This is J.Wi.S's design for a sprung chain that shortens itself when not under tension. It is a key part of the J.Wi.S derailleur system - which was a fork type derailleur, but had no chain tensioning arm. It did not need a chain tensioning arm because the chain was self-tensioning.

The key feature is that the main chain pins are oval - allowing the links, which have round holes, to move on the pins. A spring runs all around the outside of the chain. Frank Berto claims that the chain could stretch enough to allow a total capacity of 4 teeth.

This is one of three Austrian Patents that relate to the J.Wi.S derailleur system. The first was granted in 1937 and displays the double headed eagle of Austria. This one and the third one were granted in late 1938, after the Anschluss, and these show an eagle holding a Swastika.

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• Publisher: Österreichisches Patentamt
• Inventor: Robert Pollak
• Country: Austria
• Date: Priority date November 1936
• Derailleur brands: J.Wi.S, Wanderer
• Derailleurs: J.Wi.S