material having a high tensile strength has a tensile strength greater than 700 MPa. The tensile strength of a material corresponds to a limit force related to a section, to which it can be stretched before it breaks. Given the drawing depths necessary to form said parts, it is considered that beyond 700 MPa, the first edge portion cannot be drawn properly to be able to form the concave reinforcing edge.

[0025] Also, when a first material having a high tensile strength is chosen, preferably, said second portion of upright edge is extended by a second free wing curved in return. The second free curved wing extends opposite the intrados edge along a mean plane substantially perpendicular to the second portion of upright edge which it extends. Thus, the second free curved wing constitutes a rib which further stiffens the second portion of upright edge.

[0026] Furthermore, the first metallic material having a low tensile strength, has a tensile strength of less than 500 MPa. Thus, as indicated above, the breaking strength of a material corresponds to a limit force related to a section, to which it can be stretched before it breaks. And taking into account the drawing depths necessary for the formation of said parts, it is considered that below 500 MPa, the first edge portion can be drawn suitably to be able to form the concave reinforcing edge.

[0027] Also, when a first material having a low tensile strength is chosen, said first portion of upright edge is advantageously extended by a first free wing curved in return. This first free curved wing makes it possible to increase the mechanical strength of the first portion of upright edge.

[0028] Furthermore, and according to a second object, the invention provides a lower suspension arm for the front axle of a motor vehicle obtained in accordance with the production method as described above.

[0029] Other features and advantages of the invention will emerge from reading the description given below of particular embodiments of the invention, given for informational but non-limiting purposes, with reference to the appended drawings in which:

[0030] [Fig. 1] is a partial schematic top view of a motor vehicle equipped with the subject of the invention;

[0031] [Fig. 2] is a schematic perspective view from below of an element of the invention in a first implementation condition; [Fig. 3] is a schematic perspective view from below of another element of the invention in said first implementation condition;

[0032] [Fig. 4] is a schematic perspective view from below of the meeting of the two elements which are the subject of [Fig. 2] and [Fig. 3];

[0033] [Fig. 5] is a schematic perspective view from below of the element of the invention in a second implementation condition;

[0034] [Fig. 6] is a schematic perspective view from below of the other element of the invention in the second implementation condition;

[0035] [Fig. 7] is a schematic perspective view from below of the union of the two elements which are the subject of [Fig. 5] and [Fig. 6]; and,

[0036] [Fig. 8] is a flowchart showing the steps of the production process according to the invention.

[0037] [Fig. 1] partially shows the front left top of a motor vehicle 10, and it is inscribed in an orthogonal reference frame X, Y, Z, in which the X axis extends in a longitudinal front-rear direction of the motor vehicle, oriented towards the rear; the Y axis extends in a transverse direction of the vehicle, oriented from left to right when in a rolling situation; and the Z axis extends in a vertical direction, oriented away from the ground.

[0038] [Fig. 1] shows a chassis element 12 and a body element 14 of the motor vehicle. Also, the chassis element 12 receives a left front lower suspension arm 16, making it possible to connect a left front wheel 18 and the chassis element 12. A right front lower suspension arm, not shown, symmetrical to the left lower suspension arm with respect to a median plane of the motor vehicle, also connects a right front wheel and another chassis element, not shown.

[0039] [Fig. 8] illustrates steps of the method according to the invention. Also, after an initiation step 20, it is chosen to manufacture a lower suspension arm having a high resistance to deformation HR, or a lower suspension arm with a low resistance to deformation BR.

[0040] If it is desired to manufacture a lower suspension arm having a high resistance to deformation HR, then, according to a first first selection step 22, a plate of a first metallic material having a high tensile strength is chosen. It is considered here that such a metallic material has a tensile strength Rm, corresponding to the stretching limit of the material of a given section before rupture, greater than 700 MPa. The breaking strength of the chosen material is for example 1000 MPa.

[0041] The metallic material is advantageously steel and the plate has a thickness of, for example, between 0.3 mm and 3 mm.

[0042] Then, in a first stamping step 24, the plate of the first material is stamped to form a first part 26, which is illustrated in detail seen from below in [Fig. 2] and which will now be described.

