RELATED APPLICATIONS 
     This application claims priority to and the benefit from U.S. Provisional Patent Application Ser. No. 61/612,366 filed on Mar. 18, 2012, which is fully incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a vehicle drivetrain and a drive axle system for the vehicle drivetrain capable of driving two axles. 
     BACKGROUND OF THE INVENTION 
     Trucks may utilize tandem axles where the axles are connected together by an inter-axle drive shaft. The forward tandem axle has parallel axis input and output shafts with the input on the upper shaft centerline, a helical gear drop to the forward axle pinion and a parallel output on the upper centerline to the rear axle. The forward tandem axle chassis mounting must optimize the transmission to forward tandem axle drive shaft universal joint angles. The inter-axle drive shaft is then a compromise for the universal joint operating angles resultant from the front/rear axle side view installation angles. The typical tandem hypoid rear axle gear set has lower efficiency as compared to a spiral bevel gear set. The tandem rear axle can utilize spiral bevel gears to improve the inter-axle drive shaft joint angles and efficiency but the joint angles cannot be completely optimized. Alternatively, the tandem rear axle can be a negative offset hypoid to achieve the best inter-axle shaft joint angles but this arrangement necessitates several rear axle negative hypoid primary gear set performance deficiencies. The above summarized compromises associated with tandem axles typically results in poor noise, harshness and vibration in the driveline as well as decreased durability. 
     SUMMARY OF THE INVENTION 
     A drive axle system for a vehicle comprises a first axle assembly and a second axle assembly. The first axle assembly comprises an input shaft, a transfer shaft concentric with the input shaft, a drive gear and a pinion gear and a ring gear. A clutch selectively connects the input shaft with the drive gear. The drive gear is connected to a driven gear. One or both of the gears maybe a bevel-type gear so that the driven gear locates an inter-axle assembly at an angle. The inter-axle assembly is connected to a second axle assembly, comprised of a ring and pinion gear. The pinion gear is substantially collinear with the inter-axle assembly. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts a schematic side view of one embodiment of a tandem axle system; 
         FIG. 2  depicts a schematic side view of another embodiment of a tandem axle system; and 
         FIG. 3  depicts a schematic side view of another embodiment of a tandem axle system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. 
       FIG. 1  illustrates a schematic side view of a drive axle system  10  for a vehicle according to a first embodiment. The drive axle system  10  includes a first axle assembly  12 , a second axle assembly  14 , and an inter-axle shaft assembly  16 . The first axle assembly  12  and second axle assembly  14  may be part of a tandem axle system. The drive axle system  10  forms a portion of a vehicle drivetrain. As a non-limiting example, the drive axle system  10  may form a portion of the vehicle drivetrain of a semi-tractor; however it is understood the drive axle system  10  may be used in other applications. 
     The first axle assembly  12  includes an input shaft  18 , an inter-axle clutch  20 , an inter-axle differential  22 , a second axle drive gear  24 , a second axle driven gear  26 , a first axle bevel pinion  28 , and a first axle bevel gear  30 . The inter-axle differential  22  includes a first inter-axle differential output gear  32  and a second inter-axle differential output gear  34 . The first axle assembly  12  is disposed in a first housing (not shown) and is drivingly engaged with a transmission (not shown) of the vehicle, a first differential and axle (both not shown), and the inter-axle assembly  16 . 
     The input shaft  18  has a first end  36  drivingly engaged with the transmission, a middle portion  38  that may be drivingly engaged with the inter-axle clutch  20 , and a second end  40  drivingly engaged with a carrier  42  of the inter-axle differential  22 . As shown in  FIG. 1 , the input shaft  18  is substantially parallel to a horizontal axis  44 . Further, an axis of the input shaft is oriented so that it is substantially collinear with an axis of the first inter-axle differential output gear  32 , the second inter-axle differential output gear  34 , and an axis of the second axle drive gear  24 . 
     The clutch  20  is selectively engagable with the input shaft  18 , a transfer shaft gear  46  and the second axle drive gear  24 . The inter-axle clutch  20  has a shift collar  48  driven by a shift fork  50  that moves the clutch  20  into selective engagement with the second axle drive gear  24  and the transfer shaft gear  46  or with the input shaft  18 . Although a shift fork  50  is shown, it can be understood that the inter-axle clutch  20  may be any type of clutching device. 
