Jackknife Types and Causes
A jackknife occurs when a trailer swings forward relative to the tractor, creating an acute angle at the fifth wheel coupling. There are two distinct types. Drive-wheel lockup jackknife: occurs when the tractor's drive axle wheels lock (skid) during hard braking. Locked drive wheels continue forward in a straight line while the tractor cab follows them — but the trailer, connected to the fifth wheel, continues pushing forward under its own momentum. The angle between the locked-sliding tractor and the continuing trailer creates the jackknife. This is the most common jackknife type and is preventable with ABS, proper braking technique, and avoiding hard braking on slippery surfaces. Trailer swing jackknife: occurs when the trailer slides sideways (trailer wheels lock or the trailer is pushed sideways by a sudden turn). Less common than drive-wheel jackknife. Prevention: ABS prevents wheel lockup on equipped vehicles. Driving technique: avoid sudden hard braking on curves or turns. On slippery roads, brake in a straight line when possible — braking while turning maximizes jackknife risk. Understeer vs. oversteer: a tractor that understeers (front wheels lose traction) slides forward without jackknifing; a tractor that oversteers (drive wheels lose traction and the rear swings out) creates the jackknife geometry.