[0043] Thus, the first first part 26 extends along a first first average plane and it has a first first branch 28 inclined relative to a second first branch 30. The two first branches 28, 30 are for example inclined to each other by an angle of between 120° and 150°.

[0044] The two first branches 28, 30 thus inclined define a first first intrados 32 opposite a first first extrados 34. And the first first part 26 has a first first intermediate branch 36 which extends in projection from the extrados 34 in a direction opposite to the second first branch 30. In other words, the first first intermediate branch 36 forms an acute angle with the first first branch 28.

[0045] Also, the first first part 26 has a first first concave edge portion 38 extending from the first first intermediate branch 36 towards the first first branch 28. The first first concave edge portion 38 substantially defines an arc of a circle.

[0046] Also, it will be observed that the first first concave edge portion 38 is stamped so as to form a first rim portion which is extended by a first complementary rim portion 40 which is substantially rectilinear in the first first branch 28. It will be observed that the first first concave edge portion 38 has a very moderate and acceptable stamping depth with regard to the material chosen and its tensile strength Rm.

[0047] Furthermore, on the other hand, the first first part 26 has a first intrados rim 42 which extends into the two first branches 28, 30. Also, the first first part 26 has a first extrados rim 44 which extends into the second first branch 30 and into the first first intermediate branch 36 opposite the first intrados rim 42. Also, the extrados rim 44 extends into the first first intermediate branch 36, converging towards the first first concave edge portion 38 up to a first first external free end 46.

[0048] The edges 40, 42, 44 of the first part 26 define a first bottom 45 of the part 26.

[0049] It will be observed that the first concave edge portion 38 and the extrados rim 44 converge towards each other to become substantially parallel, forming a first U-shaped end 48.

[0050] Also, it will be observed that the first first exterior free end 46 of the first extrados rim 44 has a first first semi-circular notch 50, while opposite, the first first concave edge portion 38 extends to a first first interior free end 51 having a first first quarter-circle notch 52. Also, the first first interior free end 51 extends in line with the first first exterior free end 46.

[0051] We will now refer again to the flowchart of [Fig. 8] illustrating the steps of the method according to the invention. Thus, according to a second first choice step 54, a second metal plate material is chosen. In the example illustrated here, the second metal material is identical to the first metal material and it likewise has a high tensile strength.

[0052] Then, in a second first stamping step 56, the metal material plate is stamped to form a second first part 58, which is illustrated in detail seen from below in [Fig. 3] and which will be described below. The two selection steps 22, 54 and two corresponding stamping steps 24, 56 are not necessarily successive or in the order given here. They may be in reverse order or be simultaneous.

[0053] The second first part 58 illustrated in [Fig. 3], is adapted to be engaged in the first first part 26 as will be explained below.

[0054] Also, the second first part 58 extends along a second first average plane and it has a third first branch 60 inclined relative to a fourth first branch 62. They are inclined at the same angle as the two first branches 28, 30 of the first first part 26. The third and fourth first branches 60, 62 define a second first extrados 64. And the second first part 58 has a second first intermediate branch 66 which projects from the extrados 64 in a direction opposite to the fourth first branch 62.

[0055] Also, the second first part 58 has a second first concave edge portion 68 extending from the second first intermediate branch 66 towards the third first branch 60. The second first concave edge portion 68 substantially defines an arc of a circle.

[0056] It will first be observed that the second first concave edge portion 68 is stamped to a greater depth than the first first concave edge portion 38, and it is upright relative to the second first mean plane of the second first part 58. In certain circumstances, where the first metal material chosen has a very high tensile strength, a different second metal material is chosen, having a lower tensile strength. In this way, a sufficient stamping depth can be obtained. In other words, the second first concave edge portion 68 is stamped to an even greater depth so as to be able to compensate for the lesser depth of the first concave edge portion 38, and thus obtain an arm reinforcement edge having the required height.Such an arm reinforcement edge height could not be achieved if the two parts were obtained by stamping a plate of metal material with very high tensile strength. Advantageously, the stamping depth is between 2 cm and 3 cm.

