A cutter bar is provided for trimming debris away from the wheel-track interface of a tracked vehicle. A pivot attachment is provided wherein a knife having a cutting edge is removably mounted to a bracket by way of fasteners which will shear when a pre-selected force is applied to them. The bracket is mounted to the vehicle structure so that the knife is disposed in close proximity to the drive wheel and endless track of the vehicle. Accordingly, when a piece of tough debris is encountered by the knife edge some of the fasteners will shear and the knife pivots out of the way so that the debris can pass by without causing damage to the wheel or track. The knife and fasteners being readily serviceable with inexpensive parts.