material 60 between the two points 301 302 thereby shortening it this provides a means of fine adjustment as distinguished from the more coarse adjustment accomplished by the contraction binder 200 if the binding material 60 comprises a chain which is preferable the links thereof 61 conveniently comprise the insertion points 301 302 for the bolt 300 the same shearing capabilities are provided at the outboard site where the two position mode is employed as are provided at the inboard site there are numerous ways to incorporate a coupling pin 18 connection into some point of the two position assembly one way to provide a connection point is to provide a fork shaped connector into the two tines of which an extension is position both having an aligned aperture through which a pin 18 may be emplaced because of the widespread familiarity with pin connections already extant in the prior art it is unnecessary that any particular configuration be insisted upon for that purpose it is only important for this purpose that a pin 18 have an emplacement site within the outboard two position coupling assembly 57 so that it 39 s shank configured with shearing loci 32 will break upon impact to the system as discussed supra the provision of shearing capability at the outboard coupling site 50 protects the system when an object on the roadway is struck in the same way inboard coupling site 10 shearing capability offers such protection when the impact is incurred in forward snowplow movement.