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What causes the jackknife effect when driving with a trailer?

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Jackknife effect: “The car brakes more effectively than the trailer.” (Driving Licence Book, 19th Edition, page 173)

The car brakes effectively, but the trailer brakes poorly. This means that the trailer has to go somewhere (speed and force remains, as the trailer has not come to a full stop). This is called the jackknife effect, as the trailer will slide either to the right or the left and thereby fold in towards the car like the blade of a jackknife. The forces are so great that the car is also affected and will be pulled along – a very dangerous traffic hazard!

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