Patent Publication Number: US-2016244112-A1

Title: Axle lift system and method of control

Description:
TECHNICAL FIELD 
     This patent application relates to an axle lift system and a method of control. 
     BACKGROUND 
     An axle lift assembly is disclosed in U.S. Pat. No. 8,695,998. 
     SUMMARY 
     In at least one embodiment, a method of controlling an axle lift system for a vehicle is provided. A first axle assembly may have a first tire and may be coupled to a chassis via a first suspension unit. A second axle assembly may have a second tire and may be coupled to the chassis via a second suspension unit. The first suspension unit and the second suspension unit may be refracted to lower the chassis. The second axle assembly may be held in a secured position with an axle securing unit. The first suspension unit may be extended to raise the chassis such that the second tire does not support the vehicle. 
     In at least one embodiment, a method of controlling an axle lift system for a vehicle is provided. The method may include retracting the first suspension unit to lower the chassis, releasing the axle securing unit from a second axle assembly, and extending first and second suspension units to raise the chassis such that first and second tires provided with the first and second axle assemblies, respectively, support the vehicle. 
     In at least one embodiment, an axle lift system for a vehicle is provided. The axle lift system may include a first axle assembly, a second axle assembly, and an axle securing unit. The first axle assembly may have a first tire and may be configured to be coupled to a chassis with a first suspension unit. The second axle assembly may have a second tire and may be configured to be coupled to the chassis with a second suspension unit. The axle securing unit may be disposed proximate the chassis and the second axle assembly. The axle securing unit may be actuated when the first suspension unit and the second suspension unit are retracted to lower the chassis, and the first tire and the second tire support the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an exemplary vehicle having an axle lift system. 
         FIG. 2  is a perspective view of an axle securing unit that may be provided with the axle lift system. 
         FIGS. 3 and 4  are flowcharts associated with a method of controlling an axle lift system. 
         FIGS. 5-8  illustrate operation of the axle lift system. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to  FIG. 1 , an exemplary vehicle  10  is shown. The vehicle  10  may be a motor vehicle that may be used to transport cargo. For example, the vehicle  10  may be configured as a truck and may include a tractor  12  and/or a trailer  14 . The tractor  12  may receive a driver and may propel and steer the vehicle  10 . The trailer  14  may be coupled to the tractor  12  and may be configured to receive cargo. The vehicle  10  may also have an axle lift system  16  and a control system  18 . 
     The tractor  12  and the trailer  14  may each include a frame or chassis  20 . For clarity in the text below, a common reference number is used to designate the chassis  20  of the tractor  12  and the chassis  20  of the trailer  14  in the figures, although the chassis  20  of the tractor  12  may be separate from the chassis  20  of the trailer  14 . 
     The tractor  12  and the trailer  14  may include one or more axle assemblies. For example, the tractor  12  and/or the trailer  14  may include at least one non-liftable axle assembly  30  and a liftable axle assembly  32 . The non-liftable and liftable axle assemblies  30 ,  32  may be disposed proximate or may be mounted to the chassis  20 . 
     The non-liftable and liftable axle assemblies  30 ,  32  may each rotatably support one or more wheel assemblies  34  that may include a tire  36  that may be mounted on a wheel  38 . A non-liftable axle assembly  30  may be configured such that its associated wheel assemblies  34  may be disposed on a support surface  40 , like a road or the ground, during normal operation to support the vehicle  10  and facilitate movement of the vehicle  10 . A non-liftable axle assembly  30  may not be held in a lifted position or a secured position by the axle lift system  16  such that its associated wheel assemblies  34  are raised or lifted from the support surface  40  toward the chassis  20  and held in the secured position such that an associated wheel assembly  34  is lifted above and does not engage the support surface  40  or support the weight of the vehicle  10 . A liftable axle assembly  32  may be selectively held in a secured position by the axle lift system  16  such that its associated wheel assemblies  34  are raised or lifted from the support surface  40  toward the chassis  20  and do not engage the support surface  40  to support the weight of the vehicle  10 . As such, one or more non-liftable axle assemblies  30  or axle assemblies that are not held in the secured position may support the vehicle  10  when a liftable axle assembly  32  is held in the secured position. The wheel assemblies  34  of a liftable axle assembly  32  may be disposed on the support surface  40  when they are not held in a secured position by the axle lift system  16  to support the vehicle  10  and facilitate movement of the vehicle  10 . 