     In the depicted embodiment, the clutch  20  is a cone type clutch. The clutch is comprised to a first cone-shaped ring  52  and a second cone-shaped ring  54 . The first ring  52  is designed to selectively engage a complimentary shaped surface  56  mounted for rotation with the input shaft  18 . The second ring  54  is designed to selectively engage a complimentary shaped surface  58  mounted for rotation on the second axle drive gear  24 . The first and second rings are not engaged with their respective surfaces  56 ,  58 , at the same time; when one is engaged, the other is disengaged. 
     The inter-axle differential  22  includes the spider  42 , at least two driving pinions  60 , the first inter-axle differential output gear  32 , the second inter-axle differential output gear  34 , and a transfer shaft  62 . The first inter-axle differential output gear  32  and the second inter-axle differential output gear  34  disposed on the transfer shaft  64  form the side gears of the inter-axle differential  22 . The inter-axle differential  22  is a bevel gear differential as known in the art, however, it may be any other type of differential. When a rotational force is applied to the spider  42 , the driving pinions  60  apply the rotational force to the first inter-axle differential output gear  32  and the second inter-axle differential output gear  34 , which rotates the transfer shaft  62 . 
     The transfer shaft  62  is a hollow shaft rotatably disposed in the first housing and has an axis of rotation concentric with the axis of rotation of the input shaft  18 . One end portion of the transfer shaft  62  has the side gears and the other end has the transfer shaft gear  46 . The gear  46  may be replaced with splines. 
     The second axle drive gear  24  is a gear concentrically disposed on or about a portion of the transfer shaft  62 . The second axle drive gear  24  may be disposed between the output gears  32 ,  34 , and the transfer shaft gear  46 . The second axle drive gear  24  is a substantially disc shaped body having an inner diameter portion  64 , an outer diameter portion  66 , and an engagement surface  68 . The axis of the second axle drive gear  24  is concurrent with the axis of the input shaft  18 . The inner diameter portion  64  may be selectively engaged with the inter-axle clutch  20 . The engagement may be such as through a gear  70  or splines. A set of gear teeth are formed on the outer diameter portion  66 . The outer diameter portion teeth form a bevel gear, and may be one of a modified helical gear, a spiral bevel gear, a beveloid gear, and a straight bevel gear. The second axle drive gear  24  is drivingly engaged with the second axle driven gear  26 . 
     The second axle driven gear  26  is located on the outer diameter portion  66  of the second axle drive gear  24 . More particularly, the second axle driven gear  26  is not concentric with the second axle drive gear  24  but instead it is in contact with a discrete location of the outer diameter portion  66  of the second axle drive gear  24 . The second driven axle gear  26  is engaged with just the top of the second drive axle gear  24 . This location elevates a rotational axis  72  of the second driven axle gear  26  above the second drive axle gear  24  and the other components of the first axle assembly  12 . The driven gear  26  may be one of a modified helical gear, a spiral bevel gear, a beveloid gear and straight bevel gear. 
     The bevel of either, or both, of the second axle drive gear  24  and/or the second axle driven gear  26  results in the second axle driven gear axis  72  to be non-parallel to the axis of the input shaft  18 , transfer shaft or the first axle bevel pinion  28 , all of which are aligned with one another and the horizontal axis  44 . More particularly, the second axle driven gear axis  72  is at an angle A that is 1 degree to 30 degrees below a horizontal axis. 
     The first axle bevel pinion  28  is drivingly coupled to the second output gear  34 . The first axle bevel pinion  28  is rotationally supported on bearings  74 . Alternately, the first axle bevel pinion  28  may be integrally formed with the second output gear  34 . The first axle bevel pinion  28  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. 
     The first axle bevel gear  30  is drivingly engaged with the first axle bevel pinion  28 . The rotation from the first axle bevel pinion  28  is imparted to the first axle bevel gear  30 . The bevel gear  30  rotates the differential (not shown) which imparts rotation to the axle half shafts (not shown). The first axle bevel gear  30  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. 
