Abstract:
A multi-piece siderail is fabricated for use as a truck frame which has potential cost and weight reductions while also providing superior weight loading characteristics. Each section of the multi-piece siderail includes a webbed portion having longitudinal ribs that form alternating protruding and recessed sections. An upper and a lower flange are provided on each section and each flange is formed with a stepped, lipped configuration. The unique structure of each section provides increased rigidity and support to that section and also allows for a nesting arrangement between each of the three components. Furthermore, this structure provides multiple attachment zones throughout the frame—two zones on each flange and three zones on the front and rear section webs and five zones on the center section web. Each of these attachment zones are surrounded by a work hardened corner to provide reinforcement. To further increase strength and provide for sound joints, a support member is inserted between the middle component, which is often deeper, and the front and rear components. Specifically, the support member, having a Z-shaped cross-section, is inserted between the lower portions of the respective deep and shallow sections. Thus, a multi-piece siderail can be utilized in truck frame applications, allowing for reduced weight and reduced over all production costs, while providing superior performance characteristics such as greater payload and/or better fuel economy. The design also allows for potential section commonality and flexibility in assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to truck frames. More specifically, the present invention relates to a multi-piece siderail (often a three-piece siderail) suitable for use in frames for medium trucks, heavy trucks, trailers, and other transportation equipment. 
     2. Description of the Related Art 
     Traditionally, vehicle frames or truck frames are composed of two siderails interconnected by a plurality of cross members. (It will be understood that the term truck frame, as used herein, relates equally to other vehicles and equipment frames such as those used in trailers, etc.) The truck frame then forms a foundation to which all other components are directly or indirectly affixed. Thus, the truck frame must be suitably strong and sufficiently rigid to not only support a relatively large amount of weight, but it must also retain its integrity as severe forces and stresses are applied during use. It might therefore seem logical to form the siderails from relatively thick, wide, heavy gauge steel beams. However, this is simply undesirable in most modern truck frames. The use of unduly rigid and thick steel for siderails greatly and unacceptably increases both the cost and the weight of the completed frame. 
     For some time it has been known to form each siderail of a truck frame from two separate pieces of steel. That is, each siderail is composed of separate steel members that are appropriately coupled to each other. This is a standard practice for fabricating frames used on light duty trucks. Light duty trucks are those trucks that are generally used for individual/personal use such as pick-up trucks, sport utility vehicles, or minivans. Typically, with a two-piece siderail, the joint of the siderail is located near the front seat of the cab or its equivalent. Thus, the front section of the siderail is used to support the engine and at least a significant portion of the cab. The rear section of the siderail supports a portion of the cab and the remainder of the vehicle. 
     As is well understood, there is a distinction between light duty trucks as one class of vehicles and medium or heavy duty trucks as a second class of vehicles. Often products for these different classes are produced by different manufacturers, using different manufacturing techniques, and in vastly different volumes. Some medium and perhaps some heavy duty trucks have employed a two-piece siderail in constructing the truck frame. For example, a commercial delivery van may be classified as a medium duty vehicle and could be formed using a variation of the two-piece siderail. Oftentimes, with heavy trucks, such as tractor trailers, the siderail is formed from a single piece of relatively heavy gauge steel. This is due to the increased weight and towing/loading capacities required by such vehicles. Alternatively, reinforced siderails have been used where an additional layer of metal is provided at various points to provide the prescribed strength or stiffness. These siderails have been designed to accommodate the high stresses typically generated in the center portion. Thus, the front and rear portions of these siderails are typically over-designed (e.g. capable of carrying loads that will never be experienced.) Due to the commercial nature of these vehicles, the increased cost in using heavy gauge single beam siderails has been accepted. Often these rails are straight in sideview and of constant section, requiring a high vehicle profile for axle travel—this, too has been accepted. 
     Sometimes a “doubler” of either “C” or “L” shape is used to reinforce and stiffen the center siderail and a portion of the rear siderail—but this add-on piece is usually constrained to the height of the base siderail, not enjoying the greater weight efficiency of a deeper section. 
     In certain circumstances a “drop-belly” (one-piece) rail is used to achieve greater efficiency. This design includes a rail having a relatively deep (high) center portion and shallower end portions. Manufacturing such a rail requires expensive contoured tooling that is adjustable for each of the many specialized wheelbases typical of medium and heavy truck applications. Furthermore, this design lacks a desired overlap at the high stress rear spring front hanger. While this drop-belly rail is the most weight-efficient of past-practice siderails, it also has by far the highest tool and unit costs, considering the many tool set-ups for low-volume application. Additionally, this design creates scrap material in the shallower front and rear sections. 
