thoroughbrace

noun1

1. •
   (old-fashioned, historical) A leather strap supporting the body of a coach or wagon, attached to springs or serving as a spring itself.

   • Spring, tire, axle, and linchpin too, Steel of the finest, bright and blue; Thoroughbrace bison-skin, thick and wide; Boot, top, dasher, from tough old hide Found in the pit when the tanner died. — 1858