     The second axle driven gear  26  is disposed on a driven shaft  76 . The driven shaft  76  comprises a part of the inter-axle assembly  16 . The assembly  16  also comprises an inter-axle shaft  78 . 
     Bearings  80  disposed about the driven shaft  76  and the inter-axle shaft  78  support them for rotation. The driven shaft  76  is oriented substantially parallel and substantially co-axial with the axis  72  of the second axle driven gear  26 . The driven shaft  76  is drivingly engaged with the inter-axle shaft  78 , such as through a U-joint  79 . 
     The inter-axle shaft  78  comprises at least one elongate cylindrical member drivingly engaged with the driven shaft  76  and a second axle bevel pinion  82 . As illustrated, the inter-axle shaft  78  has U-joints  79  disposed on each end thereof. One joint  79  is connected to the driven shaft  76  and the other joint  79  is connected to a second axle bevel pinion  82 . The inter-axle shaft  78  is oriented substantially parallel and substantially collinear to the driven shaft  76  and the second axle bevel pinion  82 . Thus, the second axle bevel pinion  82  is angled with respect to the horizontal axis  44  and the input shaft  18 . 
     Substantially collinear as used herein preferably means collinear or so that from one shaft to the next shaft not more than 6 degrees (plus or minus) of angle is provided. More preferably, substantially collinear as used herein means that there is not more than approximately 1 to 3 degrees (plus or minus) angle from one shaft to the next. 
     The second axle assembly  14  includes the second axle bevel pinion  82  and a second axle bevel gear  84 . The second axle assembly  14  is disposed in a second housing (not shown) and is drivingly engaged with the inter-axle assembly  16  and a second differential and axle (both not shown). 
     The second axle bevel pinion  82  has gear teeth formed on an outer surface thereof. The second axle bevel pinion  82  may be one of a hypoid gear, a spiral bevel gear, a straight bevel gear, a positive offset hypoid gear, and a negative offset hypoid gear. 
     The second axle bevel gear  84  is coupled to a housing of the second differential by a plurality of fasteners or a weld and is rotatable about an axis thereof. Alternately, the second axle bevel gear  84  may be integrally formed with the second differential. As is known in the art, the second axle bevel gear  84  has gear teeth formed on an outer surface thereof. The second axle bevel gear  84  may be one of a spiral bevel gear or a straight bevel gear. The second axle bevel gear is drivingly engaged with the second axle bevel pinion  82 . 
     While  FIG. 1  depicts the input shaft  18  at no angle, an angle may be added. An angle may be added to optimize the joint angles for the main driveshaft from the transmission while maintaining the parallel and substantially collinear orientation of the inter-axle shaft  78  to the driven shaft  76  and the second axle bevel pinion  82 . It can be appreciated without the device described above, the input shaft  18  and shafts  76 ,  78  must be parallel and changing the input shaft  18  angle has a direct effect on the inter-axle shaft joint angles. 
       FIG. 2  illustrates a schematic side view of a drive axle system  86  for a vehicle according to a second embodiment. The drive axle system  86  includes a first axle assembly  88 , a second axle assembly  90 , and an inter-axle assembly  92 . The first axle assembly  88 , and the second axle assembly  90  may be part of a tandem axle system. The drive axle system  86  forms a portion of a vehicle drivetrain. As a non-limiting example, the drive axle system  86  may form a portion of the vehicle drivetrain of a semi-tractor; however it is understood the drive axle system  86  may be used in other applications. 
     The first axle assembly  86  includes an input shaft  94 , an inter-axle clutch  96 , an inter-axle differential  98 , a second axle drive gear  100 , a second axle driven gear  102 , a first axle bevel pinion  104 , and a first axle bevel gear  106 . The inter-axle differential  98  includes a first inter-axle differential output gear  108  and a second inter-axle differential output gear  110 . The first axle assembly  88  is disposed in a first housing (not shown) and is drivingly engaged with a transmission (not shown) of the vehicle, a first differential and axle (both not shown), and the inter-axle assembly  92 . 
