When Simplex do weird, they do weirdly weird.
The Selematic is one of the most surreal derailleurs that I have ever seen. It appears to work by a rigid cable pushing or pulling the lever mounted underneath it. This lever springs back to its central position awaiting the next push or pull. Each push or pull activates some mechanism, that I can only dream about, hidden inside the large rectangular body. This mechanism pushes the pulley cage out along a rod. In proper 1950’s style (an age when Simplex ruled the derailleur world) the pulley cage pivot is concentric with both the rod and the guide pulley. Just to complete the 1950’s picture there is a volute spring around the rod mechanism to keep out dirt. Wacky.
US patent # 4,504,250 describes this mechanism in detail with full exploded diagrams - but even after reading this you may be none the wiser.
One of the many mysteries surrounding this design is the issue of chain gap. The design, despite its complexity, makes no attempt to maintain a constant chain gap - so it will probably not work well with a wide ratio freewheel. Another mystery is its weight - it comes in at a stonking 525g - handy if your are cycling in a low gravity environment like, perhaps, the moon.
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