     A major improvement in light duty truck frames has been the ability to produce multi-piece siderails. This concept, as used in light duty truck frames, is more fully described in U.S. Pat. No. 5,149,132 to Ruehl et al. issued on Sep. 22, 1992 and U.S. Pat. No. 5,308,115, also issued to Ruehl et al. on May 3, 1994. Both patents are hereby incorporated by reference. With this multi-piece siderail design, the rear section is subdivided into two components. Since the front most section of the siderail often supports substantially less weight than the middle section, and the front and rear sections must accommodate appropriate amounts of axle or suspension travel, the front end section can sometimes be fabricated from lighter gauge material and both end sections can also have a smaller cross-section than the center section. In this design, the strength and stiffness requirements for the center section is not compromised. Maintaining this stiffness insures that the ride quality of the vehicle is maintained at a desirable level. Furthermore, the highest local stresses often occur at the solid front attachment of the rear suspension which must react to braking and acceleration loads—a convenient spot to overlap center and rear siderail sections of this invention. Thus locally doubling stock at that load point, while using the spring hangers&#39; fasteners to create the rail joint is an efficient rail configuration. Joining these three independent sections provides a way to form a single siderail that meets or exceeds structural integrity for the application better than the traditional one or two-piece design, while also potentially reducing both the cost and weight. Due to these potential cost, weight, quality and ride performance benefits provided by multi-piece considerations, multi-piece siderails have become an industry standard and represent a large percentage of the light duty truck frames being produced today. 
     Despite the potential cost and weight benefits provided by a multi-piece siderail, this design has not been used in medium and heavy trucks. The siderails utilized for these applications are usually much longer than those utilized in light duty trucks. Also due to the nature of the vehicle, the siderails will be required to support far greater weight. Hence, longer and more rigid siderails are required and frame designers have been reluctant to investigate the three-piece siderail. Rather, the industry has relied on one-piece and in some cases two-piece siderails constructed from heavy gauge material and/or reinforced siderails. This presents little opportunity to modify the design and weight characteristics of the vehicle&#39;s frame. This also limits any opportunity to generate manufacturing efficiencies which could be achieved using common components for various frames. 
     As mentioned, medium and heavy truck frames typically handle much heavier loads. Consequently, the frame itself must be much larger. In one aspect, the height of the frame becomes much taller in order to handle these weight loads. Along with this increase in height comes a potential for localized buckling or “oil-canning”(i.e. deflection of the web). Heavy gauge steel has historically been used to avoid this potential problem, thus eliminating one important benefit of the multi-piece siderail (i.e. potential weight reduction). This creates yet a further reason why three-piece siderails have not been used in the medium or heavy truck market. 
     As mentioned above, the markets for light duty trucks and medium/heavy duty trucks are quite different. In addition to the varying load requirements, the volume and product mix is drastically different. In the medium/heavy duty truck market, much lower volumes are involved, thus not providing as much potential for economies of scale. Additionally, multiple configurations are often desired by the vehicle manufacturers. Consequently, manufacturing costs can be very substantial due to this market characteristic. Obviously, truck frames manufactured with single piece siderails make it very difficult to achieve any economies of scale. Consequently, any potential reduction in manufacturing cost is of great benefit. 
     Therefore, there exists a need to provide a multi-piece siderail system suitable for use in medium and heavy duty trucks that will provide the required degree of rigidity and support while potentially reducing the overall cost and weight of the frame. Further, there exists a need to provide a siderail that can be efficiently manufactured, possibly using common components to create a variety of particular frame designs. 
     SUMMARY OF THE INVENTION 
     The present invention provides two side-rails that are interconnected by a plurality of cross members, thus forming a completed frame. Each siderail is formed from multiple sections (typically three) that are integrally connected to form a unitary member. Since the overall siderail is made up of smaller components, fabrication costs can be greatly reduced. In addition, the unique arrangement of the three sections provides greater strength and rigidity due to its cross-sectional shape. Hence, thinner gauge and lighter weight materials can be utilized while still providing the required degree of structural integrity and rigidity. Such multi-piece siderails thus become economically and functionally desirable due to their potential for decreased production costs, decreased material costs, increased performance, decreased weight and fuel economy consumption, and increased design and manufacturing options/flexibility. 
     Utilizing shorter pieces in combination to form the siderail, smaller and more economical stamping or press machines can be utilized. These machines are more readily available and often much cheaper to operate, thus decreasing total overall production costs. The lighter, smaller sections are more easily handled—even man-handled by two operators. This helps to simplify and speed up the manufacturing process. 
