a chain reaction that damages all cutterheads the drive train of the cutterbar and or tractor t in such case the damage is usually significant referring now to fig2 4 it can be seen that each cutterhead module 40 is provided with a forwardly positioned rock guard 65 and a skid shoe 70 that passes beneath cutterhead module 40 for engagement with the surface of the ground the rock guard 65 has a conventional semi circular configuration and is mounted to opposing forward mounting arms 35 of spacer module 32 adjacent to the corresponding cutterhead module 40 one skid shoe 70 is mounted beneath each cutterhead module 40 to protect the cutterhead module from wear due to engagement with the surface of the ground each skid shoe is formed as a generally planar body portion 71 with a mounting tab 73 affixed thereto and projecting upwardly the body portion 71 is also formed with a forward end 72 that is bent upwardly to engage the corresponding rock guard 65 modular drive mechanism 75 best seen in fig2 and 3 is fully disclosed in the 39 323 patent and reference is made thereto for a more complete description broadly within each cutterhead unit there is a two piece hub one inner hub and one outer hub normally held together by a shear mechanism the inner hub is connected to a drive shaft and the outer hub including an integral upper locking block is connected to a rotatable knife support member spaced below the outer hub is a fixed lower locking block these components are arranged such that when a knife engages a solid or fixed object and a shear force generated adequate to cause the shear mechanism to fail the outer hub rotates freely and drops preferably.