     The non-liftable axle assemblies  30  and liftable axle assemblies  32  may be provided in various quantities and locations. In  FIG. 1 , one non-liftable axle assembly  30  and one liftable axle assembly  32  are provided with the tractor  12  and the trailer  14 , although it is contemplated that a greater number of non-liftable axle assemblies  30  and/or liftable axle assemblies  32  may be provided. In  FIG. 1 , a non-liftable axle assembly  30  is located to the right of each liftable axle assembly  32  from the perspective shown; however, it is contemplated that this positioning may be reversed in one or more embodiments. In addition, it is contemplated that the either the tractor  12  or the trailer  14  may not be provided with a liftable axle assembly  32  in one or more embodiments. 
     The non-liftable axle assemblies  30  and liftable axle assemblies  32  may be provided in different configurations. For example, a non-liftable axle assembly  30  or a liftable axle assembly  32  may or may not be configured to steer the vehicle  10 . In addition, a non-liftable axle assembly  30  or a liftable axle assembly  32  and may or may not be configured as a drive axle that may provide torque to at least one wheel assembly  34  that may propel the vehicle  10 . 
     Referring to  FIG. 5 , an exemplary pair of non-liftable and liftable axle assemblies  30 ,  32  is shown in more detail. The non-liftable and liftable axle assemblies  30 ,  32  may include a housing  42  that may rotatably support one or more wheel assemblies  34 . In the case of a drive axle, the housing  42  may receive a differential and an axle that may provide torque to one or more wheel assemblies  34 . The non-liftable and/or liftable axle assemblies  30 ,  32  may also include multiple axles that may each support corresponding vehicle wheels. An example of such an axle assembly is the Meritor RideSentry™ MPA Series Trailer Air Suspension. Such an axle assembly may have two or more axles or axle assemblies that may be mounted on a common subframe, which may be referred to as a slider or slider assembly. The slider assembly may be mounted to the chassis  20  such that the slider assembly and move longitudinally with respect to the chassis  20  (e.g., back and forth in a direction extending from the front of the vehicle to the back of the vehicle). The axle assemblies will be described below primarily with reference to non-liftable and liftable axle assemblies that have a single axle; however, it is to be understood that multiple axle assemblies may be associated with a non-liftable axle assembly or a liftable axle assembly in one or more embodiments. As such, the axle lift system  16  may hold a liftable axle assembly that may include multiple axles that may each support different sets of wheel assemblies  34  in a secured position in which the wheel assemblies  34  are raised or lifted from the support surface  40  toward the chassis  20  and do not engage the support surface  40  to support the weight of the vehicle  10 . 
     The non-liftable and liftable axle assemblies  30 ,  32  may each be coupled to or connected to the chassis  20  with an axle suspension system or suspension unit  50 . The suspension unit  50  and may dampen vibrations, provide a desired level of ride quality, and control ride height or the distance between the chassis  20  and the support surface  40 . In addition, the suspension unit  50  may be part of the axle lift system  16  or may cooperate with the axle lift system  16  to raise and lower the chassis  20  with respect to the support surface  40  so that a liftable axle assembly  32  may be secured or released. The suspension unit  50  may be configured as an air suspension unit or an air ride suspension unit that may employ air springs or air bellows that receive a pressurized gas as will be discussed in more detail below. As such, the suspension unit  50  may raise or lower the chassis  20  by inflating or deflating one or more air springs. The suspension unit  50  may be provided in various configurations. For example, the suspension unit  50  may include a suspension arm  52 , a hanger  54 , a shock absorber  56 , and an air spring  58 . 