     The use of multi-piece siderails in the medium/heavy truck market affords several other manufacturing efficiencies. More specifically, this allows for standardized components which can be easily combined to create multiple designs (i.e., a modular approach). For example, more complex standardized front sections can be combined with simpler specialized center sections to create a specific design. Thus, a specific frame configuration can be achieved as desired by a customer. However, because a standardized front section is being used, these components can be manufactured in larger quantities to achieve economies of scale. 
     Different sections of the siderail have different requirements. For example, each section will be required to support differing amounts of weight and hence will be required to withstand differing loads. Further, each section may have different torsional load requirements. The center section supporting the payload and spanning the wheelbase, traditionally needs to be more rigid in vertical beaming in order to reduce deflection. Hence, this section—with no axle-travel clearance requirements—is formed as a relatively deep open member from lighter gauge materials. That is, when completed, the middle section of the siderail will have a relatively large vertical dimension. This deep section of siderail is formed by pressing, bending, rolling, extruding or otherwise forming that member so that an upper and a lower flange are formed at each extremity. The portion of the planar member remaining between the upper and lower flange is the web portion of the section. This generally C-shaped configuration (i.e., having perpendicular flanges extending from an upper and lower potion of the support member) has been used in multi-piece siderails in the past. However, this configuration (generally C-shaped) alone, especially with the deep, lighter gauge center section, would not provide sufficient support for use in a medium or heavy truck frame. A C-shaped beam made from light gauge steel will have a tendency to suffer from “oil canning” or other structural fatigue or elastic/plastic collapses when the web is relatively large (deep) and the gauge is reduced. 
     To produce the necessary level of strength, the deep center section can be formed to include stepped, lipped flanges as well as one or more web ribs extending longitudinally along the length of the center section. The stepped portion is fabricated by deforming the flange so that two separate portions of the flange extend perpendicularly from the web, but are in different planes. A crease or bend separates the two flange portions such that one portion is shifted or offset from the other portion of the flange. This shifted portion is also aligned perpendicular to the web, however, is offset from the first portion. That is, the first portion and the shifted portion exist in separate but parallel planes. This creates a “stair-step” type configuration. 
     The lip of the flange is formed by rolling or bending a terminating portion of the flange so that it is at a predetermined angle with respect to the stepped flange. In production, this configuration can be achieved by rolling, pressing, bending, stamping, weldment of, or extrusion of the material. 
     The stepped, lipped flanges provide two fastener-attachment zones on both the top and bottom flanges where any rivet or bolt in the center of the zone will have a work-hardened ridge on each side to resist fatigue. Furthermore, a fastener head on the outboard step (top flange) can not protrude above the inboard step—due to a requirement for a smooth continuous surface to attach a 5 th  wheel, or crossbar, or cab, or other structure to the flange as well as web. In current practice, fasteners are rarely used on flanges because of fear of fatigue crack to edge of flange and to keep a smooth top flange. Adding flange attachment points permits much more rigid attachment for brackets and cross members while spreading the attachment stress into multiple planes (using shear as well as tension or compression). The “lips” extending down from the top flange and up from the bottom flange transform the high stress outer fiber from a fatigue-sensitive fissured blank edge to a fatigue-resistant work-hardened corner. This flange shape also is very resistant of elastic buckling. The stability permits longer, thinner flanges so that more of the rail material is furthest from the rail centroid both under lateral and vertical loading (proportional to the cube of the centroidal distance). The multi-plane cross member and bracket attachment also greatly enhances torsional stiffness. These features have been demonstrated in the many light truck applications. 
     Ribs are formed into the web in a similar fashion. That is, the planar section of the web is transformed so that a plurality of parallel sections are formed wherein adjacent sections are offset from one another. While each of the adjacent sections are parallel they exist in different planes. Once again these ribs can be formed by stamping, rolling, extruding or pressing the raw material. While various configurations can be employed, in the preferred embodiment the front and rear web will include one rib, while the center will have two ribs. These ribs greatly reduce the tendency to buckle or oil can. These ribs will also create multiple fastener-attachment zones—three each on the front and rear webs and five on the center web—where any rivet or bolt at the zone center will have a work-hardened ridge above and below it to resist local fatigue. Together these features will permit lighter gauge material to take advantage of the greater bending efficiency of the deep sections. 
