referring now to the drawings in more detail and initially to fig1 - 3 , numeral 10 generally designates a tractor of the type used to pull semi - trailers along highways and other roadways . the tractor 10 has a rigid frame which is generally identified by numeral 12 and which is supported on front wheels 14 and rear wheels 16 . a cab 18 is mounted on the front end of the frame 12 . the tractor 10 is propelled by a conventional engine 13 ( fig6 ) which turns a drive shaft 20 ( fig1 ). the drive shaft 20 connects through a universal joint 22 with a differential 24 . the differential 24 in turn drives a rear axle 26 on which the rear wheels 16 are carried . typically , each end of the axle 26 is equipped with a pair of the rear wheels 16 . the tractor frame 12 includes a pair of rigid channel beams 28 which are generally parallel to one another on opposite sides of the frame . the beams 28 are connected at their back ends by a rigid cross member 30 . bracket plates 32 extend downwardly from the beams 28 , and a transverse tube 34 is connected between the bracket plates 32 . flanges 36 are provided on the opposite ends of tube 34 and are secured to the bracket plates 32 by bolts 38 . the tractor frame 12 is provided with a conventional fifth wheel 40 which supports the front end portion of a semi - trailer ( not shown ) which the tractor pulls . the fifth wheel 40 is mounted on a rigid pivot frame which includes a pair of parallel arms 42 . extending between the arms 42 is a rigid tube 43 ( a fragment of which appears in fig3 ). the fifth wheel 40 is carried on the back ends of the arms 42 , and the front ends of the arms are connected with a transverse shaft 44 which extends horizontally between the beams 28 on the opposite sides of the tractor frame . shaft 44 is supported by at its opposite ends by sleeve bushings 46 and provides a horizontal pivot axis about which the pivot frame arms 42 can pivot to raise and lower the fifth wheel 40 between the positions shown in broken and solid lines in fig1 . the shaft 44 is located directly above tube 34 . in accordance with the present invention , the rear axle 26 is rigidly connected with a subframe which is generally identified by numeral 48 . as best shown in fig3 the sub - frame 48 takes the form of an a frame having a pair of rigid arms 50 which may be constructed from i beams . the arms 50 converge from back to front and are connected at their front ends at an apex of the a frame . the connection between the front ends of the arms is formed by a rigid bracket 52 having top and bottom plates 54 and a thicker vertical plate 56 ( see fig4 in particular ). the a shaped subframe 48 is connected for universal pivotal movement to the tube 34 . a pair of parallel bracket plates 58 are welded to the center portion of tube 34 . the back ends of the bracket plates 58 receive and support a horizontal pin 60 in extension between them . one end of the pin 60 is secured to and covered by a cap 62 which is bolted at 64 to one of the bracket plates 58 . the pin 60 extends through a spherical bearing 66 which seats within a ring 68 having a spherical inside surface that mates with the spherical outside surface of the bearing 66 . spacer bushings 70 are located on opposite sides of the bearing 66 to center the a - frame between plates 58 , and the entire bearing assembly is contained within a bearing housing 72 which is bolted at 74 to the vertical plate 56 of bracket 52 . bearing 66 allows pin 62 to tilt up and down longitudinally as well as to rock about a vertical axis passing through the center of the pin . thus , the spherical bearing mounting arrangement permits the a shaped subframe 48 to pivot about three mutually perpendicular axes and thus mounts the a frame in a manner allowing it three degrees of freedom of movement . the arms 50 diverge from front to back and are rigidly connected with the rear axle 26 at their back ends . as best shown in fig2 the axle 26 is rigidly secured near its opposite ends between a pair of plates 76 , and the lower plate 76 in each pair of plates is rigidly secured to the back end of one of the arms 50 . bolts 78 are used to secure the axle 26 between the plates 76 . the arms 42 of the fifth wheel frame are pivoted up and down about the axis of shaft 44 by a pair of hydraulic lift cylinders 80 which are mounted at their lower or base ends to the respective subframe arms 50 . as best shown in fig5 a horizontal shaft 82 extends between the subframe arms 50 and is received at its opposite ends in sleeves 84 which are secured to the arms 50 . reinforcing plates 86 are provided on the arms 50 at the locations of the sleeves 84 . each cylinder 