     The suspension arm  52  may extend from the axle assembly  30 ,  32  and may be fixedly positioned with respect to the axle assembly  30 ,  32 . For example, at least one suspension arm  52  may be fixedly positioned on or with respect to the housing  42 . It is contemplated that the suspension unit  50  may be provided without a suspension arm in one or more embodiments. 
     The hanger  54  may be fixedly disposed or fixedly positioned with respect to the chassis  20 . For instance, a top surface of the hanger  54  may be mounted to the chassis  20 . The suspension arm  52  may be pivotally coupled to the hanger  54 . For example, the suspension arm  52  may be pivotally coupled to the hanger  54  with a pivot pin  60  that may extend through the hanger  54  and the suspension arm  52 . As such, the suspension arm  52  may pivot about the pivot pin  60  and may pivot with respect to the hanger  54 . 
     The shock absorber  56  may be provided to dampen shock impulses and dissipate kinetic energy. The shock absorber  56  may be mounted to the hanger  54  at a first end and to the suspension arm  52  at a second end. 
     An air spring  58  may be disposed proximate the suspension arm  52 . For example, the air spring  58  may be disposed proximate an end of the suspension arm  52  that may be disposed opposite the hanger  54 . The air spring  58  may support the chassis  20 . For instance, the air spring  58  may be disposed above an associated axle assembly  30 ,  32  and under the chassis  20 . The air spring  58  may have a flexible bellows  62  that may at least partially define a chamber within the air spring  58  that may receive pressurized gas from a pressurized gas supply system  70 . Providing pressurized gas to one or more air springs  58  may extend a corresponding suspension unit  50  or move the suspension unit  50  toward an extended position and may raise or lift the chassis  20  away from the support surface  40 . Venting pressurized gas from one or more air springs  58  may retract a corresponding suspension unit  50  or move the suspension unit  50  toward a retracted position and may lower the chassis  20  toward the support surface  40 . 
     Referring to  FIG. 1 , the pressurized gas supply system  70  may provide a pressurized gas or pressurized gas mixture, such as air, to the air spring  58 . The term pressurized gas is used to generically reference a single gas or a gas mixture, such as air, that may be pressurized above atmospheric pressure by the pressurized gas supply system  70 . As is best shown in  FIG. 1 , the pressurized gas supply system  70  may include a pressurized gas source  72 , one or more conduits  74 , and one or more valves  76 . 
     The pressurized gas source  72  may include a tank or reservoir that contains a volume of pressurized gas and/or a pump or compressor that provides pressurized gas. 
     A conduit  74  may fluidly connect the pressurized gas source  72  to an air spring  58 . A conduit  74  may have any suitable configuration, such as a hose, tubing, pipe, or combinations thereof. 
     One or more valves  76  may be provided to control the flow of pressurized gas to and/or from an air spring  58 . For example, at least one valve  76  may enable or disable the flow of pressurized gas from the pressurized gas source  72  to at least one air spring  58 . The valve  76  may have any suitable configuration and may be actuated in any suitable manner, such as with a solenoid. In  FIG. 1 , a single valve  76  is associated with the air springs  58  of each axle assembly  30 ,  32 ; however, a different number of valves  76  or different valve configuration may be employed. For instance, a valve  76  may be associated with each individual air spring  58  or a single valve  76  may be associated with multiple axle assemblies  30 ,  32 . In addition, it is contemplated that a common valve  76  may provide inflation and venting functionality or that separate valves  76  may be provided to control inflation and venting of an air spring  58 . An air spring  58  may be inflated by opening an associated valve  76  to provide pressurized gas from the pressurized gas source  72  to the air spring  58 . In addition, the valve  76  may enable or disable venting or the exhaust of pressurized gas from the air spring  58 . The air spring  58  may be deflated by positioning the valve  76  such that pressurized gas is vented or exhausted from the air spring  58 , such as by venting pressurized gas to the surrounding environment. 
