Patent Publication Number: US-2011057409-A1

Title: Dual Load Path Suspension Assembly with Auxiliary Roll Stiffness

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a suspension assembly with leaf springs for a vehicle with a suspension and an axle. 
     BACKGROUND 
     A Hotchkiss-type leaf spring suspension assembly is simple and effective but has limitations. One limitation is the tradeoff between roll stiffness and vertical stiffness for a comfortable ride and responsive vehicle handling. Roll stiffness of the suspension assembly relates to the vertical stiffness of the leaf springs and their lateral spacing by the relationship of the vertical stiffness multiplied by the square of the lateral spacing. Vertical stiffness, for example, may be chosen for a comfortable ride, such as providing a desired suspension natural frequency, while lateral spacing is largely dictated by available space and structural design considerations of transmitting suspension loads to the vehicle frame. The choice may result in roll stiffness lower than ideal for vehicle handling. Conversely, springs selected to meet particular roll stiffness may provide a vertical stiffness and natural frequency higher than ideal for a comfortable ride. 
     Another limitation of the Hotchkiss-type leaf spring suspension relates to the axle location. The axle location in the sense of caster angle is governed by second-order vertical bending stiffness of the leaf springs, such as the leaf spring deflection mode of a horizontal S-shape with the axle at the center of the “S”. While this is a concern primarily under braking conditions, it is also a concern under conditions of driving torque (for the case of a driven axle) and a longitudinal impact at the tire (such as a bump or pothole). Also, the reaction of braking forces and torques between the axle and the chassis/frame through the mechanism of second-order leaf spring vertical bending imposes significant constraints on the design relating to “anti-dive”, “anti-lift”, or “anti-squat” geometry. 
     A third limitation is that some of the spring leaves are not fully used for lateral stiffness. Typical heavy-duty Hotchkiss-type suspensions have leaf springs with multiple leaves. Generally, only one of the leaves has features at either end for mounting to the frame or body, such as a formed eye with an elastomeric bushing. While the remaining leaves are not directly connected to the frame-mounted leaf at the eye, they can attach to the mounted leaf at the central axle. The remaining leaves, therefore, have a lateral-longitudinal sliding relationship with frame ends of the frame-mounted leaf, governed by normal forces and friction coefficients. The remaining leaves contribute little to lateral stiffness and are inefficient relative to their weight. 
     Some leaf springs are designed with a second leaf extended and wrapping around the eye formed with the first, frame-mounted leaf. This “military wrapper” design is primarily a safety measure for retaining the axle in case of the mechanical failure of the first leaf. The second leaf still has a sliding relationship with the first leaf and the lateral stiffness drawbacks related to the sliding relationship. 
     A fourth limitation is the friction inherent in the sliding relationships between multiple spring leaves. As the suspension articulates, longitudinal (and/or lateral) force builds up at the interface between adjacent leaves until friction is overcome, then the leaves break free in a “stick-slip” manner. This cycle of force building up, abruptly releasing when an interface breaks free, and then abruptly returning to the force build-up phase when the interface “seizes” again, causes suspension hysteresis. In suspension hysteresis, the force is not consistently or smoothly related to displacement. Particularly in the case of small motions, this friction effect can significantly increase the effective stiffness of the entire suspension assembly, which is detrimental to vehicle ride quality and handling precision. To reduce hysteresis, additional components are sometimes added between spring leaves, such as low-friction polymeric liners. 
     Therefore, there is a need to develop a suspension system that improves roll stiffness without greatly increasing vertical stiffness and without relying upon additional devices such as anti-sway bars, in order to better optimize vehicle handling and ride comfort. There is an additional need for a suspension system with improved handling characteristics resulting from increased lateral stiffness and reduced castor angle change during articulation, braking torque, and driving torque, in order to improve vehicle handling precision. There is a further need to produce a suspension assembly with reduced friction and hysteresis, to improve vehicle ride comfort and handling precision. Finally, there is a need to add redundant elements to the suspension assembly to increase safety. 