     The input shaft  94  has a first end  112  drivingly engaged with the transmission, a middle portion  114  that may be drivingly engaged with the inter-axle clutch  96 , and a second end  116  drivingly engaged with a carrier  118  of the inter-axle differential  98 . 
     As shown in  FIG. 2 , the input shaft  94  is angled with respect to a horizontal axis  120 . The angle B represents the angle between the input shaft  94  and the axis  120 . It can be appreciated that the input shaft  94  is not collinear with the axis  120 . The input shaft  94  is substantially collinear with an axis of the first inter-axle differential output gear  108 , the second inter-axle differential output gear  110 , and an axis of the second axle drive gear  100 . 
     The inter-axle clutch  96  comprises a gear  122  located on the input shaft  94 . The gear  122  selectively receives a shift collar  124 . In a first position (shown above the input shaft  94 ), the shift collar  124  is engaged with just the gear  122 . In a second position (shown below the input shaft  94 ), the shift collar  124  slides in the axial direction to engage with a transfer shaft gear  126 . The gear  126  alternatively may be splines. 
     The shift collar  124  connects with the input shaft gear  122  and the transfer shaft gear  126  by being moved with a shift fork  128 . Although a shift fork  128  is shown, it can be understood that the inter-axle clutch  96  may be any type of clutching device. 
     The inter-axle differential  98  includes the spider  118 , at least two driving pinions  130 , the first inter-axle differential output gear  108 , the second inter-axle differential output gear  110 , and a transfer shaft  132 . The first inter-axle differential output gear  108  and the second inter-axle differential output gear  110  disposed on the transfer shaft  132  form the side gears of the inter-axle differential  98 . The inter-axle differential  98  is a bevel gear differential as known in the art, however, the inter-axle differential may be any other type of differential. When a rotational force is applied to the spider  118 , the driving pinions  130  apply the rotational force to the first inter-axle differential output gear  108  and the second inter-axle differential output gear  110 , which rotates the transfer shaft  132 . 
     The transfer shaft  132  is a hollow shaft rotatably disposed in the first housing and has an axis of rotation concentric with the axis of rotation of the input shaft  18 . One end portion of the transfer shaft  132  has the output gears  108 ,  110 , and the other end has the gear  126  for selectively receiving the clutch  96  thereon. 
     The second axle drive gear  100  is a gear concentrically disposed on or about a portion of the transfer shaft  132 . The second axle drive gear  100  may be disposed between the output gears  108 ,  110 , and the transfer shaft gear  126 . The second axle drive gear  100  is a substantially disc shaped body having an inner diameter portion  134 , an outer diameter portion  136 , and an engagement surface  138 . The axis of the second axle drive gear  100  is concentric with the axis of the input shaft  94 . 
     A set of gear teeth are formed on the outer diameter portion  136 . The set of gear teeth formed on the outer diameter portion  136  form a bevel gear, and may be one of a modified helical gear, a spiral bevel gear, a beveloid gear, and a straight bevel gear. The second axle drive gear  100  is drivingly engaged with the second axle driven gear  102 . 
     The second axle driven gear  102  is located on the outer diameter portion  136  of the second axle drive gear  100 . More particularly, the second axle driven gear  102  is not concentric with the second axle drive gear  100  but instead it is in contact with a discrete location on the outer diameter of the second axle drive gear  100 . The second driven axle gear  102  is engaged with just the top of the second drive axle gear  100 . This location elevates a rotational axis  140  of the second driven axle gear  102  above the second drive axle gear  100  and the other components of the first axle assembly  88 . The driven gear may be one of a modified helical gear, a spiral bevel gar, a beveloid gear and a straight bevel gear. 
     The bevel of either, or both, of the second axle drive gear  100  and/or the second axle driven gear  102  results in the second axle driven gear axis  140  to be non-parallel to the input shaft  94 . The axis  140  is located at an Angle C from a horizontal axis, which may be 1 degree to 30 degrees below the horizontal axis. 
     The first axle bevel pinion  104  is drivingly coupled to the second inter-axle differential output gear  110 . Alternately, the first axle bevel pinion  104  may be integrally formed with the second inter-axle differential output gear  110 . The first axle bevel pinion  104  is rotationally supported on bearings  142 . The first axle bevel pinion  104  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. 
