In a popular typical lawn mower design, the rider sits atop a four wheeled machine powered by an internal combustion engine. One or more blades swing about a vertical axis within a cutter deck attached to the machine, to cut grasses as the mower moves across a lawn or other turf.
The present invention is described here in terms of its application to a transmission-steerable commercial riding mower, for which it is especially useful. Such mowers are typically driven by independent hydraulic motors connected to the rear drive wheels. The front wheels are freely pivotable casters. Directional control, or steering, is obtained by differently varying the speed of the two rear wheels. It is consequently important that good traction be sustained between the rear wheels and the earth surface, especially when the mower traverses a slope, since gravity tends to pivot the front of the mower downhill. It is also important that any lawn mower maintain an evenness of grass cut. Cutter decks are typically either rigidly fixed to the underside of the mower, or they are suspended so they "float" --that is, they hang at a fixed elevation, but move upwardly upon contact with a rise in the soil surface.
Lawn mowers are generally intended for use on lawns and other surfaces which are level or gently undulating. Therefore, in most typical three and four wheel riding mowers heretofore, the wheels are directly fastened to the frame of the mower machine. However, then any vertical force or displacement on a wheel of the mower is transmitted directly to the frame (or chassis) of the machine. The frame of the mower will tilt and rise and fall according to the effects of one of the wheels. So, under certain conditions the chassis vertical motion will undesirably change the elevation of the cutter deck with respect to the lawn being cut. Another undesirable effect of lifting or tilting of the mower chassis is loss of wheel traction which is critically important in transmission steerable machines. And, undue bumpy up and down motion is discomforting to the operator.
For mowers intended for use on rougher terrains, designs have been employed which seek to mitigate the adverse effects of undulations. Among such are front and rear wheel axles which tilt about a center pivot. But, in machines with a center pivot axle, when a wheel rises, the center pivot point and the whole end of the frame necessarily also rise, albeit to a lesser degree than when the wheel is fastened directly to a frame. Therefore, there is a continuing need for further improvements in suspension systems in self-propelled lawn mowers.