Abstract: A coupler assembly for limiting lateral forces which may be generated during the highway operation of a trailer coupled to a towing vehicle. The present invention is part of the yoke of the trailer, and comprises a completely self-contained system for momentarily actuating the brakes on the towed vehicle, in response to undesirable lateral accelerations of the towed vehicle. The invention is characterized by a coupler member which itself is relatively rotatable, in a lateral plane, with respect to the trailer yoke, together with dampening spring assemblies, so there is a surge-brake actuation in response to lateral accelerations above a predetermined magnitude.