"The fifth wheel coupling provides the link between a semitrailer and the towing truck, tractor unit, leading trailer, or dolly."
Glossary of Terms, Compilation of Existing State Truck Size and Weight Limit Laws — FHWA, May 2015. Public domain.
The fifth wheel matters for permits because its kingpin is the origin of the KPRA measurement, and because sliding it changes both the weight distribution across the axle groups and the geometry the bridge formula is computed on. Repositioning the fifth wheel is one of the cheapest ways to bring a marginal axle group back inside its allowance.
Need a Fifth Wheel carrier?
QuoteFleet lists FMCSA-verified drayage and intermodal carriers, and gives shippers an instant freight quote. Find a carrier or price your move now.