     The remaining sections of the siderails have many of the same structural features as the center section. That is, both the front and rear section of the siderail will include a stepped, lipped flange as well as including a rib in the central web. However, since neither the front nor rear sections need to support the same bending movements as the center section, they need not have the same vertical dimensions. Furthermore, their depth is limited by suspension or axle travel. In the preferred embodiment, both the front and rear section will include one rib that is medially disposed along the web portion of the section. 
     The ribs are specifically designed so that the particular ribs of the front or rear rail will mate with one of the ribs located on the center section of the siderail. Furthermore, the lower web portion of the front and rear sections is dimensioned such that it nests between the ribs of the deep center section. An upper flange of both the front and rear section is configured similar to the upper flange of the center section. Thus, when properly positioned, the upper flanges of the front and/or rear section will nest within the upper flange of the deep center section. Similarly, the lower flanges could be mated with the lower flange of the center section, if so desired. 
     Additionally, the torsional stiffness of the front section, under hood and cab, contributes to better driver comfort, and thus to his endurance, on long runs. Therefore a boxed section, even an aluminum extruded box section for very low tooling cost and very low weight, might be considered for this smallest and lightest of the three rail sections. A boxed section is also easier to tune for “accordion” collapse and energy absorption in barrier crash. 
     The rear section must also be shorter to clear suspension travel, but it shares support of the payload with the center section—thus requiring comparatively heavy gage/high strength steel. Torsional stiffness is less significant to driver comfort than in the front. Thus a less expensive open section would be best. 
     Any appropriate fastener may be used to secure the front and rear section to the center section such as utilizing bolts, rivets, crimping or welding. When so secured, a portion of the front and rear section will extend into the center section, thus forming an area of overlap. It is in this area of overlap that the two sections are attached to one another. Since there is often a substantial difference in the dimensions of the various components (due to variations in rail height), a gap is formed between the lower flange of the center section and the lower flanges of the front or rear sections. Rather than simply relying on the web and top flange attachment mechanism (bolts, rivets, welds, etc.) a support member is inserted between the lower flange of the center section and the relevant lower flange of either the front section or the rear section. Optimally, this support is configured in the shape of a Z-bar. That is, the support has a generally vertical main body with an upper support arm extending in one direction perpendicular from the main body, and a lower support arm extending perpendicularly from the main body in a direction opposite that of the upper support arm. Thus, when properly positioned the lower support arm of the Z-bar is firmly engaged against the lower flange of the deep center section. The upper support arm of the Z-bar is in contact with the lower flange of either the front section or the rear section. This creates a boxed section at this portion of the frame—a structure that imparts considerable strength and rigidity. 
     By providing both the stepped, lipped flange as well as the ribs extending along the webs of the various sections, dramatic increases in strength and rigidity are achieved for each of these sections. Thus, it becomes possible to use even lower gauge materials to form siderails while providing strength and yield characteristics exceeding traditional siderails. The use of lighter gauge materials is extremely advantageous in that significant weight and cost reductions can be achieved, as well as better fuel economy and/or greater payload. 
     Various dimensional requirements also exist for each section of the frame. The center section is required to carry the largest bending load. In order to utilize lighter gauge material, the vertical portion of the center section must remain relatively deep. Correspondingly, the front and rear sections must have relatively shallow vertical center sections in order to properly accommodate the front and rear axle travel. Thus, the multi-piece siderail of the present invention is extremely advantageous in that it allows for potential reductions in weight and/or cost by using lower gauge materials while maintaining the desired dimensional requirements of each portion. 
     The use of the stepped, lipped flange as well as the ribs along the webbing also advantageously provides a superior mechanism for joining the various cross members and brackets required to form a completed frame, as well as superior attachment points for mounted systems (e.g. steering gear, fuel tank, battery box, etc.). Namely, the supports for the cross members are configured to mate with the corresponding sections of siderail. In addition, where appropriate, the cross member or other attachment mechanisms can be coupled to the siderails at the point of overlap between the sections and the siderail. Thus, increased strength is achieved at these attachment points by doubling or tripling the thickness of material, while the attachments might also serve to join the siderail sections at no added cost. 
     The “stepped, lipped” flanges and the “ribbed” webs will input a considerable amount of strategically placed work-hardening throughout the rail sections. In some cases this may permit use of less expensive low-carbon steels. Further, for lighter gauge sections more expensive high-strength-low-alloy steels (H.S.L.A.) can be used while achieving the same finished strength. In other cases, H.S.L.A. steels may be substituted for die-quenched material permitting processing and equipment savings. In further situations, aluminum may be sufficient for the front section, where front axle loads are often one half to one third that of rear axle loads. This might permit simple extruded sections under the cab—even boxed extrusions, giving much higher torsional resistance. These extruded sections can be produced at much lower tool costs than similar stampings. Alternately, steel boxed weldments might be used in front for stiffness, while open “C” channels would suffice for center and rear sections. Together smaller presses, smaller parts and avoidance of heat-treat equipment may permit small remote frame manufacturing plants near the O.E.M. vehicle assembly plant—reducing the transportation costs and complications. 
