FORM 2
THE PATENTS ACT 1979
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION
IMPROVED SEMI TRAILING ARM SUSPENSION SYSTEM
FOR A VEHICLE
APPLICANTS
TATA MOTORS LIMITED, an Indian company having its registered office at Bombay House, 24 Homi Mody Street, Hutatma Chowk, Mumbai 400 001 Maharashtra, India
INVENTORS
Soheb Ahmed, Abhay Deshpande, Umesh Abhyankab
& Mandar Hajare, all Indian nationals
of TATA MOTORS LIMITED an Indian company
having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF INVENTION
This invention relates to an improved semi trailing arm suspension system for a vehicle and more particularly relates to a suspension of an individual wheel of a vehicle.
BACKGROUND OF THE INVENTION
The conventional independent rear suspensions system normally uses semi trailing arm suspension. This suspension system comprises of two arms pivoted at an angle to front plane of vehicle and the arms may also be pivoted at an angle to the horizontal plane. This is done in order to achieve desired kinematic behavior from the suspension. The semi trailing arm is secured at the rear end to a hub carrier which carries a rear wheel hub. A vertically extending strut is provided between rear end portion of the semi trailing arm and the vehicle body. It has, however been recognized that these suspensions systems suffer from defects like low lateral rigidity. During cornering of a vehicle the centrifugal force exerted on the vehicle body acts on the suspension as lateral force at tyre contact patch. This lateral force causes the tyre on the rear inner wheel to toe out. This decreases the under steer level of the vehicle which translates into decreased driving stability of the vehicle.
US 4537420, US4715615 & US 4815755 have addressed this issue through various means. In addition to the issue of low lateral rigidity, these suspension systems also suffer from problems like geometry distortion during assembly/joining. Conventionally, the two arms are joined together most commonly by arc welding. Arc welding leads to distortions in the geometry of the suspension which is unacceptable. The joint portion of the two arms is subjected to forces and torques in multiple directions which have to be endured by the welding, which is a concern from durability of the suspension system.

OBJECTS OF INVENTION
The main object of this invention is to provide a suspension system which can provide sufficient lateral rigidity without adding additional parts in order to improve the stability of the vehicle.
Another object of this invention is to provide a suspension system having improved joint between two arms.
Yet another object of this invention is to provide a suspension system which is simple in construction and cost effective.
Yet another object of this invention is to provide a suspension system which is easy to assemble and is dimensionally accurate in series production.
SUMMARY OF INVENTION
The semi trailing arm suspension system comprises of two semi trailing arms made by hydro forming process. The shape of the arms is such that it gives improved stiffness in camber and toe. The outer arm and inner arm are given profiles such that in assembled conditions there is interlocking between the two. The interlocking is provided either by having depression on outer arm and corresponding projection on inner arm or by having projection on outer arm and corresponding depression on inner arm. This ensures that only part of twisting loads is endured by the welding and remaining is taken care by the interlocking between the two arms. This improves durability of the suspension system apart from increasing rigidity.
STATEMENT OF INVENTION
Accordingly the invention provides an improved semi trailing arm suspension system for a vehicle comprising an outer arm (2) and an inner arm (3), front ends of said outer arm (2) and inner arm (3) are provided with sleeves wherein outer arm (2) has a depression (8) or projection (7) and the inner arm (3) has a corresponding projection (7) or depression (8) respectively and said outer and inner arms are