80 is provided at its lower end with a spherical bearing 88 which mounts the cylinder on the shaft 82 at a location adjacent to the corresponding arm 50 . bearing 88 is held in position on shaft 82 between a spacer ring 85 adjacent the corresponding sleeve 84 , and a spacer tube 87 on the opposite side of the bearing . the cylinders 80 have extensible and retractable piston rods 89 and connect at their top or rod ends with the opposite sides of the pivot frame for the fifth wheel 40 . each arm 42 of the fifth wheel frame is provided with a bracket 90 ( see fig5 ) which is located immediately behind the transverse tube 43 . a horizontal shaft 92 extends between blocks 93 mounted on arm 42 and the side member 95 of each bracket 90 . the rod 89 of the corresponding cylinder 80 is connected with the shaft 92 by a spherical bearing 94 which is centered between blocks 93 by spacer rings 97 . in this manner , the hydraulic cylinders 80 connect the fifth wheel frame arms 42 with the a frame arms 50 , and the loads that are applied to the fifth wheel 40 are thus transmitted to the cylinders and from the cylinders to the a frame 48 . since the a frame is rigidly connected with the axle 26 , the loads on the fifth wheel 40 are transferred to the axle 26 and are not applied in significant part to the tractor frame 12 . at the same time , the cylinders 80 can be extended in unison to raise the fifth wheel 40 to the position shown in phantom lines in fig1 and the cylinders can be retracted to lower the fifth wheel to the position shown in solid lines in fig1 . the cylinders 80 are displaced only a short distance forwardly from the rear axle 26 and they have nearly vertical orientations . because the semi - trailer load is thus applied almost entirely to the axle 26 and not to the tractor frame 12 , a suspension system can be provided between the axle 26 and the frame 12 in order to provide a comfortable ride to the tractor 10 and the occupants of the cab 18 . the suspension system preferably includes a pair of pneumatic cushions 96 . as best shown in fig2 each cushion 96 is mounted on a plate 98 which is in turn bolted at 100 to a mounting bracket 102 . the two mounting brackets 102 are secured to the upper plates 76 which are located adjacent to the opposite sides of the frame 12 . another plate 104 overlies each air cushion 96 and is suitably secured to the corresponding beam 28 of the tractor frame . in addition to the pneumatic cushions 96 , a pair of conventional shock absorbers 106 are connected between the tractor frame 12 and the subframe 48 . the shock absorbers 106 are pivotally connected at their base ends with the respective subframe arms 50 and at their upper ends with the beams 28 on the opposite sides of the tractor frame 12 . the shock absorbers 106 serve both a dampening function and a protective function in that they prevent excessive expansion (&# 34 ; rebound &# 34 ;) of the pneumatic cushions 96 . as shown in fig3 a transverse link 108 is connected at one end with one side of the tractor frame 12 and at the opposite end with a bracket 110 which is connected with the rear axle 26 and the subframe 48 . the link 108 is able to pivot about the horizontal axes identified by numerals 112 and 114 in fig3 and the link thus permits the subframe to move up and down as required to function as intended . at the same time the link 108 transfers side loads that are applied to the subframe 48 to the frame 12 , thus maintaining the subframe 48 in the proper alignment and preventing it from wandering unduly from side to side . the presence of the link 108 prevents side load forces from being borne in significant part by the spherical bearing 66 and thus avoids the application of high stresses to the bearing . in operation , the suspension system of the present invention provides the tractor 10 with a comfortable ride both when the tractor is traveling with and without a loaded semi - trailer . when the semi - trailer is attached to the fifth wheel 40 , its load is applied through the cylinders 80 to the rear axle 26 . consequently , the air cushions 96 provide substantially the same cushioning effect for the tractor frame 12 when the semi - trailer is attached and when it is not attached , since the load that is applied to the frame 12 does not differ appreciably in the two situations . the a frame configuration of the subframe 48 provides it with sufficient strength to withstand the loads to which it is subjected in service . in addition , the a frame configuration provides the subframe with an apex which can be mounted at a single location for universal pivotal movement of the a frame . the cylinders 80 are