When did Paul de Vivie (Vélocio) formally patent the derailleur concept?

1895.

The fundamental concept for the derailleur mechanism, a device designed specifically to push a bicycle chain laterally from one sprocket to another, was formally captured and patented by Paul de Vivie, known by his nom de plume Vélocio, in 1895. This predates the introduction of successful internal hub gears by several years, indicating that the idea for external shifting was present quite early in bicycle history. However, this initial patent represented only the theoretical concept; the actual physical implementation required significant refinement over the following decades to become reliable enough for widespread use, especially concerning chain tensioning and effective chain throwing.