     Referring to  FIGS. 2 and 5 , the axle lift system  16  may include an axle securing unit  80 . The axle securing unit  80  may be associated with or may be provided with a liftable axle assembly  32 , but may not be provided with a non-liftable axle assembly  30 . The axle securing unit  80  may be disposed proximate the chassis  20 . For example, the axle securing unit  80  may be disposed on a cross member  82  that may be fixedly positioned on or with respect to the chassis  20 . The axle securing unit  80  may include a hook  84  and an actuator  86 . 
     The hook  84  may be configured to secure an associated axle assembly in a secured position. The hook  84  may be configured move with respect to the chassis  20  and/or the cross member  82 . For example, the hook  84  may be movably disposed on the cross member  82  with a mounting bracket and may be configured to engage and receive the liftable axle assembly  32 . The hook  84  may be configured to move between a first position, which may also be called a released position, and a second position, which may also be called a secured position. The hook  84  may be disengaged from a liftable axle assembly  32  when in the released position as is shown in  FIGS. 5 and 6 . As such, movement of the liftable axle assembly  32  may be limited by the suspension unit  50 , but not the axle securing unit  80  when the hook  84  is in the released position. The hook  84  may engage or receive the liftable axle assembly  32  when in a secured position, such as is shown in  FIGS. 7 and 8 . As such, movement of the liftable axle assembly  32  may be limited by axle securing unit  80  when the hook  84  is in the secured position. For example, the axle securing unit  80  may inhibit or limit movement of the liftable axle assembly  32  away from the chassis  20  or toward the support surface  40  when in the secured position. Thus, the liftable axle assembly  32  may be free to move further away from the chassis  20  when the hook  84  is in the released position as compared to the secured position. The hook  84  may move in any suitable manner. For example, the hook  84  may be configured to move linearly or rotate or pivot about a pivot axis  88 . 
     The actuator  86  may be configured to move or actuate the hook  84  between the released position and the secured position. The actuator  86  may be of any suitable type, such as a pneumatic, hydraulic, electrical, or electromechanical actuator. In at least one embodiment, the actuator  86  may include an actuator shaft that may be operatively connected to the hook  84 . In the embodiment shown in  FIG. 2 , the actuator shaft extends through a hole in the cross member  82 . 
     The axle securing unit  80  may not include an actuator that lifts an axle assembly toward the chassis  20 . As such, the axle lift system  16  described herein may be smaller, lighter, less expensive, easier to package, and easier to install than an axle lift assembly that has an actuator that lifts a heavy axle assembly toward the chassis rather than lowering the chassis toward the support surface  40  prior to securing or releasing an axle assembly. 
     Referring to  FIG. 1 , the control system  18  may monitor and control operation of components and systems of the vehicle  10 . The control system  18  may include at least one microprocessor-based controller or control module that may monitor and/or control various components or systems of the vehicle  10 , such as the axle lift system  16  and/or the pressurized gas supply system  70 . For example, the control system  18  may be configured to control the operation of the valves  76  to control the flow of pressurized gas to the air spring  58  and to control venting of pressurized gas from the air spring  58 . The connection or communication between the control system  18  and the valves  76  is represented with connection nodes A, B, C and D. 
     The control system  18  may also communicate with various sensors or input devices. For instance, the control system  18  may be configured to receive a signal or data from a speed sensor  90 , a gear selector sensor  92 , a load sensor  94 , and an operator communication device  96 . 
     The speed sensor  90  may be configured to detect or provide data indicative of the speed of the vehicle  10 . For example, the speed sensor  90  may detect the rotational speed of a drivetrain component or a wheel assembly  34 . The speed sensor  90  may be of any suitable type and may provide data indicative of whether the vehicle  10  is stationary or moving. 
     The gear selector sensor  92  may be configured to detect or provide data indicative of the selection of a transmission gear ratio or whether a transmission drive gear has not been selected or engaged (e.g., the transmission is in a neutral or park position). As such, the gear selector sensor  92  may provide data that may be indicative of whether the vehicle  10  is stationary (e.g., a neutral or park position is selected). A gear selector sensor  92  may be associated with a gearshift lever or similar operator input device for selection of a transmission gear ratio. 