preferably provided with a valving arrangement that allows them to &# 34 ; float &# 34 ;. consequently , when the wheels 16 on one end of the rear axle 26 move through a depression or over a raised area , the rear axle 26 can tilt longitudinally without applying undue stresses to the axle 26 or to the subframe 48 . if the back end of one of the arms 50 moves up or down relative to the back end of the other arm 50 , the entire subframe 48 is able to tilt by virtue of the universal joint mounting arrangement provided by the spherical bearing 66 . thus , the pin 60 is able to tilt longitudinally as permitted by the bearing . in addition , the back end of the subframe 48 can move up and down to follow the contour of the roadway , and the air cushions 96 and shock absorbers 106 transmit the resulting forces to the frame 12 in a manner providing the occupants of the cab 18 with a smooth and comfortable ride . the hydraulic system which permits the aforementioned &# 34 ; float &# 34 ; of the cylinders 80 is schematically illustrated in fig6 of the drawing . a hydraulic pump 116 operably coupled with a motor which can be the vehicle engine 13 , is operably connected by the hydraulic lines shown between a hydraulic reservoir 118 and the cylinders 80 . a valve 120 controls the direction of flow of hydraulic fluid in the lines . valve 120 is preferably a manually operated , four position valve having internal configurations for effecting the connections illustrated in fig6 with a first position shown above the broken line 122 , a second position shown between line 122 and broken line 124 , a third position between line 124 and broken line 126 and a fourth position shown below broken line 126 . preferably , valve 120 is spring biased to the second position when it is in either the first or third positions . however , when valve 120 is shifted to the fourth position , it will remain in that position until it is manually shifted to a different position . a pressure relief valve 128 is schematically illustrated interposed in a relief line extending from the pressure line from the pump to the return line from the cylinders to the reservoir . the operation of the hydraulic system illustrated in fig6 will be readily apparent to those skilled in the art . valve 120 is shifted to the first position connecting the pump outlet to the lifting side of cylinders 80 to raise the frame upon which fifth wheel 40 is mounted . at the same time , the outlet sides of cylinders 80 are connected through valve 120 to reservoir 118 via the return hydraulic line . when valve 120 is shifted to the second position , flow of fluid to the cylinders from the pump and from the cylinders to the reservoir is blocked . this holds the cylinders and consequently the fifth wheel in whatever elevated condition may be desired . it should be pointed out that the ports on the sides of the cylinders for extending the cylinders are hydraulically connected together as are the ports on the sides of the cylinders for retracting the cylinders . this permits one cylinder to automatically retract as the other cylinder extends as might result from the tractor wheels encountering an uneven road surface condition such as a bump or depression . float as mentioned above is achieved as a result of the two cylinders working together in this manner when the fifth wheel is elevated . when the cylinders are retracted however , neither cylinder would be free to extend or retract , with the valve in the second position , negating the ability of the axle to pivot with respect to the rigid frame when one side of the truck drives over a depression in the road surface . this is avoided by shifting the valve to the fourth position when driving with the cylinders retracted . the fourth valve position interconnects all of the cylinder ports with the fluid line to the fluid reservoir . this permits a single cylinder , or both if necessary to extend or retract independently of one another . this maintains the float condition , even with the fifth wheel lowered . thus , the valve is normally shifted to the fourth position when the truck is driven with the cylinders retracted . from the foregoing , it will be seen that this invention is one well adapted to attain all the ends and objects hereinabove set forth together with other advantages which are obvious and which are inherent to the structure . it will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations . this is contemplated by and is within the scope of the claims . since many possible embodiments may be made of the invention without departing from the scope thereof , it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense .