Jackknife (truck tractor and trailer)

Jackknifing refers to the folding of an articulated vehicle i.e. one towing a trailer, such that it resembles the acute angle of a folding pocket knife. If a vehicle towing a trailer skids, the trailer can push the towing vehicle from behind until it spins the vehicle around and faces backwards. This may be caused by equipment failure, improper braking, or adverse road conditions such as an icy road surface. In extreme circumstances, a driver may attempt to jackknife the vehicle deliberately in order to halt it following brake failure. A jackknife event of a heavy vehicle can be fatal to the driver as the top of the cab can be crushed or sliced off as it swings round violently and tries to fold under the trailer.

When a trailer skids to one side, this is known as a trailer swing or trailer slew. This could happen on a slippery road surface, often where there is a cant. This is not the same as "jackknifing" and is not as serious since the trailer moves back into line as the vehicle continues forwards. The driver must be aware, however, that the trailer could slide up against parked cars or a guard rail, or the wheels could slide into a ditch. This situation can occur especially when the trailer is empty or lightly loaded, and weather conditions cause violent gusts of crosswind.

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