     Another added feature is that the complex mounting structures required for the steering gear, front sheet metal, cab, engine, and front suspension mounting can often be confined to the front siderail section—often of common length for all truck “family” members. Likewise the rear siderail section containing rear suspension attachment is often restricted to one or two lengths. Variance in models are often confined to the relatively simple center rail where the myriad wheelbase shifts can be handled. Together, these requirements suggest the use of common front and rear rail sections for a “family,” or even front and rear stub frame assemblies joined by relatively simple center rail sections. This, in turn, may enable outsourcing of complete or partial frame assemblies—thus simplifying vehicle assembly plants—and perhaps eventually leading to complete chassis modules or at least partial modules. 
     It is an object of the present invention to provide a multi-piece siderail system for use in medium and heavy truck frames. 
     It is yet another object of the present invention to provide interlocking siderail members from light gauge material, having sufficient strength and rigidity for use in vehicle frames. 
     It is another object of the present invention to provide a multi-piece siderail system wherein a middle section has relatively large vertical dimensions in comparison to the front and rear sections. 
     It is still another object of the present invention to provide siderail components having a stepped, lipped flange. 
     It is a further object of the present invention to provide a multi-piece frame with increased stiffness, thus improving ride performance. 
     It is still a further object of the present invention to provide a multi-piece siderail system wherein each component has longitudinally extending ribs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top planar view of a truck frame constructed from the three-piece siderails of the present invention. 
     FIG. 2 is a side planar view of a three-piece siderail with a representative front cab and rear load member shown in phantom. 
     FIG. 3 is a perspective view of one overlap section of the three-piece siderail. 
     FIG. 4 is a perspective view illustrating the attachment of cross members to the right and left three-piece siderail. 
     FIG. 5 is a cross-sectional view taken along line  5 — 5  of FIG. 3 with an internal attachment plate or bracket shown. 
     FIG. 6 is perspective view illustrating the interconnection of the right center deep section with the right front section with the use of an elongated Z-bar. 
     FIG. 7 is a perspective view of the right center deep section interconnected to the right rear shallow section by use of an elongated Z-bar. 
     FIG. 8 is a perspective view of the right center deep section interconnected with the right rear shallow section by use of two similar Z-bars. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a truck frame according to the teachings of the present invention is shown and generally referred to as  10 . Truck frame  10  is formed from right siderail  12  and left siderail  14  which are interconnected by a plurality of cross-members  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 . In addition to joining right siderail  12  and left siderail  14 , the various cross members  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32  serve various independent functions. For example, cross member  20  also acts as a primary support for the engine once truck frame  10  has been formed. FIG. 2 illustrates the relative positions of a front cab  16  and a rear load member  18 . Front cab  16  will include the passenger cab as well as the engine, while rear load member  18  can be in various forms depending upon the nature of the vehicle. For example, in a tractor-trailer configuration, rear load member  18  would be the attachment mechanism for the fifth wheel. In various medium duty vehicles, rear load member  18  may be a flatbed or an enclosed compartment and may be integral with front cab  16 . Front cab  16  is attached to truck frame  10  through the use of a plurality of cab supports  40 . (See FIG. 4 for an illustration of a cab support.) Cab supports  40  are positioned at suitable points along the truck frame  10  and form a mounting platform to attach the front cab  16 . Many other attachment brackets/supports may be necessary, including fuel tank attachment brackets  41  (See FIGS.  1  and  2 ). 
     Referring again to FIG. 1, generally illustrated towards the rear of truck frame  10  are a first right front spring hanger  42 , a second right front spring hanger  44 , a first left front spring hanger  46 , and a second left front spring hanger  48 . The front ends of various springs or suspension members (not shown) would typically be coupled to the numerated spring hangers and thus interconnected with the axles of the vehicles. The arrangement illustrated would be applicable to a tractor having two rear axles. Alternative configurations are contemplated to accommodate vehicles having more or less axles. 
     Described below are further details regarding left siderail  14 . It is to be understood that right siderail  12  is substantially similar and most of the description below applies equally to both siderails  12  and  14 . Referring primarily to FIG. 2, left siderail  14  includes left front section  56 , left center section  52 , and left rear shallow section  60 . Each of these sections is formed from an elongated piece of sufficiently rigid material, such as steel. The steel member is stamped, pressed, extruded, rolled or otherwise formed into the proper configuration. 
