Semi-Truck with Triple Trailer Swaying Oscillation

triple_trailerSome U.S. states allow semi-trucks with three trailers. The truck configuration can total up to 105 feet  by adding a rear trailer.

Terry Sumter, a UPS driver for 30 years, has experienced the back trailer moving into the oncoming lane. “The most significant condition is the sway of the rear trailer,” Sumter states.

This effect is referred to as “rearward amplification” in technical literature.

Rearward amplification is more precisely the ratio of the lateral acceleration of the last unit in a multi-unit combination to the lateral acceleration of the first unit, and it is directly related to rollover risk.

The amplification and swaying can be caused by a blustery crosswind, rapid steering maneuvers, or some other source.  Any vortex shedding would also provoke lateral oscillation.

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I recently drove down a highway in Ohio where I encountered a semi-truck pulling three trailers.  I recorded this video which almost shows a lateral standing wave pattern.  The resulting effect was more dramatic than appears in the video.

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– Tom Irvine

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