assembled by meshing the depression (8) with the projection (7) of said arms and a spring seat (4) fitted at the rear end (R) of said meshed arms.
DETAILED DESCRIPTION OF INVENTION
Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting the same,
Figure 1 shows an isometric top view of a suspension system in accordance with this invention.
Figure 2 shows an isometric bottom view of a suspension system in accordance with this invention
Figure 3 shows isometric view of outer arm used in this invention
Figure 4 shows isometric view of inner arm used in this invention
Figure 5 shows plan view of suspension system in accordance with this invention
Figure 6 shows details of section A-A taken from fig.5
Referring to figs.l to 6 semi trailing arm suspension (1) consists of outer arm (2), inner arm (3), spring seat (4), damper bracket (5) and bush sleeves (6) on front end (F) of both inner and outer arms. Wherein outer arm (2) has a depression (8) and the inner arm (3) has a corresponding projection (7), said outer arm (2) and inner arm (3) are made by hydro forming process and are assembled in such a way that the depression (8) of outer arm (2) meshes with projection (7) of inner arm (3) and outer arm (2) and inner arm (3) are joined together by welding process. The spring seat (4) is fitted at rear end (R) of the meshed arms. Outer arm (2) has depressions (10) at rear end (R) which is shown in fig.2. Spring seat (4) has two side walls (13, 14) and top surface (11), which is either an integrated unit or an assembly of

different or multiple parts to form assembly. Top surface (11) of spring seat (4) is slanted from outer side of the vehicle towards the inner side of the vehicle with respect to base surface (12) of spring seat (4), The projection (7) and depression (8) forms the male and female type fit. It can be in many combinations of shapes of such male female fit.
Damper bracket (5) is welded to inner arm (3) and two bush sleeves (6) are respectively welded to outer arm (2) and inner arm (3) at front end (F). Spring seat (4) is welded to both outer arm (2) and inner arm (3). Outer arm 2 has axle hub mounting holes (9) which is shown in fig. 3
During vehicle running on rough roads various forces act on suspension. In case of semi trailing arm suspension (1) these forces act on the pivot centers, hub end (H) and damper mounting point (D). Vehicle fitted with suspension system wherein bush sleeves on front end of inner and outer arms of semi trailing arm suspension are fitted onto vehicle body and rear end (R) of semi trailing arm suspension is fitted on wheel hub. The twisting forces have to be endured by the joint between the two suspension arms. In accordance with this invention, outer arm (2) and inner arm (3) are provided with curvilinear profiles such that in assembled conditions the depression (8) of outer arm (2) meshes with projection (7) of inner arm (3), thus creating interlocking between the two. When twisting forces are exerted at the suspension during vehicle driving the interlocking resists the same. Thus, substantial part of the twisting forces is endured by the interlocking between the two arms. This not only improves durability of the suspension system but also increases rigidity. The welding length /joint length requirement in such system is also reduced. In addition, since the semi trailing arms are made by hydro forming process, there is improvement in dimensional accuracy and repeatability of the suspension system.
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those

skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

WE CLAIM
1. An improved semi trailing arm suspension system for a vehicle comprising an outer arm (2) and an inner arm (3), front ends of said outer arm (2) and inner arm (3) are provided with sleeves wherein outer arm (2) has a depression (8) or projection (7) and the inner arm (3) has a corresponding projection (7) or depression (8) respectively and said outer and inner arms are interlocked by meshing the depression (8) with the projection (7) of said arms and said interlocked outer and inner arms are joined by welding process and a spring seat (4) fitted at the rear end (R) of said meshed arms.
2. Improved suspension system as claimed in claim 1 wherein said outer (2) and inner arms (3), are made by hydroforming process.
3. Improved suspension system as claimed in claim 1 wherein said outer arm (2) has curvilinear profile to have clearance with tyre of vehicle and at least two depressions (10) at rear end (R).
4. Improved suspension system as claimed in claim 1 wherein said spring seat (4) has two side walls and top surface, which is either an integrated unit or multiple parts joined together.
5. Improved suspension system as claimed in claim 4 wherein one side wall of spring seat (4) is fitted to inner arm and other side wall and top surface are fitted to outer arm (2).

6. Improved suspension system as claimed in either claim 1 or 5 wherein the top surface of spring seat (4) is slanted from outer side of the vehicle towards the inner side of the vehicle with respect to base surface of spring seat (4).
7. Improved suspension system as claimed in claims 1 to 6 wherein said sleeves are fitted onto vehicle body and rear end is fitted with wheel hub.