TECHNICAL FIELD OF THE INVENTION This invention concerns a suspension system for an automotive vehicle comprising at least a leaf spring. The invention also concerns an automotive vehicle, in particular a truck, equipped with such a suspension system.

BACKGROUND OF THE INVENTION It is known to mount a leaf spring on an automotive vehicle in order to form a part of a suspension system. As shown on figure 1 , which corresponds to the prior art, a leaf spring 1 is made of a number of metallic leaves, only two such leaves being represented on figure 1 , with reference 11 and 12, for the sake of simplicity. Leaf spring 1 is bended at the level of its front end in order to form a front eye 13 which surrounds a shaft 2 extending along an axis Y ₂ which is perpendicular to the longitudinal axis X-X' of a truck represented by its chassis 3. Shaft 2 is directly mounted on chassis 3, whereas a lever 4 connects the back end or free end 14 of leaf spring 1 to chassis 3. Lever 4 is articulated on two pins 41 and 42 respectively fast with back end 14 and chassis 3. When a transient effort E is exerted on a central part 15 of leaf spring 1 , e.g. by a front axle of the truck, this induces a deformation of leaf spring 1 which takes the configuration represented in dashed lines. With this deformation, a central point A of leaf spring 1 follows a curved path C which is substantially in the form of an arc of a circle centered on a specific point P called Ross point. This curved path C of point A induces undesired movements in the steering system of the truck. Indeed, the movement of point A on path C tends to move the front axle of the truck in a frontward-backward direction, which induces a corresponding movement in the steering link connecting a steering unit to a steering knuckle of a steered wheel. This induces undue pivoting movements of the wheel with respect to the chassis, which may be dangerous.

A solution to solve this problem is to try to fix the end of the steering link at the so called "Ross point" of the spring lever, that is at the center of rotation of the substantially circular path of a central point of the leaf spring. However, such a Ross point is difficult to identify. Moreover, this Ross point can vary from one spring leaf to another. Such an approach requires to install a steering gear unit as close as possible to the Ross point, which is not always possible. Because of the general shape of a leaf spring, the location of its Ross point is such that the steering gear unit must be installed above the leaf spring. This is sometimes difficult.

The same kind of problem arises with the back axle of a truck when it is subject to roll when turning Indeed, the deformation of the leaf springs for suspending the left and right sides of the axle will, because of transient efforts E of opposite directions on the left and right sides of the axle, also tend to make the rear axle of the truck to pivot with respect to a vertical axis of the chassis.

It is known from FR-A-682 635 to use an eyelet where a cylindrical block is mounted within a bush with a possibility of rotation in order to allow a movement of an eye of a leaf spring, along a longitudinal direction of a chassis. Such an approach does not allow to actually control the movement of a central part of a leaf spring. It is also known from JP-A-6206414 to prevent rear axle steering by using an eccentric pin to mount a leaf spring on a chassis. The eyelet of the leaf spring must have a sliding movement with respect to the eccentric part in order to allow an adaptation of their relative position. This implies that the movement of a central part of the leaf spring must be monitored by a lever which drives a rotating lever whose movement is transferred to an end of the leaf spring. This is complicated and, therefore, expensive.

SUMMARY OF THE INVENTION

The invention aims at providing a suspension system where the movement of a central part of a leaf spring can be precisely controlled, so that its path avoids or substantially limits undue movements of the corresponding steering knuckle and steered wheel, or of the rear axle of a truck, when a transient effort is applied on the leaf spring.

To this purpose, the invention concerns a suspension system of an automotive vehicle, this system comprising at least a leaf spring and mounting means to connect the leaf spring to a chassis of the vehicle. This system is characterized in that at least one end of the leaf spring is fixedly connected to an external surface of an eccentric bushing articulated on the chassis around an axis perpendicular to a longitudinal axis of the leaf spring.

Thanks to the invention, the rigid connection between the end of the leaf spring and the external surface of the eccentric bushing makes it possible to control a longitudinal movement of the leaf spring when the eccentric bushing rotates.

