1.  Field of the Invention. 
The present invention relates generally to suspension systems, and in particular to a suspension system for stabilizing a slab saw. 
2.  Description of the Prior Art. 
A variety of different types of suspension systems have heretofore been devised to meet the requirements of various vehicles and mobile equipment.  The primary purpose of a suspension system is to transfer the weight of a particular vehicle or piece of mobile equipment to a surface upon which it is supported.  Important considerations for an effective suspension system include stability, vibration damping and adaptability to various service conditions. 
Specific tasks in the construction, maintenance and demolition industries are often accomplished with specially designed vehicles and mobile equipment.  For example, self-propelled slab saws are commonly used for sawing horizontal slabs of concrete, asphalt and similar materials.  A typical self-propelled slab saw includes a chassis mounting a prime mover, a circular saw blade mounted on a transverse arbor shaft at the front end and a suspension system.  Such a slab saw or concrete sawing machine is disclosed in the Tucker U.S. Pat.  No.  2,973,950, and includes a suspension system with a pair of rear stabilizing wheels that are raised and lowered by a hydraulic cylinder. 
A common type of suspension system for a self-propelled slab saw includes a pair of driven rear wheels mounted on a live axle and a pair of extendable and retractable front wheels.  An operator walks behind the machine and guides it with a pair of handles.  Extending the front wheels raises the front of the machine so that its blade can be positioned over a slab surface.  As the front wheels are retracted, the blade lowers into the slab.  By adjusting the extension of the front wheels, the depth of cut can be controlled. 
Slab saws are presently available for cutting concrete slabs to depths of three feet.  As the saw advances, the front edge of the circular blade removes slab material in the saw kerf.  In a downcut mode, which is generally preferred, the saw blade rotates in the same direction as the wheels whereby the blade cuts downwardly into the slab material.  The resistance of the slab material produces an upward reactionary force on the saw blade at the front of the machine.  This upward, reactionary force tends to unload the retracted vehicle front wheels whereby the saw is substantially supported on the three points of a triangle defined by the rear wheels and the saw blade.  As long as the center of gravity of the machine is located directly over this triangle, the saw will tend to remain fairly stable.  However, if the saw's center of gravity shifts to a position where it is outside of the rear wheels/blade triangle, the machine can become unstable.  When the saw becomes unstable, the operator may experience difficulty in following a straight cut line, and the blade may wobble or bind.  Such conditions tend to greatly increase blade wear and can lead to blade failure. 
The parameters that control the operation of a slab saw include the reactionary force associated with the resistance of the material being cut, the depth of cut, the torque applied to the saw blade, the forward travel speed of the saw, the weight of the saw and the location of the saw's center of gravity.  A typical slab saw is designed to travel on all four wheels with the center of gravity located over a rectangle defined thereby.  However, the saw blade torque is often sufficient to produce an upward reactionary force through the blade and arbor shaft that unloads the front wheels whereby the aforementioned operating parameters become unbalanced.  In this condition the saw is substantially supported on three points defined by the two rear wheels and the blade.  This three point support makes it difficult to maintain a constant depth of cut and a uniform forward travel speed, because the upward reactionary force of the saw blade is sufficient to raise the blade and front end of the saw thereby resulting in a shallower cut.  The normal operator response to this condition is to reduce travel speed, which reduces the upward reactionary force and permits the saw blade to cut at a desired depth.  However, reducing the travel speed is undesirable because of the corresponding reduction in efficiency and machine capacity utilization. 
Unloading the front wheels of the saw can render it unstable if its center of gravity is located outside of the triangle defined by the rear wheels and saw blade.  In this condition the saw will tend to tip diagonally towards its unsupported front corner transversely opposite the blade.  Upon sensing this condition, the operator can reduce speed and thus reload or increase the load on the front wheels.  However, the condition of instability, even if only encountered intermittently, can cause premature blade failure, reduced cutting performance and/or stalling of the prime mover and the blade.  A particularly serious consequence is that the blade can actually rise from its kerf and skip across the pavement, or the entire saw may tip over. 
A previous solution was to concentrate as much weight as far rearwardly as possible.  For example, ballast could be added to the rear of the saw, but the added weight made the saw more cumbersome and difficult to operate.  Designers of previous slab saws have been somewhat limited by the aforementioned center of gravity parameters, which often dictated the location of major components like the engine. 
Another solution to the blade reactionary force problem is to increase the length of the transverse arbor shaft upon which the saw blade is mounted to increase the area of the rear wheels/blade support triangle.  A greater stabilizing reactionary moment can thus be created to counter the moment produced by the reactionary force of the saw blade.  However, this solution requires an increase in the overall width of the saw, which makes access to certain work areas more difficult.  For example, slab saws are often used inside existing structures and must pass through existing doorways.  Thus, to accomodate such field conditions slab saw width is normally limited whereby it is not practical to solve the aforementioned stability problems by increasing the arbor shaft length. 
The present invention addresses the aforementioned problems.