Abstract:
Concepts and technologies described herein provide for a modular and adjustable axle system for vehicles. According to one aspect of the disclosure provided herein, an axle housing system includes a central differential housing, a pair of retainers attached to the central differential housing, a pair of bell housings attached to the retainers, a pair of axle tube housings attached to the bell housings, and a pair of inner steering knuckles attached to the axle tube housings. Other aspects further include a drivetrain system within the axle housing system that includes a differential, a pair of inner constant velocity (CV) joints attached to opposing sides of the differential, a pair of axle shafts attached to the inner CV joints, and a pair of outer CV joints attached to the axle shafts, wherein the CV joints are maintained in a fixed angular relationship.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/179,750, filed on May 20, 2009, and entitled “Modular and Adjustable Axle Systems for Vehicles,” which is expressly incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Off-road vehicles are commonly subjected to extreme conditions and terrain. Ground clearance, or the distance between the lowest component of the vehicle and the ground, is extremely important to off-road vehicle utility. The greater the ground clearance of a vehicle, the larger the obstacles that the vehicle is capable of traversing without contacting the underside of the vehicle. Typically, the lowest vehicle component is the driven axles&#39; differential housing that encompasses the vehicle differential that translates the longitudinal rotation of the driveshaft to the lateral rotation of the vehicle axle shafts. A conventional vehicle drivetrain configuration includes a differential housing having axle shafts extending horizontally and collinearly outwards from the differential housing to the vehicle wheels. Because a traditional differential housing protrudes below the axle shafts, it is the lowest drivetrain component and can be first to contact ground obstacles, rendering the differential highly susceptible to impeded forward motion and/or damage. 
     Typically, to increase the ground clearance of an off-road vehicle, the vehicle may be fitted with tires having a larger radius than an on-road tire. The larger radius translates into raising the vehicle and corresponding drivetrain components by an amount equal to the radius increase of the tire. However, the larger the tire, the greater the power requirement to turn it, which translates into a greater engine capacity. The amount of shear forces exerted against the drivetrain components also increases with the increase in tire radius. Ultimately, the size of the tire is limited by the strength of the wheel hub and axle. Typically, the stronger the axles, the greater the weight and cost of the axle assembly. 
     Another typical solution to increasing the ground clearance of a vehicle is to utilize a portal axle. Portal axles add housings at the outside ends of the axles that have a vertical component hanging down from the conventional axle. Typically, this component contains reduction gears and corresponding stub axles such that the axle rotation is transferred to the lower stub axles to drive the wheels. However, portal axles are significantly heavier and weaker than traditional axle configurations due to the gearing and stub axle tangential forces. 
     With respect to these considerations and others, the disclosure made herein is presented. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to be used to limit the scope of the claimed subject matter. 
     Systems described herein provide for a vehicle axle configuration that allows for the vehicle axles to be angularly displaced from a differential housing to increase the ground clearance under the differential housing or to otherwise offset the wheels in any orientation relative to the differential housing. According to aspects presented herein, an axle system for a vehicle includes a central differential housing, a pair of retainers attached to the central differential housing, a pair of bell housings attached to the retainers, a pair of axle tube housings attached to the bell housings, and a pair of inner steering knuckles attached to the axle tube housings. 
     According to other aspects of the disclosure, an axle system for a vehicle includes a fixed axle housing system and a drivetrain system installed within the fixed axle housing system. The fixed axle housing system includes a central differential housing, a pair of bell housings attached to opposing sides of the central differential housing, a pair of axle tube housings attached to the bell housings, and a pair of steering knuckles attached to the axle tube housings. The drivetrain system includes a differential within the central differential housing, a pair of inner constant velocity (CV) joints attached to opposing sides of the differential and positioned within the bell housings, a pair of axle shafts attached to the inner CV joints and positioned within the axle tube housings, and a pair of outer CV joints attached to the axle shafts. These CV joints may be of any design that accomplishes the required constant rotational velocity. 
     According to yet other aspects, an axle system for a vehicle includes a fixed axle housing system and a drivetrain system installed within the fixed axle housing system. The fixed axle housing system includes a central differential housing, and from each side of the central differential housing, includes a bell housing, an axle tube housing attached to the bell housing, and a steering knuckle attached to the axle tube housing. The bell housing is clockably attached to the central differential housing via a retainer and encompasses an inner CV joint and an end of an axle shaft attached to the inner CV joint. The clockable bell housing defines a fixed drop angle corresponding to a non-zero angle from horizontal from which the axle shaft is directed from the central differential housing. The drivetrain system includes a differential positioned within the central differential housing and from each side, includes an attached inner CV joint, an axle shaft attached to the inner CV joint at the non-zero angle defined by the bell housing, and an outer CV joint attached to the axle shaft and configured to attach to a wheel spindle. 
     The features, functions, and advantages discussed herein can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating components suitable for constructing the axle systems in a drop-axle arrangement. 
         FIG. 2  is a diagram illustrating a differential housing in more detail, as viewed from the side and in isolation from other housing components shown in  FIG. 1 . 
         FIG. 3  is a diagram illustrating in more detail certain aspects of housing components and drivetrain components shown in  FIG. 1 . 
         FIG. 4  is a diagram illustrating in more detail bell housings as shown in  FIGS. 1 and 3 . 
         FIG. 5  is a diagram illustrating axle systems that may incorporate housing components that provide an alternative straight-axle configuration. 
