Lucien Juy, of Simplex, and indeed many other derailleur designers, spent the 1940s and 1950s obsessing about chain tension. Today we cannot quite understand this, but we live in a world of effective derailleur geometries and strong long-lived springs. Perhaps, if our derailleurs were imprecise and our springs were weak and rapidly got weaker, we might believe in all kinds of engineering black magic.

This patent describes a mechanism by which the rider pulls the cable and this pulling of the cable cable increases the tension in the main spring in the derailleur.

• Publisher: L’Institut national de la propriété industrielle
• Inventor: Lucien-Charles-Hippolyte Juy
• Country: France
• Date: Priority date April 1948
• Derailleur brands: Simplex
• Derailleurs: An experimental variant of the Simplex Tour de France