     The various sections are fabricated to nest against one another. Right front section  50  will nest with right center section  52  at a right front overlap joint  62 . Similarly, left front section  56  will nest with left center section  58  at left front overlap joint  64 . As illustrated in FIG. 2, left front overlap joint  64  is defined as the area of overlap between the two sections. As such, left front overlap joint  64  begins at front end  80  and extends to rear end  82 . Due to the differing vertical dimensions between front sections  50 ,  56 , and their corresponding center sections  52 ,  58 , it is advantageous to utilize a spacer  88 ,  90  to help support front sections  50 ,  56 , within their respective front overlap joints  62 ,  64 . Spacer  88 ,  90  is referred to as a Z-bar and is configured to nest between a lower portion of the respective deep section and a lower portion of the respective front section. Right front Z-bar  88  and left front Z-bar  90  are illustrated schematically in FIGS. 1 and 2. As illustrated, the relative positions of the front Z-bars  88 ,  90  abut the rear end of right front overlap joint  80  and left front overlap joint  82 . Z-bars  88 ,  90  create a boxed section when attached to the respective rail section, thus imparting considerable strength—both torsionally and transversely. Similar results could be achieved using similarly configured mounting brackets and/or differently configured spacers (e.g. hollow or “C” shaped components). 
     Similarly, rear shallow sections  54 ,  56  are configured to nest within center sections  52  and  58 . Right rear overlap joint  66  is formed in the area where right center section  52  overlaps with right rear section  54 . Similarly, left rear overlap joint  68  is defined as the area of overlap between left center section  52  and left rear section  60 . More specifically, left rear overlap joint  68  is defined between front end  84  and rear end  86 . It is also desirable to utilize spacers  92 ,  94 ,  96 ,  98  in the rear overlap areas to bridge the gap between the lowermost portion of center sections  52 ,  58  and the lowermost portion of the respective rear sections  54 ,  60 , and to provide a sound joint capable of easily transferring loads. 
     Since more weight will be supported and more forces will be applied in the rear of the vehicle, it is often desirable to have any spacers be more structurally significant than in the front of the vehicle. One way to accomplish this is to use dual spacers  92 ,  94 ,  96 ,  98  within the larger overlap joints  66 ,  68 . Another solution is to use elongated spacers in a larger overlap joints  66 ,  68 . As shown in FIG. 1, a first right rear Z-bar  92  and a second right rear Z-bar  94  are positioned within right rear overlap joint  66 . Likewise a first left rear Z-bar  96  and a second left rear Z-bar  98  are positioned within left rear overlap joint  68 . First left rear Z-bar  96  is positioned near the front end  84  of left rear overlap joint  68 . In other words, first left rear Z-bar  96  is positioned to support a front end of left rear shallow section  60 . The second left rear Z-bar  98  is positioned towards rear end  86  of left rear overlap joint  68  and is thus intended to support left rear shallow section  60  throughout the rearmost portion of left rear overlap joint  68 . The particular arrangement of the various Z-bars, including their sizes, numbers, and positions can be varied depending upon the intended use of the vehicle frame  10 . For example, more than one Z-bar can also be used in front overlap joints  62 ,  64 , and if sufficient support is provided, only one Z-bar may be required in the rear overlap joints  66 ,  68 . Additionally, the length of these Z-bar spacers can easily be varied to span appropriate portions of the overlap joint. 
     It should also be noted that center sections  52 ,  58  are provided with front and rear transition areas  70 ,  72 . As shown in FIG. 2, left center section  52  includes a left front transition area  70 . Thus, a small part of left center section  52  has a vertical dimension substantially equal to that of the left front section  56 . Front transition area  70  represents an angled taper between the areas of differing vertical dimensions. Likewise, left rear transition area  72  provides a smooth transition between areas of differing vertical dimensions. It is desirable to provide areas of the left center section  52  that correspond to the dimensions of the front section  56  and rear section  60  so that as these components are nested, maximum support is achieved. The transition areas  70 ,  72  are provided to minimize the stress and fracture points that could occur if a more abrupt transition is made. In manufacturing, these transitions are created by changing blanks while continuing to use the same tooling/forms. Obviously, this will reduce tooling and set-up costs. 
     Referring to FIG. 3, right rear overlap joint  66  is illustrated with portions of right center section  52  and right rear section  54 . It should be noted that only one Z-bracket  92  is utilized in this overlap joint. As discussed above, two Z-brackets could easily be used. FIG. 3 illustrates some of the structural detail that allows for a multi-piece siderail system to be utilized with medium and heavy trucks. The various sections differ in their vertical (as shown) dimensions, however, they are formed to be similar in their overall shape and structure. With right center section  52 , web  112  is disposed between an upper flange  116  and a lower flange  118 . In general, web  112 , upper flange  116 , and lower flange  118  form a generally C-shaped configuration. In addition to this generally C-shaped configuration, upper flange  116  and lower flange  118  have their terminating sections bent towards one another thus forming upper lip  120  and lower lip  122 . Providing upper lip  120  and lower lip  122  adds significant structural reliability to the configuration of the right center section  52 . Further adding to the structural integrity is a step in both the upper flange  116  and lower flange  118 . In FIG. 3 the step is visible as lower step  115  of lower flange  118 . Lower step  115  is defined by a crease or bend longitudinally disposed along lower flange  118  and medially subdividing lower flange  118  into two separate planar areas wherein each of the planar areas are parallel, but offset from one another. 
     Similarly, a plurality of ribs  124 ,  125  are longitudinally disposed along web  112 . Deformation of web  112  forms first rib  124  and second rib  125  extending along web  112 . The placement and spacing of ribs  124 ,  125  transform planar web  112  into a series of protruding and retracting planar sections wherein each one of these sections is substantially similar to the next. Both first rib  124  and second rib  125  increase the strength of web  112  and help to prevent oil-canning, and withstand localized fatigue stresses at fasteners. 
     Thus, each section of siderail  12 ,  14  is generally C-shaped having a stepped, lipped flange, and at least one rib extending longitudinally along it. These structural characteristics serve two distinct purposes. First these structural features provide for an integrated nesting between right rear section  54  and right center section  52 . For example, right rear section  54  includes an upper flange  126 , a lower flange  130 , upper lip  128  and a lower lip  140 , and a single rib  142  extending longitudinally therealong wherein these components correspond to similar components on right center section  52 . As will be more clearly shown in later views, upper flange  126  nests tightly against upper flange  116 . Furthermore, the deformation formed by rib  142  provides a recess into which rib  124  of right center section  52  interconnects. Thus, the first important feature that the structural characteristics provide is for the integrated nesting of the various sections to form an integral siderail  12 ,  14 . 
     The second important feature provided by the structural characteristics is the increased strength which they provide to the various sections. For example, web  112  of right center section  52  must have a relatively large vertical (as shown) dimension in order to provide the required degree of stiffness. Simply providing this deep web  112  as a planar member would subject right center section  52  to a substantial risk of buckling and/or oil canning, especially when using reduced gauge material. This would render the deep web  112  unacceptable for use in medium and heavy truck frame structures. As mentioned above, the inclusion of ribs helps to avoid this problem. Further, the ribs provide reinforced fastener attachment zones at the ribs&#39; vertical center. The stepped, lipped portions of the flanges also provide for increased rigidity and structural support without requiring heavier gauge materials to be used. Thus, each of these characteristics help to increase rigidity and strength, while the combination resulting in the unique configuration of these structural members allows relatively light gauge materials to be used without surrendering the required degree of support and integrity. 
     Referring to FIG. 3, right rear Z-bar  92  is shown in greater detail. Right rear Z-bar  92  includes a main body  146  with an upper support arm  148  extending in a perpendicular direction away from main body  146 . Opposite upper support arm  148  is lower support arm  150  which also extends in a perpendicular direction away from main body  146 , but in an opposite direction from upper support arm  148 . Hence the cross-section of rear Z-bar  92  is a generally Z-shaped configuration. Lower support arm  150  rests atop lower flange  118  abutting web  112 . Upper support arm  146  is positioned to support the lower flange  130  of right rear section  54 . Upper support arm  148  and lower support arm  150  are substantially planar sections that are intended to flushly abut respective portions of right center section  52  and right rear section  54 . In this manner, lower flange  130  is supported substantially throughout right rear overlap joint  66 . Due to the configuration of Z-bar  92 , any force applied by lower flange  130  is transferred through Z-bar  92  and is applied to lower flange  118 . As previously mentioned, the boxed portion created by the combination of Z-bar  92  and associated sections (center section  52  and rear section  54 ) transfers these loads very efficiently. 
