knife

Jack-knife

1) Term for very dangerous accident situation where semi-trailer of articulated combination pushes rear axle of tractive unit out of line so whole combination forms a 'jack-knife', usually completely out of driver's control.

Anti-jack knife (device)

1) A device fitted to articulated vehicles to prevent jack-knifing whereby the tractive unit and semi-trailer hinge in the middle causing the semi-trailer to push the towing unit round in a circular movement out of control. Such devices may be designed to lock the tractive unit and trailer in the straight-ahead position but more commonly prevent the tractive unit rear wheels from locking under braking, which is a fundamental cause of jack-knifing accidents.