Originally Posted by
Kimmo
Hm, haven't come across this idea before; I thought it was enough for the teeth to miss each other. Could you say a bit more about the thinking behind it?
You want to avoid any direct interaction between the pulley and sprocket (through the chain). The pulley has to lead the chain onto the sprocket, and off when shifting. If the pulley is too close the chain's stiffness resists the motion of the pulley and increases the force needed to shift.
If you have a clothesline you can experiment derailing the rope. If you try from too far you have to push the rope sideways to get any action, but if you're too close you can't move the rope over the flange. The key is to be as close as possible, without being too close. On a derailleur, you'll find that happy medium to be with about 2 links of chain between them. BTW- it rou struggle through the Shimano adjustment data you'll see a similar guideline for "B" screw adjustment.
Direct contact though the chain must be avoided because it prevents the cage from rotating fully, and can push the RD off the B stop. That risks the entire RD being pushed back, and can cause a breakaway hanger to let go.