According to further aspects of the invention which are advantageous but not compulsory, such a system might incorporate one or several of the following features: - The end of the leaf spring forms an eye which surrounds the bushing on more than 180° preferably more than 270°.

- The end of the leaf spring is welded on the eccentric bushing. Alternatively, the end of the leaf spring is press-fitted on the eccentric bushing or locked on the eccentric bushing by cooperation of shapes. - A second end of the leaf spring, which is opposite to the end fast with the eccentric bushing, is connected to the chassis via a lever articulated on this second end and on the chassis, around two axes perpendicular to the longitudinal axis of the leaf spring.

The invention also concerns an automotive vehicle, in particular a truck, equipped with a suspension system as mentioned here-above.

Advantageously, the leaf spring extends along a longitudinal axis parallel to a longitudinal axis of the chassis of the vehicle and is fixedly connected to the external surface of the eccentric bushing, at its front end.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on the basis of the following description which is given in correspondence with the annexed figures and as an illustrative example, without restricting the object of the invention. In the annexed figures: - figure 1 is a schematic representation of some parts of a suspension system according to the prior art;

- figure 2 is a view similar to figure 1 , for a suspension system according to the invention; - figure 3 is an enlarged view of detail III on figure 2;

- figure 4 is a view similar to figure 2 when a transient effort is exerted on the leaf spring of the suspension system;

- figure 5 is an enlarged view of detail V on figure 4; and - figure 6 is an enlarged view similar to figure 5 when a transient effort is exerted on the leaf spring in a direction opposite to the effort shown on figure 4.

DETAILED DESCRIPTON OF SOME EMBODIMENTS The suspension system 100 represented on figures 2 to 6 includes a leaf spring 101 mounted on a chassis 103 of a truck.

X-X' denotes a longitudinal axis of chassis 103 which extends in a frontward/backward direction of the truck.

Leaf spring 101 is mounted on chassis 103 via a pin 102 centered on an axis Yio ₂ which is perpendicular to axis X-X' and horizontal when the truck rests on a horizontal surface.

Xioi denotes a longitudinal axis of leaf spring 101. Axis X101 is parallel to axis X-X' and perpendicular to Y ₁₀₂ -

Leaf spring 101 is made of one or several metallic leaves, only two such leaves being represented on figures 2 to 6, with references 111 and 112, for the sake of simplicity. The front end 113 of leaf spring 101 has the form of an eye or eyelet which surrounds pin 102 on an angle α of about 315°.

The back end or free end 114 of leaf spring 111 is articulated on a lever 104 which is articulated around a pin 142 fast on chassis 103 and extending parallely to axis Yio2- A pin 141 mounted on back end 114 allows the articulation of lever

104 on leaf spring 101. Pins 141 and 142 form articulation axles for lever 104. The invention is not limited to such type of connection of the free end of the spring.

An eccentric metallic bushing 105 is mounted around pin 102 with a possibility of rotation. The internal radial surface 151 of bushing 105 is cylindrical with a circular cross section adapted to the circular cross section of the outer surface 121 of pin 102.

The outer surface 152 of bushing 105 has an eccentric shape and the eye formed by front end 113 is fixedly connected to this surface 152. Three welding points 16 make end 113 fast with bushing 105. The number of welding points can be different from three.

The relatively large value of angle α allows efficient fastening of front end 113 on surface 152. Angle α is chosen larger than 180° preferably larger than 270° the value 315°giving good results.