     One or more load sensors  94  may be provided to detect or provide data indicative of axle load and/or vehicle load. The load sensor  94  may be of any suitable type. For example, the load sensor  94  may include one or more physical sensors that may be disposed on the vehicle  10  that may detect or provide data indicative of the pressure of pressurized gas that is disposed in or supplied to an air spring  58 . As such, axle load or vehicle load may be based on data indicative of pressure in an air spring  58 . A load sensor  94  may be associated with or provided with one or more non-liftable axle assemblies  30  or with one or more non-liftable axle assemblies  30  and one or more liftable axle assemblies  32 . Alternatively, the load sensor  94  may be a virtual sensor that may receive vehicle load data that may be wirelessly transmitted to the vehicle  10 , such as from a scale that may be equipped with suitable communication equipment. Communication between the control system  18  and each load sensor  94  is represented by connection nodes L 1  through L 4  in  FIG. 1 . 
     The operator communication device  96  may be provided to receive an input from an operator or an operator command. The operator communication device  96  may be of any suitable type or types, such as a switch, button, sensor, display, touchscreen, keypad, voice command or speech recognition system, or the like. The operator communication device  96  may be used to input data that may not be predetermined or provided by a sensor or other input device, such as may be the case when a vehicle  10  is not equipped with one or more of the sensors discussed herein. In addition, the operator communication device  96  may be used to allow manual entry of vehicle load data and/or a command to lift and secure or lower and release one or more liftable axle assemblies  32 . It is also contemplated that the operator communication device  96  may provide a warning message when the operator attempts to secure or release a liftable axle assembly  32  under conditions that fall outside of target axle load levels or predetermined operating parameters. 
     Referring to  FIG. 3 , a flowchart of an exemplary method of controlling an axle lift system is shown. The method may be used to inflate or deflate one or more air springs to extend or retract one or more suspension units to facilitate securing and lifting of one or more liftable axle assemblies and/or releasing one or more liftable axle assemblies so that its associated wheel assemblies may engage the support surface and support the vehicle. As will be appreciated by one of ordinary skill in the art, the flowchart represents control logic which may be implemented or affected in hardware, software, or a combination of hardware and software. For example, the various functions may be affected by a programmed microprocessor. The control logic may be implemented using any of a number of known programming and processing techniques or strategies and is not limited to the order or sequence illustrated. For instance, interrupt or event-driven processing may be employed in real-time control applications rather than a purely sequential strategy as illustrated. Likewise, parallel processing, multitasking, or multi-threaded systems and methods may be used. 
     Control logic may be independent of the particular programming language, operating system, processor, or circuitry used to develop and/or implement the control logic illustrated. Likewise, depending upon the particular programming language and processing strategy, various functions may be performed in the sequence illustrated, at substantially the same time, or in a different sequence while accomplishing the method of control. The illustrated functions may be modified, or in some cases omitted, without departing from the spirit or scope intended. 
     Referring to  FIG. 3 , a flowchart is shown that illustrates steps associated with lifting a liftable axle assembly  32  and holding the liftable axle assembly  32  in a secured position with the axle securing unit  80 .  FIGS. 5-8  are provided help illustrate associated method steps. For simplicity,  FIGS. 5-8  illustrate the method in the context of a vehicle  10  that has one non-liftable axle assembly  30  and one liftable axle assembly  32 . For illustration purposes, the method will be described beginning with the vehicle  10  in an initial configuration shown in  FIG. 5  in which liftable axle assembly  32  is released and not secured by the axle lift system  16  with the tires  36  of the non-liftable axle assembly  30  and the liftable axle assembly  32  disposed on the support surface  40  such that the non-liftable axle assembly  30  and the liftable axle assembly  32  support the chassis  20 . 
