When is a trailer most likely to jackknife?

A trailer is most likely to jackknife when it is empty due to the reduced weight on the wheels, which can lead to decreased traction and stability. When a trailer is empty, the rear wheels have less weight pushing down on them, which can make it easier for the trailer's back end to swing out if the driver makes a sudden maneuver or applies the brakes sharply. This is compounded in situations where the vehicle is turning or changing lanes, as an unloaded trailer may not follow as smoothly and can pivot or swing out.

In contrast, a fully loaded or unevenly loaded trailer would generally be more stable because the added weight increases the traction on the wheels and reduces the likelihood of the trailer swinging out of alignment. A lightly loaded trailer could also be prone to some instability, but it would not have the same pronounced risk as an empty trailer.