         FIG. 6  is a diagram illustrating additional details of housing components for the alternative straight-axle configurations. 
         FIG. 7  is a diagram illustrating further examples of straight-axle configurations. 
         FIG. 8  is a diagram illustrating additional examples of straight-axle configurations. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to modular and adjustable axle systems for vehicles. This description is most readily understood with reference to the attached drawings, which include reference numbers whose first digits indicate the drawing in which the corresponding elements are first introduced. 
     In general, these modular and adjustable axle systems may be configured in a drop-axle arrangement or in a straight-axle arrangement. For conciseness of description, and to avoid duplication, some features of the axle systems may be described in connection with one configuration or the other. However, these features may also be incorporated into the other configuration, unless expressly noted to the contrary. Note that the use of CV joints is not indicated for the alternative straight axle configuration due to the horizontal alignment of the components. 
     Whether configured in the drop-axle arrangement or in the straight-axle arrangement, the axle systems as described herein may be incorporated into a variety of different vehicles. For example, these vehicles may provide all-terrain or off-road travel capabilities, but may also enhance on-road capability as well. As described in further detail below, some implementations of these axle systems may also be incorporated into vehicles that provide specific lifting and/or carrying capabilities (e.g., forklifts, or other suitable machinery operating within warehouse environments, or the like). 
       FIG. 1  illustrates components, denoted generally at  100 , suitable for constructing the axle systems in a drop-axle arrangement. More specifically,  FIG. 1  includes individual housing components, denoted collectively at  102 , as well as individual drivetrain components, denoted collectively at  104 .  FIG. 1  also illustrates an assembled axle system  106  that includes the housing components  102  and the drivetrain components  104 . The assembled axle system  106  may include wheels  108   a  and  108   b  (collectively, wheels  108 ), which are shown installed onto the ends of the axle system  106 . 
     Turning to the individual housing components  102  and drivetrain components  104  in more detail, these housing components  102  may include a central differential housing  110 . The term “central” as used in this context refers to locating the differential housing  110  somewhere between the ends of the axle system  106 . However, the differential housing  110  may or may not be in the exact center of the axle system  106 , but instead may be offset to one side or the other as appropriate or suitable in different implementations. For example, if the axle system  106  is implemented as a rear axle that is driven by a driveshaft originating at the transmission output, the differential housing  110  may be approximately in the center of the axle system  106 . However, if the axle system  106  is implemented as a front axle that is driven by a driveshaft in a four-wheel drive or all-wheel drive vehicle, the differential housing  110  may be offset to one side or the other, to align with the output of an offset transfer case. 
     Turning to the differential housing  110  in more detail, this housing may be sized as appropriate to contain a differential  112 . The differential  112  may be available commercially from a variety of different vendors, and chosen as appropriate for different applications. For example, the differential  112  may be characterized as open, locked, lockable, or limited slip. In addition, the differential  112  may be further characterized as having forward rotation or reverse rotation, depending on whether the axle system  106  is serving as a front axle or a rear axle. The differential  112  may also be characterized as a high-pinion or low-pinion differential, depending on the height of the point of origin of the input driveshaft. 
     The differential housing  110  may define an opening  114 , through which the input driveshaft may pass.  FIG. 1  omits the input driveshaft, only for clarity of illustration. The differential housing  110  may also include any flanges, seals, or other appropriate mechanisms suitable for connecting to and receiving the input driveshaft. 
     Regarding the interior of the differential housing  110 , this housing may define or include any suitable mechanisms for mounting the differential  112  within the differential housing  110 . In addition, the interior of the differential housing  110  may incorporate mechanisms for lubricating the pinion gear within the differential  112 . If the differential  112  is a low-pinion gear set, the pinion gear may be bathed in lubricant (e.g., gear oil). However, if the differential  112  is a high-pinion gear set, the pinion gear may be mounted above the ambient level of the lubricant. In these latter scenarios, the differential housing  110  may include structure for capturing the lubricant as splashed by the ring gear during its rotation. The differential housing  110  may also define oil grooves for directing the splashed lubricant so to lubricate the pinion gear. 
     The housing components  102  may also include retainers  116   a  and  116   b  (collectively, retainers  116 ). These retainers  116  may bolt or otherwise join to the sides of the differential housing  110 . Although not shown in  FIG. 1  for clarity, gaskets or other suitable structure may seal the gear oil within the differential housing, so that the gear oil does not leak at the junction of the retainers  116  and the differential housing  110 . 
     Turning to the retainers  116  in more detail, the retainers may define respective apertures  118   a  and  118   b  (collectively retainer apertures  118 ), through which shafts of inner constant-velocity (CV) joints  120   a  and  120   b  (collectively, inner CV joints  120 ) may pass. These retainer apertures  118  may incorporate seals, O-rings, or other mechanisms to prevent gear oil within the differential housing  110  from leaking through the retainers  116 . In addition, the retainers  116  may mechanically isolate or separate the inner CV joints  120  from the differential  112 , such that if the differential experiences mechanical failure, the retainers  116  may reduce the risk that fragments of the differential would reach and damage the CV joints  120 . Similarly, if the inner CV joints  120  fail mechanically, the retainers  116  may minimize the risk that fragments of the CV joints  120  may reach and damage the differential  112 . In this manner, the retainers  116  may reduce the risk that failures in the differential  112  may cause the inner CV joints  120  also to fail, and vice versa. 
