What is a jackknife and what driving action most commonly causes it?

JACKKNIFE describes the dangerous folding of a tractor-trailer when the trailer slides sideways relative to the tractor. There are two main types with different causes: TRACTOR JACKKNIFE (drive axle jackknife): The tractor's rear drive axles lock (typically from overbraking on slippery pavement); the locked wheels can no longer provide directional control; the tractor's rear slides sideways while the trailer continues pushing straight; the tractor pivots at the fifth wheel; TRAILER JACKKNIFE: The trailer's brakes lock excessively (often from trailer brake imbalance or improper hand-valve use); the trailer slides sideways while the tractor continues forward; the trailer swings out from behind the tractor. COMMON CAUSES: Excessive brake force in a turn or on slippery pavement; improper use of the trailer hand valve (which applies only trailer brakes); brake imbalance between tractor and trailer; driving too fast for conditions; over-braking in a curve. PREVENTION: Maintain balanced braking; avoid using the trailer hand valve except for pre-trip inspection; reduce speed before curves; use brakes smoothly and progressively; if skid begins, release brakes to restore directional control. ABS SYSTEMS dramatically reduce jackknife risk by preventing wheel lockup.