     At block  100 , the method may determine whether any liftable axle assemblies  32  are available to lift. A liftable axle assembly  32  may be available to lift when it is not held in a secured position with an associated axle securing unit  80 . As such, a tire  36  of a liftable axle assembly  32  may be disposed on the support surface  40  and the hook  84  of the axle securing unit  80  may be disengaged from the liftable axle assembly  32 . A determination as to whether a liftable axle assembly  32  is available to lift may be based on data associated with the axle securing unit  80 , such as whether the hook  84  is in the secured position or the actuator  86  has been actuated to move the hook  84  to the secured position. Alternatively or in addition, a determination as to whether a liftable axle assembly  32  is available to lift may be based on data from an associated load sensor  94 . For instance, a liftable axle assembly  32  may not be available to lift when a corresponding axle load is not detected (i.e., no axle load may be detected when a liftable axle assembly  32  is secured with an associated axle securing unit  80  and the liftable axle assembly  32  does not support the vehicle  10 ). If a liftable axle assembly is not available to lift (e.g., all liftable axle assemblies  32  are secured by an associated axle securing unit  80 ), then the method or iteration of the method may end at block  102 . If a liftable axle assembly is available to lift (e.g., at least one liftable axle assembly  32  is not secured by an associated axle securing unit  80 ), then the method may continue at block  104 . 
     At block  104 , the method may determine whether the vehicle  10  is stationary. A determination as to whether the vehicle  10  is stationary may be based on data from the speed sensor  90  and/or the gear selector sensor  92 . If the vehicle is not stationary, then the method or iteration of the method may end at block  102 . If the vehicle is stationary, then the method may continue at block  106 . It is also contemplated that block  104  may be omitted in one or more embodiments to facilitate raising or lowering of an axle assembly if the vehicle is not stationary. As such, block  100  may proceed to block  106  instead of block  104 . 
     At block  106 , one or more axle loads may be compared to an axle load threshold. An axle load may be based on data from a load sensor  94  that may support the vehicle  10 . As such, an axle load may be determined for a non-liftable axle assembly  30  or a non-liftable axle assembly  30  and one or more liftable axle assemblies  32  that are not secured by the axle securing unit  80  and are supporting the vehicle  10 . The axle load threshold may be a predetermined value that may be based on vehicle development testing. For example, the axle load threshold may be set at a predetermined value, such as 80% of a maximum load rating of the axle assembly. If the axle load or axle loads are not less than the axle load threshold (e.g., at least one of a non-liftable axle load and a liftable axle load is greater than or equal to a corresponding axle load threshold), then the method or method iteration may end at block  102 . If an axle load exceeds the axle load threshold (e.g., at least one of a non-liftable axle load and a liftable axle load is less than a corresponding axle load threshold), then the method may continue at block  108 . 
     At block  108 , one or more liftable axle assemblies  32  may be selected to lift. A liftable axle assembly  32  may be manually or automatically selected. A liftable axle assembly  32  may be manually selected by an operator or based on a user input that may be provided via the operator communication device  96 . For example, the operator communication device  96  may permit an operator to select one or more liftable axle assemblies  32  to lift and secure. A liftable axle assembly  32  may be automatically selected by the control system  18 . For example, the control system  18  may automatically select a single available liftable axle assembly  32  when the axle load on the non-liftable axle assembly  30  does not exceed the axle load threshold. If more than one liftable axle assembly  32  is available to lift, then the method may determine whether to secure one or more liftable axle assemblies  32  based on load data from one or more load sensors  94 . For example, if there are two liftable axle assemblies  32  and one non-liftable axle assembly  30  and none of the liftable axle assemblies  32  are secured with a corresponding axle securing unit  80 , then the two liftable axle assemblies  32  and one non-liftable axle assembly  30  support the weight of the vehicle  10 . As such, the load sensors  94  associated with each axle assembly may detect an axle load. The total axle load or the maximum axle load may be compared to additional axle load thresholds or may be used to reference data in a lookup table to determine whether one or more liftable axle assemblies  32  may be lifted. As an example, two liftable axle assemblies  32  may be selected to lift when the maximum axle load detected is less than 50% of an associated maximum axle load rating. One liftable axle assembly  32  may be selected to lift when the maximum axle load detected between 50% and 80% of an associated maximum axle load rating. These values and ranges are examples and it is contemplated that additional or different values or ranges may be provided depending on the number of axle assemblies and the configuration of the vehicle. 