     The retainers  116  may also provide a measure of structural reinforcement for the differential housing  110 , once the retainers are bolted or otherwise joined onto the differential housing  110 . As discussed throughout this description, the differential housing  110  may be generally cylindrical in nature, and thus benefits from the structural rigidity inherent in curved, arched, or circular structures. The retainers  116  may be generally circular in configuration. Thus, the inherent structural rigidity of the differential housing  110  may be further improved by attaching the retainers  116  to the open sides of the differential housing  110 . This inherent structural rigidity can also reduce or eliminate the need for additional armoring or skid plates. 
     The housing components  102  may also include bell housings  122   a  and  122   b  (collectively, bell housings  122 ). As shown in  FIG. 1 , the bell housings  122  may bolt or otherwise attach to the retainers  116 . In addition, the bell housings  122  may also define interior cavities  124   a  and  124   b  (collectively, cavities  124 ), sized as appropriate to contain the CV joints  120 , and to permit the CV joints  120  to rotate freely therein. The exact dimensions and configuration of the cavities  124  may vary in different implementations. 
     The bell housings  122  may also define receptacles  126   a  and  126   b  (collectively, receptacles  126 ) for axle housings  128   a  and  128   b  (collectively, axle housings  128 ). More specifically, the receptacles  126  may receive inner ends  130   a  and  130   b  (collectively, inner ends  130 ) of the axle housings  128 . 
     The axle housings  128  may include flanges  131   a  and  131   b  (collectively, flanges  131 ), for attaching respectively to the bell housings  122   a  and  122   b . The flanges  131  may be positioned as appropriate along the length of the axle housings, with the positions shown in  FIG. 1  chosen only for convenience of illustration. The flanges  131  may be separate parts that are welded or otherwise joined to the axle housings, or may be manufactured as integral components of the axle housings  128 . 
     In addition, the flanges  131  may cooperate with bolts or other mechanisms that are suitable for joining the flanges  131  (and thus, the axle housings  128 ) to the bell housings  122 . Thus, the flanges  131  may define apertures through which the bolts may pass, and the bell housings  122  may define threaded passageways for receiving the bolts. 
     In the drop-axle configuration shown in  FIG. 1 , the bell housings  122  may provide the overall axle system  106  with an angled configuration, such that the axle housings  128  are non-linear, or not parallel to one another. However, in the straight-axle configuration shown in other Figures, the axle housings  128  are co-linear or parallel with one another. 
     Turning to the axle housings  128  in more detail, these housings may be generally elongated and tubular in configuration. The axle housings  128  may define interior passageways  132   a  and  132   b  (collectively, interior passageways  132 ) that contain corresponding axle shafts  134   a  and  134   b  (collectively, axle shafts  134 ), with the interior passageways  132  being dimensioned so as to permit the axle shafts  134  to rotate freely therein. The axle shafts  134  may include inner ends  136   a  and  136   b  (collectively, inner ends  136 ) that are splined or otherwise adapted to engage the inner CV joints  120 . In this manner, torque passes from the differential  112 , through to the inner CV joints  120  and to the axle shafts  134 . 
     The housing components  102  may also include inner steering knuckles  138   a  and  138   b  (collectively, inner steering knuckles  138 ), which attach to outer ends  140   a  and  140   b  (collectively, outer ends  140 ) of the axle housings  128 . The inner steering knuckles  138  may be bolted or otherwise joined onto the outer ends  140 . In addition, the inner steering knuckles  138  may define apertures or passageways  142   a  and  142   b  (collectively, apertures  142 ) through which the axle shafts  134  may pass. 
     Outer CV joints  144   a  and  144   b  (collectively, outer CV joints  144 ) may receive outer ends  156   a  and  156   b  (collectively, outer ends  156 ) of the axle shafts  134 . More specifically, the outer ends  156  may be splined or otherwise adapted to engage the outer CV joints  144 , such that the axle shafts  134  supply torque to the outer CV joints  144 . 
     Outer steering knuckles  146   a  and  146   b  (collectively, outer steering knuckles  146 ) may pivotally attach to the inner steering knuckles  138 . More specifically, the outer steering knuckles  146  may pivot relative to the inner steering knuckles  138  through kingpins, ball joints, or other suitable mechanisms. 
     Although not shown in  FIG. 1  in the interests of clarity, the outer steering knuckles  146  may include tabs or other attachment points for steering linkage or mechanisms, or scenarios in which the axle system  106  is a steerable axle. However, in cases where the axle system  106  is a non-steerable axle, the outer steering knuckles  146  may be in a fixed relationship to the inner steering knuckles  138 . For example, in the latter scenario, if the outer steering knuckles  146  include tabs for attaching the steering linkage, the same tabs could provide an attachment point for linkage that locks the outer steering knuckles  146  in fixed relationship with the inner steering knuckles  138 . 
     The outer steering knuckles  146  may define passageways or apertures  148   a  and  148   b  (collectively, passageways  148 ) through which shafts of the outer CV joints  144  may pass. In addition, wheel spindles  150   a  and  150   b  (collectively, wheel spindles  150 ) may ride within the passageways  148 , and engage the shafts of the outer CV joints  144 . As described above with other elements of the drivetrain components  104 , the shafts of the outer CV joints  144  may be splined or otherwise adapted to transmit torque to the wheel spindles  150 . In turn, the wheels  108  may bolt or otherwise attach to the wheel spindles  150 . 