     Referring to FIG. 4, right front overlap joint  62  is illustrated along with the area of right front section  50  that intersects right center section  52 . A portion of right front section  50  is supported by right front Z-bar  88  in the same manner as described with reference to FIG.  3 . Also illustrated in this view are cross member brackets  100  used to support cross members  22 ,  24 . Cross member bracket  100  is configured to have a shape similar to that of web  112  to allow for easy attachment. In other words, cross member bracket  100  includes a ribbed web section  102  and bracket attachment flanges  110 . In this manner, cross member brackets  100  are uniquely mated with a corresponding section of frame  10 , thus providing increased support and reliability. 
     FIG. 5 illustrates a cross-sectional view taken along line  5 — 5  of FIG.  3  and clearly illustrates the nested configuration of the various components. Right center section  52  is provided with upper flange  116  and lower flange  118  wherein upper flange  116  includes upper step  151  while lower flange  118  includes lower step  115 . Upper step  151  and lower step  115  produce planar flange portions that are spatially offset from one another while remaining generally parallel. Upper lip  120  and lower lip  122  are inwardly turned portions of upper flange  116  and lower flange  118  that are substantially parallel with webbing  112 . As previously described, webbing  112  also includes a first rib  124  and a second rib  125  that form a series of protrusions and recessed portions along webbing  112 . Right rear section  54  is shown nested against right center section  52  so that rib  142  of right rear section  54  is nested over first rib  124  of right center section  52 . As illustrated, the planar potions of right rear section  54  are substantially flush with the corresponding planar sections of right center section  52 . This nesting continues with upper flange  126  which abuts upper flange  116  (i.e. upper step  158  abuts upper step  151 ). The dimensions of right rear section  54  are selected so that the upper lip  128  of right rear section  54  abuts against and within upper lip  120  of right center section  52 . 
     Although not illustrated in FIG. 3, a support plate  154  as shown in FIG. 5, having a configuration similar to that of right rear section  54  can be inserted to provide additional rigidity and support through part or whole of the overlap joint  66 . Alternatively, the end portion of a cross member could be similarly configured to provide support. Since support plate  154  has a similar configuration to that of right rear section  54 , the attachment of support plate  154  in this manner will serve to sandwich webbing  132  of right rear section  54  flushly against webbing  112  of right center section  52 . Upper support arm  148  of right rear Z-bar  92  supports lower flange  130  of right rear section  54 . Lower support arm  150  of right rear Z-bar  92  rests against lower flange  118 . Other configurations could be used to provide similar support outside the siderail, or on any portion thereof (e.g. an L-shaped support plate configured to mate with webbing  132  and lower flange  130 ). 
     Each exposed planar portion of the illustrated components represents a potential mounting location  160 . For example, bolt  152  is shown connecting a portion of support plate  154  with the respective upper lips of right rear section  54  and right center section  52 . Bolt  153  connects the respective upper flanges of the deep and shallow sections. The various illustrated components can be held together through bolting as illustrated or any other suitable attachment means such as welding, riveting, or crimping. The potential mounting locations  160  are also utilized to attach various components to the completed frame such as cross member bracket  100  and cab support  40  (as illustrated in FIG.  4 ). 
     FIG. 6 illustrates right front overlap joint  62  as well as a portion of right front section  50  and right center section  52 . Supporting right front section  50  through right front overlap joint  62  is an elongated Z-bar  164 . Structurally, elongated Z-bar  164  is substantially similar to right front Z-bar  88  as previously illustrated. The distinction is that the overall length of elongated Z-bar  164  is substantially greater than right front Z-bar  88 . The use of such an elongated Z-bar  164  provides more support to right front section  50  through the right front overlap joint  62 . Likewise FIG. 7 illustrates the use of an elongated Z-bar  164  in the right rear overlap joint  66 . Thus, for use in frames where more support is required in the rear overlap joints, it is acceptable to use two (or more) independent rear Z-bars  92 ,  94  or a single elongated Z-bar  164 . FIG. 8 illustrates the use of two independent and shorter right rear Z-bars  92 ,  94 . 
     Although the above discussion has related to medium and heavy trucks, all of the features involved may well be beneficial with light trucks. Consequently, the invention is not intended to be limited to any particular class or type of vehicle. 
     Those skilled in the art will further appreciate that the present invention may be embodied in other specific forms without departing from the spirit or central attributes thereof. In that the foregoing description of the present invention discloses only exemplary embodiments thereof, it is to be understood that other variations are contemplated as being within the scope of the present invention. Accordingly, the present invention is not limited in the particular embodiments which have been described in detail therein. Rather, reference should be made to the appended claims as indicative of the scope and content of the present invention. The use of two shorter Z-bars allows for the use of a common part in multiple locations on the frame, or the use of common parts throughout a single frame family.