The present invention relates to four wheel steering tractors, and particularly to a four wheel steering tractor having a mower or grass cutting system mounted in a lower spece thereof between front and rear wheels. 
A tractor having this type of construction is disclosed in U.S. Pat.  No.  4,671,376, for example.  As apparent from this patent, gauge wheels are provided forwardly and rearwardly of a blade housing forming part of the grass cutting system.  The front gauge wheel, generally, does not constnatly engage the ground but performs an ancillary function during a grass cutting operation.  In this U.S. patent, however, the front gauge wheel has a somewhat different construction since the blade housing includes a sensor for enabling automatic running of the tractor.  The rear guage wheels usually comprise caster type wheels which constantly engage the ground during a grass cutting operation and follow turning round of the tractor in a natural way.  However, the caster type guage wheels turn 180 degrees each time the tractor is reversed from forward running to backward runnig or vice vice, thereby marring the lawn or digging the ground.  There occurs a further inconvenience of the gauge wheels spinning on their swivels when the blade housing is raised from the ground.  The gauge wheels turning to an unnecessary extend could interfere with the tires when the tractor makes a sharp small turn. 
In order to eliminate these disadvantages, the gauge wheels may be fixed against oscillation relative to the blade housing.  However, when the tractor makes a small turn, a very small radius of turning being possible particularly in a four wheel sterring mode, such gauge wheels will have difficulties in following the movement of the tractor.  The gauge wheels will tend to make side slips thereby to mar the lawn all the same. 
In addition to the above problem, a four wheel steering tractor having a mid-mount grass cutting system has the following problem which is related to a steering link mechanism for enabling the four wheel steering mode.  As disclosed in Japanese Patent Publication Kokai No.  64-44072, a construction for steering the front and rear wheels in opposite directions includes a bell crank disposed adjacent a rear axle case and connected at opposite ends thereof to knuckle arms of the right and left rear wheels through right and left rear drag links, respectively.  One end of the bell crank is connected to a pitman arm through a front drag link.  This construction is complicated since it requires a bell crank for relaying purposes, and two drag links connected to the right and left knuckle arms, respectively.  Where a grass cutting system is mounted in a lower space of the tractor between the front and rear wheels, it is desirable to keep a drive input section at the center of a blade housing out of contact with the tractor body when the blade housing is raised.  However, the presence of the pitman arm and associated components centrally of the bottom of the tractor body allows only a small range for vertical movement of the blade housing.  With the limited range of vertical movement, the grass cutting system could collide with earth surfaces when the tractor runs over uneven ground.