     Although not shown explicitly in  FIG. 1 , the axle systems  106  may include any suitable wheel bearings, adapted to facilitate the rotation of the wheel spindles  150  and/or axle shafts  134  as the vehicle travels. The retainers  116  may also include passageways or apertures, which allow lubricants to flow to and from a reservoir in the differential housing  110 , so as to lubricate these wheel bearings. 
     Having described the various housing components  102  and drivetrain components  104 , several observations are noted. Turning first to the housing components  102 : the retainers  116 , the bell housings  122 , the axle housings  128 , and the inner steering knuckles  138  may be symmetrical, and may be interchangeable across opposite sides of a given axle system  106 . Because of these different symmetrical and interchangeable parts, the design of the axle system  106  may minimize the number of spare parts involved in repairing the axle system. In a more specific scenario, if the axle system  106  is incorporated into an all-terrain or off-road vehicle, and the operator of this vehicle often ventures far off-road and away from repair facilities, such operators may wish to transport as few spare parts as possible in order to repair and maintain the axle system  106  while in remote locations. Thus, these operators may carry along as spare parts: one retainer  116 , one bell housing  122 , one axle housing  128 , and/or one inner steering knuckle  138 . With these spare parts, the operator could replace either one of the retainers  116   a  or  116   b , the bell housings  122   a  or  122   b , the axle housings  128   a  or  128   b , or the inner steering knuckles  138   a  or  138   b.    
     Similar considerations apply to the drivetrain components  104 . For example, the CV joints  120  and  144  may be all interchangeable, so that the operator may repair any of the CV joints  120   a ,  120   b ,  144   a , or  144   b  with a given spare CV joint. In addition, the axle shafts  134  may be interchangeable, assuming that the differential housing  110  is located in the center of the axle system  106  and that the axle shafts  134  thus have equal lengths. Similar description applies to the axle housings  128 , which may or may not be interchangeable side-to-side on a given axle, depending on their lengths relative to one another. 
     In addition, the housing components  102  include a relatively small number of sub-components that are interchangeable with one another. These sub-components may be connected to one another with common interchangeable hardware, such that an operator may repair and replace any of these components with a relatively small number of common hand tools (e.g., wrenches, socket sets, and the like). For example, the housing components  102  may be joined to one another by bolts or other fasteners of the same size and that are operable using the same basic tools. 
     The foregoing characteristics of the axle systems may further simplify repair of the axle systems  106  in remote locations. In an example scenario, all sub-components of the axle systems  106  may be assembled and/or disassembled using only one size of wrench or socket. In general, the various housing and drivetrain components are bolted together so as to be readily repairable in the field, as compared to being welded or press-fit together. 
     In the fully-assembled axle systems  106 , the drivetrain components  104  are incorporated into the housing components  102 . More specifically, when the housing components  102  are assembled with one another, the housing components  102  as a whole provide a rigid structure for containing the various drivetrain components  104 . Accordingly, the housing components  102  may maintain the various drivetrain components  104  in a captive relationship with one another. For example, turning to the CV joints  120  and  144 , the housing components  102  maintain these CV joints in a fixed angular relationship to one another. If the CV joints  120  and  144  are fixed at a given angle, and do not flex through their full range of motion, the CV joints  120  and  144  may be able to transmit increased torque with greater reliability. Put differently, fixing the CV joints  120  and  144  in a fixed angular relationship may extend the operational lifetimes of these CV joints, while also providing greater torque-handling capability. 
     Turning to the differential housing  110 , it may contain different types of differentials  112 . In addition, the differential housing  110  may be interchangeable between different axle systems  106  that serve as front or rear axles within a given vehicle. Thus, a given differential housing  110  that is carried as a spare part may replace a front or rear differential housing that is incorporated into a front or a rear axle system  106 . 
     Referring to the axle system  106  in the drop-axle configuration shown in  FIG. 1 , this axle system may provide increased ground clearance, as compared to a straight-axle system.  FIG. 1  represents this increased ground clearance generally by the arrow  152 . The exact amount of this ground clearance may depend upon the angular relationship between the axle housings  128   a  and  128   b . Generally, as the angle between the axle housings  128  increases, the ground clearance decreases, and as the angle between the axle housings  128  decreases, the ground clearance increases. Specific configurations of the housing components  102  may be chosen as appropriate for different implementations. 
     Turning now to  FIG. 2 , this Figure illustrates the differential housing  110  in more detail, as viewed from the side and in isolation from the rest of the housing components  102 . As shown in  FIG. 2 , the differential housing  110  may define an interior area  202  for containing the differential  112 . In addition, the differential housing  110  may define an aperture or passageway  204 , through which one end of a driveshaft  206  may pass to interact with the differential  110 , as shown generally at  208 . 
     Viewing the differential housing  110  from the side as shown in  FIG. 2 , the differential housing  110  may be generally cylindrical in configuration, and circular in cross-section. In addition, as detailed further below, the exterior of the differential housing may be generally smooth in character, as distinguished from previous differential housings that may include ribbing or other mechanical reinforcement along their exteriors. 
