What is the term for when a trailer swerves back and forth due to being too light?

The term that describes when a trailer swerves back and forth due to being too light is known as "fishtail." This phenomenon occurs when the weight distribution on the trailer is not sufficient to maintain stability, particularly while towing. A lightweight trailer can easily become unstable when it experiences changes in speed or direction, leading to a sideways motion that resembles a fish swimming back and forth in the water.

This instability can be exacerbated by factors such as road conditions, wind, or abrupt inputs from the tow vehicle. Recognizing and addressing fishtailing is crucial for safe towing practices, as it can lead to loss of control over the trailer and vehicle. Proper loading techniques, ensuring adequate tongue weight, and using the right towing equipment are essential to mitigate this issue.