     SUMMARY 
     Accordingly, a suspension assembly is provided with a vehicle with a chassis and an axle. The suspension assembly has upper and lower leaf springs that are spaced apart from each other, a shackle assembly and a shackle hanger attaching to the chassis. The shackle assembly has first and second shackles that pivotally attach to the shackle hanger. The first ends of each of the leaf springs pivotally fasten to the shackle assembly spaced apart from each other. The second ends of each of the leaf springs pivotally fasten to a spring hanger spaced apart from each other. 
     The upper and lower leaf springs combined with the spaced-apart first and second shackles provide dual load paths. These load paths are supported by a load transfer assembly, shackle assembly, and spring hanger and contribute to auxiliary roll stiffness, to lateral stiffness, better control of axle caster angle, improved management of braking and driving torques, and improved safety by increased redundancy. Additionally, these linkages produce torque reactions between the axle and the frame, for improved anti-dive, anti-lift, and anti-squat suspension behavior relative to braking and driving torques. 
     The inner and outer load transfer plates maintain alignment with each other by means of the spring and shackle fasteners clamping the pivot bushing inner metal columns at four locations. The inner and outer load transfer plates transmit force and torque between the upper and lower leaf springs and the first and second shackles. Because the load transfer assembly is pivotally fastened to the two nearly vertically-oriented shackles and to the two nearly horizontally-oriented leaf springs, suspension articulation causes rotations about the laterally-oriented pivot axes only. The load transfer assembly can provide very high stiffness in regards to rotation about longitudinal and vertical axes and in regards to translation about the lateral axis. 
     The suspension assembly exhibits a roll center intermediate between the upper and lower leaf springs, such that suspension articulation in roll introduces a small amount of first-order lateral bending in the upper leaf spring and an approximately equal-but-opposite amount of lateral bending in the lower leaf spring. This opposed lateral bending of upper and lower leaf springs generates opposed lateral forces that are coupled by the vertical offset between the leaf springs to produce a restoring moment that resists vehicle roll displacement. The suspension assembly, therefore, provides an additional mechanism to furnish auxiliary roll stiffness while adding little extra mass compared to traditional leaf spring suspensions. 
     The vertical offset between the upper and lower leaf springs can adjust the auxiliary roll stiffness of the suspension assembly, as the restoring moment is directly proportional to the square of the vertical offsets at the front and rear mounting points of the upper and lower leaf springs multiplied by the lateral stiffness of the leaf springs. This mechanism resists vehicle roll equally on both the left and right sides of the suspension. This auxiliary roll stiffness mechanism operates in addition to the normal roll stiffness resulting from the vertical stiffness of the springs and their lateral spacing. The auxiliary roll stiffness mechanism can affect suspension assembly roll stiffness independently from vertical stiffness, decoupling the customary relationship between roll stiffness and vertical stiffness inherent in traditional leaf spring suspensions and removing a significant constraint from the suspension design process. 
     The suspension assembly provides additional load paths to increase lateral stiffness by replacing sliding joints with friction reducing joints that engage leaf springs which would otherwise make little contribution to lateral stiffness in traditional leaf spring suspensions. Lateral stiffness may increase by a factor approaching two times normal lateral stiffness simply by incorporating the lower leaf spring into the lateral load path, when compared to traditional leaf spring suspensions with a sliding connection between the first and second spring leaves. 
     The suspension assembly significantly improves axle location in the sense of reduced caster angle variation, under conditions of braking torque, driving torque, and longitudinal tire inputs. By incorporating dual load paths and effective four-bar linkages, braking torque, driving torque, and longitudinal forces are transmitted through the four-bar linkages as tension and compression force pairs to the vehicle chassis. In contrast, in a traditional leaf spring suspension, such torques and forces cause rotational wind-up of the leaf spring as seen in the side view; this allows the caster angle of the axle to change, altering steering geometry with some detriment to steering feel and directional stability. This suspension assembly improves axle location in the sense of reduced caster angle variation, under conditions of braking torque, driving torque, and longitudinal force inputs. 