[0057] In addition, the second first concave edge portion 68 is extended by a second free wing curved in return 70.

[0058] Furthermore, on the other hand, the second first part 58 has a first intrados edge 72 which extends into the third and fourth first branches 60, 62. Also, it has a first extrados edge 74 which extends into the fourth first branch 62 and into the second first intermediate branch 66 opposite the first intrados edge 72. Also, the first extrados edge 74 extends into the second first intermediate branch 66 converging towards the second first concave edge portion 68 up to a second first outer free end 76. In addition, the second first part 58 has a first complementary extrados edge 75 which extends into the third first branch 60 in the extension of the second first concave edge portion 68.

[0059] Furthermore, the second first concave edge portion 68 extends, in the second first intermediate branch 66, to a second first inner free end 78. And the latter has a second first quarter-circle notch 80.

[0060] Thus, the second first part 58 is engaged inside the first first part 26 as illustrated in [Fig. 4] to which reference will now be made, and is welded there according to a first assembly step 82.

[0061] More specifically, the third and fourth first branches 60, 62 are respectively connected to the first first branch 28 and second first branch 30, while the second first intermediate branch 66 is connected to the first first intermediate branch 36.

[0062] The first intrados edge 72 of the second first part 58 is continuously secured to the first intrados rim 42, and on the other hand, on the one hand the first extrados edge 74 is secured to the first extrados rim 44, and on the other hand, the first complementary extrados edge 75 is secured to the first complementary rim portion 40.

[0063] Thus, the second first part 58 is held inside the first first part 26 in a position spaced from the bottom 45 so that the two average planes of the two parts are substantially parallel.

[0064] In addition, the first concave edge portion 38 is also secured to the second first concave edge portion 68 on which it rests. They thus together form a first concave arm reinforcing edge 84. Such a reinforcing edge 84 makes it possible to give the reinforcing arm thus produced high rigidity.

[0065] Also, the second first quarter-circle notch 80 of the second first part 58 extends in the extension of the first first quarter-circle notch 52 of the first first part 26, forming a semi-circle notch coming to the right of the first first semi-circle notch 50 of the first first part 26.

[0066] In this way, the notches can accommodate a bushing, which is then welded onto the edge of the notches. Such a lower suspension arm can then be installed in a motor vehicle as shown in [Fig. 1], for which the wheels 18 have a large width.

[0067] On the other hand, if the lower suspension arm requires less resistance to deformation BR, then, according to a first second choice step 22', as illustrated in [Fig. 8], a plate of a first metallic material having a low tensile strength is chosen.

[0068] The references of the elements used to implement the lower suspension arm with low resistance to deformation common to that used to implement the arm with high resistance to deformation, will be identical and will have a prime sign: “'”.

[0069] A first metallic material having a low tensile strength Rm is here a material whose tensile strength Rm, corresponding to the stretching limit of a given section before rupture, is less than 500 MPa. The breaking strength of the chosen material is for example 360 ​​MPa.

[0070] The steel plate of the first metallic material has a thickness also between, for example, 0.3 mm and 3 mm.

[0071] Then, in a first second stamping step 24', the plate is stamped to form a first second part 26', which is illustrated in detail seen from below in [Fig. 5] and which will now be described.

[0072] The first second part 26' extends along a first second average plane and it has a first second branch 28' inclined relative to a second second branch 30'. The two second branches 28', 30' are for example inclined to each other at an angle of between 120° and 150°.

[0073] The two second branches 28', 30' thus inclined define a first second intrados 32' opposite a first second extrados 34'. And the first second part 26' has a first second intermediate branch 36' which extends projecting from the extrados 34' in a direction opposite to the second second branch 30'. In other words, the first second intermediate branch 36' forms an acute angle with the first second branch 28'.

[0074] Also, the first second part 26' has a first second concave edge portion 38' extending from the first second intermediate branch 36' towards the first second branch 28'. The first second concave edge portion 38' substantially defines an arc of a circle.

[0075] Also, the first second portion of concave edge 38' also extends by a second portion of complementary edge 40' which is substantially rectilinear in the first second branch 28'.