     At block  110 , the chassis  20  may be lowered. The chassis  20  may be lowered by deflating the air spring or air springs  58  associated with one or more suspension units  50  which in turn may retract those suspension units  50  toward the chassis  20 . An air spring  58  may be deflated by venting pressurized gas to the surrounding environment as previously described. In at least one embodiment, all air springs  58  of the tractor  12  and the trailer  14  except for those provided with the front steering axle assembly of the tractor  12  may be deflated when the vehicle  10  includes a tractor  12  and a trailer  14 . Similarly, all air springs  58  of the tractor  12  except for those provided with front steering axle assembly may be deflated when the vehicle  10  includes a tractor  12  without a trailer  14 . Lowering of the chassis  20  is best shown by comparing  FIG. 5  to  FIG. 6 . In  FIG. 6 , the air springs  58  of the non-liftable axle assembly  30  and the liftable axle assembly  32  are deflated, thereby allowing the associated suspension units  50  to retract or move toward a retracted position in which the suspension arm  52  may pivot about the pivot pin  60  and move closer to the chassis  20 . 
     At block  112 , the selected liftable axle assembly or assemblies  32  may be secured with the axle lift system  16 . Securing of a liftable axle assembly  32  is best shown in  FIG. 7 . In  FIG. 7 , the actuator  86  of the axle lift system  16  may move the hook  84  into engagement with the liftable axle assembly  32  to hold the liftable axle assembly  32  in the secured position. 
     At block  114 , the chassis  20  may be raised. The chassis  20  may be raised by inflating the deflated air springs  58  of the suspension units  50  of the axle assemblies that are not secured by an associated axle securing unit  80 . As such, the air springs  58  of the non-liftable axle assembly or assemblies  30  that were deflated and the air springs  58  of the liftable axle assemblies  32  that were deflated but are not currently secured by an associated axle securing unit  80  may receive pressurized gas and may be inflated. Inflation of the air springs  58  may extend the suspension units  50  and may move the chassis  20  away from the support surface  40  and may lift the secured liftable axle assemblies  32  away from the support surface  40  such that the wheel assemblies  34  and tires  36  of the secured liftable axle assemblies  32  may disengage the support surface  40  and not support the vehicle  10 . An illustration of raising of the chassis  20  is best shown in  FIG. 8 . In  FIG. 8 , the air spring  58  of the non-liftable axle assembly  30  is inflated while the air spring  58  of the liftable axle assembly  32  that is held in the secured position by the axle lift system  16  is not inflated. Inflation of the air spring  58  of the non-liftable axle assembly  30  causes the chassis  20  and the liftable axle assembly  32  to move away from the support surface  40 , thereby allowing the tire or tires  36  of the secured liftable axle assembly  32  to lift away from and disengage the support surface  40 . Lifting of one or more liftable axle assemblies  32  may help improve vehicle fuel economy by reducing or eliminating frictional drag forces between one or more tires  36  and the support surface  40  that may otherwise be present if the tires  36  remained in engagement with the support surface  40 . 
     Referring to  FIG. 4 , a flowchart is shown that illustrates steps associated with releasing a liftable axle assembly  32  from the secured position. Releasing a liftable axle assembly  32  from a secured position generally reverses many of the method steps shown in  FIG. 3 . The steps associated with this flowchart will be described beginning with the vehicle  10  in the configuration shown in  FIG. 8 . 