     When the differential housings  110  are incorporated into vehicles, these housings may travel relative to the ground in either direction, as represented by the arrow  210 . The exact direction of travel may depend on whether the differential housing is incorporated into the front or the rear axle of the vehicle, as well as which direction the vehicle travels primarily. However, referring to the cylindrical configuration of the differential housing  110 , it is noted that the axis of this cylinder (denoted generally by the point  212  in  FIG. 2 , and the line  154  in  FIG. 1 ) is orthogonal to the primary line of travel  210 . Thus, as the differential housing  110  travels over terrain, a smooth bottom portion  214  of the differential housing is more able to traverse rough terrain features with less risk of snagging or experiencing damage. 
     Typically, the bottom portion  214  does not include a flange for mounting a differential cover. Previous differential housings may incorporate exterior ribbed features for structural reinforcement, and may also include mounting flanges for gaskets and differential covers, and the like. In such previous differential housings, these flanges and differential covers are generally perpendicular to the line of travel, and may sometimes snag on protruding terrain features, potentially damaging these previous housings. However, the relatively smooth profile of the bottom portion  214  may reduce the risk of snagging or damaging the overall housing  110 , as compared to previous differential housings. 
     Referring once again to the generally circular cross-section of the differential housing  110 , this housing  110  provides a substantially continuous structure around the circumference of this circular cross-section (aside from a relatively small aperture through which the driveshaft  206  passes). In contrast, previous differential housings typically incorporate a removable differential access plate covering the back of the differential housing. Thus, these previous differential housings typically define a relatively large cut-out for accessing and servicing these previous differentials, with the access plate covering this cut-out. Typically, this cut-out is defined on the side of the differential housing, opposite the side where the input driveshaft enters the differential. However, this relatively large cut-out weakens the overall structure of these previous differential housings. Previous differential housings are often referred to as “pumpkins”, due to their visual resemblance to pumpkins. However, like pumpkins, these differential housings may be weakened considerably by removing a substantial portion of their outer shell. This provides at least one reason why some previous differential housings have incorporated ribbed reinforcements along their exteriors. 
     In contrast to previous “pumpkin” differential housings, the cylindrical or circular structure of the differential housings  110  is substantially continuous and uninterrupted, thereby resulting in a stronger overall structure owing to the inherent strength of substantially uninterrupted curved structures. Therefore, the stronger overall structure enables the differential housings  110  to dispense with the exterior ribbing or reinforcements typical of previous differential housings, resulting in a relatively smooth external profile shown in  FIG. 2 . Thus, the differential housings  110  may provide additional ground clearance, as compared to previous differential housings that incorporate exterior ribbing and exterior flanges. In addition, while the differential housings  110  and the retainers  116  may be joined by seams, the seams would generally run parallel to the line of travel  210 , and would be less likely to snag on terrain features and result in damage. 
     As described previously, the driveshaft  206  transmits incoming torque to the differential  112 , as disposed within the differential housing  110 . In some scenarios, this incoming torque may result in a torque vector, which is represented by the arrow  216 . When the differential is under a torque load, the differential may be subject to shifting somewhat, for example, in response to the torque vector  216 . In addition, the differential housing may flex in response to this torque vector. Access covers or plates as provided by previous differential housings may incorporate blocks or other structure for contacting the differential. This structure may counteract the torque vector by supporting the differential from the “back” of the differential housing (i.e., that side of the housing opposite where the drive shaft enters the housing). However, referring to the differential housing  110  as shown in  FIG. 2 , the inherent strength and rigidity in the substantially uninterrupted and continuous cylindrical housing  110  is better able to resist the torque vector. In addition, because the differential housing  110  is not weakened by a substantial cut-out for the access plate, the differential housing  110  is better able to supprt better to support the differential  112  by relying on the inherent strength of the circular cross-section of the housing  110 . 
       FIG. 3  illustrates in more detail certain aspects of the housing components  102  and the drivetrain components  104  from  FIG. 1 . For ease of reference, but not to limit possible implementations,  FIG. 3  carries forward the differential housing  110 , the differential  112 , the aperture  114  for receiving the input driveshaft (not shown), the retainer  116   b , the bell housing  122   b , the CV joint  120   b , the axle housing  128   b , the axle shaft  134   b , and the inner steering knuckle  138   b.    
     Turning to these housing components and drivetrain components in more detail, the retainer  116   b  (as well as other retainers  116 ) may define an aperture  302  through which a shaft  304  provided by the CV joint  120   b  may pass. In addition, the retainer  116   b  may define threaded holes  306   a  and  306   b  (collectively, threaded holes  306 ), which receive bolts  308   a  and  308   b  (collectively, bolts  308 ) for attaching the bell housing  122  to the retainer  116   b . As shown in  FIG. 3 , the bell housing may include a flange  309 , through which the bolts  308  may pass. 
     Turning to the bell housing  122   b  in more detail, as described above, the bell housing may define a passageway  302  through which the end of the axle shaft  134  may pass, to engage the CV joint  120   b . In addition, the bell housing  122   b  may define threaded holes  310   a  and  310   b  (collectively, threaded holes  310 ), which may receive bolts  312   a  and  312   b  (collectively, bolts  312 ). The bolts  312  may secure the axle housing  128   b  to the bell housing  122   b , with the bolts  312  passing through a flange  314  (which represents the flanges  131  shown in more detail) that is provided by the axle housing  128   b.    
       FIG. 4  illustrates in more detail the bell housings  122  as shown in  FIGS. 1 and 3 . As shown in  FIG. 4 , the bell housings  122  may be generally circular in shape, with the flange  309  providing a mounting surface by which the bell housings  122  may be secured to the retainers  116 , using bolts  308 . The flange  309  may define any number of slotted apertures  402   a  and  402   n  (collectively, slotted apertures  402 ), which may receive bolts  308 . 