[0076] On the other hand, the first second concave edge portion 38' has a significant drawing depth which is entirely acceptable given the material chosen and its tensile strength Rm of 360 MPa. The drawing depth is significantly greater than that of the second first concave edge portion 68 of the second first part 58.

[0077] Thus, the first second portion of concave edge 38' is raised relative to the first second average plane of the first second part 26'.

[0078] Furthermore, it extends, just like the second first portion of concave edge 68, by a first free wing curved in return 70'.

[0079] Furthermore, on the other hand, the first second part 26' has a second intrados rim 42' which extends into the two second branches 28', 30'. Also, the first second part 26' has a second extrados rim 44' which extends into the second second branch 30' and into the first second intermediate branch 36' opposite the second intrados rim 42'. Also, the second extrados rim 44' extends into the first second intermediate branch 36', converging towards the first second concave edge portion 38' up to a first second outer free end 46'.

[0080] The edges 40', 42', 44' of the first second part 26' define a second bottom 45' of the part 26'.

[0081] Also, the first second concave edge portion 38' and the second extrados rim 44' converge towards each other to become substantially parallel, forming a second U-shaped end 48'.

[0082] Furthermore, the first second outer free end 46' of the second extrados rim 44' has a first second semicircular notch 50', and opposite, the first second concave edge portion 38' extends to a first second inner free end 5T also having a second second semicircular notch 52'. The second second semicircular notch 52', and the first second semicircular notch 50' have substantially the same dimensions. This is made possible by the stamping depth of the first second concave edge portion 38'. Also, the first second inner free end 51' extends in line with the first second outer free end 46'.

[0083] With regard to the flowchart of [Fig. 8] showing the steps of the process, according to a second second choice step 54', a second metal plate material is chosen. The second metal material is also identical to the first metal material and it then has a low tensile strength. In other words, its tensile strength Rm is 360 MPa.

[0084] Then, in a second stamping step 56', the metal material plate is stamped to form a second second part 58', which is illustrated in detail seen from below in [Fig. 6] and which will be described below. Here too, the two selection steps 22', 54' and the two corresponding stamping steps 24', 56' are not necessarily successive or in the order given here.

[0085] The second second part 58' illustrated in [Fig. 6], is adapted to be engaged in the first second part 26' like the second first part 58 in the first first part 26.

[0086] Also, the second second part 58' extends along a second second average plane and it has a third second branch 60' inclined relative to a fourth second branch 62'. They are inclined at the same angle as the two second branches 28', 30' of the first second part 26'.

[0087] The third and fourth second branches 60', 62' define a second second extrados 64'. And the second second part 58' has a second intermediate second branch 66' which extends projecting from the extrados 64' in a direction opposite to the fourth second branch 62'.

[0088] Also, the second second part 58' has a second second concave edge portion 68' extending from the second second intermediate branch 66' towards the third second branch 60'. The second second concave edge portion 68' substantially defines an arc of a circle.

[0089] Unlike the second first concave edge portion 68 of the second first part 58, the second second concave edge portion 68' extends substantially in the second second average plane of the part 58'. Conversely, the second second part 58' has a second intrados edge 72' which extends in the third and fourth second branches 60', 62'. Also, it has a second extrados edge 74' which extends in the fourth second branch 62' and in the second second intermediate branch 66' opposite the second intrados edge 72'. Also, the second extrados edge 74' extends in the second second intermediate branch 66' converging towards the second second concave edge portion 68' up to a second second outer free end 76'.In addition, the second second part 58' has a second complementary extrados edge 75' which extends, substantially longitudinally, in the third second branch 60' in the extension of the second second concave edge portion 68'.

[0090] Furthermore, the second second concave edge portion 68' extends, in the second second intermediate branch 66', to a second second inner free end 78'. The second second concave edge portion 68' extends in the form of a substantially curved tab 86 to the second second inner free end 78' and to the second second outer free end 76'.

[0091] Thus, the second second part 58' is engaged inside the first second part 26' as illustrated in [Fig. 7] to which reference will now be made, and is welded there according to a second assembly step 82'.