     The suspension assembly provides dual load paths and four-bar linkages that allow for control of suspension anti-dive, anti-lift, and anti-squat characteristics. Anti-dive, anti-lift, and anti-squat behaviors all involve the reaction of braking or driving torques from the axle through the suspension to the vehicle frame. The angles of individual links and their moment-arm distances from the axle govern the magnitudes and directions of the forces developed in the links, and reactions at the frame, due to the braking or driving torques. Adjustment of linkage geometry allows the braking or driving torque reactions to be controlled to achieve desired anti-dive, anti-lift, or anti-squat characteristics. The ability to adjust these characteristics removes an intrinsic design constraint of traditional leaf spring suspensions, thus allowing suspension performance to be better optimized. 
     The suspension assembly reduces suspension friction by replacing sliding joints with friction reducing joints, such as bushing- or bearing-type joints, providing better ride comfort and handling precision. It also provides greater redundancy in the connection of the axle to the chassis compared to the traditional leaf spring suspension and “military wrappers” to increase the safety margin in the event of a leaf spring mechanical failure. 
     As described above, the Dual Load Path Suspension Assembly and a vehicle made with this system provide a number of advantages, some of which have been described above and others of which are inherent in the invention. Also, modifications may be proposed to the Dual Load Path Suspension Assembly or a vehicle made with this system without departing from the teachings herein. Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a side plan view of one of the embodiments of the suspension assembly; 
         FIG. 2  is a sectional view along line A-A′ of  FIG. 1  of the shackle assembly; 
         FIG. 3  is a close up sectional view of  FIG. 1  of the shackle assembly using a bushing; 
         FIG. 4  is a close up sectional view of  FIG. 1  of the shackle assembly using a bearing; 
         FIG. 5  is a side plan view of one of the embodiments of the suspension assembly; 
         FIG. 6  is a partial bottom view of one of the embodiments of the suspension assembly; 
         FIG. 7  is a side plan view of one of the embodiments of the suspension assembly; 
         FIG. 8  is a side plan view of one of the embodiments of the suspension assembly; 
         FIG. 9  is a side plan view of one of the embodiments of the suspension assembly; and 
         FIG. 10  is a side plan view of one of the embodiments of the suspension assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Turning to the Figures where like reference numerals refer to like structures, a vehicle, such as a medium or heavy duty truck or school bus, trailer, agricultural implement, construction equipment, and the like, has a suspension assembly  10  mounted to the vehicle&#39;s chassis  12 . The suspension assembly  10  engages an axle  14 . The vehicle can have additional suspension assemblies, for example a second suspension assembly  11  mounted to the other side of the chassis  12  lateral to suspension assembly  10 . 
     Turning to  FIGS. 1-4  and  9 - 10 , the suspension assembly  10  engages the axle  14  at about the center of the suspension assembly  10 , although other locations can be used. The suspension assembly  10  has an upper leaf spring  16  and a lower leaf spring  18  located in a spaced apart relation and each pivotally fasten to a shackle assembly  20 . The upper leaf spring  16  has an opposite first upper spring end  22  and a second upper spring end  23 , and the lower leaf spring  18  has an opposite first lower spring end  24  and a second lower spring end  25 . The first upper spring end  22  and first lower spring end  24  each pivotally fasten to the shackle assembly  20  with spring fasteners  26  passing through an upper spring eye  48  and a lower spring eye  50  respectively. The second upper spring end  23  and second lower spring end  25  each pivotally fasten with spring fasteners  26  to a first spring hanger  30  attached to the chassis  12 . Alternately, the second upper spring end  23  or second lower spring end  25 , or both, may slidably engage the first spring hanger  30  attached to the chassis in a vari-rate type of arrangement (not shown), requiring an additional locating link (not shown) between the axle  14  and the first spring hanger  30  if both the second upper spring end  23  and the second lower spring end  25  are slidably engaged to the first spring hanger  30 . Also, the first upper spring end  22  or first lower spring end  24 , or both, may slidably engage the shackle assembly  20  in a vari-rate type of arrangement (not shown), requiring an additional locating link (not shown) between the axle  14  and the shackle assembly  20  if both the first upper spring end  22  and the first lower spring end  24  are slidably engaged to the shackle assembly  20 . 
     The axle  14  is located between the upper leaf spring  16  and lower leaf spring  18 . Both upper leaf spring  16  and lower leaf spring  18  connect to the axle  14  using a U bolt  32  or other suitable mechanical fastener(s). A spacer block or bracket (not shown) may be used between the axle  14  and the upper leaf spring  16 , or between the axle  14  and the lower leaf spring  18 , or in both locations, to provide adequate separation distance between the springs and control the relative position of the axle  14 . Alternatively, the axle  14  can be located above ( FIG. 9 ) or below ( FIG. 10 ) the suspension assembly  10 , and spacer blocks  49  or brackets may be used above the upper leaf spring  16  or below the lower leaf spring  18 , as well as between the upper leaf spring  16  and the lower leaf spring  18 , to adjust the relative positions of the axle  14  to springs  16  and  18 . 
     The shackle assembly  20  has a shackle hanger  28  mounted to the chassis  12 . First shackle  34  and second shackle  35  are spaced apart from each other and pivotally fasten to the shackle hanger  28  using shackle fasteners  36 . The first shackle  34  and second shackle  35  in this embodiment are compression shackles with the second shackle  35  behind the first shackle  34 . A load transfer assembly  33  of the shackle assembly  20  pivotally fastens to first shackle  34  and second shackle  35 . The load transfer assembly  33  has substantially parallel first load transfer plate  38  and second load transfer plate  40 , having shackle fasteners  37  passing through first load transfer plate  38 , second load transfer plate  40 , and through first shackle  34  or second shackle  35 . The load transfer assembly  33  also has spring fasteners  26  passing though first load transfer plate  38 , second load transfer plate  40 , and through upper spring eye  48  or lower spring eye  50 . The first load transfer plate  38  and second load transfer plate  40  may be polygonal, and may further be triangular. 
     The first load transfer plate  38  and second load transfer plates  40  may also have additional structures, such as holes, sculpted outer profiles, flanges, reinforcing ribs, and the like, for improved weight and stiffness. A friction reducing device  45 , such as a bushing  44 , sleeve, bearing  47 , roller bearing, ball bearing, tapered roller bearing, or equivalent device, may surround the spring fasteners  26 , shackle fastener  36 , or shackle fastener  37 . The bushing  44 , for example, has a bushing inner metal sleeve  90 , an elastomeric intermediate material  92  surrounding the bushing inner metal sleeve  90 , and a bushing outer metal sleeve  94 , all located within the leaf spring eye  50 , the leaf spring eye  48 , or the end of shackles  34  or  35 , and located by the fastener  26  or  37 . 
     Side-view linkages can form between the suspension system  10 , the axle  14  and the chassis  12 . A side-view linkage can form between the load transfer assembly  33 , the upper leaf spring  16  and lower leaf spring  18  and the axle  14  as a quasi-four-bar linkage. The axle  14 , the upper leaf spring  16 , the first spring hanger  30 , and the lower leaf spring  18  can also combine to form a quasi-four-bar linkage. Another side-view linkage can form between the load transfer assembly  33 , the first shackle  34  and second shackle  35 , and the shackle hanger  28  as a four-bar linkage. 
     In this disclosure, a quasi-four-bar linkage is similar in function to a conventional four-bar linkage. The degrees of freedom conventionally provided by two adjacent revolute joints, however, are provided by the vertical bending of the two leaf springs  16  and  18 . These quasi-four-bar linkages can be thought of as partially-flexible quadrilaterals, essentially rigid at the axle  14 , with the leaf springs  16  and  18  gradually becoming less rigid at the first spring hanger  30  and load transfer assembly  33  connections. The first spring hanger  30  and load transfer assembly  33  maintain the distance between the upper and lower leaf spring connections while the suspension articulates. 
     The embodiment of the suspension assembly  52  shown in  FIGS. 5 and 6  has a shackle assembly  56  using a first tension shackle  54  and a second tension shackle  55  fastened to the first upper spring end  22  and first lower spring end  24  respectively. First tension shackle  54  and second tension shackle  55  pivotally fasten to shackle hanger  58 . The first tension shackle  54  is located above the second tension shackle  55 . Substantially parallel inner and outer load transfer plates  63  pivotally fasten to the first tension shackle  54  and second tension shackle  55  with shackle fasteners  37 . Inner and outer load transfer plates  63  are pivotally fastened to the upper leaf spring  16  and to the lower leaf spring  18  with spring fasteners  26 . A friction reducing device  45  such as a bushing  44  or a bearing  47  (not shown) may surround the shackle fasteners  37  or the spring fasteners  26 . The inner and outer load transfer plates  63  may be polygonal and may have two pairs of opposite parallel sides, such as a parallelogram, and may have additional structures, such as holes, sculpted outer profile, flanges, reinforcing ribs, and the like, to improve weight and stiffness. A crossmember  60  may connect the shackle assembly  56  with a second shackle assembly  57  (not shown) located laterally on the opposite side of the chassis  12 . The crossmember  60  may connect to the rear, bottom, or inside faces of the shackle hangers  58  and  59  of the shackle assemblies  56  and  57 . Similarly, a crossmember  61  (not shown) may connect the first spring hanger  30  with a second spring hanger  31  (not shown) located laterally on the opposite side of the chassis  12 . 
     The embodiment of the suspension assembly  66  shown in  FIG. 7  has a shackle assembly  68  with first compression shackle  70  and second compression shackle  71  pivotally fastened to the shackle hanger  28  with shackle fasteners  36 . First compression shackle  70  and second compression shackle  71  pivotally fasten to the first upper spring end  22  and first lower spring end  24  respectively with spring fasteners  26 . 
     In  FIG. 8 , the suspension assembly  74  has a shackle assembly  76  with first tension shackle  78  and second tension shackle  79  pivotally fastened to the shackle hanger  80  with shackle fasteners  36 . First tension shackle  78  and second tension shackle  79  pivotally fasten to the first upper spring end  22  and first lower spring ends  24  with spring fasteners  26 . The first tension shackle  78  is located above the second tension shackle  79 . A crossmember  60  may connect the shackle hanger  80  of shackle assembly  76  to a similar hanger located laterally on the opposite side of the chassis  12 . 
     Spring fasteners  26  and shackle fasteners  36  and  37  may be any type of fastener known in the art, such a fastener with a head  42  and a shank  43 , or a pin. The spring fasteners  26  and shackle fasteners  36  and  37  may be surrounded by a friction reducing device  45  to reduce friction during movement. The friction reducing device  45  may be metal or may be at least partially elastomeric or polymeric. A bushing inner metal sleeve  90  may permit the first load transfer plate  38  and second load transfer plate  40  to be solidly clamped together by the spring fasteners  26  or shackle fasteners  36  or  37 . 
     Other potential embodiments may include the axle  14  located either above ( FIG. 9 ) or below ( FIG. 10 ) the upper and lower leaf springs  16  and  18  instead of between the upper and lower leaf springs  16  and  18 , with a spacer  84  located between the leaf springs  16  and  18 . In addition, the upper leaf spring  16  may be comprised of multiple individual leafs, forming an upper leaf spring assembly  81 , or the lower leaf spring  18  may be comprised of multiple individual leafs, forming a lower leaf spring assembly  82 , or both the upper leaf spring  16  and the lower leaf spring  18  may be comprised of multiple individual leafs. The upper and lower leaf spring assemblies  81  and  82  may further be comprised of less than full leaf springs or half leaf springs connected to the upper or lower leaf spring assemblies  81  or  82  at the axle  14 . More than two spaced apart leaf springs, such as leaf spring  83  in  FIG. 10 , may also be used in the suspension assembly, with spacers  84  located between the leaf springs  16 ,  18 , and  83 . 
     While specific embodiments have been described in detail in the foregoing detailed description and illustrated in the accompanying drawings, those with ordinary skill in the art will appreciate that various permutations are possible without departing from the teachings disclosed herein. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the claims, which include all equivalents thereof. Other advantages to a Dual Load Path Suspension Assembly and a vehicle made with this assembly may also be inherent in the invention, without having been described above.