Abstract:
A utility vehicle that has a bogey beam suspension for the front steered wheels is also provided with all wheel drive and steered rear wheels for greater maneuverability. The steering of the rear wheels is coordinated with the steering of the front wheels to have the rear wheels follow in the tracks of the front wheels, thereby avoiding the need to disable the front drive on turns.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention deals generally with off road vehicles and more specifically with all wheel drive vehicles that have both front and rear drive wheels that are also both steered. 
         [0002]    Off road utility vehicles are becoming quite common for recreational and some occupational purposes. During some seasons of the year they are heavily advertised on television, and those commercials almost always show them taking sharp turns and cresting hills at high speeds. That is all very well for the camera, but it is not easily accomplished in the real world without careful design of the vehicle. If the suspension system of such an off road vehicle is not designed to accommodate rough terrain, it is easy to end up with one or more wheels off the ground or with very light loading on some wheels. In fact, even on level ground, the steering on some vehicles can be negatively affected by merely placing a heavy load in the cargo carrying area. 
         [0003]    This problem has been essentially solved by a suspension structure disclosed in U.S. Pat. Nos. 6,629,699; 6,536,545; 6,557,661; and 6,601,665 by Joseph C. Hurlburt, who is also the inventor of the invention described herein. The suspension system includes a “bogey beam” which is a beam parallel to the axis of the vehicle. The bogey beam distributes the vehicle load among all the wheels of the vehicle, even when rough terrain puts individual wheels at different heights. For a four wheeled vehicle, the front axle is mounted on the forward end of the bogey beam and a suspension strut connects the rear end of the bogey beam to the frame. On a six wheel vehicle, instead of the suspension strut the middle axle is attached at the rear end of the bogey beam. 
         [0004]    U.S. patent application Ser. No. 10/766,144 also by Hurlburt, extends the use of a bogey beam to all wheel drive vehicles, those in which the front steering wheels are also driven. 
         [0005]    However, front wheels that are both steered and powered raise new problems. Steering angles of driven axles are limited, and vehicles with full time powered steering axles have difficulty turning sharply. Rather than having the front steering axle properly pull the vehicle around turns, a steered and driven front axle acts as if it were being braked while fighting the turn unless the speed of such steered wheels is increased. This is because the limited steering angle causes the front driven wheels to have a greater turning radius and travel much farther than the non-steered axles. This wastes power, is hard on the drive train, causes severe tire wear, and tears up turf below the tires. These problems are aggravated by sharper turns. 
         [0006]    Since an important goal of utility vehicle is good maneuverability, the steering angle problem may be the reason that prior art utility vehicles have avoided front wheel drive. A limited number of other types of vehicles have attacked the steering angle problem. U.S. patent application Ser. No. 10/766,144 by Hurlburt, suggests automatic disengagement of the steering axle drive during tight turns, and some high end automobiles have added only limited steering to the rear wheels to counteract the problem. 
         [0007]    It would be very beneficial to provide utility vehicles that not only include all wheel drive, but also overcome the problems caused by the limited steering angle of driven front wheels. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention solves the problem of limited front wheel steering angles on all wheel drive utility vehicles such as the vehicle disclosed in U.S. patent application Ser. No. 10/766,144 by Hurlburt, the disclosure of which is incorporated herein and made a part of this application. The limitation on maneuverability of an all wheel drive utility vehicle is overcome by adding steering to the rear wheels. With the proper selection of the rear steering angle, the drive to the rear wheels can be maintained at the same speed as the drive to the front wheels, and the turning circle diameter can even be reduced from that of a front wheel drive utility vehicle with standard steering. 
         [0009]    In the preferred embodiment of the present invention the steering angles of the rear wheel are approximately the same as those for the front wheels resulting in an inner rear wheel maximum angle of 45 degrees and an outer rear wheel angle of 26 degrees. This results in a fully powered turning circle diameter of only 10½ feet. 
         [0010]    The present invention provides almost identical travel paths and distances for all the axles of an all wheel drive utility vehicle and thereby allows maintaining full power in turns and improves maneuverability. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective side view of a typical six wheel utility vehicle upon which the preferred embodiment of the invention is installed. 
           [0012]      FIG. 2  is a bottom plan view of the frame and drive apparatus of a six wheel utility vehicle provided with the rear wheel steering of the preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  is a perspective side view of a typical six wheel utility vehicle  10  upon which the preferred embodiment of the invention is installed. Vehicle  10  has all its wheels powered and it is supported above the ground G by front steered and driven wheels  12  mounted on front steering axle  14 , by rear driven wheels  16  mounted on rear drive axle  18 , and by middle driven wheels  20  mounted on middle axle  22 . Vehicle  10  includes an operator compartment  24 , with seats and the typical conventional controls, and a load bed  26  behind operator compartment  24 . Except for the fact that vehicle  10  has all its wheels powered and includes rear wheel steering, which is discussed below, it is quite conventional. 