     The slotted apertures  402  provide for rotational adjustment between the bell housings  122  and the retainers  116  (as attached to the differential housing  110 ). This rotational adjustment can serve several different functions. In a first scenario, referring briefly back to  FIGS. 1 and 3 , the bell housings  122 , the axle housings  128 , the inner knuckles  138  and the outer knuckles  146  may be assembled together and considered as one integral sub-assembly. As described previously, the outer knuckles  146  may pivot relative to the inner knuckles  138 , along an axis. In implementations where a kingpin joins the inner and outer knuckles, the kingpin would lie along this pivot axis. As also described above, the wheels  108  are attached to the wheel spindles  150 , which are joined in rotating relation to the outer knuckles  148 . The caster of the wheels  108  refers to the angle between this pivot axis and the vertical. The slotted apertures  402  enable adjustment of the caster of the wheels  108 , by allowing the bell housings  122  to rotate relative to the retainers  116 . 
     In another scenario, the slotted apertures  402  may enable rotational alignment of the retainers  116  and the differential housing  110  (considered as one sub-assembly), relative to the bell housings  122 . Referring briefly back to  FIG. 2 , the driveshaft  206  may enter the differential housing  110  at a given angle. This angle may be chosen or specified as appropriate in different applications, so as to minimize wear on any joints in the driveshaft  206 , to minimize vibrations within the driveshaft  206 , and/or to minimize stress placed upon the pinion gear within the differential  112 . The axle systems  106  as described herein enable adjustment of this driveshaft angle, by enabling the differential housing  110  to be clocked, relative to the bell housings  122 . The slotted apertures  402  and the bell housings  122  enable this clocking or angular adjustment of the differential housing  110 . 
     In still other scenarios, the slotted apertures  402  may enable two degrees of adjustment within the axle systems  106 . First, a specified angle of caster may be achieved by rotating the bell housings  122  relative to the retainer/differential housing sub-assembly  116 / 110 . In addition, a specified driveshaft angle may be achieved by further rotating the retainer/differential housing sub-assembly  116 / 110  relative to the bell housings  122 . Once these adjustments are complete, the bolts  308  may be tightened to an appropriate torque specification, to complete assembly of the axle systems  106 . It is noted that the wheel caster and the driveshaft angle may be adjusted individually or together, as appropriate in different implementation scenarios. 
     While  FIG. 4  illustrates the slotted apertures  402  as defined by the bell housings  122 , the flanges  314  and  131  may also define similar apertures. These additional apertures as defined by the flanges  314  and  131  may provide additional degrees of adjustment. 
       FIG. 5  illustrates axle systems, denoted generally at  500 , that may incorporate housing components  502  to provide a straight-axle configuration. For ease of description, but without limiting possible implementations, the differential housing  110 , the retainers  116   a  and  116   b , the axle housings  128   a  and  128   b , the inner knuckles  138   a  and  138   b , the outer knuckles  146   a  and  146   b , and the wheel spindles  150  are carried forward into  FIG. 5  from previous Figures. In general, the previous descriptions of these components apply equally to  FIG. 5 . 
     The housing components  502  may include the differential housing  110 , the retainers  116 , and the axle housings  128 . In addition, bell housings  504   a  and  504   b  (collectively, bell housings  504 ) provide the axle systems  500  with the straight-axle configuration, as compared to the drop-axle configurations discussed above with  FIGS. 1-4 . In all other respects, the above descriptions of the bell housings  122  apply equally to the bell housings  504  as shown in  FIG. 5 . 
       FIG. 6  illustrates additional details of some of the housing components  502  as shown in  FIG. 5 . For ease of reference, but not to limit possible implementations, the differential housing  110 , the retainer  116   b , the bell housing  504   b , the axle housing  128   b , the inner knuckle  138   b , the outer knuckle  146   b , and the wheel spindle  150   b  are carried forward into  FIG. 6 .  FIG. 6  also illustrates an example steering pivot axis at  602 . 
     As shown in  FIG. 6 , the bell housings  504   b  (and more generally, any bell housings  504 ) may include a flange  604  around the circumference of the bell housings  504 . This flange  604  may be similar to the flange  309  shown in  FIGS. 3 and 4 . The flange  604  may also define a plurality of slotted apertures (not shown in  FIGS. 5 and 6 ) that are similar to those shown in  FIG. 4  at  402   a  and  402   n . As discussed above in  FIG. 4  with the drop-axle configurations, these slotted apertures  402  may facilitate adjustment of wheel caster and/or adjustment of driveshaft entry angles. In the straight-axle configurations shown in  FIG. 6 , slotted apertures defined by the flange  604  may facilitate similar adjustments. 
     Previous axle housings typically included fixed spring perches, for attaching suitable spring mechanisms to the axles. If these previous axle housings are not rotatable, such fixed spring perches may be suitable. However, the axle housings  128  as described herein may be rotatable, as facilitated by the slotted apertures  402 . As shown in  FIG. 6 , the axle systems as described herein provide spring perches that may accommodate these rotatable axle housings  128 . 
     Turning more specifically to the axle housing  128   b  as shown in  FIG. 6 , the exterior of the axle housing may define or include convex splines  606 . The axle systems may also include spring perch components  608   a  and  608   b  (collectively, spring perch components or spring perches  608 ). The spring perch components  608   a  may provide a top or upper block, while the spring perch components  608   b  may provide a bottom or lower block. Inner surfaces of these spring perch components  608  may define or include respective concave splines  610   a  and  610   b  (collectively, concave splines  610 ) that correspond with and engage the convex splines  606 . At  609   a  and  609   b ,  FIG. 6  illustrates the spring perches  608  as oriented for placement on the axle housing  128   b , so as to engage the convex splined surface  606 . However, to illustrate the concave splines  610  more clearly,  FIG. 6  also includes views  611   a  and  611   b , which illustrate the spring perches  608  rotated 90°, so as to present the concave splines  610 . 
     In example implementations, the individual convex slines  606  and concave splines  610  may occur in 1° increments, such that the spring perches  608  may be adjusted in 1° increments around the axle housing  128 . In providing this example, however, it is noted that the splines may be implemented in any suitable pitch, without departing from the scope and spirit of the present description. 
     In an operational scenario, once the bell housing  504   b , the axle housing  128   b , the inner knuckle  138   b , and the outer knuckle  146   b  are assembled together, they may be considered as a consolidated sub-assembly. Once this sub-assembly is oriented or rotated into proper alignment with the retainer  116   b  and the differential housing  110 , the spring perches  608  may be mated onto the appropriate splines  606  on the axle housing  128   b . In turn, suitable spring systems may be fitted onto the perches  608 , with the springs then being secured to the axle housing  128   b  using any appropriate mechanism (e.g., U-bolts, or the like). 
     In this manner, the splined axle housing  128   b  as shown in  FIG. 6  may be attached to suitable spring mechanisms, regardless of how the axle housing  128   b  is rotated or aligned relative to the retainer  116   b  and the differential housing  110 . It is noted that the splined axle housing may be incorporated into the straight-axle configuration, as well as the drop-axle configuration, even though  FIG. 6  illustrates a straight-axle configuration only for example. 
     For clarity of illustration,  FIG. 6  does not illustrate any particular spring system. However, it is noted that the various axle configurations described herein (whether characterized as straight-axle or drop-axle configurations) may cooperate with any type of spring system. Examples of such spring systems may include, but are not limited to, leaf springs, coil springs, torsion springs, or any other suitable type of spring system, chosen as appropriate for different applications. Accordingly, the upper or top blocks  608   a , as well as the lower or bottom blocks  608   b  in some cases, may be adapted as appropriate to accommodate different types of spring systems. For example, the spring perches  608  may include posts  612   a  and  612   b  (collectively, posts  612 ) or other structure that is suitable for engaging the spring systems. Without limiting possible implementations, the posts  612  may engage apertures provided by leaf springs. 
       FIG. 7  illustrates additional examples of straight-axle configurations, denoted generally at  700 . For ease of reference, but not to limit possible implementations,  FIG. 7  carries forward the differential housing  110 , as well as the inner CV joints  120   a  and  120   b , the axle shafts  134   a  and  134   b , the outer CV joints  144   a  and  144   b , the wheel spindles  150   a  and  150   b , and the outer knuckles  146   a  and  146   b.    
     In the straight-axle configurations  700 , the axle housings  132  and the inner knuckles  138  that were shown in previous drawings are integrated into combined axle housing and inner knuckle components.  FIG. 7  provides examples of such combined housing/knuckle components at  702   a  and  702   b  (collectively, combined housing/knuckle components  702 ). 
     In addition, the straight-axle configuration  700  may also integrate or combine the retainers  116  and the bell housings  504  into combined retainer/bell housings  704   a  and  704   b  (collectively, combined retainer/bell housings  704 ). These combined retainer/bell housings  704  may also include flanges  706   a  and  706   b  (collectively, flanges  706 ). The flanges  706  may define slotted apertures (not shown in  FIG. 7 ), which provide rotation and alignment capabilities similar to those capabilities discussed above with the slotted apertures  402  shown in  FIG. 4 . 
     In some implementations of the straight-axle configurations  700 , the inner CV joints  120  may serve as a type of mechanical fuse. More specifically, for a variety of reasons, the differential  112  may experience severe torque loads or overloads. Without the inner CV joints  120  in place, the severe torque load may be transmitted directly to the axle shafts  134   a  and  134   b , potentially breaking axle shafts  134 . In typical scenarios, the axle shafts  134  are more expensive and more difficult to replace than the inner CV joints  120 . Therefore, if the inner CV joints  120  are rated to fail at a torque rating that is lower than what would damage the axle shafts  134 , then the inner CV joints  120  would fail before the axle shafts  134  are damaged. Once the inner CV joints  120  fail, any torque overload would then dissipate. 
     If the inner CV joints  120  serve as mechanical fuses, and fail due a torque overload, these failed CV joints  120  may be replaced by removing the combined retainer/bell housings  704  from the differential housing  110 , replacing any broken CV joints  120 , and reinstalling the retainer/bell housings  704  onto the differential housing  110 . The mechanical-fuse capabilities provided by the straight-axle configurations  700  may be particularly suitable when the axle shafts  134   a  and  134   b  have unequal lengths and are thus not interchangeable with one another. In these scenarios, operators may replace any failed CV joints  120  relatively easily, as compared to replacing the axle shafts  134 , allowing the operators to carry extra CV joints  120  as spare parts, rather than carrying extra axle shafts  134  (and possible axle shafts having different lengths). 
     As shown in  FIG. 7 , the straight-axle configurations  700  may include four CV joints  120  and  144  in total. In addition to the mechanical fuse capabilities described above, the straight-axle configurations  700  may also be suitable when a given vehicle is equipped with uniform front and rear axles. In such vehicles, for example, both axles may be steerable, or at least may be made steerable. Accordingly, the straight-axle configurations  700  may include the outer CV joints  144  if the axle is steerable. In these scenarios, parts may be swapped from front to rear axles, to repair either of the axles as appropriate. As described above, a given axle may be converted selectively to non-steerable mode by locking the outer knuckles  146  to the inner knuckles  138  or  702 . 
       FIG. 8  illustrates additional examples of straight-axle configurations, denoted generally at  800 . For ease of reference, but not to limit possible implementations,  FIG. 8  carries forward the differential housing  110 , as well as the differential  112 , the combined bell housing and retainers  704   a  and  704   b , the axle shafts  134   a  and  134   b , the combined inner knuckles and axle housings  702   a  and  702   b , the outer CV joints  144   a  and  144   b , the outer knuckles  146   a  and  146   b , and the wheel spindles  150   a  and  150   b.    
     The straight-axle configurations  800  omit the inner CV joints  120   a  and  120   b  shown in  FIG. 7 , trading-off the mechanical fuse capabilities of these inner CV joints in favor of reduced part count and cost. In the examples shown in  FIG. 8 , the axle shafts  134  engage directly with the differential  112 . 
     As described above with  FIG. 1 , the retainers  116   a  and  116   b  may separate the differential  112  from the inner CV joints  120   a  and  120   b . In this manner, the retainers  116  may reduce the risk of fragments from a failed differential  112  contaminating the inner CV joints  120 , and vice versa. However, in the scenarios shown in  FIG. 8 , there are no inner CV joints  120  to be contaminated by fragments from a failed differential  112 . Accordingly, the retainers  116  may be integrated into the bell housings  122 , to provide the integrated retainers/bell housings  704 . 
     In implementations in which the straight-axle configurations  800  are steerable, or may be made steerable, these configurations  800  may include the outer CV joints  144 . However, in cases where steering capability is not desired, the straight-axle configurations  800  may omit the outer CV joints  144 , engaging the axle shafts  134  directly with the wheel spindles  150 . 
     Although some of the above description relates to axle systems incorporated into on-road or off-road vehicles, these axle systems may also be incorporated into other vehicles, without departing from the scope and spirit of this description. Examples of such other vehicles may include vehicles that are designed specifically to perform certain roles (e.g., lifting and moving heavy payloads, or the like). More specific examples may include forklifts or other similar vehicles. 
     When designing such specialized, weight-lifting vehicles, one design factor often considered is moving the front wheels of such vehicles forward within the vehicle, so as to reduce the amount of counterweight provided at the rear of the vehicles. By reducing the amount of counterweight, the cargo-moving capacity of the vehicle may be increased, and the fuel consumption of the vehicle may be decreased. The drop-axle configuration may be particularly suitable for some of these other types of vehicles. 
     As a more concrete example of applying portions of this description to such weight-moving vehicles, the discussion returns to the drop-axle configuration as shown in  FIG. 1 . This Figure illustrates how the drop-axle configuration may provide increased ground clearance, as represented at  152 . The drop-axle configuration achieves this increased ground clearance by dropping the wheels  108  “downwards”, thereby raising the differential housing  110  “upwards” away from the ground and providing the increased ground clearance. However, if the wheels  108  are moved “forwards” rather than “downwards”, the drop-axle configuration may provide a wheels-forward arrangement suitable for forklifts or other types of specialized, weight-lifting vehicles. 
     This wheels-forward arrangement may be achieved by rotating the bell housings  122  relative to the differential housing  110 , so that the axle shafts  134  and axle housings  128  are generally horizontal. This arrangement of the vehicle would place the wheels  108  ahead of the differential housing  110  and differential  112 . In addition, a lift mechanism provided by the vehicle may be located between the differential housing  110  and the wheels  108 , further shifting the vehicle&#39;s center of mass toward the rear of the vehicle, and possibly allowing further reduction of the counterweight at the rear. 
     As described above, the CV joints as incorporated into the various axle systems provided in this description may be “captive”, in the sense that these CV joints do not flex throughout their entire range of motion, but are instead locked in one angular configuration. As also described above, such captive CV joints may offer longer life and increased torque-handling capacity. In the wheels-forward arrangement, these captive CV joints may increase the load carrying capacity provided by these weight-lifting vehicles. 
     Throughout this discussion, this description refers to “CV joints” as a general term of reference, but use of this term does not limit possible implementations of this description. In some implementation scenarios, universal joints (i.e., U-joints) may be suitable. Turning to CV joints more specifically, a variety of different CV joints may be suitable, according to the circumstances of different particular applications. Suitable CV joints may be available from a variety of different vendors. Examples of such CV joints may include, but are not limited to, Thompson couplings, Rzeppa CV joints, Tripod CV joints, Double Cardan CV joints and the like. 
     It is noted that the various drawing Figures provided with this description are not drawn to scale. Instead, these Figures illustrate various features only for the purposes of facilitating this description. Thus, it is noted that items shown in these various Figures are not drawn to show exact proportions, sizes, or scale. 
     The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described.