Originally Posted by
Kuromori
You know, I imagine it would work decent enough. As far as I can tell, the primary purpose of the drop parallelogram is to shorten the distance between the A/B pivots while maintaining a reasonable linkage length for the parallelogram. When these designs first started appearing, there was a pivot bolt that went all the way through the A/B knuckle, so one of them had to be offset and the blind pivot design only came later. The Athena RD uses a blind pivot so it can be behind the linkage plates, so you still get the shorter A/B distance. The Suntour XCT is of a similar design as well.
Shorten the distance, or keep the distance more uniform with the freewheel? Part of the problem with the straight paralleogram design is that it effectively pivots the entire pulley cage
closer to the freewheel as the cogs get larger - thus the pulley cage and upper jockey wheel need to pivot too far backwards for crisp shifting, just to clear the cogs.
What I
think I see in the Athena's engineering is that the blind lower pivot allowed the pulley cage to be brought closer to the cogs, and the angled parallelogram pulls the derailer rearwards, so to move the upper jockey wheel backwards, roughly in parallel with the increasing cog size. At least, that's what it
looks like it's trying to achieve; in practice, it probably just doesn't wrap as much chain as ideal, specifically because of this.
-Kurt