[0092] More specifically, the third and fourth second branches 60', 62' are respectively connected to the first second branch 28' and second second branch 30', while the second intermediate second branch 66' is connected to the first intermediate second branch 36'.

[0093] The second intrados edge 72' of the second second part 58' is continuously secured to the first intrados rim 42', and on the other hand, on the one hand the second extrados edge 74' is secured to the second extrados rim 44', and on the other hand, the second complementary extrados edge 75' is secured to the second complementary rim portion 40'.

[0094] Thus, the second second part 58' is held inside the first second part 26' in a position spaced from the second bottom 45' so that the two mean planes of the two parts are substantially parallel. Here, the second second concave edge portion 68' of the second second part 58' is integral with the first second concave edge portion 38' against which it is pressed at the rear. They thus together form a second concave arm reinforcing edge 84'. Such a reinforcing edge 84' makes it possible to give the reinforcing arm thus produced a rigidity lower than that of the reinforcing arm as described above, but entirely compatible with the use made of it.

[0095] Also, the tab 86 extends partially along the second semicircular notches 50', 52'. The second semicircular notch 52' and the first semicircular notch 50', in line with each other, make it possible to accommodate a socket which is thus welded to the edge of the notches. In addition, the socket then partially rests on the tab 86 and a weld bead can also be applied between the two. In this way, the socket is perfectly secured to the second intermediate branch 66' and the first intermediate branch 36'.

Claims

CLAIMS

1. [A method for producing a lower suspension arm for the front axle of a motor vehicle, characterized in that it comprises the following steps: - a first metallic material is chosen and a plate of said first material is stamped to form a first part (26; 26') comprising two first branches (28, 30; 28', 30') inclined relative to each other, defining an extrados (34; 34'), and a first intermediate branch (36; 36') extending projecting from said extrados opposite one (30; 30') of said two first branches, said first part having a first concave edge portion (38; 38') extending from said first intermediate branch (36; 36') towards the other (28; 28') of said two first branches; - a second metallic material is chosen and a plate of said second material is stamped to form a second part (58; 58') comprising two second branches (60, 62; 60', 62') and a second intermediate branch (66; 66'), adapted to be connected respectively to said two first branches (28, 30; 28', 30') and to said first intermediate branch (36; 36'), said second part having a second concave edge portion (68; 68') adapted to cooperate with said first edge portion (38; 38') to form a concave arm reinforcing edge (84; 84'); and when a first material having a low tensile strength is selected, said first edge portion (38') is dressed to form said reinforcing edge (84'), while the second edge portion (68) is dressed to form said reinforcing edge (84) when a first material having a high tensile strength is selected.

2. Production method according to claim 1, characterized in that the first (38; 38') and second (68; 68') edge portions are connected to each other.

3. Production method according to any one of claims 1 to 2, characterized in that said first material and said second material are identical.

4. Production method according to any one of claims 1 to 3, characterized in that said first part (26; 26') has an intrados rim (42; 42') extending in said two first branches (28, 30; 28', 30') and an opposite extrados rim (34; 34') extending from said one (30; 30') of said two first branches to a first free end (46; 46') in said first intermediate branch (36; 36').

5. Production method according to claim 4, characterized in that said second concave edge portion (68; 68') extends to a second free end (78; 78'), and in that said second free end comes in line with said first free end (46; 46'), when the first material having a high tensile strength is chosen.

6. A production method according to any one of claims 1 to 5, characterized in that the first metallic material having high tensile strength has a tensile strength greater than 700 MPa.

7. Production method according to any one of claims 1 to 6, characterized in that when a first material having a high tensile strength is chosen, said second upright edge portion (68) is extended by a second free wing (70) curved in return.

8. A production method according to any one of claims 1 to 7, characterized in that the first metallic material having low tensile strength has a tensile strength of less than 500 MPa.

9. Production method according to any one of claims 1 to 8, characterized in that when a first material having a low tensile strength is chosen, said first upright edge portion (68') is extended by a first (70') free wing curved in return.

10. Lower suspension arm for the front axle of a motor vehicle obtained in accordance with the production method according to any one of claims 1 to 9.]

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