When a transient effort E is exerted on leaf spring 101 , as shown on figure 4, the upward movement of a point A of the central part 115 of leaf spring 101 involves a rotation of bushing 105 around pin 102, as shown by arrow A ₁ on figure 5. Y ₁₅₂ denotes a central geometrical axis of surface 152. Axes Y102 and Y152 are parallel but, because of the eccentricity of surface 152, they are not in alignment. Y ₁₅₂ forms a reference axis for the front end 113 of leaf spring 101. When bushing 105 rotates from the configuration of figure 3 to the configuration of figure 5, axis Y ₁₅₂ is displaced with respect to axis Y102, that is to the front of the truck in the example shown. In the configuration of figure 3, axes Y ₁₀₂ and Y ₁₅₂ are aligned on a same vertical plane P ₀ , with Y ₁₅₂ being above Y ₁₀₂ . In the configuration of figure 5, a straight line L ₁ joining axes Y _{Ϊ O2} and Y ₁₅₂ in the plane of figure 5 makes a positive angle βi, of about 15° with respect to plane P o- In the configuration of figure 3, the angle βo between line L ₁ and plane Po equals 0. When leaf spring 101 changes from the configuration of figures 2 and 3 to the configuration of figures 4 and 5, reference axis Y ₁₅₂ is moved to the left of figures 2 to 5, which involves a corresponding movement of front end 113 and the remaining parts of leaf spring 101 , in particular its central part 115 which is displaced with respect to the chassis with a movement parallel to axis X ₁₀₁ . The fact that front end 113 of leaf spring 101 is fixedly connected to the outer surface 152 of eccentric bushing 105 induces that the offset of reference axis Y ₁₅₂ induces a corresponding offset of a point A belonging to a central part 115 of leaf spring 101.

Therefore, the path of point A resulting from transient effort E takes the configuration of curve C shown in dotted line on figure 4, which is bended to the left above the central part 115 of spring 101 in the configuration of figure 3. In other words, the effect of effort E is that point A follows a curve C centered on a Ross point P' which is lower than point P on figure 1. Thanks to an appropriate choice of the distance between axes Y102 and Y152, Ross point P can even be below axis Y101, as shown on figure 4.

Curve C is represented in dashed line on figure 4, for the sake of comparison.

If an effort opposite to the effort E is exerted on central part E, then spring 101 takes the configuration of figure 6 where line Li makes a negative angle β ₂ of about 15° with respect to plane P 0 and axis Y152 moves towards back end 114, so that front end 113 and all other parts of spring 101 move to the right on figures 4 and 6. In particular, point A moves to the right on figure 4. This is why curve C is on the right of curve C in its part below leaf spring 101 on figure 4. Therefore, curve C can be approximated as an arc of circle centered on a equivalent Ross point P' located below the Ross point P of the spring considered alone, possibly even below leaf spring 101. The fact that the resulting equivalent

Ross point P' is lower than the Ross point P of the spring on its own allows to install the steering gear in a lower position, while still limiting pivoting parasitic pivoting movement.

The values of angles βi and β ₂ given here above correspond to an example of the amplitude of the oscillation movement of bushing 105 around axis Y102. This amplitude is generally less than 20° in both direct ions.

It must be noted that, in the example shown, the initial setting of the eccentric bushing is such that, in the absence of dynamic load, and for a given static load, axis Yi ₅₂ is above axis Y ₁₀₂ Such initial setting results in a lower equivalent Ross point P'. Nevertheless, depending on the exact result which one seeks to obtain, it could be chosen to have a different initial setting. For example, having an initial setting where axis Y ₁₀₂ is above axis Yi ₅₂ , which is equivalent to initial angle β ₀ equal to 180 degrees, would result in having an equivalent Ross point P' higher than the Ross point of the spring considered alone. Other initial settings could also be chosen, with an initial angle βo ranging between 0 and 360 degrees.

The same kind of advantage can be obtained on a suspension system for a rear axle of a truck: pivoting of the rear axle around a vertical axis when the vehicle is subject to roll can be avoided or substantially decreased by fixedly connecting one end of the leaf spring to the outer surface of an eccentric bushing articulated on an axis perpendicular to a longitudinal axis of the leaf spring.

The invention can also be implemented in a suspension system of a trailer. According to a non represented embodiment of the invention, front end 113 of leaf spring 101 can be press-fitted on surface 152 or mechanically locked on eccentric bushing 105 by cooperation of shapes. In such a case, it is not necessary to use welding points 16.

The invention has been represented when implemented with a metallic bushing 105. One can also make use of a rubber bushing when the front end of the leaf spring is locked or press-fitted in the bushing.