Originally Posted by
BTDisciple
Never seen that before. It sounds like its an older version of a modern derailleur with clutch. Setting the return force to high would help chain retention on a single chainring setup but probably not required on a double chainring setup unless you are going cyclo crossing.
The modern clutch is controlling
chain tension only.
This screw adjusts the derailer's return force acting on the
cable.
It is a fault of the design of slanted parallelogram derailers that causes the cage to bounce inward toward the larger cogs in response to bumpy roads taken at speed.
The higher return spring tension combats this, but makes for more shifting effort and friction/stress/elasticity going on with the cable, none of which is good for shifting effort or performance.
Recent 1X rear derailershave returned to non-slanted parallelogram designs, with only the cage pulley offset being used to track the conical/concave profile of the cassette.
This has allowed some lowering of the return spring tension, but the design can't really cope with the chain tension variations that occur when shifting chainrings.
Modern 2x and 3x rear derailers have dealt with the cable tension issue to some degree by using longer-travel, lower-rate return springs, which at least see less drop off of cable tension as the derailer moves toward the smallest cogs. This somewhat negates the issues with bounce and with the high spring tensions that used to occur as the derailer approached the largest cogs, but riding off-road can generate high enough chain sag forces that traditional slanted derailers may really struggle to control.