Suspension systems in which the front axle is hydro-pneumatically sprung are known in agricultural vehicles, such as tractors, for example. Front axle suspension affords the driver greater ride comfort and increased working efficiency, since both the speed of travel and the traction of the vehicles can be significantly increased compared to unsprung vehicles. The hydraulic design configuration of such a front axle suspension generally comprises hydraulic accumulators, which are connected to hydraulic cylinders arranged on the front axle, which in turn connect a vehicle frame to the front axle. In such vehicles the front axle is embodied as a fully floating axle, so as to provide a sprung suspension. Such vehicles furthermore have a rigid rear axle, in order to counter the risk of rolling movements or even overturning of the vehicle. The use of a floating rear axle is therefore not possible with such suspension systems for only sprung front axles.
Agricultural vehicles are also known, which have both a sprung floating front axle and sprung wheel suspensions. Such vehicles are known, for example, in the form of John Deere, 8000-series tractors. In these the problem of rolling movements or a lateral tipping of the frame of the vehicle is resolved in that a hydraulic interconnection is made between the rod side of the suspension cylinders of one side of the rear axle and the piston side of the opposing suspension cylinder. The advantage of this interconnection is that a mechanical roll stabilizer can be dispensed with, since when one suspension cylinder is compressed, the opposing suspension cylinder is bound to be likewise retracted due to the increasing pressure on its rod side. In this way, rolling movements can to a certain extent be suppressed. An uneven loading of the vehicle, which would lead to tipping of the frame, can be compensated for by different pressures of the hydraulic accumulators contained in the suspension system. Such a suspension system, however, cannot be used on vehicles that do not have hydraulic cylinders on a floating rear axle.
In the case of loader vehicles, the front axle is generally designed as a rigid axle and the rear axle is floating. The reason for this is that the payload of a loader vehicle is primarily situated in front of the front axle, for which reason a greater stiffness is required in this area, in order to prevent the vehicle overturning. A suspension system for such a loader vehicle is also known and is used, for example, on the Turbofarmer ‘P41.7 Top’ produced by Merlo and marketed under the name ‘EAS’—Electronic Active Suspension. The suspension system produced by Merlo comprises a front axle suspended on two hydraulic cylinders, the hydraulic cylinders being connected to hydraulic accumulators and the hydraulic cylinders and hydraulic accumulators being correspondingly controlled by an electronic control unit according to the attitude of and forces acting on the vehicle. The rear axle is in this case floating, affording both a suspension function for the front axle and a correction of the tilting of the vehicle frame. A roll stabilization implemented in the electronic control unit is intended to ensure that the vehicle always remains in a horizontal position. The roll stabilization, however, functions relatively sluggishly and requires improvement, since the ride comfort for an operator is less than ideal. A further disadvantage is that the suspension system can be activated irrespective of the load state of the vehicle and that critical vehicle states with regard to the positional stability can thereby occur, if the suspension is activated in correspondingly unfavorable load states or in an inclined position of the vehicle, which ultimately poses a safety risk. Furthermore, with the suspension activated, adjustments of the suspension system to varying load states are feasible only when the vehicle is at a standstill, so that an operator is obliged to interrupt his working process in order to obtain an optimum suspension effect, when load states change.