     At block  200 , the method may determine whether any liftable axle assemblies  32  are lifted or held in a secured position by an associated axle securing unit  80 . If no liftable axle assemblies are lifted (e.g., no liftable axle assembly  32  is secured by an associated axle securing unit  80 ), then the method or method iteration may end at block  202 . If at least one liftable axle assembly is lifted (e.g., at least one liftable axle assembly  32  is secured by an associated axle securing unit  80 ), then the method may continue at block  204 . 
     At block  204 , the method may determine whether the vehicle  10  is stationary as previously discussed with respect to block  104 . If the vehicle is not stationary, then the method or iteration of the method may end at block  202 . If the vehicle is stationary, then the method may continue at block  206 . It is also contemplated that block  204  may be omitted in one or more embodiments to facilitate raising or lowering of an axle assembly if the vehicle is not stationary. As such, block  200  may proceed to block  206  instead of block  204 . 
     At block  206 , one or more axle loads may be compared to an axle load threshold. An axle load may be based on data from one or more load sensors  94  as previously described. The axle load threshold may be a predetermined value as previously described. If the axle load or axle loads are less than or equal to the axle load threshold, then the method or method iteration may end at block  202 . If an axle load exceeds the axle load threshold, then the method may continue at block  208 . 
     At block  208 , one or more liftable axle assemblies  32  may be selected to release. A liftable axle assembly  32  may be manually or automatically selected. A liftable axle assembly  32  may be manually selected by an operator as previously discussed. A liftable axle assembly  32  may be automatically selected by the control system  18 . For example, the control system  18  may automatically select a single available liftable axle assembly  32  for release when the axle load on the non-liftable axle assembly  30  is less than the axle load threshold. If more than one liftable axle assembly  32  is available to release, then the method may determine whether to release one or more liftable axle assemblies  32  based on load data from one or more load sensors  94 . For example, if there are two liftable axle assemblies  32  and one non-liftable axle assembly  30  and both of the liftable axle assemblies  32  are secured with a corresponding axle lift system  16 , then the one non-liftable axle assembly  30  supports the weight of the vehicle  10 . As such, the load sensor  94  associated with the non-liftable axle assembly  30  may detect an axle load. The axle load may be compared to one or more axle load thresholds or may be used to reference data in a lookup table to determine whether one or more liftable axle assemblies  32  may be released. As an example, two liftable axle assemblies  32  may be selected for release when the axle load detected is less than 50% of an associated maximum axle load rating. One liftable axle assembly  32  may be selected for release when the axle load detected between 50% and 80% of an associated maximum axle load rating. These values and ranges are examples and it is contemplated that additional or different values or ranges may be provided depending on the number of axle assemblies and the configuration of the vehicle. 
     At block  210 , the chassis  20  may be lowered by deflating the air springs  58  associated with one or more suspension units  50  as previously discussed in with respect to block  110 . As such, the chassis  20  may move from the position shown in  FIG. 8  to the position shown in  FIG. 7 . 
     At block  212 , the selected liftable axle assembly or assemblies  32  may be released by the axle lift system  16 . The axle lift system  16  may release one or more liftable axle assemblies  32  by moving the hook  84  out of engagement with a selected liftable axle assembly  32 . Releasing a liftable axle assembly  32  is best shown in  FIG. 6 . 
     At block  214 , the chassis  20  may be raised. The chassis  20  may be raised by inflating the air springs  58  of the suspension units  50  of the axle assemblies that are deflated and are not secured by an associated axle securing unit  80 . Raising of the chassis  20  is best shown in  FIG. 5 . 
     Inflation of the air springs  58  of the may move the chassis  20  away from the support surface  40  and may lift any remaining secured liftable axle assemblies  32  away from the support surface  40  such that the wheel assemblies  34  and tires  36  of the secured the liftable axle assembly or assemblies  32  may disengage the support surface  40  and not support the vehicle  10 . Released liftable axle assemblies  32  may remain on the support surface  40  and may support the vehicle  10 . The release of a liftable axle assembly  32  may provide additional axle assemblies to support vehicle loads and may help avoid exceeding maximum axle load ratings and/or may help better distribute loads between multiple axle assemblies. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.