     The first axle bevel gear  106  is drivingly engaged with the first axle bevel pinion  104 . The rotation from the first axle bevel pinion  104  is imparted to the first axle bevel gear  106 . The bevel gear  106  rotates the differential (not shown) which imparts rotation to the axle half shafts (not shown). The first axle bevel gear  106  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. 
     The second axle driven gear  102  is disposed on a driven shaft  144 . The driven shaft  144  comprises a part of an inter-axle shaft assembly  92 . The assembly  92  also comprises an inter-axle shaft  146 . 
     Bearings  148  disposed about the driven shaft  144  and inter-axle shaft  146  support them for rotation. The driven shaft  144  is oriented substantially co-axial with the axis  140  of the second axle driven gear  102 . The driven shaft  144  is drivingly engaged with the inter-axle shaft  146 , such as through a U-joint  149 . 
     The inter-axle shaft  146  comprises at least one elongate cylindrical member drivingly engaged with the driven shaft  144  and a second axle bevel pinion  150 . As illustrated, the inter-axle shaft  146  has U-joints  149  disposed on each end thereof. One joint is connected to the driven shaft  144  and the other joint is connected to the second axle bevel pinion  150 . The inter-axle shaft  146  is oriented substantially parallel and substantially collinear to the driven shaft  144  and the second axle bevel pinion  150 . 
     Substantially collinear as used herein preferably means collinear or so that from one shaft to the next shaft not more than 6 degrees (plus or minus) of angle is provided. More preferably, substantially collinear as used herein means that there is not more than approximately 1 to 3 degrees (plus or minus) angle from one shaft to the next. 
     The second axle assembly  90  includes the second axle bevel pinion  150  and a second axle bevel gear  152 . The second axle assembly  90  is disposed in a second housing (not shown) and is drivingly engaged with the inter-axle shaft  146  and a second differential and axle (not shown). The second axle bevel pinion  150  is axially aligned with the inter-axle shaft  146 ; the second axle bevel pinion  150  is not parallel to the input shaft  94 . 
     The second axle bevel pinion  150  has gear teeth formed on an outer surface thereof. The second axle bevel pinion  150  may be one of a hypoid gear, a spiral bevel gear, a straight bevel gear, a positive offset hypoid gear, and a negative offset hypoid gear. 
     The second axle bevel gear  152  is coupled to a housing of the second differential by a plurality of fasteners or a weld and is rotatable about an axis thereof. Alternately, the second axle bevel gear  152  may be integrally formed with the second differential. As is known in the art, the second axle bevel gear  152  has gear teeth formed on an outer surface thereof. The second axle bevel gear  152  may be one of a spiral bevel gear or a straight bevel gear. The second axle bevel gear  152  is drivingly engaged with the second axle bevel pinion  150 . 
     While  FIG. 2  depicts the input shaft  18  at an angle, any angle may be used. The angle may be used to optimize the joint angles for the main driveshaft from the transmission while maintaining the parallel and substantially collinear orientation of the inter-axle shaft  146  to the driven shaft  144  and the second axle bevel pinion  148 . 
       FIG. 3  illustrates a schematic side view of a drive axle system  154  for a vehicle according to a third embodiment. The drive axle system  154  includes a first axle assembly  156 , a second axle assembly  158 , and an inter-axle shaft assembly  160 . The first axle assembly  156  and the second axle assembly  158  may be part of a tandem axle system. The drive axle system  160  forms a portion of a vehicle drivetrain. As a non-limiting example, the drive axle system  10  may form a portion of the vehicle drivetrain of a semi-tractor; however it is understood the drive axle system  10  may be used in other applications. 
     The first axle assembly  156  includes an input shaft  162 , an inter-axle clutch  164 , an inter-axle differential  166 , a second axle drive gear  168 , a first axle bevel pinion  170 , and a first axle bevel gear  172 . The inter-axle differential  166  includes a ring gear  174 , a plurality of driving planet gears  176 , a sun gear  178 , a first inter-axle differential output gear  180 , and a transfer shaft  182 . The first axle assembly  156  is disposed in a first housing (not shown) and is drivingly engaged with a transmission (not shown) of the vehicle, a first differential and axle (not shown), and the inter-axle shaft assembly  160 . 
     The input shaft  162  is drivingly engaged with the transmission. As shown in  FIG. 3 , the input shaft  162  is shaft is oriented at a low angle D with respect to a horizontal axis  184 . As a non-limiting example, the angle may be between 1° and 30°. Further, the input shaft  162  is oriented to be substantially collinear with the first axle bevel pinion  170 . 
     The input shaft  162  is at least partially disposed in the first housing. Bearings  186  disposed between the input shaft  162  and the first housing permit the input shaft  162  to rotate within the housing. 
     The input shaft  162  is directly rotationally connected to a planetary housing  188 . The housing  188  contains the ring gear  174 , the planets  176  and the sun gear  178 . The housing  188  has a cone clutch surface  190  thereon and gear  180 , or splines, for selectively receiving the inter-axle clutch  164 . 
     The ring gear  174  is directly connected to the first axle bevel gear  172  via a carrier shaft  194 . The carrier shaft  194  is substantially collinear with the input shaft  162 . The transfer shaft  182  is drivingly connected to the sun gear  178  and is concentric with the sun gear  178 . A gear  196 , or splines, are located on an outer diameter of the transfer shaft  182 . A second axle gear shaft  198  is concentric with the transfer shaft  182  and the carrier shaft  194 . The second axle gear shaft  198  at one end has a cone clutch surface  200  and a gear  202 , or set of splines. The other end of the second axle gear shaft  198  has the second axle drive gear  168  mounted for rotation therewith. 
     The inter-axle clutch  164  comprises a shift collar  204  which may be drivingly engaged with the cone clutch surface  190  on the planetary housing  188  and the first inter-axle differential output gear  180 . The shift collar  204  may also be engaged with the transfer shaft gear  196 . Additionally, the shift collar  204  may be engaged with the gear  202  on the second axle drive gear shaft  198 . The shift collar  204  has first and second cones  206 ,  208 , for selectively contacting the planetary housing surface  190 , and the second axle drive gear shaft surface  200 . A shift fork  210  selectively axially moves the shift collar  204  between the above-described locations. 
     Bearings  212  disposed between the first axle pinion gear  170  and the first housing support the pinion gear  170  and permit it to rotate substantially collinear with the input shaft  162 . 
     The second axle drive gear  168  is a gear concentrically disposed on or about a portion of the transfer shaft  182 . The second axle drive gear  168  may be disposed between the transfer shaft gear  196  and the first axle bevel pinion  170 . The second axle drive gear  168  is a substantially disc shaped body having an inner diameter pinion  214 , an outer diameter portion  216 , and an engagement surface  218 . The axis of the second axle drive gear  168  is concurrent with the axis of the input shaft  162 . 
     A set of gear teeth are formed on the outer diameter portion  216 . The set of gear teeth formed on the outer diameter portion  216  form a bevel gear, and may be one of a modified helical gear, a spiral bevel gear, a beveloid gear, and a straight bevel gear. The second axle drive gear  168  is drivingly engaged with a second axle driven gear  220 . 
     The second axle driven gear  220  is located on the outer diameter portion  216  of the second axle drive gear  168 . More particularly, the second axle driven gear  220  is not concentric with the second axle drive gear  168  but instead it is in contact with a discrete location on the outer diameter portion  216  of the second axle drive gear  168 . The second driven axle gear  220  is engaged with just the top of the second drive axle gear  168 . This location elevates a rotational axis  222  of the second driven axle gear  220  above the second drive axle gear  168  and the other components of the first axle assembly  156 . The driven gear  220  may be one of a modified helical gear, a spiral bevel gear, a beveloid gear and a straight bevel gear. 
     The bevel of either, or both, of the second axle drive gear  156  and/or the second axle driven gear  220  results in the second axle driven gear axis  222  to be non-parallel to the input shaft  162  axis. The second axle driven gear axis  222  is depicted in  FIG. 3  as being located below a horizontal axis at angle E. Angle E may be 1 degree to 30 degrees below the input shaft axis  162  and/or the horizontal axis. 
     The first axle bevel pinion  170  is drivingly coupled to the shaft  194 . The first axle bevel pinion  170  has gear teeth formed on an outer surface thereof. The first axle bevel pinion  170  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. The first axle bevel pinion  170  is drivingly coupled to the first axle bevel gear  172 . 
     The first axle bevel gear  712  is coupled to a housing of the first differential (not shown) and axle by a plurality of fasteners or a weld and is rotatable about an axis thereof. Alternately, the first axle bevel gear  172  may be integrally formed with the first differential. The first axle bevel gear  172  has gear teeth formed on an outer surface thereof. The first axle bevel gear  172  may be one of a spiral bevel gear, a straight bevel gear, and a hypoid gear. The rotation from the first axle bevel pinion  170  is imparted to the first axle bevel gear  172 . The bevel gear  172  rotates the differential which imparts rotation to the axle half shafts (not shown). 
     The second axle driven gear  220  is disposed on a driven shaft  224 . The driven shaft  224  comprises a part of the inter-axle assembly  160 . The assembly  160  also comprises an inter-axle shaft  226  and U-joints  234 . 
     Bearings  228  disposed about the driven shaft support  224  and the inter-axle shaft  226  support them for rotation. The driven shaft  224  is oriented substantially parallel and substantially co-axial with the axis  222  of the second axle driven gear  220 . The driven shaft  224  is drivingly engaged with the inter-axle shaft  226 , such as through a U-joint  234 . 
     The inter-axle shaft  226  comprises at least one elongate cylindrical member drivingly engaged with the driven shaft  224  and a second axle bevel pinion  230 . As illustrated, the inter-axle shaft  226  comprises an elongate cylindrical member having U-joints  234  disposed on each end thereof. One joint  234  is connected to the driven shaft  224  and the other joint  234  is connected to the second axle bevel pinion  230 . The inter-axle shaft  226  is oriented substantially parallel and substantially collinear to the driven shaft  224  and the second axle bevel pinion  230 ; the second axle bevel pinion  230  is not parallel to the input shaft  162 . 
     Substantially collinear as used herein preferably means collinear or so that from one shaft to the next shaft not more than 6 degrees (plus or minus) of angle is provided. More preferably, substantially collinear as used herein means that there is not more than approximately 1 to 3 degrees (plus or minus) angle from one shaft to the next. 
     The second axle assembly  158  includes the second axle bevel pinion  230  and a second axle bevel gear  232 . The second axle assembly  158  is disposed in a second housing (not shown) and is drivingly engaged with the inter-axle shaft assembly and a second differential and axle (not shown). The second axle bevel pinion  230  is substantially axially aligned with the inter-axle shaft  226 . Thus, an axis of the second axle bevel pinion  230  is at an angle between 1 and 30 degrees elevated from the horizontal. 
     The second axle bevel pinion  230  has gear teeth formed on an outer surface thereof. The second axle bevel pinion  230  may be one of a spiral bevel gear, a straight bevel gear, a positive offset hypoid gear, and a negative offset hypoid gear. 
     The second axle bevel gear  232  is coupled to a housing of the second differential by a plurality of fasteners or a weld and is rotatable about an axis thereof. Alternately, the second axle bevel gear  232  may be integrally formed with the second differential. As is known in the art, the second axle bevel gear  232  has gear teeth formed on an outer surface thereof. The second axle bevel gear  232  may be one of a hypoid, a spiral bevel gear or a straight bevel gear. The second axle bevel gear  232  is drivingly engaged with the second axle bevel pinion  230 . 
     While  FIG. 3  depicts the input shaft  18  at one angle, other angles are permissible. An angle may be used to optimize the joint angles for the main driveshaft from the transmission while maintaining the parallel and substantially collinear orientation of the inter-axle shaft  226  to the driven shaft  224  and the second axle bevel pinion  228 . 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.
Various drive axle systems are described each having a bevel gear system associated with the front axle system that is designed to minimize the angle of the inter-axle driveline with respect to the rear axle pinion gear.