A combine harvester comprising a pair of track units at a front of the harvester. The track units are suspended so that a frame of the units is pivotable about a centrally placed pivot axis, the axis dividing the track units between a front and rear portion. For each of the track units, the harvester comprises at least one variable length actuator mounted between the chassis of the harvester and the frame of the track unit, the actuator being mounted so that it is capable of actuating an upward or downward tilting of the front portion of the track unit. The actuator is operated to counteract an excessive downward or upward tilting of the front portion of the unit. The actuator may be operated by a control mechanism working on the basis of measurements performed by suitable sensor arrangements that monitor or predict the downward or upward tilting.