         [0014]      FIG. 2  is a bottom plan view of the frame and drive apparatus of six wheel utility vehicle  10  with the rear wheel steering of the preferred embodiment of the invention. 
         [0015]    Many parts of the structure of utility vehicle  10  shown in the figures have been previously disclosed, and are more fully described in U.S. patent application Ser. No. 10/766,144 by Hurlburt. However, a brief summary of some of those previously disclosed features follows. 
         [0016]    All the wheels of the utility vehicle  10  are powered. Power is first supplied to rear wheels  16  by an engine (not shown) through transmission  30  and rear axle  18 . Transmission  30  also provides power to middle wheels  20  through universal drive assembly  32 . Universal drive assembly  32  accommodates to variations in the relative positions of rear wheels  16  and middle wheels  20 . As more fully described in U.S. patent application Ser. No. 10/766,144 by Hurlburt, universal drive assembly  32  comprises an assembly of telescoping shafts interconnecting universal joints attached to transmission  30  and middle wheel differential  38 . 
         [0017]    Bogey beam  44  is a suspension structure in which bogey beam  44  is a pivoting, longitudinally oriented beam parallel to the axis of the vehicle. Bogey beam  44  supports front wheel axle  14  at bogey beam forward end  46  and middle wheel axle  22  at bogey beam rear end  48 . Bogey beam  44  permits predictable sharing of the frame load by front wheels  12  and middle wheels  20  while keeping all four of the front and middle wheels in contact with the ground. The load on the rear of the vehicle is conventionally applied to rear axle  18 , usually by shock absorbers of conventional construction at connection points (not shown) on rear axle  18 . Thus, the entire vehicle load is shared by bogey beam pivot point  54  near the front of the vehicle and connection points near the rear. 
         [0018]    Motive power is delivered to front wheels  12  by powering middle axle  22  with universal drive assembly  32  and transferring power from middle axle  22  to the front axle  14 . Middle axle  22  and middle wheel differential  38  are mounted at rear end  48  of bogey beam  44  in a pivotal relationship to bogey beam  44 , and front axle  14  and front differential  56  are similarly mounted in a pivotal relationship to bogey beam  44  at its front end  46 . It is therefore practical to interconnect middle differential  38  to front differential  56  with a simple drive shaft  58 . As shown in  FIG. 2  when bogey beam  44  is hollow, drive shaft  58  can be completely enclosed within it. However, a solid bogey beam can also be used and the drive shaft can be mounted outside of and parallel to the bogey beam. 
         [0019]      FIG. 2  shows conventional front steering assembly  60  attached to front wheels  12 . Such a steering assembly is of conventional construction for steered and driven front wheels as is well known in the automotive art.  FIG. 2  also shows a typical outline for frame  66  of such a vehicle. 
         [0020]    It is at rear wheels  16  that the present invention deviates from previously disclosed utility vehicles. As shown in  FIG. 2 , rear wheels  16  are furnished with fully operating rear steering assembly  70 . Rear steering assembly  70  is of conventional construction, but is controlled to operate in conjunction with front steering assembly  60 . Since the goal of the dual steering of the front and rear wheels is to have the rear wheels follow in the tracks of the front wheels, rear wheels  16  actually steer at angles opposite to that of front wheels  12 . Arrows A at front wheels  12  and arrows B at rear wheels  16  show the direction of the steering angles for a typical turn. Furthermore, to have rear wheels  16  follow in the tracks of front wheels  12 , the steering angles of the two sets of wheels should also be approximately equal for any turn. 
         [0021]    This coordinated control can be accomplished by cable  72  or a linkage (not shown). Cable  72  is perhaps the simplest of many devices that can coordinate rear steering assembly  70  with front steering assembly  60 . Cable  72  is connected to front axle steering arm  74  on one side of front steering assembly  60  and connected  2  to rear axle steering arm  76  on the opposite side of rear steering assembly  70 . Thus, steering the front axle in any direction results in the rear wheels steering in the opposite direction. Cable  72  is typically a sheathed cable so that it can push as well as pull rear steering arm  76  to perform its full function. 
         [0022]    The present invention can also be used on a four wheel vehicle with a bogey beam. Such a vehicle is more fully described in U.S. patent application Ser. No. 10/766,144 by Hurlburt, and actually eliminates middle wheels  20 , so that the steering of rear wheels  16  is not affected. 
         [0023]    The present invention thereby overcomes the limitations of limited front wheel steering angles on all wheel drive utility vehicles by placing both the driven front wheels and the driven rear wheels on the same turning circle. 
         [0024]    It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.