Abstract:
An integrally cast aluminum subframe for a vehicle suspension system has increased stiffness for an overall vehicle frame while minimizing the thickness of the main aluminum casting by using tension/compression struts to resist flexing. Various strut attachments are shown that simplify manufacturing and lower the cost. The invention results in further improvement of weight reduction while simultaneously achieving a desired amount of stiffness.

Description:
BACKGROUND OF THE INVENTION 
     This application is related to co-pending application U.S. Ser. No. 10/006,327, filed concurrently herewith. 
     The present invention relates in general to a rear subframe for a multi-link vehicle suspension, and, more specifically, to increasing stiffness of the subframe using tension/compression struts. 
     The vehicle frame supports the vehicle body and, together with other components, such as control arms, springs, and shock absorbers, comprises the suspension system which permits up and down wheel movement without up and down movement of the body. Due to the many forces to which the frame is subjected, it is important that the frame have high stiffness for purposes of structural integrity, vehicle geometry, and creation of noise. 
     The vast majority of vehicle frames have been fabricated from steel because of its high strength, high stiffness, and reasonable cost. However, there are also concerns for minimizing the weight of a frame, based mainly on a desire to improve fuel economy. 
     Integral castings of aluminum or aluminum alloys may be used as vehicle frames or more typically subframes, cradles, and cross members (i.e., frame sections). Aluminum is able to provide good stiffness and can provide a significant reduction in weight. A hollow cross section (e.g., box or tubular) of the subframe members is used to further improve stiffness and reduce weight. 
     Although the characteristic stiffness of aluminum or aluminum alloy is good, the e-module or longitudinal stiffness is about three times less than it is for steel. Therefore, the aluminum casting has to maintain a certain thickness in order to maintain sufficient stiffness, thus limiting the amount of weight reduction that could be obtained in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention has the advantage of increased stiffness of a vehicle frame while minimizing the thickness of the main aluminum casting, resulting in further improvement of weight reduction while simultaneously achieving a desired amount of stiffness. 
     In one aspect of the present invention, a frame apparatus for a vehicle comprises an aluminum subframe including a substantially hollow left side-rail, a substantially hollow right side-rail, a front cross-member, a rear cross-member, a left-side upper control arm attachment, a left-side lower control arm attachment, a right-side upper control arm attachment, and a right-side lower control arm attachment. The substantially hollow side-rails and cross-members each has a respective cross-sectional diameter over a majority of their respective lengths about equal to a first predetermined diameter. A tension/compression strut element has a first end and a second end, the first and second ends being affixed between a first locus and a second locus on the cast aluminum subframe. The tension/compression strut element has a substantially straight body between the first and second ends and has a respective cross-sectional diameter over a majority of its substantially straight body about equal to a second predetermined diameter, the second predetermined diameter being less than about one-half of the first predetermined diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one preferred embodiment of a subframe according to the present invention. 
     FIG. 2 is a perspective view of another embodiment of a subframe according to the present invention. 
     FIG. 3 is a perspective view of yet another embodiment of a subframe according to the present invention. 
     FIG. 4 is a perspective view showing passages in the cross-members for receiving a strut. 
     FIG. 5 is a partially cutaway, perspective view showing a passage in greater detail. 
     FIGS. 6-8 are each a partial cross section showing preferred embodiments of a strut attachment. 
     FIG. 9 is a partially cutaway, side cross-section of a mechanical lock for rigidly retaining a strut. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a cast aluminum subframe  10  includes a left side-rail  11 , right side-rail  12 , rear cross-member  13 , and front cross-member  14 . Bosses  18  located at each corner of subframe  10  provide connection points to the vehicle body and/or to other frame components. 
     Subframe  10  also includes a left lower control arm connection  15  in the form of two clevis arrangements, one for each link of the control arm (not shown). A left upper control arm connection  16  and a right upper control arm connection  17  utilize in this one example mounting studs as described in copending U.S. application Ser. No. 10/006,327. 
     The present invention supplements the stiffness of cast subframe  10  with strategically located tension/compression struts. Specifically, for any undesirable flexibility present in the subframe, the struts are affixed rigidly between a first locus on the subframe and a second locus on the subframe such that flexing would tend to stretch or compress the strut so that the flexing motion is opposed by the strut. In order to avoid the addition of significant weight, each strut has a cross-sectional diameter or width that is less than about one-half of the cross-sectional diameter or width of the side-rails or cross-members of the main casting. Due to the size and added structural complexity that would result to the subframe, one preferred embodiment of the present invention does not form the struts integrally as part of the main casting. Instead, the struts are formed separately and then one or more struts are affixed between respective locus points where needed. 
     As shown in FIG. 1, a strut  20  extends between a first locus  21  on rear cross-member  13  and a second locus  22  on front cross-member  14 . An attachment structure such as a hole, passage, threaded receptacle, or clamp may preferably be located at each locus. Strut attachment may be achieved in a variety of ways including bolting, riveting, welding, clamping, mechanical locking, screwing, and other ways known to those skilled in the art. The strut itself may be formed of metallic material by stamping, forging, casting, and extruding, etc. Preferred materials for the strut include aluminum, aluminum alloys, and steel. The strut may be a solid rod or a hollow tube, for example. 
     A strut  23  is affixed between side-rails  11  and  12 . A weld  24  affixes strut  23  at one locus and a mechanical lock or clamp  25  affixes strut  23  at the other locus. One non-limiting example of a mechanical lock is shown in greater detail in FIG.  9 . Strut  23  has a collar or disk  95  attached to its end by a weld  96 , for example. A cover  97  has an inner recess sized for retaining collar  95  and has a central opening through which strut  23  passes. Cover  97  is bolted to side-rail  12  by bolts  98  in order to rigidly retain strut  23 . 
     FIG. 2 shows additional embodiments for the struts. A strut  27  is affixed to front cross-member  14  by a rivet  28  at one locus and to rear cross-member  13  by a rivet  29  at a second locus. Another strut  30  is affixed to front cross-member  14  by a rivet  31  at one locus and to rear cross-member  13  by a rivet  32  at a second locus. Struts  27  and  30  cross each other to brace the subframe in a transverse direction. 
     Stiffness may also be added to the control arm connecting structures. Thus, a strut  33  is connected at one end by a rivet  34  to one locus on front cross-member  14 . The other end of strut  33  includes a bolt-hole  35  and is connected by a bolt  36  to a second locus at a mounting stud  37  of left upper control arm connection  16 . Preferably, a bushing for the link of an upper control arm would also be retained by bolt  36 , but this is not shown. 
     A strut  40  is connected at one end by a rivet  41  to a hole  42  located at a first locus. The second end of strut  40  is connected to a second locus, which is at a clevis flange  44  of the left lower control arm connection. A bolt  43  couples strut  40  with the clevis and a bushing  45  of a lower control arm articulation. A nut  46  secures bolt  43 . 
     Multi-piece struts are also contemplated by the present invention as shown in FIG. 3. A triangular multi-strut  47  has vertices  50 ,  51 , and  52 , each with a respective connection hole and each connected to a respective locus  53 ,  54 , and  55  by a screw or bolt (not shown). Since multi-strut  47  includes straight body portions between each pair of vertices, it functions at a tension/compression strut between each pair of loci. 
     An x-shaped multi-strut  56  has four strut ends, each connected to a respective locus  57 ,  58 ,  59 , and  60 . 
     FIG. 4 illustrates a preferred strut attachment method of the present invention. The side-rails and cross-members of the cast subframe have hollow cross sections. Passages  61  and  62  are integrally cast and are coaxially aligned so that a straight strut can be inserted through one passage and into the other. Passages  61  and  62  preferably have continuous walls surrounding the strut and may have various attachment means as described below. 
     Referring to FIG. 5, front cross-member  14  is shown in partial section. Integral with cross-member  14  is passage  61  which has threads  63  on its interior side wall. A strut  64  is formed as a circular tube and has a threaded outside surface  65 , the threads of which engage threads  63  after strut  64  is installed. 
     Cross-member  14  is a hollow tube (with a square cross-sectional shape in this embodiment) and has a cross-sectional diameter or width d which is the same as the cross-sectional diameter or width of all the side-rails and cross-members generally, over a majority of their lengths. Strut  64  has a diameter d s  which stays approximately constant over its length and which is less than about one-half of diameter d c . As a consequence of the structure of the present invention, the tension/compression struts provide an optimal combination of static and dynamic stiffness and significant weight reductions for the overall structure. 
     A preferred embodiment for affixing strut  64  to both passages  61  and  62  is shown in FIG.  6 . Strut  64  has a maximum diameter at threads  65 . The central portion and other end of strut  64  have a slightly reduced diameter allowing them to pass through passage  61 . The other end is inserted into passage  62  for affixing at that second locus. The other end of strut  64  has internal threads  66 . Strut  64  is screwed into passage  61  until the other end of strut  64  contacts a shoulder  67  at the end of passage  62 . A slot (not shown) in the first end of strut  64  near passage  61  may be provided to facilitate turning of strut  64  to screw it into passage  61 . Shoulder  67  creates a central opening which receives a bolt  68  for screwing into threads  66 , thereby securing strut  64 . 
     FIG. 7 shows an alternative embodiment for either or both of the passage connections. A strut  69  is received in passage  61  which includes pockets  70 ,  71 , and  72 . Originally, strut  69  has a substantially constant diameter along the portion of its length that is received inside passage  61 . Strut  69  includes expansion areas  73 ,  74 , and  75  which are expanded under pressure applied inside strut  69  in order to bulge the expansion areas into pockets  70 ,  71 , and  72 . 
     Strut  69  has a closed end  76  and an open end  77 . Open end  77  is connected to a pressurization system  78  including a pump  80 . A reservoir  81  contains a fluid  82  (which in a preferred embodiment is water). Pump  80  pumps fluid  82  into a high pressure line  83 . A seal  84  couples high pressure line  83  with open end  77 . By introducing pressurized fluid to the interior of strut  69 , expansion areas  73 ,  74 , and  75  fill pockets  70 ,  71 , and  72 , thereby creating stops against movement of strut  69 . Once the stops are created, the pressure is released and the pressurization system removed. 
     FIG. 8 shows an alternative embodiment for creating stops in situ when a strut is installed on the subframe. A strut  86  includes expansion areas  87  and  88 . In this embodiment, expansion is achieved by the application of longitudinal forces F 1  and/or F 2  to strut  86  while passage  61  is immobilized. Bevels  90  and  91  may be used to facilitate formation of stops  92  and  93 , respectively. 
     In view of the foregoing description, a cast aluminum subframe has been shown with improved stiffness through the use of tension/compression struts. Thus, the subframe achieves the advantages of low weight and high stiffness. Either a casting material of lower stiffness or a reduced thickness of casting material can be employed using the present invention since stiffness can be restored by the struts.